Research Profile

ISI Web of Science Total Citations: 7479 H-Index: 37
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Google Scholar Total Citations: 10936 H-Index: 42 i10-index: 89
Select: 2019 2018 2017 2016 2015 Earlier Books

Selected Publications

    Journal Papers - 2019

    Engineering the dispersion properties of multilayered periodic segmented waveguides and nanowire waveguides Opt. Eng. 58(9), 097107 (2019)pdf Oliveira, A.J., Rodriguez-Esquerre, V.F. and Liu, Z.
    Abstract (click)

    We proposed and systematically analyzed the propagation properties of periodic segmented waveguides and nanowire waveguides where the segments and nanowires have been considered to be composed of multilayers of silicon and silica with subwavelength thickness. We demonstrated, through a comprehensive number of numerical simulations involving modal analysis, that the artificial uniaxial negative anisotropy introduced by the multilayered segments or nanowire can be used to engineer waveguides with independent polarization propagation characteristic over the entire band of optical communication frequencies.

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    Spin controlled wavefront shaping metasurface with low dispersion in visible frequencies Nanoscale 11, 17111-17119 (2019)pdf Zhou, J., Qian, H., Luo, H., Wen, S. and Liu, Z.
    Abstract (click)

    Similar to amplitude and phase, optical spin plays an important and nontrivial role in optics, which have been widely demonstrated in wavefront engineering, creation of new optical components, and sensitive optical metrology. In this work, we propose and experimentally demonstrate a new type of spin controlled wavefront shaping metasurface. The proposed geometric phase metasurface is designed by employing the integrated and interleaved structures to independently control the left and right-handed spin components. As an exemplary demonstration, our experimental results show that such a composite metasurface can convert a plane wave into a vortex beam and a Hermite beam for left-handed and right-handed polarized light, respectively. Because such metasurface is made by non-resonance dielectric structures, it can work for broadband frequencies with very low dispersion. The proposed metasurface is fabricated by the laser writing method in a transparent glass with a low-cost, which avoids the typical high-resolution lithography process. This spin dependent broadband wavefront shaping metasurface may find potential applications in optical communications, information processing, and optical metrology.

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    Photothermal Modulation of Propagating Surface Plasmons on Silver Nanowires ACS Photonics 6(8), 2133-2140 (2019)pdf Li, Q., Chen, L., Xu, H., Liu, Z., and Wei, H.
    Abstract (click)

    Nanoplasmonic devices have received much attention in recent years due to their ability to confine light beyond the diffraction limit. Manipulating the propagation of surface plasmons (SPs) is of vital importance for the creation of nanometer-scale integrated photonic devices. In this work we exploit the photothermal property of a silver nanowire (NW) to optically modulate the propagation of SPs on it. Under the excitation of a control laser beam, the rise of local temperature induced by the photothermal effect of silver NW results in the dramatic increase or decrease of the intensity of the transmitted SPs generated by a probe laser beam, depending on the Fabry-Pérot resonance conditions of the SPs on the NW. The amplitude of the photothermal modulation depth is found to be strongly dependent on the focal positions, polarizations, and power of the control beam. The simulations reveal that the high modulation depth at the NW end is mainly caused by the additional heat generated by the propagating SPs on the NW. The analytical solutions for the transmissivity and modulation depth are presented. Both numerical simulations and theoretical analysis agree well with the experimental results. Our work provides not only a new kind of all-optical modulation method for the propagating SPs in ultra-compact plasmonic devices, but also the basic understanding about the influence of environmental temperature on the propagating SPs.

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    Localized plasmonic structured illumination microscopy with gaps in spatial frequencies Opt. Lett. 44(11), 2915-2918 (2019)pdf Bezryadina, A., Zhao, J., Xia, Y., Lee, Y.U., Zhang, X. and Liu, Z.
    Abstract (click)

    Localized plasmonic structured illumination microscopy (LPSIM) is a super-resolution fluorescent microscopy method to image samples at a high speed with a wide field of view and low phototoxicity. Here we propose a methodology to extend the resolution capability of LPSIM by shifting spatial frequencies farther away from the diffraction-limited cutoff frequency with a plasmonic nano-array. We analyze the performance and accuracy of image reconstruction by using simulations of standard structured illumination microscopy (SIM) and blind-LPSIM. LPSIM experiments were also performed by using various LPSIM substrates and different microscope objectives. The experiments and simulations show that by shifting spatial frequencies farther away, resolution improvement can be extended up to 5 times beyond the diffraction limit with minimal deformation and artifacts in the reconstructed image.

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    Optical edge detection based on high-efficiency dielectric metasurface Proc. Natl. Acad. Sci. 116(23), 11137-11140 (2019)pdf Zhou, J., Qian, H., Chen, C., Zhao, J., Li, G., Wu, Q., Luo, H., Wen, S. and Liu, Z.
    Abstract (click)

    Edge detection is a fundamental tool in image processing, computing, and machine vision. Compared with digital processes, optical analog approaches show enormous advantages owing to its intrinsic parallel nature for high-speed operation. Recently, optical metamaterials and metasurfaces have performed edge detection via analog spatial differentiation, which shows superior integration capability compared with the traditional bulky system. Unfortunately, experimental realization of optical-edge detection with metamaterials and metasurfaces remains challenging based on previous theoretical proposals. Here, we demonstrated a mechanism to realize an optical spatial differentiator consisting of a designed metasurface sandwiched by two orthogonally aligned linear polarizers. This approach relies on spin-orbit interaction of light and the metasurface, showing versatile edge-detection capability with exceptional quality.

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    Plasmonically Enhanced Amorphous Silicon Photodetector with Internal Gain IEEE PHOTONIC. TECH. L. 31(12), 959-962 (2019)pdf Yu, Y., Xu, Z., Li, S., Zhang, A.C., Yan, L., Liu, Z. and Lo, Y.
    Abstract (click)

    Photodetectors made of amorphous materials enable low cost optical imaging and communications over non-semiconductor platforms. The key challenges are to improve efficiency, sensitivity, and frequency response. Using the localized surface plasmon resonance (LSPR) effect and an efficient carrier multiplication process, cycling excitation process (CEP), the plasmonically enhanced amorphous silicon photodetector (PEASP) with a thin (60 nm) absorption layer achieves a high external quantum efficiency with a record fast impulse response of 170 ps (FWHM). This approach offers the possibility of making detectors out of amorphous material for high frame rate imaging and optical communications in spite of the material's low carrier mobility.

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    Organic Bulk Heterojunction Infrared Photodiodes for Imaging Out to 1300nm ACS Appl. Electron. Mater. 1(5), 660-666 (2019)pdf Yao. W., Wu, Z., Huang, E., Huang, L., London, A.E., Liu, Z., Azoulay, J.D. and Ng, T.N.
    Abstract (click)

    This work studies organic bulk heterojunction photodiodes with a wide spectral range capable of imaging out to 1.3 um in the shortwave infrared. Adjustment of the donor-to-acceptor (polymer:fullerene) ratio shows how blend composition affects the density of states (DOS) which connects materials composition and optoelectronic properties and provides insight into features relevant to understanding dispersive transport and recombination in the narrow bandgap devices. Capacitance spectroscopy and transient photocurrent measurements indicate the main recombination mechanisms arise from deep traps and poor extraction from accumulated space charges. The amount of space charge is reduced with a decreasing acceptor concentration; however, this reduction is offset by an increasing trap DOS. A device with 1:3 donor-to-acceptor ratio shows the lowest density of deep traps and the highest external quantum efficiency among the different blend compositions. The organic photodiodes are used to demonstrate a single-pixel imaging system that leverages compressive sensing algorithms to enable image reconstruction.

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    Array atomic force microscopy for real-time multiparametric analysis Proc. Natl. Acad. Sci. 116(13), 5872-5877 (2019)pdf 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.
    Abstract (click)

    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.

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

    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.

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    Journal Papers - 2018

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

    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.

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

    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.

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    Experimental Demonstration of Hyperbolic Metamaterial Assisted Illumination Nanoscopy ACS Nano 12(11), 11316-11322 (2018) pdf 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.
    Abstract (click)

    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.

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    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 Zhao, C., Zhu, Y., Chen, L., Zhou, S., Su, Y., Ji, X., Chen, A., Gui, X., Tang, Z. and Liu, Z.
    Abstract (click)

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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    Controlled Homoepitaxial Growth of Hybrid Perovskites Adv. Mater. 30(20), 1705992 (2018)pdf 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.
    Abstract (click)

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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    Journal Papers - 2017

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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

    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.

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

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

    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.

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    Journal Papers - 2016

    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 Han, D.Y., Safigholi, H., Soliman, A., Ravi, A., Leung, E., Scanderbeg, D.J., Liu, Z., Owrangi, A. and Song, W.Y.
    Abstract (click)

    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.

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

    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.

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

    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.

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

    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.

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

    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.

    google_scholar
    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 Kim, T.K., VanSaders, B., Caldwell, E., Shin, s., Liu, Z., Jin, S. and Chen, R.
    Abstract (click)

    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 750oC operating temperature and a solar concentration ratio of 1000. More importantly, the coating showed unprecedented durability, as demonstrated from long-term isothermal annealing at 750oC in air as well as rapid thermal cycling between room temperature and 750oC.

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

    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.

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

    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.

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

    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.

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

    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.

    google_scholar

    Journal Papers - 2015

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

    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 Schrodinger equation and the Poisson equation is proposed and its predictions agree well with experimental results.

    google_scholar
    Enhancement of electroluminescence from embedded Si quantum dots/SiO2 multilayers film by localized- surface-plasmon and surface roughening Scientific Reports 5, 11881 (2015)pdf Li, W., Wang, S., Hu, M., He, S., Ge, P., Wang, J., Guo, Y.Y., and Liu, Z.
    Abstract (click)

    In this paper, we prepared a novel structure to enhance the electroluminescence intensity from Si quantum dots/SiO2 multilayers. 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.

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

    We report successful growth of a uniform and scalable nanocomposite 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.

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

    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.

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

    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.

    google_scholar
    Coherent Four-Fold Super-Resolution Imaging with Composite Photonic-Plasmonic Structured Illumination ACS Photonics 2(3), 341-348 (2015)pdf Fernandez-Dominguez, A.I., Liu, Z., and Pendry, J.B.
    Abstract (click)

    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.

    google_scholar
    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 Moon, J., Kim, T.K., VanSaders, B., Choi, C., Liu, Z., Jin, S., and Chen, R.
    Abstract (click)

    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 750oC 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.

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

    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.

    google_scholar
    Tandem structured spectrally selective coating layer of copper oxide nanowires combined with cobalt oxide nanoparticles Nano Energy 11, 274-259 (2015)pdf 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.
    Abstract (click)

    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.

    google_scholar

    Journal Papers - earlier

    Localized surface plasmon assisted contrast microscopy for ultrathin transparent specimens Applied Physics Letters 105, 163102 (2014)pdf Wei, F., Lu, D., Aguinaldo, R., Ma, Y., Sinha, S.K., and Liu, Z.
    Si boride-coated Si nanoparticles with improved thermal oxidation resistance Nano Energy 9, 32-40 (2014)pdf 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.
    Wide Field Super-Resolution Surface Imaging through Plasmonic Structured Illumination Microscopy Nano Letters 14, 4634-4639 (2014)pdf Wei, F., Lu, D., Shen, H., Ponsetto, J.L., Huang, E., and Liu, Z.
    High performance multi-scaled nanostructured spectrally selective coating for concentrating solar power Nano Energy 8, 238-246 (2014)pdf Moon, J., Lu, D., VanSaders, B., Kim, T.K., Kong, S.D., Jin, S., Chen, R., and Liu, Z.
    From Fano-like interference to superscattering with a single metallic nanodisk Nanoscale 6, 9093-9102 (2014)pdf Wan, W., Zheng, W., Chen, Y., and Liu, Z.
    TIRF microscopy with ultra-short penetration depth Opt. Express 22(9), 10728-10734 (2014)pdf Shen, H., Huang, E., Das, T., Xu, H., Ellisman, M., and Liu, Z.
    Ultralow Thermal Conductivity of Multilayers with Highly Dissimilar Debye Temperatures Nano Letters 14(5), 2448-2455 (2014)pdf Dechaumphai, E., Lu, D., Kan, J.J., Moon, J., Fullerton, E.E., Liu, Z. and Chen, R.
    Localized plasmon assisted structured illumination microscopy for wide-field high-speed dispersion- independent super resolution imaging Nanoscale 6, 5807-5812 (2014)pdf (correction ) Ponsetto, J.L., Wei, F., and Liu, Z.
    Enhanced spontaneous emission inside hyperbolic metamaterials Opt. Express 22(4), 4301-4306 (2014)pdf Ferrari, L., Lu, D., Lepage, D., and Liu, Z.
    Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials Nat. Nanotech. 9, 48-53 (2014)pdf Lu, D., Kan, J.J., Fullerton, E.E., and Liu, Z.
    Motion-map constrained image reconstruction (MCIR): Application to four-dimensional cone-beam computed tomography Med. Phys. 40, 121710 (2013)pdf Park, J.C., Kim, J.S., Park, S.H., Liu, Z., Song, B. and Song , W.Y.
    Three-dimensional ZnO/Si broom-like nanowire heterostructures as photoelectrochemical anodes for solar energy conversion Phys. Status Solidi A 210(12), 2561-2568 (2013)pdf Kargar, A., Sun, K., Kim, J.S., Lu, D., Jing, Y., Liu, Z., Pan, X. and Wang , D.
    Control the dispersive properties of compound plasmonic lenses Opt. Commun. 291, 390-394 (2013) pdf Wan, W., Ma, C. and Liu, Z.
    Hyperlenses and metalenses for far-field super-resolution imaging Nature Commun. 3, 1205 (2012)pdf Lu, D., and Liu, Z.
    Liver mothion during cone beam computed tomography guided stereotactic body radiation therapy Med. Phys. 39(10), 6431-6442 (2012)pdf 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.
    Organic light-emitting-diode-based plasmonic dark-field microscopy Opt. Lett. 37(21), 4359-4361 (2012)pdf Wei, F., O, Y. W., Li, G., Cheah, K. W., and Liu, Z.
    3D branched nanowire heterojunction photoelectrodes for high-efficiency solar water splitting and H2 generation Nanoscale 4(5), 1515-1521 (2012)pdf 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.
    Fast compressed sensing-based CBCT reconstruction using Barzilai-Borwein formulation for application to on-line IGRT Med. Phys. 39(3), 1207-1217 (2012)pdf Park, J. C., Song, B., Kim, J. S., Park, S. H., Liu, Z. Suh, T. S., and Song, W. Y.
    Breaking the imaging symmetry in negative refraction lenses Opt. Express 20(3), 2581-2586 (2012)pdf Ma, C., and Liu, Z.
    Extraordinary light focusing and Fourier transform properties of gradient-index metalenses Phys. Rev. B 84, 195142 (2011)pdf Ma, C., Escobar, M. A. and Liu, Z.
    Direct observation of plasmonic index ellipsoids on a deep-subwavelength metallic grating Appl. Opt. 50(31), G1-G6 (2011)pdf Feng, L., Liu, Z., and Fainman, Y.
    Design, fabrication and characterization of indefinite metamaterials of nanowires Phil. Trans. R. Soc. A 369(1950), 3434-3446 (2011)pdf Yao, J., Wang, Y., Tsai, K-Ti, Liu, Z., Yin, X., Bartal, G., Stacy, A. M., Wang, Y. L., and Zhang, X.
    Ultra-fast digital tomosynthesis reconstruction using general-purpose GPU programming for image-guided radiation therapy Technology in Cancer Research & Treatment 10(4), 295-306 (2011)pdf Park, J. C., Park, S. H., Kim, J. S., Han, Y., Cho, M. K., Liu, Z., Jiang, S. B., Song, B., Song, W. Y.,
    Metamaterials for enhanced polarization conversion in plasmonic excitation ACS Nano. 5(6), 5100-5106 (2011)pdf Feng, L., Mizrahi, A., Zamek, S., Liu, Z., Lomakin, V., and Fainman, Y.
    Tunable surface plasmon polaritons in Ag composite films by adding dielectrics or semiconductors Appl. Phys. Lett. 98(24), 243114 (2011)pdf Lu, D., Kan, J., Fullerton, E. E. and Liu, Z
    Designing super-resolution metalenses by the combination of metamaterials and nanoscale plasmonic waveguide couplers J. Nanophotonics. 5(1), 051604 (2011)pdf Ma, C., and Liu, Z.
    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 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.
    Spherical hyperlens for two-dimensional sub-diffractional imaging at visible frequencies Nature Commun. 1, 143 (2010)pdf Rho, J., Ye, Z., Xiong, Y., Yin, X., Liu, Z., Choi, H., Bartal, G., and X. Zhang
    Advances in the hyperlens Chinese Sci. Bull. 55(24), 2618-2624 (2010)pdf Ma, C., Aguinaldo, R. and Liu, Z.
    Plasmonic Structured Illumination Microscopy Nano Lett. 10, 2531-2536 (2010)pdf Wei, F., Liu, Z.
    A super resolution metalens with phase compensation mechanism Appl. Phys. Lett. 96(18), 183103 (2010)(also selected for Virtual Journal of Nanoscale Science & Technology, May 14, 2010)pdf Ma, C., Liu, Z.
    Plasmonic dark field microscopy Appl. Phys. Lett. 96, 113107 (2010)pdf Hu, H., Ma, C., Liu, Z.
    Focusing light into deep subwavelength using metamaterial immersion lenses Opt. Express 18(5), 4838-4844 (2010)pdf Ma, C., Liu, Z.
    Form birefringence metal and its plasmonic anisotropy Appl. Phys. Lett. 96, 041112 (2010)pdf Feng, L., Liu, Z., Lomakin, V., and Fainman, Y.
    Focusing surface waves with an inhomogeneous metamaterials lens Appl. Opt. 49(7), A18-A22 (2010)pdf Escobar, M. A., Berthome, M., Ma, C. and Liu, Z.
    Combined surface plasmon and classical waveguiding through metamaterial fiber design Nano. Lett. 10(1), 1-5 (2010)pdf Smith, E. J., Liu, Z., Mei, Y. F. and Schmidt, O. G.
    Imaging visible light using anisotropic metamaterial slab lens Opt. Express 17(25), 22380-22385 (2009)pdf Yao, J., Tsai, K. T., Wang, Y., Liu, Z., Bartal, G., Wang, Y. L., and Zhang, X.
    System investigation of a rolled-up metamaterial optical hyperlens structure Appl. Phys. Lett. 95, 083104 (2009)pdf Erratum:pdf Smith, E. J., Liu, Z., Mei, Y. F. and Schmidt, O. G.
    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 Xiong, Y,. Liu, Z., and Zhang, X.
    Broad band two-dimensional manipulation of surface plasmons Nano. Lett. 9(1), 462-466 (2009)pdf Liu, Z., Wang, Y., Yao, J., Lee, H., Srituravanich, W., and Zhang, X.
    Ray optics at a deep-subwavelength scale: a transformation optics approach Nano. Lett. 8(12), 4243-4247 (2008)pdf Han, S., Xiong, Y., Genov, D., Liu, Z., Bartal, G., and Zhang, X.
    Projecting deep-subwavelength patterns from diffraction-limited masks using metal-dielectric multilayers Appl. Phys. Lett. 93, 111116 (2008)pdf Xiong, Y., Liu, Z., and Zhang, X.
    Negative refraction in bulk metamaterials at visible frequencies Science 321(5891), 930 (2008)pdf Yao, J., Liu, Z., Liu, Y., Wang, Y., Sun, C., Bartal, G., and Zhang X.
    Superlenses to overcome the diffraction limit Nature Materials. 7(6), 435 (2008)pdf Zhang, X., and Liu, Z.
    Super-resolution imaging by random adsorbed molecule probes Nano. Lett. 8(4), 1159-1162 (2008)pdf Wu, D., Liu, Z., Sun. C., and Zhang, X.
    Design, fabrication and characterization of far-field superlens Solid State Commun. 146, 202-207 (2008)pdf Lee, H., Liu, Z., Durant, S., Xiong, Y., Sun, C., and Zhang, X.
    All optical interface for parallel, remote and spatiotemporal control of neuronal activity Nano. Lett. 7(12), 3859-3863 (2007)pdf Wang, S., Szobota, S., Wang, Y., Volgraf, M., Liu, Z., Sun, C., Trauner, D., Isacoff, E. Y., and Zhang, X.
    Development of optical hyperlens for imaging below the diffraction limit Opt. Express 15, 15886 (2007)pdf Lee, H., Liu, Z., Xiong, Y., Sun, C., and Zhang, X.
    Two dimensional imaging by far-field superlens at visible wavelength Nano. Lett. 7(11), 3360-3365 (2007)pdf Xiong, Y., Liu, Z., Sun, C., and Zhang, X.
    Magnetic plasmon hybridization and optical activity at optical frequencies in metallic nanostructures Phys. Rev. B 76, 073101 (2007)pdf Liu, H., Genov, D. A., Wu, D. M., Liu, Y. M., Liu, Z. W., Sun, C., Zhu, S. N., and Zhang, X.
    Tuning the far-field superlens: from UV to visible Opt. Express 15, 7095 (2007)pdf Xiong, Y., Liu, Z., Durant, S., Lee, H., Sun, C., and Zhang, X.
    Experimental studies of far-field superlens for sub-diffractional optical imaging Opt. Express 15(11), 6947-6954 (2007)pdf Liu, Z., Durant, S., Lee, H., Pikus, Y., Xiong, Y., Sun, C., and Zhang, X.
    Optical hyperlens magnifying sub-diffraction-limited objects Science 315(5819), 1686 (2007)pdf Liu, Z., Lee, H., Xiong, Y., Sun, C. and Zhang, X.
    Far field optical superlens Nano. Lett. 7(2), 403-408 (2007)pdf Liu, Z., Durant, S., Lee, H., Pikus, Y., Fang, N., Xiong, Y., Sun, C. and Zhang, X.
    Near-field Moire effect mediated by surface plasmon polariton excitation Opt. Lett. 32(6), 629-631 (2007)pdf Liu, Z., Durant, S., Lee, H., Xiong, X., Pikus, Y., Sun, C., and Zhang, X.
    Enhanced backward scattering by surface plasmons on silver film Appl. Phys. A 87(2), 157 (2007)pdf Liu, Z., Xi, D., Pile, D., Luo, Q, Fang, N., and Zhang X.
    Resonant and non-resonant generation and focusing of surface plasmons with circular gratings Opt. Express 14(12), 5664-5670 (2006) (also selected for The Virtual Journal for Biomedical Optics, July 17, 2006)pdf Steele, J. M., Liu, Z., Wang, Y., 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 Liu, Z., Steele, J. M., Lee, H., 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 Durant, S., Liu, Z., Steele, J. M., and Zhang, X.
    Focusing surface plasmons with a plasmonic lens Nano. Lett. 5(9), 1726-1729 (2005)pdf Liu, Z., Steele, J. M., Srituravanich, W., Pikus, Y., Sun, C., and Zhang, X.
    Surface plasmon interference nanolithography Nano. Lett. 5(5), 957-961 (2005)pdf Liu, Z., Wei, Q. H, and 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 Fang, N., Liu, Z., Yen, T. J., Zhang, X.
    Large positive and negative lateral optical beam displacements due to surface plasmon resonance Appl. Phys. Lett. 85, 372 (2004)pdf Yin, X., Hesselink, L., Liu, Z., Fang, N., and Zhang, X.
    Rapid Growth of Evanescent Wave with a Silver Superlens Appl. Phys. Lett. 83, 5184 (2003)pdf Liu, Z., Fang, N., Yen, T.-J., and Zhang, X.
    Regenerating evanescent waves from a silver superlens Opt. Express 11, 682 (2003)pdf Fang, N., Liu, Z., Yen, T. J., and Zhang, X.
    Parametric and cascaded parametric interactions in a quasiperiodic optical superlattice Appl. Phys. Lett. 81, 1573 (2002)pdf 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.
    Engineering of a Dual-periodic optical superlattice used in a coupled optical parametic interaction J. Opt. Soc. Am. B. 19, 1676 (2002)pdf 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.
    Quasi-cw ultraviolet generation in a dual-periodic LiTaO3 superlattice by frequency tripling Jap. J. Appl. Phys. 40, 6841 (2001)pdf 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.
    A scheme to realize three-fundamental-colors laser based on quasi-phase matching Solid State Commun. 119, 363 (2001)pdf 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.
    Simultaneously efficient blue and red light generations in a periodically poled LiTaO3 Appl. Phys. Lett. 78, 3006 (2001)pdf Luo, G. Z., Zhu, S. N., He, J. L., Zhu, Y. Y., Wang, H. T., Liu, Z. W., Zhang, C. and Ming, N. B.
    Ultraviolet generation in a dual-periodic domain inverted structure in LiTaO3 crystal by frequency tripling a 1.064 mu m laser Ferroelectrics 253(1), 263-270 (2001)pdf Liu, Z., Qin, Y., Zhu, Y., Wang, H., Zhang, C., Zhu, S., and Ming, N.
    Red and blue light generation in an LiTaO3 crystal with a double grating domain structure Chin. Phys. Lett. 18(4), 539 (2001)pdf 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.

Books

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

Book Chapters

Ponsetto, J. L., and Liu, Z.,"The far-field superlens" in Plasmonics and Super-Resolution Imaging Liu, Z. (Editor) Pan Stanford (2017)
Wei, F., Ponsetto, J. L., and Liu, Z., "Plasmonic structured illumination microscopy" in Plasmonics and Super-Resolution Imaging Liu, Z. (Editor) Pan Stanford (2017)
Lu, D., and Liu, Z., "Hyperlenses and metalenses" in Plasmonics and Super-Resolution Imaging Liu, Z. (Editor) Pan Stanford (2017)
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