Zizhuo Liu

Localized surface plasmons in nanostructured monolayer black phosphorus

We theoretically investigate localized surface plasmon resonances (LSPR) in a monolayer, nanostructured black phosphorus (BP). Using finite-difference time-domain simulations, we demonstrate LSPRs at mid-infrared and far-infrared wavelength regime in BP nanoribbon arrays. Black phosphorus nanostructures provide polarization dependent, anisotropic plasmonic response.

Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films

Plasmonic and metamaterial based nano/micro-structured materials enable spectrally selective resonant absorption, where the resonant bandwidth and absorption intensity can be engineered by controlling the size and geometry of nanostructures. Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition. Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from ~90% to ~30% in measured absorption) over the entire mid-wavelength infrared spectrum. Our numerical and experimental results indicate that the bandwidth of the absorption bands can be controlled by changing the dielectric spacer layer thickness. Broadband tunable absorbers can find applications in absorption filters, thermal emitters, thermophotovoltaics and sensing.

Quantifying Plasmon-Enhanced Light Absorption in Monolayer WS2 Films

Transition metal dichalcogenide semiconductors hold great promise in photonic and optoelectronic applications, such as flexible solar cells and ultrafast photodetectors, because of their direct band gap and few-atom thicknesses. However, it is crucial to understand and improve the absorption characteristics of these monolayer semiconducting materials. In this study, we conducted a systematic numerical and experimental investigation to demonstrate and quantify absorption enhancement in WS2 monolayer films, in the presence of silver plasmonic nanodisk arrays. Our analysis combining full-field electromagnetic simulations and optical absorption spectroscopy measurements indicates a fourfold enhancement in the absorption of an WS2 film near its band edge, close to the plasmonic resonance wavelength of Ag nanodisk arrays. The proposed Ag/WS2 heterostructure exhibited a 2.5-fold enhancement in calculated short-circuit current. Such hybrid plasmonic or two-dimensional (2D) materials with enhanced absorption pave the way for the practical realization of 2D optoelectronic devices, including ultrafast photodetectors and solar cells.

Broadband asymmetric light transmission through tapered metallic gratings at visible frequencies

Asymmetric transmission phenomenon has attracted tremendous research interest due to its potential applications in integrated photonic systems. Broadband asymmetric transmission (BAT) is a highly desirable but challenging functionality to achieve in the visible regime due to the limitation of material dispersion. In this paper, we propose and numerically demonstrate that a tapered-metal-grating structure (TMGS) can achieve high-contrast BAT spectra covering the entire visible region. The transmission efficiency reaches ~95% for the forward illumination and ~35% for the backward illumination at the same wavelengths, respectively, and the corresponding transmission ratio is larger than 2.5 over a broadband wavelength regime. Such a design with high performance suggests applications for unidirectional optical transmission, optical diode, and so on.

Enhanced infrared transmission through gold nanoslit arrays via surface plasmons in continuous graphene

Graphene is a monolayer plasmonic material that has been widely studied in the area of plasmonics and nanophotonics. Combining graphene with traditional plasmonic structures provides new opportunities and challenges. One particular application for nanostructured metals is enhanced optical transmission. However, extraordinary transmission (EOT) is known to have a frequency-selective performance due to size and periodicity of the nanohole arrays. Here, we propose to use a continuous graphene layer to enhance transmission through gold nanoslit arrays at mid-infrared (mid-IR) wavelengths. Although graphene absorbs 2.3% of light, by exciting surface plasmon polaritons (SPPs) at the graphene/gold nanoslit arrays interface, we have theoretically demonstrated enhanced infrared transmission over broad range of wavelengths in the mid-IR region. Our analyses of the effects of various structure parameters on the transmittance spectra shows that surface plasmon polaritons excited at the graphene/metal interface is responsible for enhanced transmission behavior. Moreover, calculated steady-state electric field distribution supports our predictions. Our work opens new directions to study 2D plasmonics using a continuous graphene film without the need of structuring it and also employs the broadband optical response of graphene to enable broadband extraordinary transmission enhancement.

Chiral-selective plasmonic metasurface absorbers operating at visible frequencies

We demonstrate theoretically and experimentally a chiral-selective plasmonic absorber by utilizing eng -shaped-resonators in the visible frequencies. Our metasurface design enables chiral-selective absorption bands, in which absorption peaks for left-handed circularly polarized and right-handed circularly polarized occur at different resonance wavelengths resulting in significant circular dichroism (CD). Both simulated and measured absorption spectra exhibit maximum absorptions exceeding 80% associated with a CD value of ~0.5. Such a chiral plasmonic metasurface absorber with high performance could find applications in optical filters, non-linear optics, thermal emitters, and hot-electron collection devices.

DNA-Mediated Size-Selective Nanoparticle Assembly for Multiplexed Surface Encoding

Multiplexed surface encoding is achieved by positioning two different sizes of gold nanocubes on gold surfaces with precisely defined locations for each particle via template-confined, DNA-mediated nanoparticle assembly. As a proof-of-concept demonstration, cubes with 86 and 63 nm edge lengths are assembled into arrangements that physically and spectrally encrypt two sets of patterns in the same location. These patterns can be decrypted by mapping the absorption intensity of the substrate at λ = 773 and 687 nm, respectively. This multiplexed encoding platform dramatically increases the sophistication and density of codes that can be written using colloidal nanoparticles, which may enable high-security, high-resolution encoding applications.

Wideband zero-index metacrystal with high transmission at visible frequencies [Invited]

Materials with zero refractive index exhibit unprecedented optical properties including no spatial phase change and infinitely large phase velocity. Several zero-index material designs including artificial layered metallic/dielectric medium were proposed and demonstrated at microwave, terahertz, and IR wavelengths. However, realizing a zero-index material with low-losses, none-dispersion, and relatively wide bandwidth operation at visible frequencies is quite challenging due to optical losses in metals. Here, we propose and numerically demonstrate a three-dimensional zero-index metacrystal (ZIM) with low loss, low dispersion, and wide bandwidth at visible frequencies. The ZIM simply consists of periodic Ag nanocube arrays embedded inside a dielectric medium with same lattice constant in all directions. The calculated effective refractive index using a parameter retrieval method reveals a relatively wide band (∼40  nm) of near-zero index (<0.02) and achromatic behavior for designed metacrystal in the visible frequency. Using full-field electromagnetic (EM) simulations, we have theoretically demonstrated that the EM wave always propagates normal to the ZIM–air interface in spite of oblique incidence cases or any arbitrary wavefront of illumination. Our proposed zero-index metacrystal for visible frequencies could find use in many practical applications of wide-bandwidth and low-loss achromatic photonic devices for steering light propagation, arbitrary wavefront conversion, directional emission, and obstacle-free light guiding.

Extrinsic polarization-controlled optical anisotropy in plasmon-black phosphorus coupled system

Two-dimensional black phosphorus (BP) has drawn extensive research interest due to its promising anisotropic photonic and electronic properties. Here, we study anisotropic optical absorption and photoresponse of exfoliated BP flakes at visible frequencies. We enhance this intrinsic optical anisotropy in BP flakes by coupling plasmonic rectangular nanopatch arrays that support localized surface plasmon resonances. In particular, by combining extrinsic anisotropic plasmonic nanostructures lithographically aligned with intrinsically anisotropic BP flakes, we demonstrate for the first time a combined anisotropic plasmonic-semiconductor coupling that provides significant control over the polarization-dependent optical properties of the plasmon-BP hybrid material system, enhancing polarization-sensitive responses to a larger degree. This hybrid material system not only unveils the plasmon-enhanced mechanisms in BP, but also provides novel controllable functionalities in optoelectronic device applications involving polarization-sensitive optical and electrical responses.

Enhanced infrared transmission from gold wire-grid arrays via surface plasmons in continuous graphene (Presentation Recording)

Enhanced transmission of light through nanostructures has always been of great interest in the field of plasmonics and nanophotonics. With the aid of near-field effects, the transmission of the electromagnetic waves can be enhanced or suppressed. Much of the work on enhanced transmission has been shown to be frequency-selective. However it is possible to increase the transmission over a large frequency range by using graphene, which has shown broadband properties in many applications. Here, we propose enhanced transmission in wire grid gold structure making use of continuous graphene sheets. We use finite-difference time-domain simulations to study the optical properties of this graphene-metal hybrid structure at mid infrared (mid-IR) wavelengths. The grating structure in wire grid gold provides an ideal platform to match the momentum and excite the surface plasmon polaritons (SPPs) in monolayer graphene. Our numerical calculations show that the local electromagnetic field around the graphene is largely enhanced due to surface plasmons. Moreover, with the highly confined SPPs coupling with the incident light, the transmission through the whole structure can be broadly enhanced in the mid infrared region. We also analyze the effect of the spectrum with different periods and gold nanowire widths to evaluate the size effects of the plasmons in graphene. In addition, by tuning the Fermi level, one can control the wavelength range at which the transmission is enhanced. The mechanism of the enhancement will be explained in the calculated electric field distribution. And we will also highlight the opportunities of graphene for applications such as tunable transmission and active photonic modulator.