JOURNAL PUBLICATIONS

Publication statistics (as of November 2018)
	ISI Web of Science Total citations: 7080 H-index: 36			Google Scholar Total citations: 10140 H-index: 41 i10-index: 87

• Lu, D., Kan, J.J., Fullerton, E.E., and Liu, Z., "Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials," Nature Nanotechnology 9, 48-53 (2014) pdf
• Lu, D., and Liu, Z., "Hyperlenses and metalenses for far-field super-resolution imaging", Nature Communications 3, 1205 (2012) pdf
• Wei, F., and Liu, Z., "Plasmonic structured illumination microscopy", Nano Letters 10, 2531 (2010) pdf
• Rho, J., Ye, Z., Xiong, Y., Yin, X., Liu, Z., Choi, H., Bartal, G., and X. Zhang, "Spherical hyperlens for two-dimensional sub-diffractional imaging at visible frequencies", Nature Communications 1, 143 (2010) pdf
• Yao, J., Liu, Z., Liu, Y., Wang, Y., Sun, C., Bartal, G., and Zhang X., "Negative refraction in bulk metamaterials at visible frequencies", Science 321, 930 (2008). pdf
• Zhang, X., and Liu, Z., "Superlenses to overcome the diffraction limit", Nature Materials 7, 435 (2008). pdf
• Liu, Z., Lee, H., Xiong, Y., Sun, C. and Zhang, X., "Optical hyperlens magnifying sub-diffraction-limited objects", Science 315, 1686 (2007).pdf
• Liu, Z., Durant, S., Lee, H., Pikus, Y., Fang, N., Xiong, Y., Sun, C. and Zhang, X., "Far field optical superlens", Nano Letters 7, 403 (2007). pdf
• Liu, Z., Steele, J. M., Srituravanich, W., Pikus, Y., Sun, C., and Zhang, X., "Focusing surface plasmons with a plasmonic lens", Nano Letters 5, 1726 (2005). pdf
• Liu, Z., Wei, Q., and Zhang, X., "Surface plasmon interference nanolithography", Nano Letters 5, 957 (2005). pdf

117. Yang, Q., Ma, Q., Herum, K.M., Wang, C., Patel, N., Lee, J., Wang, S., Yen, T.M., Wang, J., Tang, H., Lo, Y., Head, B.P., Azam, F., Xu, S., Cauwenberghs, G., McCulloch, A.D., John, S., Liu, Z. and Lal, R.,"Array atomic force microscopy for real-time multiparametric analysis," Proc. Natl. Acad. Sci. DOI: 10.1073/pnas.1813518116 (2019) pdf

     High-resolution multipoint simultaneous structure–function analysis is becoming of great interest in a broad spectrum of fields for deciphering multiscale dynamics, especially in biophysics and materials science. However, current techniques are limited in terms of versatility, resolution, throughput, and biocompatibility. Here, a multifunctional imaging platform is introduced that shows high sensitivity, minimum cross-talk, and a variety of probe-based sensing. This is demonstrated by parallel multiparametric studies in air and liquid, including mechanical wave propagation in a soft polymer film, imaging of live neurons, and cooperative activities of living coupled cardiac muscle cells. As an experimental demonstration of array atomic force microscopy for multiparametric analysis in dynamic systems this work sheds light on the study of emergent properties in wide-ranging fields.

116. Qian, H., Li, S., Chen, C., Hsu, S., Bopp, S.E., Ma, Q., Tao, A.R. and Liu, Z.,"Large optical nonlinearity enabled by coupled metallic quantum wells," Light Sci. Appl. 8, 13 (2019) pdf

     Here, we introduce a metal/dielectric heterostructured platform, i.e., TiN/Al2O3 epitaxial multilayers, to overcome that limitation. This platform has an extremely high χ(2) of approximately 1500 pm/V at NIR frequencies. By combining the aforementioned heterostructure with the large electric field enhancement afforded by a nanostructured metasurface, the power efficiency of second harmonic generation (SHG) achieved 10−4 at an incident pulse intensity of 10 GW/cm2, which is an improvement of several orders of magnitude compared to that of previous demonstrations from nonlinear surfaces at similar frequencies. The proposed quantum-engineered heterostructures enable efficient wave mixing at visible/NIR frequencies into ultracompact nonlinear optical devices.

115. Zeng, Y., Qian, H., Rozin, M.J., Liu, Z. and Tao, A.R.,"Enhanced Second Harmonic Generation in Double‐Resonance Colloidal Metasurfaces," Adv. Funct. Mater. 28 (51), 1803019 (2018) pdf

     A key challenge for optical circuits is the ability to integrate nonlinear optical signal processing components such as optical modulators and frequency mixers at the chip scale. Optical antennas that focus light into nanoscale volumes can be utilized to shrink the footprint and increase the efficiency of these components. Multiresonant antennas that enhance both optical absorption and emission process are recently demonstrated to enable efficient nonlinear frequency conversion at the nanoscale and are promising as structures for second harmonic generation (SHG) and upconversion. Here, the ability of colloidal metasurfaces fabricated by self‐assembly as on‐chip platforms for enhanced SHG is demonstrated. These metasurfaces exhibit high spatial overlap of multiple surface plasmon modes whose frequencies can be independently tuned through appropriate design of colloidal and metasurface geometries. It is demonstrated that these bottom‐up structures rival lithographic nonlinear optical antennas in SHG efficiency, suggesting the potential for these colloidal metasurfaces in integrated on‐chip architectures.

114. Lu, D., Ferrari, L., Kan, J.J., Fullerton, E.E. and Liu, Z.,"Optimization of Nanopatterned Multilayer Hyperbolic Metamaterials for Spontaneous Light Emission Enhancement," Phys. Status Solidi A 215(24), 1800263 (2018) pdf

     Nanopatterned multilayer hyperbolic metamaterials (HMMs) with engineerable material property are promising in enhancing spontaneous emission rates at desired frequencies with improved far‐field radiative power. In this work, the authors study the optimization process for spontaneous emission enhancement by using nanopatterned HMMs. By theoretically investigating the Purcell effect on HMMs compared with traditional metals, the authors choose better material combinations for stronger Purcell enhancement. Different decay channels in the HMM are analyzed against the emitter distance and their wavelengths. Systematic optimization of achieving large emission intensity is demonstrated by comparing performance of nanopatterned HMMs with different geometry parameters. The promise in achieving light emission with both high decay rates and brightness has various potential applications including light‐emitting devices, single‐molecule detection, and surface‐enhanced Raman scattering.

113. Ma, Q., Qian, H., Montoya, S., Bao, W., Ferrari, L., Hu, H., Khan, E., Wang, Y., Fullerton, E.E., Narimanov, E.E., Zhang, X. and Liu, Z.,"Experimental Demonstration of Hyperbolic Metamaterial Assisted Illumination Nanoscopy," ACS Nano 12(11), 11316–11322 (2018) pdf

     An optical metamaterial is capable of manipulating light in nanometer scale that goes beyond what is possible with conventional materials. Taking advantage of this special property, metamaterial- assisted illumination nanoscopy (MAIN) possesses tremendous potential to extend the resolution far beyond conventional structured illumination microscopy. Among the available MAIN designs, hyperstructured illumination that utilizes strong dispersion of a hyperbolic metamaterial (HMM) is one of the most promising and practical approaches, but it is only theoretically studied. In this paper, we experimentally demonstrate the concept of hyperstructured illumination. A ∼80 nm resolution has been achieved in a well-known Ag/SiO2 multilayer HMM system by using a low numerical aperture objective (NA = 0.5), representing a 6-fold resolution enhancement of the diffraction limit. The resolution can be significantly improved by further material optimization.

112. Zhao, C., Zhu, Y., Chen, L., Zhou, S., Su, Y., Ji, X., Chen, A., Gui, X., Tang, Z. and Liu, Z.,"Multi-layer nanoarrays sandwiched by anodized aluminium oxide membranes: an approach to an inexpensive, reproducible, highly sensitive SERS substrate," Nanoscale 10, 16278-16283 (2018) pdf

     A large-scale sub-5 nm nanofabrication technique is developed based on double layer anodized aluminium oxide (AAO) porous membrane masking. This technique also provides a facile route to form multilayer nano-arrays (metal nanoarrays sandwiched by AAO membranes), which is very challenging for other techniques. The preserved AAO layers as the support for the second/third layer of the metal arrays provide a high-refractive index background for the multilayer metal arrays. This background concentrates the local E-field more significantly and results in a much higher Surface-Enhanced Raman Spectroscopy (SERS) signal than single layer metal arrays. This technique may lead to the advent of an inexpensive, reproducible, highly sensitive SERS substrate.

111. Ferrari, L., Smalley, J., Qian, H., Tanaka, A., Lu, D., Dayeh, S.A., Fainman, Y. and Liu, Z.,"Design and analysis of blue InGaN/GaN plasmonic LED for high-speed, high-efficiency optical communications," ACS Photonics 5(9), 3557-3564 (2018) pdf

     We design, fabricate and analyze a nanostructured plasmonic light emitting diode (LED) that simultaneously increases the modulation speed and radiative efficiency, compared to conventional LEDs and unpatterned plasmonic LEDs respectively. Our structure, optimized to ensure its integrability with electrical contacts, couples an InGaN/GaN blue LED with a Ag nanohole grating. Through spatio-temporally resolved photoluminescence measurements, we determine a 40-fold decrease in spontaneous emission lifetime, which sets an upper bound to the direct modulation bandwidth in the GHz regime. Additionally, through careful optimization of the plasmonic nanohole grating, we demonstrate a 10-fold increase in outcoupling efficiency relative to an LED with an unstructured plasmonic film. Our work bridges the plasmonic metamaterial and III-nitride semiconductor communities, laying the groundwork for high-speed, high-efficiency blue plasmonic LEDs for applications in visible light communication and beyond.

110. Bezryadina, A., Zhao, J., Xia, Y., Zhang, X. and Liu, Z.,"High Spatiotemporal Resolution Imaging with Localized Plasmonic Structured Illumination Microscopy," ACS Nano 12(8), 8248-8254 (2018) pdf

     Localized plasmonic structured illumination microscopy (LPSIM) provides multicolor wide-field super-resolution imaging with low phototoxicity and high-speed capability. LPSIM utilizes a nanoscale plasmonic antenna array to provide a series of tunable illumination patterns beyond the traditional diffraction limit, allowing for enhanced resolving powers down to a few tens of nanometers. Here, we demonstrate wide-field LPSIM with 50 nm spatial resolution at video rate speed by imaging microtubule dynamics with low illumination power intensity. The design of the LPSIM system makes it suitable for imaging surface effects of cells and tissues with regular sample preparation protocols. LPSIM can be extended to much higher resolution, representing an excellent technology for live-cell imaging of protein dynamics and interactions.

109. Qian, H., Hsu, S., Gurunatha, K., Riley, C.T., Zhao, J., Lu, D., Tao, A.R. and Liu, Z.,"Efficient light generation from enhanced inelastic electron tunnelling," Nat. Photonics 12, 485–488 (2018) pdf

     Light emission from biased tunnel junctions has recently gained much attention owing to its unique potential to create ultracompact optical sources with terahertz modulation bandwidth. The emission originates from an inelastic electron tunnelling process in which electronic energy is transferred to surface plasmon polaritons and subsequently converted to radiation photons by an optical antenna. Because most of the electrons tunnel elastically, the emission efficiency is typically about 10−5-10−4. Here, we demonstrate efficient light generation from enhanced inelastic tunnelling using nanocrystals assembled into metal–insulator–metal junctions. The colour of the emitted light is determined by the optical antenna and thus can be tuned by the geometry of the junction structures. The efficiency of far-field free-space light generation reaches ~2%, showing an improvement of two orders of magnitude over previous work. This brings on-chip ultrafast and ultra- compact light sources one step closer to reality.

108. Ma, Q. and Liu, Z., "Metamaterial-assisted illumination nanoscopy," Natl. Sci. Rev. 5(2), 141-143 (2018) pdf

     Structured illumination microscopy (SIM) is one of the most versatile super-resolution techniques. Compared with other methods, SIM has shown its advantages in high temporal resolution and low photodamage, but it only has a 2-fold increase in resolution. We review the recent developments of metamaterial assisted illumination nanoscopes (MAIN), which combines near-field patterned illumination generated by metamaterials to extend the resolution of SIM. MAIN addresses three of the most important imaging aspects simultaneously: resolution, frame rate, and phototoxicity opening up tremendous new opportunities for future developments and applications.

107. Lei, Y., Chen, Y., Gu, Y., Wang, C., Huang, Z., Qian, H., Nie, J., Hollett, G., Choi, W., Yu, Y., Kim, N., Wang, C., Zhang, T., Hu, H., Zhang, Y., Li, X., Li, Y., Shi, W., Liu, Z., Sailor, M.J., Dong, L., Lo, Y., Luo, J. and Xu, S., " Controlled Homoepitaxial Growth of Hybrid Perovskites," Adv. Mater. 30(20), 1705992 (2018) pdf

     Organic–inorganic hybrid perovskites have demonstrated tremendous potential for the next‐generation electronic and optoelectronic devices due to their remarkable carrier dynamics. Current studies are focusing on polycrystals, since controlled growth of device compatible single crystals is extremely challenging. Here, the first chemical epitaxial growth of single crystal CH3NH3PbBr3 with controlled locations, morphologies, and orientations, using combined strategies of advanced microfabrication, homoepitaxy, and low temperature solution method is reported. The growth is found to follow a layer‐ by‐layer model. A light emitting diode array, with each CH3NH3PbBr3 crystal as a single pixel, with enhanced quantum efficiencies than its polycrystalline counterparts is demonstrated.

106. Wang, L., Li, S., Zhang, B., Qin, Y., Tian, Z., Fang, Y., Li, Y., Liu, Z. and Mei, Y., "Asymmetrically Curved Hyperbolic Metamaterial Structure with Gradient Thicknesses for Enhanced Directional Spontaneous Emission," ACS Appl. Mater. Interfaces, 10(9), 7704–7708 (2018) pdf

     We demonstrate hyperbolic metamaterials (HMMs) on a curved surface for an efficient outcoupling of nonradiative modes, which lead to an enhanced spontaneous emission. Those high-wavevector plasmonic modes can propagate along the curved structure and emit into the far field, realizing a directional light emission with maximal fluorescent intensity. Detailed simulations disclose a high Purcell factor and a spatial power distribution in the curved HMM, which agrees with the experimental result. Our work presents remarkable enhancing capability in both the Purcell factor and emission intensity, which could suggest a unique structure design in metamaterials for potential application in, e.g., high-speed optical sensing and communications.

105. Lu, D., Qian, H., Wang, K., Shen, H., Wei, F., Jiang, Y., Fullerton, E. E., Yu, P. K. and Liu, Z., "Nanostructuring Multilayer Hyperbolic Metamaterials for Ultrafast and Bright Green InGaN Quantum Wells," Adv. Mater. 30(15), 1706411 (2018) pdf

     Here, nanopatterned Ag-Si multilayer HMMs are utilized for enhancing spontaneous carrier recombination rates in InGaN/GaN QWs. An enhancement of close to 160-fold is achieved in the spontaneous recombination rate across a broadband of working wavelengths accompanied by over tenfold enhancement in the QW peak emission intensity, thanks to the outcoupling of dominating HMM modes. The integration of nanopatterned HMMs with InGaN QWs will lead to ultrafast and bright QW LEDs with a 3 dB modulation bandwidth beyond 100 GHz for applications in high-speed optoelectronic devices, optical wireless communications, and light-fidelity networks.

104. Qin, Y., Fang, Y., Wang, L., Tang, S., Sun, S., Liu, Z. and Mei, Y., "Surface wave resonance and chirality in a tubular cavity with metasurface design," Opt. Commun. 417, 42–45 (2018) pdf

     Optical microcavities with whispering-gallery modes (WGMs) have been indispensable in both photonic researches and applications. Besides, metasurfaces, have attracted much attention recently due to their strong abilities to manipulate electromagnetic waves. Here, combining these two optical elements together, we show a tubular cavity can convert input propagating cylindrical waves into directed localized surface waves (SWs), enabling the circulating like WGMs along the wall surface of the designed tubular cavity. Finite element method (FEM) simulations demonstrate that such near-field WGM shows both large chirality and high local field. This work may stimulate interesting potential applications in e.g. directional emission, sensing, and lasing.

103. Xiao, Y., Qian, H. and Liu, Z., "Nonlinear Metasurface Based on Giant Optical Kerr Response of Gold Quantum Wells," ACS Photonics 5(5), 1654-1659 (2018) pdf

     A nonlinear metasurface is demonstrated numerically based on the recently developed quantum-sized gold film. The active functionality of the metasurface is realized by varying the incident optical power through the ultrahigh Kerr nonlinearity of the quantum-sized gold films. In the low power region, the device acts as a normal reflecting surface, while it becomes a phase grating with most energy in the ±1 diffraction modes when the optical power increases and the nonlinear effect plays a dominating role. Unlike previously demonstrated nonlinear metasurfaces focusing on nonlinear frequency generation, the functionality of our device may be modulated by the power of incident light. As the first nonlinear metasurface that is based on optical Kerr nonlinearity, our design may lead to various applications, such as optical limiters and tunable phase gratings.

102. Tang S., Fang, Y., Zhou, L., Liu, Z. and Mei, Y., "Anomalous scaling laws of hyperbolic metamaterials in a tubular geometry," J. Opt. Soc. Am. B. 35(2), 391-395 (2018) pdf

     Hyperbolic metamaterials (HMM) can be used to control light propagations in emerging meta-devices and thus lead to various functionalities (e.g., hyperlens and cloaking devices). Here we propose a kind of exotic tubular cavity by using multilayered HMM, which contrasts with traditional materials with elliptical dispersion. In such tubular microcavities, the calculations reveal that they have anomalous scaling laws, such as that the higher-order resonance mode oscillates at a longer wavelength and the resonant wavelengths hold their positions with changing the tube wall thickness and diameter. These findings can help the understanding of tubular metamaterials and could inspire interesting optical experiments and metadevices.

101. Zhou, J., Qian, H., Hu, G., Luo, H., Wen, S. and Liu, Z., "Broadband Photonic Spin Hall Meta-Lens," ACS Nano 12(1), 82-88 (2018) pdf

     Meta-lens represents a promising solution for optical communications and information processing owing to its miniaturization capability and desirable optical properties. Here, spin Hall meta-lens is demonstrated to manipulate photonic spin-dependent splitting induced by spin–orbital interaction in transverse and longitudinal directions simultaneously at visible wavelengths, with low dispersion and more than 90% diffraction efficiency. The broadband dielectric spin Hall meta-lens is achieved by integrating two geometric phase lenses with different functionalities into one single dynamic phase lens, which manifests the ultracompact, portable, and polarization-dependent features. The broadband spin Hall meta-lens may find important applications in imaging, sensing, and multifunctional spin photonics devices.

100. Labno, A., Gladden, C., Kim, J., Lu, D., Yin, X., Wang, Y., Liu, Z. and Zhang, X., " Three-dimensional nanoscale imaging by plasmonic Brownian microscopy," Nanophotonics 7(2), 489–495 (2018) pdf

     Three-dimensional (3D) imaging at the nanoscale is a key to understanding of nanomaterials and complex systems. We demonstrate plasmonic Brownian microscopy (PBM) as a way to improve the imaging speed of SPM. Unlike photonic force microscopy where a single trapped particle is used for a serial scanning, PBM utilizes a massive number of plasmonic nanoparticles (NPs) under Brownian diffusion in solution to scan in parallel around the unlabeled sample object. The motion of NPs under an evanescent field is three-dimensionally localized to reconstruct the super-resolution topology of 3D dielectric objects. Our method allows high throughput imaging of complex 3D structures over a large field of view, even with internal structures such as cavities that cannot be accessed by conventional mechanical tips in SPM.

99. Ma, Q., Hu, H., Huang, E. and Liu, Z., "Super-resolution imaging by metamaterial-based compressive spatial-spectral transformation ," Nanoscale 9, 18268-18274 (2017) pdf

    We present a new far-field super-resolution imaging approach called compressive spatial to spectral transformation microscopy (CSSTM). The transformation encodes high-resolution spatial information to a spectrum through illuminating sub-diffraction-limited and wavelength-dependent patterns onto an object. The object is reconstructed from scattering spectrum measurements in the far field. The resolution of the CSSTM is mainly determined by the materials used to perform the spatial-spectral transformation. As an example, we numerically demonstrate sub-15nm resolution by using a practically achievable Ag/SiO2 multilayer hyperbolic metamaterial.

98. Bezryadina, A., Li, J., Zhao, J., Kothambawala, A., Ponsetto, J., Huang, E., Wang, J. and Liu, Z., "Localized plasmonic structured illumination microscopy with an optically trapped microlens ," Nanoscale 9, 14907-14912 (2017) pdf

    Localized plasmonic structured illumination microscopy (LPSIM) is a recently developed super resolution technique that demonstrates immense potential via arrays of localized plasmonic antennas. Microlens microscopy represents another distinct approach for improving resolution by introducing a spherical lens with a large refractive index to boost the effective numerical aperture of the imaging system. In this paper, we bridge together the LPSIM and optically trapped spherical microlenses, for the first time, to demonstrate a new super resolution technique for surface imaging. By trapping and moving polystyrene and TiO2 microspheres with optical tweezers on top of a LPSIM substrate, the new imaging system has achieved a higher NA and improved resolution.

97. Ferrari, L., Smalley, J., Fainman, Y., and Liu, Z., "Hyperbolic metamaterials for dispersion-assisted directional light emission ," Nanoscale 9, 9034-9048 (2017) pdf

    A novel method is presented to outcouple high spatial frequency (large-k) waves from hyperbolic metamaterials (HMMs) without the use of a grating. This approach relies exclusively on dispersion engineering, and enables preferential power extraction from the top or from the side of a HMM. A 6-fold increase in laterally extracted power is predicted for a dipole–HMM system with a Ag/Si ML operating at λ = 530 nm, when metallic filling ratio is changed from an unoptimized to the optimized one. This new design concept supports the cost-effective mass production of high-speed HMM optical transmitters.

96. Ponsetto, J.L., Bezryadina, A., Wei, F., Onishi, K., Shen, H., Huang, E., Ferrari, L., Ma, Q., Zou, Y., and Liu, Z., "Experimental Demonstration of Localized Plasmonic Structured Illumination Microscopy ," ACS Nano 11, 5344−5350 (2017) pdf

    Super-resolution imaging methods such as structured illumination microscopy and others have offered various compromises between resolution, imaging speed, and biocompatibility. Here we experimentally demonstrate a physical mechanism for super-resolution that offers advantages over existing technologies. Using finely structured, resonant, and controllable near-field excitation from localized surface plasmons in a planar nanoantenna array, we achieve wide-field surface imaging with resolution down to 75 nm while maintaining reasonable speed and compatibility with biological specimens.

95. Shen, H., Chen, L., Ferrari, L., Lin, M., Mortensen, N.A., Gwo, S.,and Liu, Z., "Optical Observation of Plasmonic Nonlocal Effects in a 2D Superlattice of Ultrasmall Gold Nanoparticles," Nano Letters 17, 2234-2239 (2017) pdf

    Here we present a nanosystem, a superlattice monolayer formed by sub-10 nm gold nanoparticles. Plasmon resonances are spectrally well-separated from interband transitions, while exhibiting clearly distinguishable blueshifts compared to predictions by the classical local-response model. Our far-field spectroscopy was performed by a standard optical transmission and reflection setup, and the results agreed excellently with the hydrodynamic nonlocal model, opening a simple and widely accessible way for addressing quantum effects in nanoplasmonic systems.

94. Riley, C. T., Smalley, J. S., Brodie, J. R., Fainman, Y., Sirbuly, D. J., & Liu, Z., "Near-perfect broadband absorption from hyperbolic metamaterial nanoparticles," Proc. Natl. Acad. Sci. 114(6), 1264-1268 (2017) pdf

    Here, a class of materials, transferrable hyperbolic metamaterial particles (THMMP), is introduced. When closely packed, these materials show broadband, selective, omnidirectional, perfect absorption. This is demonstrated with nanotubes made on a silicon substrate that exhibit near-perfect absorption at telecommunication wavelengths even after being transferred to a mechanically flexible, visibly transparent polymer.

93. Huang, E., Ma, Q. and Liu, Z., "Etalon Array Reconstructive Spectrometry," Scientific Reports 7, 40693 (2017) pdf

    Here, we demonstrate a novel method for compact spectrometry that uses an array of etalons to perform spectral encoding, and uses a reconstruction algorithm to recover the incident spectrum. This spectrometer has the unique capability for both high resolution and a large working bandwidth without sacrificing sensitivity, and we anticipate that its simplicity makes it an excellent candidate whenever a compact, robust, and flexible spectrometry solution is needed.

92. Smalley, J.S.T., Vallini, F., Montoya, S.A., Ferrari, L., Shahin, S., Riley, C.T., Kanté, B., Fullerton, E.E., Liu, Z. and Fainman, Y., "Luminescent hyperbolic metasurfaces," Nature Commun. 8, 13793 (2017) pdf

    Using an unconventional multilayer architecture, we demonstrate luminescent hyperbolic metasurfaces, wherein distributed semiconducting quantum wells display extreme absorption and emission polarization anisotropy. Through normally incident micro-photoluminescence measurements, we observe absorption anisotropies greater than a factor of 10 and degree-of-linear polarization of emission >0.9. We observe the modification of emission spectra and, by incorporating wavelength-scale gratings, show a controll ed reduction of polarization anisotropy. Finally, we experimentally demonstrate >350% emission intensity enhancement relative to the bare semiconducting quantum wells.

91. Han, D.Y., Safigholi, H., Soliman, A., Ravi, A., Leung, E., Scanderbeg, D.J., Liu, Z., Owrangi, A. and Song, W.Y. "Direction Modulated Brachytherapy for Treatment of Cervical Cancer. II: Comparative Planning Study With Intracavitary and Intracavitary–Interstitial Techniques," International Journal of Radiation Oncology• Biology• Physics, 96(2), 440-448 (2016) pdf

    A novel DMBT-concept tandem applicator that enables enhanced capacity to sculpt the 3D dose distributions in HDR brachytherapy was proposed in 2014. Subsequently, a comprehensive comparative planning study was performed on 45 cervical cancer patients, enrolled in the EMBRACE trial, treated with various intracavitary and intracavitary–interstitial techniques. All cases were replanned with an in-house–developed inverse optimization code. The proposed applicator was found to enhance the plan quality across various clinical scenarios.

90. Kwon, S., Lu, D., Sun, Z., Xiang, J., and Liu, Z., "Highly stretchable, printable nanowire array optical polarizers," Nanoscale 8, 15850-15856 (2016) pdf

    Here, we demonstrate fabrication of highly flexible and stretchable wire grid polarizers (WGPs) by printing bottom-up grown Ge or Ge/Si core/shell nanowires (NWs) on device substrates in a highly dense and aligned fashion. The maximum contrast ratio of 104 between transverse electric (TE) and transverse magnetic (TM) fields and above 99% (maximum 99.7%) of light blocking efficiency across the visible spectrum range are achieved. Further systematic analyses are performed both in experimental and numerical models to reveal the correspondence between physical factors (coverage ratio of NW arrays and diameter) and polarization efficiency.

89. Qian, H., Xiao, Y., and Liu, Z., "Giant Kerr response of ultrathin gold films from quantum size effect," Nature Commun. 7, 13153 (2016) pdf

    Here we study the optical nonlinear properties of a nanometre scale gold quantum well by using the z-scan method and nonlinear spectrum broadening technique. The quantum size effect results in a giant optical Kerr susceptibility, which is four orders of magnitude higher than the intrinsic value of bulk gold and several orders larger than traditional nonlinear media. Such high nonlinearity enables efficient nonlinear interaction within a microscopic footprint, making quantum metallic films a promising candidate for integrated nonlinear optical applications.

88. Ma, Q., Khademhosseinieh, B., Huang, E., Qian, H., Bakowski, M.A., Troemel, E.R. and Liu, Z., " Three-dimensional fluorescent microscopy via simultaneous illumination and detection at multiple planes ," Scientific Reports 6, 31445 (2016) pdf

    The conventional optical microscope is an inherently two-dimensional (2D) imaging tool. In this paper, we present a 3D optical microscopy method based upon simultaneously illuminating and detecting multiple focal planes. This is implemented by adding two diffractive optical elements to modify the illumination and detection optics. We demonstrate that the image quality of this technique is comparable to conventional light sheet fluorescent microscopy with the advantage of the simultaneous imaging of multiple axial planes and reduced number of scans required to image the whole sample volume.

87. Tserkezis, C., Maack, J.R., Liu, Z., Wubs, M. and Mortensen, N.A., "Robustness of the far-field response of nonlocal plasmonic ensembles," Scientific Reports 6, 28441 (2016) pdf

    For a normal distribution of free-electron nanoparticles, and within the simple nonlocal hydrodynamic Drude model, both the nonlocal blueshift and the plasmon linewidth are shown to be considerably affected by ensemble averaging. Size-variance effects tend however to conceal nonlocality to a lesser extent when the homogeneous size-dependent broadening of individual nanoparticles is taken into account, either through a local size-dependent damping model or through the Generalized Nonlocal Optical Response theory. The role of ensemble averaging is further explored in realistic distributions of isolated or weakly-interacting noble-metal nanoparticles, as encountered in experiments, while an analytical expression to evaluate the importance of inhomogeneous broadening through measurable quantities is developed.

86. Kim, T.K., VanSaders, B., Caldwell, E., Shin, s., Liu, Z., Jin, S. and Chen, R., "Copper-alloyed spinel black oxides and tandem-structured solar absorbing layers for high-temperature concentrating solar power systems," Solar Energy 132, 257-266 (2016) pdf

    In this work, we have developed tandem-structured solar absorbing layers with CuFeMnO4 and CuCr2O4 black oxide nanoparticles (NPs). These tandem structures exhibited a remarkably high solar-to-thermal conversion efficiency, or figure of merit (FOM), of 0.903, under the condition of 750C operating temperature and a solar concentration ratio of 1000. More importantly, the coating showed unprecedented durability, as demonstrated from long-term isothermal annealing at 750 C in air as well as rapid thermal cycling between room temperature and 750 C.

85. Huang, E., Ma, Q., and Liu, Z., "Ultrafast Imaging using Spectral Resonance Modulation," Scientific Reports 6, 1-7 (2016) pdf

    One potential way to improve the imaging speed of CCD cameras is with compressive sensing (CS), a technique that allows for a reduction in the number of measurements needed to record an image. However, most CS imaging methods require spatial light modulators (SLMs), which are subject to mechanical speed limitations. Here, we demonstrate an etalon array based SLM without any moving elements that is unconstrained by either mechanical or electronic speed limitations. This novel spectral resonance modulator (SRM) shows great potential in an ultrafast compressive single pixel camera.

84. Marin, B.C., Hsu, S., Li, C., Lo, A., Zwissler, D.W., Liu, Z., and Tao, A., "Plasmon-Enhanced Two-Photon Absorption in Photoluminescent Semiconductor Nanocrystals," ACS Photonics 3, 526-531 (2016) pdf

    In this work, we demonstrate the two-photon fluorescence of covellite-phase copper sulfide nanodisks and investigate the role of the surface plasmon resonance on emission. Using selenium doping, we blue-shift the plasmon resonance toward the two-photon absorption edge. We observed a 3-fold enhancement of emission in these samples and report two-photon action cross sections that are an order of magnitude greater than conventional fluorophores. These nanomaterials offer a novel “all-in-one” platform for engineering plasmon-exciton coupling in the absence of a physical or chemical interface.

83. Riley, C.T., Smalley, J.S.T., Post, K.W., Basov D.N., Fainman, Y., Wang, D., Liu, Z., and Sirbuly, D.J., "High-Quality, Ultraconformal Aluminum-Doped Zinc Oxide Nanoplasmonic and Hyperbolic Metamaterials," Small 12(7), 892-901 (2016) pdf

    Aluminum-doped zinc oxide (AZO) is a tunable low-loss plasmonic material capable of supporting dopant concentrations high enough to operate at telecommunication wavelengths. Here a simple procedure is devised to tune the optical constants of AZO and enable plasmonic activity at 1550 nm with low loss. The high-quality AZO is then used to make a layered AZO/ZnO structure that displays negative refraction in the telecommunication wavelength region due to hyperbolic dispersion. Finally, a novel synthetic scheme is demonstrated to create AZO embedded nanowires in ZnO, which also exhibits hyperbolic dispersion.

82. Tang, S., Fang, Y., Liu, Z., Zhou, L., and Mei, Y., "Tubular optical microcavities of indefinite medium for sensitive liquid refractometers," Lab on a Chip 1, 182-187 (2016) pdf

    Based on Mie scattering theory, we propose a tubular metamaterial device for liquid sensing, which utilizes anisotropic metamaterials with hyperbolic dispersion called indefinite media (IM). Compared with traditional dielectric media (DM), the IM tubular cavity exhibits a higher sensitivity (S), which is close to that of a metal tubular cavity. However, compared with metal media, such an IM cavity can achieve higher quality (Q) factors similar to the DM tubular cavity. Therefore, the IM tubular cavity can offer the highest figures of merit for the sensing performance among the three types of materials.

81. Qian, H., Xiao, Y., Lepage, D., Chen, L., and Liu, Z., "Quantum Electrostatic Model for Optical Properties of Nanoscale Gold Films," Nanophotonics 4, 413-418 (2015) pdf

    The optical properties of thin gold films with thickness varying from 2.5 nm to 30 nm are investigated. Due to the quantum size effect, the optical constants of the thin gold film deviate from the Drude model for bulk material as film thickness decreases, especially around 2.5 nm, where the electron energy level becomes discrete. A theory based on the self-consistent solution of the Schrödinger equation and the Poisson equation is proposed and its pre- dictions agree well with experimental results.

80. Li, W., Wang, S., Hu, M., He, S., Ge, P., Wang, J., Guo, Y.Y., and Liu, Z., "Enhancement of electroluminescence from embedded Si quantum dots/ SiO2multilayers film by localized- surface-plasmon and surface roughening," Scientific Reports 5, 11881 (2015) pdf

    In this paper, we prepared a novel structure to enhance the electroluminescence intensity from Si quantum dots/SiO2multilayers. An amorphous Si/SiO2 multilayer film was fabricated by plasma- enhanced chemical vapor deposition on a Pt nanoparticle (NP)-coated Si nanopillar array substrate. By thermal annealing, an embedded Si quantum dot (QDs)/SiO2 multilayer film was obtained. The result shows that electroluminescence intensity was significantly enhanced. And, the turn-on voltage of the luminescent device was reduced to 3V.

79. Kargar, A., Cheung, J.S., Liu, C., Kim, T.K., Riley, C., Shen, S., Liu, Z.,Sirbuly, D.J., Wang, D., and Jin, S., "NiOx-Fe2O3-coated p-Si photocathodes for enhanced solar water splitting in neutral pH water," Nanoscale 7, 4900-4905 (2015) pdf

    We report successful growth of a uniform and scalable nanocom- posite film of Fe2O3 nanorods (NRs) and NiOx nanoparticles (NPs), their properties and application for enhanced solar water reduction in neutral pH water on the surface of p-Si photocathodes.

78. Wan, W., Ponsetto, J.L., and Liu, Z., "Numerical study of hyperlenses for three-dimensional imaging and lithography," Optics Express 23, 18502-18510 (2015) pdf

    In this letter, we numerically demonstrate a hyperlens with unprecedented radial-resolution at 5 nm scale for both imaging and lithography applications. Both processes are shown to have accuracy that surpasses the Abbe diffraction limit in the radial direction, which has potential applications for 3D imaging and lithography. Design optimization is discussed with regards to several important hyperlens parameters.

77. Shen, H., Lu, D., VanSaders, B., Kan, J.J., Xu, H., Fullerton, E., and Liu, Z., "Anomalously Weak Scattering in Metal-Semiconductor Multilayer Hyperbolic Metamaterials," Physical Review X 5, 021021 (2015) pdf

    In contrast to strong plasmonic scattering from metal particles or structures in metal films, we show that patterns of arbitrary shape fabricated out of multilayer hyperbolic metamaterials become invisible within a chosen band of optical frequencies. This is due to anomalously weak scattering when the in-plane permittivity of the multilayer hyperbolic metamaterials is tuned to match with the surrounding medium. This anomalously weak scattering is insensitive to pattern sizes, shapes, and incident angles, and has potential applications in scattering cross-section engineering, optical encryption, low-observable conductive probes, and optoelectric devices.

76. Fernandez-Dominguez, A.I., Liu, Z., and Pendry, J.B., "Coherent Four-Fold Super-Resolution Imaging with Composite Photonic−Plasmonic Structured Illumination," ACS Photonics (2015) pdf

    We present a far-field super-resolution imaging scheme based on coherent scattering under a composite photonic−plasmonic structured illumination. The 4-fold super-resolution power of the scheme, able to resolve 60 nm feature sizes at the operating wavelength, is demonstrated against both Abbe’s (imaging a single object) and Rayleigh’s (imaging two closely spaced objects) criteria.

75. Moon, J., Kim, T.K., VanSaders, B., Choi, C., Liu, Z., Jin, S., and Chen, R., "Black oxide nanoparticles as durable solar absorbing material for high-temperature concentrating solar power system," Solar Energy Materials and Solar Cells 134, 417-424 (2015) pdf

    In this work, a black oxide material, made of cobalt oxide nanoparticles, is synthesized and utilized as a high-temperature solar absorbing material. After the surface modification of cobalt oxide coating, we achieved a high thermal efficiency of 88.2%. More importantly, the coating shows no degradation after 1000-h annealing at 750 1C in air, while the existing commercial light absorbing coating was reported to degrade by long- term exposure at high temperature. Our findings suggest that the materials and processes developed here are promising for solar absorbing coating for future high-temperature CSP systems.

74. Ferrari, L., Wu, C., Lepage, D., Zhang, X., and Liu, Z., "Hyperbolic metamaterials and their applications," Progress in Quantum Electronics 40, 1-40 (2015) pdf

    This review aims at providing a comprehensive and updated picture of the field of hyperbolic metamaterials, from the foundations to the most recent progresses and future perspectives. The topics discussed embrace theoretical aspects, practical realization and key challenges for applications such as imaging, spontaneous emission engineering, thermal, active and tunable hyperbolic media.

73. Kim, T.K., VanSaders, B., Moon, J., Kim, T., Liu, C.-H., Khamwannah, J., Chun, D., Choi, D., Kargar, A., Chen, R., Liu, Z., and Jin, S., "Tandem structured spectrally selective coating layer of copper oxide nanowires combined with cobalt oxide nanoparticles," Nano Energy 11, 274-259 (2015) pdf

    We report novel tandem structures combing two different materials with complementary optical properties and microstructures: copper oxide (CuO) nanowires (NWs) and cobalt oxide (Co3O4) nanoparticles (NPs). Tandem structures of spectrally selective coating (SSC) layer are built with three different methods: spray-coating, dip-coating of cobalt oxide NPs into copper oxide NWs forest, and transferring of copper oxide NWs layer onto cobalt oxide NPs layer. Our results demonstrate the efficacy of using novel tandem structures for enhanced light absorption of solar spectrum, which will find broad applications in solar energy conversion.

72. Wei, F., Lu, D., Aguinaldo, R., Ma, Y., Sinha, S.K., and Liu, Z., "Localized surface plasmon assisted contrast microscopy for ultrathin transparent specimens," Applied Physics Letters 105, 163102 (2014) pdf

    We demonstrate a high contrast imaging technique, termed localized surface plasmon assisted contrast microscopy, by combining localized surface plasmon resonances (LSPR) and dark-field microscopy technique. Due to the sensitive response of LSPR to the refractive index of the surrounding media, this technique is capable of converting a small refractive index difference to a change in scattering intensity, resulting in a high-contrast, diffraction limited image of a thin unstained specimen with small, gradual refractive-index variation.

71. Kim, T.K., Moon, J., VanSaders, B., Chun, D., Gardner, C.J., Jung, J.-Y., Wang, G., Chen, R., Liu, Z., Qiao, Y., and Jin, S., "Si boride-coated Si nanoparticles with improved thermal oxidation resistance," Nano Energy 9, 32-40 (2014) pdf

    A new fabrication technique is devised to synthesize conformal Si core–shell nanoparticles (NPs) with Si boride nanoshell by reacting the surface atoms of Si NPs (50–200 nm diameter) with a molten salt of sodium borohydride. TGA analysis reveals that the core–shell structured Si–Si boride NPs exhibit a remarkably improved resistance to thermal oxidation by a factor of 4.6 at 750 C and at by a factor of 3.5 at 850 C compared to bare Si. These surface-protected, oxidation-resistant semiconductor materials and their novel fabrica- tion methods exhibit further broad applicability of boride nanolayers which can be used for high temperature applications such as solar thermal power generation.

70. Wei, F., Lu, D., Shen, H., Ponsetto, J.L., Huang, E., and Liu, Z., "Wide Field Super-Resolution Surface Imaging through Plasmonic Structured Illumination Microscopy," Nano Letters 14, 4634-4639 (2014) pdf

    We experimentally demonstrate a wide field surface plasmon (SP) assisted super-resolution imaging technique, plasmonic structured illumination microscopy (PSIM), by combining tunable SP interference (SPI) with structured illumination microscopy (SIM). By replacing the laser interference fringes in conventional SIM with SPI patterns, PSIM exhibits greatly enhanced resolving power thanks to the unique properties of SP waves. This PSIM technique is a wide field, surface super-resolution imaging technique with potential applications in the field of high-speed biomedical imaging.

69. Moon, J., Lu, D., VanSaders, B., Kim, T.K., Kong, S.D., Jin, S., Chen, R., and Liu, Z., "High performance multi-scaled nanostructured spectrally selective coating for concentrating solar power," Nano Energy 8, 238-246 (2014) pdf

    Spectrally selective coatings (SSCs) are a critical component that enables high-temperature and high-efficiency operation of concentrating solar power (CSP) systems. In this Letter, we describe a novel design for a high-performance SSC based on multi-scaled nanostructures. Optimal design of the new structure for high optical performance of the SSC is predicted by the effective medium theory. To demonstrate the feasibility of the design, we fabricate the SSCs using fractal nanostructures with characteristic sizes ranging from ~10 nm to ~10 μm. Optical measurements on these structures show unprecedentedly high performance with 90–95% solar absorptivity and <30% infrared emissivity near the peak of 500 C black body radiation.

68. Wan, W., Zheng, W., Chen, Y., and Liu, Z., "From Fano-like interference to superscattering with a single metallic nanodisk," Nanoscale 6, 9093-9102 (2014) pdf

    By numerical simulation, we design a subwavelength nanodisk as a simple candidate to achieve superscattering. By changing the height and diameter of the nanodisk, we show high tunability of the mode interaction and evolution of the resulting spectral features from Fano-like resonance to superscattering. A model of two-driven coupled oscillators is proposed to quantitatively analyze the spectral evolution.We show that superscattering doubles the near-field intensity, potentially enhancing the signal 16 times for SERS and 4 times for SEIRS, and doubles the far-field intensity and decreases the peak linewidth, improving the figure of merit for plasmonic refractometric sensing.

67. Shen, H., Huang, E., Das, T., Xu, H., Ellisman, M., and Liu, Z., "TIRF microscopy with ultra-short penetration depth," Opt. Express 22, 10728-10734 (2014) pdf

    Total internal reflection fluorescence microscopy (TIRF), in both commercial and custom-built configurations, is widely used for high signal-noise ratio imaging. The imaging depth of traditional TIRF is sensitive to the incident angle of the laser, and normally limited to around 100 nm. In our paper, using a high refractive index material and the evanescent waves of various waveguide modes, we propose a compact and tunable ultra-short decay length TIRF system, which can reach decay lengths as short as 19 nm, and demonstrate its application for imaging fluorescent dye-labeled F-actin in HeLa cells.

66. Dechaumphai, E., Lu, D., Kan, J.J., Moon, J., Fullerton, E.E., Liu, Z. and Chen, R., "Ultralow Thermal Conductivity of Multilayers with Highly Dissimilar Debye Temperatures," Nano Letters 14(5), 2448-2455 (2014) pdf

    In this work, we experimentally demonstrated an ultralow thermal conductivity of 0.33 ± 0.04 W m−1 K−1 at room temperature in MLs made of Au and Si with a high interfacial density of ∼0.2 interface nm−1. The measured thermal conductivity is significantly lower than the amorphous limit of either Si or Au and is also much lower than previously measured MLs with a similar interfacial density. With a Debye temperature ratio of ∼3.9 for Au and Si, the Au/Si MLs represent the highest mismatched system in inorganic MLs measured to date. In addition, we explore the prior theoretical prediction that full phonon dispersion could better model the interfacial thermal resistance involving materials with low Debye temperatures.

65. Ponsetto, J.L., Wei, F., and Liu, Z., "Localized plasmon assisted structured illumination microscopy for wide-field high-speed dispersion- independent super resolution imaging," Nanoscale 6, 5807-5812 (2014) pdf (correction)

    A new super resolution imaging method, i.e. Localized Plasmon assisted Structured Illumination Microscopy (LPSIM), is proposed. LPSIM uses an array of localized plasmonic antennas to provide dynamically tunable near-field excitations resulting in finely structured illumination patterns, independent of any propagating surface plasmon dispersion limitations. The illumination pattern feature sizes are limited only by the antenna geometry, and a far-field image resolved far beyond the diffraction limit is obtained. This approach maintains a wide field of view and the capacity for a high frame-rate. The recovered images for various classes of objects are presented, demonstrating a significant resolution improvement over existing methods.

64. Ferrari, L., Lu, D., Lepage, D., and Liu, Z., "Enhanced spontaneous emission inside hyperbolic metamaterials," Opt. Express 22, 4301-4306 (2014) pdf

    Hyperbolic metamaterials can enhance spontaneous emission, but the radiation-matter coupling is not optimized if the light source is placed outside such media. We demonstrate a 3-fold improvement of the Purcell factor over its outer value and a significant enlargement in bandwidth by including the emitter within a Si/Ag periodic multilayer metamaterial. To extract the plasmonic modes of the structure into the far field we implement two types of 1D grating with triangular and rectangular profile, obtaining a 10-fold radiative enhancement at visible frequencies.

63. Lu, D., Kan, J.J., Fullerton, E.E., and Liu, Z., "Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials," Nat. Nanotech. 9, 48-53 (2014) pdf

    Plasmonic nanostructures have been extensively used to manipulate the spontaneous light emission rate of molecules and their radiative efficiency. Here, we show that by nanopatterning a hyperbolic metamaterial made of Ag and Si multilayers, the spontaneous emission rate of rhodamine dye molecules is enhanced 76-fold at tunable frequencies and the emission intensity of the dye increases by ∼80-fold compared with the same hyperbolic metamaterial without nanostructuring. We explain these results using a dynamic Lorentzian model in the time domain.

62. Park, J.C., Kim, J.S., Park, S.H., Liu, Z., Song, B. and Song , W.Y., "Motion-map constrained image reconstruction (MCIR): Application to four-dimensional cone-beam computed tomography," Med. Phys. 40, 121710 (2013) pdf

    The purpose of this study is to develop a novel 4DCBCT reconstruction algorithm framework called motion-map constrained image reconstruction (MCIR), that allows reconstruction of high quality and high phase resolution 4DCBCT images with no more than the imaging dose as well as projections used in a standard free breathing 3DCBCT (FB-3DCBCT) scan

61. Kargar, A., Sun, K., Kim, J.S., Lu, D., Jing, Y., Liu, Z., Pan, X. and Wang , D., "Three-dimensional ZnO/Si broom-like nanowire heterostructures as photoelectrochemical anodes for solar energy conversion," Phys. Status Solidi A, 1–8 (2013) pdf

    We report a low-cost solution fabrication of three-dimensional (3D) ZnO/Si broom-like nanowire (NW, “nanobroom”) heterostructures, consisting of Si NW “backbones” and ZnO NW “stalls”, and their application as photoelectrochemical anodes for solar water splitting and energy conversion. The nanobroom morphology and atomic structure are characterized using the scanning, transmission, and scanning transmission electron microscopies. This study reveals the promise of the use of simply fabricated and low-cost 3D heterostructured NW photoelectrodes for clean solar energy harvesting and conversion.

60. Wan, W., Ma, C. and Liu, Z., "Control the dispersive properties of compound plasmonic lenses," Opt. Commun. 291, 390 (2013) pdf

    We propose novel compound plasmonic lenses, which consist of metal–insulator–metal waveguides (MIMWGs) and phase zone plates (PZPs), with controllable dispersive properties. Numerical simulation results show that this new type of compound plasmonic lens is capable of not only minimizing the chromatic aberration but also rearranging the order of focal positions for incident light at visible frequencies. One chosen wavelength can be designed to have the shortest or the longest focal length. This controllable dispersive light-focusing behavior opens up new applications in the fields of hyper spectral imaging, binary optics and holographic devices.

59. Lu, D., and Liu, Z., "Hyperlenses and metalenses for far-field super-resolution imaging," Nature Commun. 3, 1205 (2012) pdf

    The resolution of conventional optical lens systems is always hampered by the diffraction limit. Recent developments in artificial metamaterials provide new avenues to build hyperlenses and metalenses that are able to image beyond the diffraction limit. Hyperlenses project super-resolution information to the far field through a magnification mechanism, whereas metalenses not only super-resolve subwavelength details but also enable optical Fourier transforms. Recently, there have been numerous designs for hyperlenses and metalenses, bringing fresh theoretical and experimental advances, though future directions and challenges remain to be overcome.

58. Park, J. C., Park, S. H, Kim, J. S., Yoon, S. M., Song, S. Y., Liu, Z. Song, B., Kauweloa, K., Webster, M. J., Sandhu, A., Mell, L. K., Jiang, S. B., Mundt, A. J., and Song, W. Y. "Liver mothion during cone beam computed tomography guided stereotactic body radiation therapy," Med. Phys. 39, 6431 (2012) pdf

    The purpose of this study was to investigate motion characteristics based on fiducial markers tracked with the x-ray projections of the CBCT scans, taken immediately prior to the treatments.This study analyzed the liver motion characteristics of 20 patients undergoing SBRT. A large variation in motion was observed, interfractionally and intrafractionally, and that as the distance between the markers increased, the difference in the absolute range of motion also increased. This suggests that marker(s) in closest proximity to the target be used.

57. Wei, F., O, Y. W., Li, G., Cheah, K. W., and Liu, Z., "Organic light-emitting-diode-based plasmonic dark-field microscopy," Opt. Lett. 37, 4359 (2012) pdf

    We propose and demonstrate a compact, alignment-free dark-field microscopy technique, termed as organic light-emitting-diode-based plasmonic dark-field microscopy. Experimental results show that it is capable of forming dark-field images of the specimens utilizing a highly integrated chip-scale plasmonic condenser. This technique has high z resolution and high imaging contrast and is suitable for the dynamics study near the contact regions of living cells and the substrate.

56. Sun, K., Jing, Y., Li, C., Zhang, X., Aguinaldo, R., Kargar, A., Madsen, K., Banu, K., Zhou, Y., Bando, Y., Liu, Z., and Wang, D., "3D branched nanowire heterojunction photoelectrodes for high-efficiency solar water splitting and H2 generation," Nanoscale 4, 1515 (2012) pdf

    We report the fabrication of a 3D branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. We demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature.

55. Park, J. C., Song, B., Kim, J. S., Park, S. H., Liu, Z. Suh, T. S., and Song, W. Y. , "Fast compressed sensing-based CBCT reconstruction using Barzilai-Borwein formulation for application to on-line IGRT," Med. Phys. 39, 1207 (2012) pdf

    In this paper, we proposed a novel, fast, low-dose CBCT reconstruction algorithm using the Barzilai-Borwein step-size calculation. A clinically viable head-and-neck image can be obtained within ~34–78 s while simultaneously cutting the dose by approximately 67%. This makes our GP-BB algorithm potentially useful in an on-line image-guided radiation therapy (IGRT).

54. Ma, C., and Liu, Z., "Breaking the imaging symmetry in negative refraction lenses", Opt. Express 20 , 2581 (2012) pdf

    Optical lenses are pervasive in various areas of sciences and technologies. It is well known that conventional lenses have symmetrical imaging properties along forward and backward directions. In this letter, we show that hyperbolic plasmonic metamaterial based negative refraction lenses perform as either converging lenses or diverging lenses depending on the illumination directions. New imaging equations and properties that are different from those of all the existing optical lenses are also presented. These new imaging properties, including symmetry breaking as well as the super resolving power, significantly expand the horizon of imaging optics and optical system design.

53. Ma, C., Escobar, M. A. and Liu, Z., "Extraordinary light focusing and Fourier transform properties of gradient-index metalenses", Phys. Rev. B 84, 195142 (2011) pdf

    We propose and demonstrate a new type of metalenses that are phase compensated by gradient index (GRIN) or inhomogeneous permittivity metamaterials. Both elliptically and hyperbolically dispersive GRIN metalenses for both internal and external focusing are studied. The requirements for the GRIN metalenses and the light focusing characteristics are analyzed and numerically verified. The GRIN metalenses can achieve super resolution and have ordinary or extraordinary Fourier transform functions, thus enabling exotic applications.

52. Feng, L., Liu, Z., and Fainman, Y., "Direct observation of plasmonic index ellipsoids on a deep-subwavelength metallic grating", Appl. Opt. 50, G1 (2011) pdf

    We constructed a metallic grating on a deep-subwavelength scale and tested its plasmonic features in visible frequencies. The deep-subwavelength metallic grating effectively acts as an anisotropic homogeneous uniaxial form-irefringent metal, exhibiting different optical responses for polarizations along different optical axes. Therefore, this form-birefringent metal supports anisotropic surface plasmon polaritons that are characterized by directly imaging the generated plasmonic index ellipsoids in reciprocal space. The observed plasmonic index ellipsoids also show a rainbow effect, where different colors are dispersively distributed in reciprocal space.

51. Yao, J., Wang, Y., Tsai, K-Ti, Liu, Z., Yin, X., Bartal, G., Stacy, A. M., Wang, Y. L., and Zhang, X., "Design, fabrication and characterization of indefinite metamaterials of nanowires", Phil. Trans. R. Soc. A 369, 3434 (2011) pdf

    Indefinite optical properties, which are typically characterized by hyperbolic dispersion relations, have not been observed in naturally occurring materials, but can be realized through a metamaterial approach. We present here the design, fabrication and characterization of nanowire metamaterials with indefinite permittivity, in which allangle negative refraction of light is observed. The bottom-up fabrication technique, which applies electrochemical plating of nanowires in porous alumina template, is developed and demonstrated in achieving uniform hyperbolic optical properties at a large scale.

50. Park, J. C., Park, S. H., Kim, J. S., Han, Y., Cho, M. K., Liu, Z., Jiang, S. B., Song, B., Song, W. Y., "Ultra-fast digital tomosynthesis reconstruction using general-purpose GPU programming for image-guided radiation therapy," Technology in Cancer Research & Treatment 10, 295 (2011) pdf

    The purpose of this work is to demonstrate an ultra-fast reconstruction technique for digital tomosynthesis (DTS) imaging based on the algorithm proposed by Feldkamp, Davis, and Kress (FDK) using standard general-purpose graphics processing unit (GPGPU) programming interface. To this end, the FDK-based DTS algorithm was programmed “in-house” with C language with utilization of 1) GPU and 2) central processing unit (CPU) cards.

49. Feng, L., Mizrahi, A., Zamek, S., Liu, Z., Lomakin, V., and Fainman, Y., "Metamaterials for enhanced polarization conversion in plasmonic excitation," ACS Nano. 5, 5100 (2011) pdf

    Surface plasmons efficient excitation is expected to be strongly constrained to transverse magnetic (TM) polarized incidence, as demonstrated so far, due to its intrinsic TM polarization. We report a designer plasmonic metamaterial that is engineered in a deep subwavelength scale in visible optical frequencies to overcome this fundamental limitation, and allows transverse electric (TE) polarized incidence to be strongly coupled to surface plasmons. The experimental verification, which is consistent with the analytical and numerical models, demonstrates this enhanced TE-to-plasmon coupling with efficiency close to 100%, which is far from what is possible through naturally available materials. This discovery will help to efficiently utilize the energy fallen into TE polarization and drastically increase overall efficiency of plasmonic devices.

48. Lu, D., Kan, J., Fullerton, E. E. and Liu, Z, "Tunable surface plasmon polaritons in Ag composite films by adding dielectrics or semiconductors," Appl. Phys. Lett. 9, 243114 (2011) pdf

    We demonstrate that the surface plasmon polariton (SPP) properties of the silver composite films can be tuned by modest additions of silicon oxide or silicon. The dispersion relations deviate from that of pure silver films, and exhibit the capability to shift the surface plasmon frequency and provide larger SPP wave vectors at longer wavelengths. The effective permittivities are modeled phenomenologically by taking into account both filling ratios and size effects. These types of tunable composite films have various useful applications in areas, such as superlens imaging, SPP based sensing, enhanced photoluminescence, and SPP based photovoltatics.

47. Ma, C., and Liu, Z, "Designing super-resolution metalenses by the combination of metamaterials and nanoscale plasmonic waveguide couplers," J. Nanophotonics. 5, 051604 (2011) pdf

    We recently demonstrated a phase compensated metalens that cannot only achieve super-resolution, but also possesses the Fourier transform capability. The metalens consists of a metamaterial slab and a plasmonic waveguide coupler (PWC). We have now ascertained the requirements for the metamaterial and the detailed design principles for the PWCs. Simulations of metalenses with a new type of PWC geometry have confirmed that the new metalenses also possess super-resolution and the Fourier transform function. The hyperbolic metalens shows an anomalous focus shifting behavior, which may be used to design exotic optical systems with new functionalities.

46. Park, J. C., Park, H., Kim, J. H., Yoon, S.M., Kim, S. S., Kim, J. S., Liu, Z., Watkins, T., and Song, W. Y., "Four-dimensional cone-beam computed tomography and digital tomosynthesis reconstructions using respiratory signals extracted from transcutaneously inserted metal markers for liver SBRT," Med. Phys . 38, 1038 (2011) pdf

    Respiration-induced intrafraction target motion is a concern in liver cancer radiotherapy, especially in stereotactic body radiotherapy (SBRT), and therefore, verification of its motion is necessary. An effective means to localize the liver cancer is to insert metal fiducial markers to or near the tumor with simultaneous imaging using cone-beam computed tomography (CBCT). Utilizing the fiducial markers, the authors have demonstrated a method to generate breath-induced motion signal of liver for reconstructing 4D digital tomosynthesis (4DDTS) and 4DCBCT images based on phasewise and/or amplitudewise sorting of projection data.

45. Rho, J., Ye, Z., Xiong, Y., Yin, X., Liu, Z., Choi, H., Bartal, G., and X. Zhang, "Spherical hyperlens for two-dimensional sub-diffractional imaging at visible frequencies," Nature Commun. 1, 143 (2010) pdf

    Hyperlenses have generated much interest recently because of their intriguing physics as well as their ability to achieve sub-diffraction imaging in the far field in real time. Here, we report the first experimental demonstration of far-field imaging at a visible wavelength, with resolution beyond the diffraction limit in two lateral dimensions. The spherical hyperlens is designed with flat hyperbolic dispersion that supports wave propagation with very large spatial frequencies. This allows us to resolve features down to 160 nm, much smaller than the diffraction limit at visible wavelengths. The hyperlens can be integrated into conventional microscopes, expanding their capabilities beyond the diffraction limit and opening a new realm in real-time nanoscopic optical imaging.

44. Ma, C., Aguinaldo, R. and Liu, Z., "Advances in the hyperlens", Chinese Sci. Bull., 25, 2618 (2010) pdf

    As a superlens to overcome the well-known diffraction limit, the hyperlens has received much attention due to its super resolving power and magnifying capabilities. In this article, we review the recent developments, including theoretical and experimental studies on the hyperlens. We also discuss its limitations and potential.

43. Wei, F., Liu, Z., "Plasmonic Structured Illumination Microscopy", Nano Lett., 10, 2531-2536 (2010) pdf

    We propose a super resolution imaging technique called plasmonic structured illumination microscopy (PSIM), which combines the structured illumination microscopy technique with the tunable surface plasmon interference. Because of the highresolution enabled by using surface plasmon interference as an illumination source, PSIM possesses higher image resolving power compared with conventional structured illumination microscopy. To demonstrate the technique, we present two specific types of plasmonic structure designs for PSIM. The final images from the simulations show 3-fold and 4-fold resolution improvement compared with conventional epi-fluorescence microscopy.

42. Ma, C., Liu, Z., “A super resolution metalens with phase compensation mechanism”, Appl. Phys. Lett. 96, 183103 (2010). (also selected for Virtual Journal of Nanoscale Science & Technology, May 14, 2010) pdf

    We propose a metalens consisting of a metamaterial slab that can support the propagation of waves with high wave vectors and a nonperiodic plasmonic waveguide coupler atop that can provide phase compensation and wave vector matching. The principle of the metalens is analyzed and numerically verified. We show that the metalens not only can achieve super resolution focusing but also has the fundamental properties of a conventional optical lens, such as Fourier transform and imaging. The proposed metalens may offer exceptional opportunities for optical system design and information processing.

41. Hu, H., Ma, C., Liu, Z., “Plasmonic dark field microscopy”, Appl. Phys. Lett. 96, 113107 (2010). pdf

    We propose plasmonic dark field microscopy, which utilizes a chip-scale integrated plasmonic multilayered structure to substitute the bulky and expensive conventional condenser optics. Experimental results show that we can get high contrast image using the compact, low-cost, and alignment free plasmonic dark field microscopy.

40. Ma, C., Liu, Z., “Focusing light into deep subwavelength using metamaterial immersion lenses”, Opt. Express 18, 4838 (2010). pdf

    We propose and demonstrate metamaterial immersion lenses by shaping plasmonic metamaterials. The convex and concave shapes for the elliptically and hyperbolically dispersive metamaterials are designed using phase compensation method. Numerical simulations verify that the metamaterial immersion lenses possess exceptionally large effective numerical apertures thus can achieve deep subwavelength resolution focusing. We also discuss the importance of the losses in modulating the optical transfer function and thus in enhancing the performance of the metamaterial immersion lenses.

39. Feng, L., Liu, Z., Lomakin, V., and Fainman, Y., “Form birefringence metal and its plasmonic anisotropy”, Appl. Phys. Lett. 96, 041112 (2010). pdf

    We constructed a uniaxial “form birefringence” metal that exhibits different dielectric polarizabilities along different optical axes as well as its supported optical anisotropy of surface plasmon polariton waves. The generated plasmonic index ellipsoid that exists in reciprocal space has been directly mapped and characterized in our experiment. The discovery of this anisotropic plasmonic metamaterial further completes analogy between artificial plasmonic metamaterials and conventional optical crystals, thereby providing opportunities to miniaturize myriad existing optical devices on-a-chip with plasmonics into nanometers scale.

38. Escobar, M. A., Berthome, M., Ma, C. and Liu, Z., “Focusing surface waves with an inhomogeneous metamaterials lens”, Appl. Opt. 49, A18 (2010). pdf

    We propose a new type of surface wave lens that is made of a circular in-plane inhomogeneous metamaterial slab and numerically demonstrate its capability to focus surface waves at optical frequencies. This approach can achieve a smaller focal spot size than the previously demonstrated Ag plasmonic lens. The use of inhomogeneous metamaterials is to decrease the high losses that are usually associated with metamaterials that support large surface k vectors by reducing the propagation distance in high loss metamaterials.

37. Smith, E. J., Liu, Z., Mei, Y. F. and Schmidt, O. G., “Combined surface plasmon and classical waveguiding through metamaterial fiber design”, Nano. Lett. 10, 1 (2010). pdf. Cover of Nano Letters

    A metamaterial integration for fiber optics, leading to a dual effect of surface plasmon and classical waveguiding, is presented along with experimental potentiality. We theoretically propose a metamaterial fiber in which, depending on the wavelength (from ultraviolet to infrared) and the particular metamaterial composition, one can transmit information through surface plasmon mediated or classical waveguidance. The metamaterial can be used as the core or cladding of a fiber which allows waveguidance through a subwavelength geometry.

36. Yao, J., Tsai, K. T., Wang, Y., Liu, Z., Bartal, G., Wang, Y. L., and Zhang, X. “Imaging visible light using anisotropic metamaterial slab lens”, Opt. Express 17, 22380 (2009). pdf

    It has been shown that an anisotropic metamaterial made of nanowire array can realize negative refraction of light even without a negative phase index of refraction. Such non-resonant bulk material can be fabricated by bottom-up electrochemical method. Using this material, we were able to achieve lensing action with micron-thick slab and demonstrate imaging of a slit object. The details of the focused light beam in 3- dimensional space have been mapped with near field scanning optical microscope (NSOM).

35. Smith, E. J., Liu, Z., Mei, Y. F. and Schmidt, O. G., “System investigation of a rolled-up metamaterial optical hyperlens structure”, Appl. Phys. Lett. 95, 083104 (2009). pdf Erratum: pdf

    An investigation of the material makeup and surrounding medium of an optical rolled-up hyperlens is presented. A working spectral range of the hyperlens for different material combinations is studied along with an examination of hyperlens immersion, which suppresses the diffraction of waves exiting the lens due to impedance matching, leading to a higher intensity output. This hyperlens immersion technique can be implemented into cell culture and molecular analysis.

34. Xiong, Y,. Liu, Z., and Zhang, X., "A simple design of flat hyperlens for lithography and imaging with half-pitch resolution down to 20 nm", Appl. Phys. Lett. 94, 203108 (2009). pdf

    We propose that a hyperlens can be used for photolithography to generate deep subwavelength arbitrary patterns from diffraction-limited masks. Numerical simulation shows that half-pitch resolution down to 20 nm is possible from a mask with 280 nm period at working wavelength 375 nm. We also extend the hyperlens projection concept from cylindrical interfaces to arbitrary interfaces. An example of a flat interface hyperlens is numerically demonstrated for lithography purposes.

33. Liu, Z., Wang, Y., Yao, J., Lee, H., Srituravanich, W., and Zhang, X., "Broad band two-dimensional manipulation of surface plasmons", Nano. Lett. 9, 462, (2009). pdf

    A plasmonic interference pattern can be formed when multiple surface plasmon waves overlap coherently. Utilizing a sharp edge coupling mechanism, we experimentally demonstrate plasmonic interference patterns that can be designed at will by shaping the edges in a metallic film. The patterns can also be dynamically tailored by adjusting the wavelength, the polarization, and the incident angle of the excitation light beam. Possessing the subdiffraction limited feature resolution, this dynamical manipulation method of surface plasmon patterns will have profound potentials in nanolithography, particle manipulation, and other related fields.

32. Han, S., Xiong, Y., Genov, D., Liu, Z., Bartal, G., and Zhang, X., "Ray optics at a deep-subwavelength scale: a transformation optics approach", Nano. Lett. 8, 4243, (2008). pdf

31. Xiong, Y., Liu, Z., and Zhang, X.,"Projecting deep-subwavelength patterns from diffraction-limited masks using metal-dielectric multilayers", Appl. Phys. Lett. 93, 111116 (2008). pdf

30. Yao, J., Liu, Z., Liu, Y., Wang, Y., Sun, C., Bartal, G., and Zhang X., "Negative refraction in bulk metamaterials at visible frequencies", Science 321, 930 (2008). pdf

29. Zhang, X., and Liu, Z., "Superlenses to overcome the diffraction limit", Nature Materials. 7, 435 (2008). pdf

28. Wu, D., Liu, Z., Sun. C., and Zhang, X., "Super-resolution imaging by random adsorbed molecule probes", Nano. Lett. 8, 1159 (2008). pdf

27. Lee, H., Liu, Z., Durant, S., Xiong, Y., Sun, C., and Zhang, X., "Design, fabrication and characterization of far-field superlens", Solid State Commun. 146, 202 (2008). pdf

26. Wang, S., Szobota, S., Wang, Y., Volgraf, M., Liu, Z., Sun, C., Trauner, D., Isacoff, E. Y., and Zhang, X., "All optical interface for parallel, remote and spatiotemporal control of neuronal activity", Nano. Lett. 7, 3859 (2007). pdf

25. Lee, H., Liu, Z., Xiong, Y., Sun, C., and Zhang, X., "Development of optical hyperlens for imaging below the diffraction limit", Opt. Express 15, 15886 (2007). pdf

24. Xiong, Y., Liu, Z., Sun, C., and Zhang, X., "Two dimensional imaging by far-field superlens at visible wavelength", Nano. Lett. 7, 3360 (2007). pdf

23. Liu, H., Genov, D. A., Wu, D. M., Liu, Y. M., Liu, Z. W., Sun, C., Zhu, S. N., and Zhang, X., "Magnetic plasmon hybridization and optical activity at optical frequencies in metallic nanostructures", Phys. Rev. B 76, 073101 (2007). pdf

22. Xiong, Y., Liu, Z., Durant, S., Lee, H., Sun, C., and Zhang, X., "Tuning the far-field superlens: from UV to visible", Opt. Express 15, 7095 (2007). pdf

21. Liu, Z., Durant, S., Lee, H., Pikus, Y., Xiong, Y., Sun, C., and Zhang, X., "Experimental studies of far-field superlens for sub-diffractional optical imaging", Opt. Express 15, 6947 (2007). pdf

20. Liu, Z., Lee, H., Xiong, Y., Sun, C. and Zhang, X., "Optical hyperlens magnifying sub-diffraction-limited objects", Science 315, 1686 (2007). pdf

19. Liu, Z., Durant, S., Lee, H., Pikus, Y., Fang, N., Xiong, Y., Sun, C. and Zhang, X., "Far field optical superlens", Nano. Lett. 7, 403 (2007). pdf

18. Liu, Z., Durant, S., Lee, H., Xiong, X., Pikus, Y., Sun, C., and Zhang, X., "Near-field Moire effect mediated by surface plasmon polariton excitation", Opt. Lett. 32, 629 (2007). pdf

17. Liu, Z., Xi, D., Pile, D., Luo, Q, Fang, N., and Zhang X., "Enhanced backward scattering by surface plasmons on silver film", Appl. Phys. A 87, 157 (2007). pdf

16. Steele, J. M., Liu, Z., Wang, Y., and Zhang, X., "Resonant and non-resonant generation and focusing of surface plasmons with circular gratings", Opt. Express 12, 5664 (2006). (also selected for The Virtual Journal for Biomedical Optics, July 17, 2006) pdf

15. Liu, Z., Steele, J. M., Lee, H., and Zhang, X., "Tuning the focus of a plasmonic lens by the incident angle", Appl. Phys. Lett. 88, 171108 (2006). (also selected for Virtual Journal of Nanoscale Science & Technology, May 15, 2006) pdf

14. Durant, S., Liu, Z., Steele, J. M., and Zhang, X., "Theory of the transmission properties of an optical far-field superlens for imaging beyond the diffraction limit", J. Opt. Soc. Am. B. 23, 2383 (2006). pdf

13. Liu, Z., Steele, J. M., Srituravanich, W., Pikus, Y., Sun, C., and Zhang, X., "Focusing surface plasmons with a plasmonic lens", Nano. Lett. 5, 1726 (2005). pdf

12. Liu, Z., Wei, Q. H, and Zhang, X., "Surface plasmon interference nanolithography", Nano. Lett. 5, 957 (2005). pdf

11. Fang, N., Liu, Z., Yen, T. J., Zhang, X., "Experimental study of transmission enhancement of evanescent waves through silver films assisted by surface plasmon excitation", Appl. Phys. A 80, 1315 (2005). pdf

10. Yin, X., Hesselink, L., Liu, Z., Fang, N., and Zhang, X., "Large positive and negative lateral optical beam displacements due to surface plasmon resonance", Appl. Phys. Lett. 85, 372 (2004). pdf

9. Liu, Z., Fang, N., Yen, T.-J., and Zhang, X., "Rapid Growth of Evanescent Wave with a Silver Superlens", Appl. Phys. Lett. 83, 5184 (2003). pdf

8. Fang, N., Liu, Z., Yen, T. J., and Zhang, X., "Regenerating evanescent waves from a silver superlens", Opt. Express 11, 682 (2003). pdf

7. Du, Y., Zhu, S. N., Zhu, Y. Y., Xu, P., Zhang, C., Chen, Y. B., Liu, Z. W., Ming, N. B., Zhang, X. R., Zhang, F. F., and Zhang, S. Y., "Parametric and cascaded parametric interactions in a quasiperiodic optical superlattice", Appl. Phys. Lett. 81, 1573 (2002). pdf

6. Liu, Z. W., Du, Y., Liao, J., Zhu, S. N., Zhu, Y. Y., Qin, Y. Q., Wang, H. T., He, J. L., Zhang, C. and Ming, N. B., "Engineering of a Dual-periodic optical superlattice used in a coupled optical parametic interaction", J. Opt. Soc. Am. B. 19, 1676 (2002). pdf

5. Liu, Z. W., Zhu, S. N., Zhu, Y. Y., Liu, H., Lu, Y. Q., He, J. L., Zhang, C., Wang, H. T., Ming, N. B., Liang, X. Y., and Xu, Z. Y., "Quasi-cw ultraviolet generation in a dual-periodic LiTaO3 superlattice by frequency tripling", Jap. J. Appl. Phys. 40, 6841 (2001). pdf

4. Liu, Z. W., Zhu, S. N., Zhu, Y. Y., Liu, H., Lu, Y. Q., Wang, H. T., Ming, N. B., Liang, X. Y., and Xu, Z. Y., "A scheme to realize three-fundamental-colors laser based on quasi-phase matching", Solid State Commun. 119, 363 (2001). pdf

3. Luo, G. Z., Zhu, S. N., He, J. L., Zhu, Y. Y., Wang, H. T., Liu, Z. W., Zhang, C. and Ming, N. B., "Simultaneously efficient blue and red light generations in a periodically poled LiTaO3", Appl. Phys. Lett. 78, 3006 (2001). pdf

2. Liu, Z., Qin, Y., Zhu, Y., Wang, H., Zhang, C., Zhu, S., and Ming, N., "Ultraviolet generation in a dual-periodic domain inverted structure in LiTaO3 crystal by frequency tripling a 1.064 mu m laser". Ferroelectrics 253, 819 (2001). pdf

1. Liu, Z. W., Zhu, S. N., Zhu, Y. Y., Wang, H. T., Luo, G. Z., Liu, H., Min, N. B., Liang, X.Y., and Xu, Z. Y., "Red and blue light generation in an LiTaO3 crystal with a double grating domain structure", Chin. Phys. Lett. 18, 539 (2001). pdf

• Plasmonics and Super-Resolution Imaging, Liu, Z. (Editor) Pan Stanford (2017)

4. Ponsetto, J. L., and Liu, Z., “The far-field superlens” in Plasmonics and Super-Resolution Imaging, Liu, Z. (Editor) Pan Stanford (2017)
3. Wei, F., Ponsetto, J. L., and Liu, Z., “Plasmonic structured illumination microscopy” in Plasmonics and Super-Resolution Imaging, Liu, Z. (Editor) Pan Stanford (2017)
2. Lu, D., and Liu, Z., “Hyperlenses and metalenses” in Plasmonics and Super-Resolution Imaging, Liu, Z. (Editor) Pan Stanford (2017)
1. Zhang, X., Ambati, M., Fang, N., Lee, H., Liu, Z., Sun, C. and Xiong, Y., "Optical superlens" in Surface Plasmon Nanophotonics, Kik, P. G. and Brongersma, M. L. (Editors) Springer (2007)