Graphene has been demonstrated to be a promising photodetection material because of its atomic-thin nature, broadband and uniform optical absorption, etc. Photovoltaic and photothermoelectric, which are considered to be the main contributors to photo current/voltage generation in graphene, enable photodetection through driving electrons via built-in electric field and thermoelectric power, respectively. Graphene photovoltaic/photothermoelectric detectors are ideal for ultrafast photodetection applications due to the high carrier mobilities in graphene and ultrashort time the electrons need to give away heat. Despite all the advantages for graphene photovoltaic/photothermoelectric detectors, the sensitivity in such detectors is relatively low, owing to the low optical absorption in the single atomic layer. In the past, our research group has used delicately designed snowflake-like fractal metasurface to realize broadband photovoltage enhancement in the visible spectral range, on SiO2 thin film backed by Si substrates. We have also demonstrated that the enhancement from the proposed fractal metasurface is insensitive to the polarization of the incident light. In this current work, we have carried out experiments of the same fractal metasurface on transparent SiO2 substrates, and obtained higher enhancement factor on the fractal metasurface than that achieved on SiO2/Si substrates. Moreover, the device allows more than 70% of the incident light to transmit during the detection, enabling photodetection in the optical path without any significant distortion. Another possibility to make use of the large portion of transmitted light is to stack multiple such devices along the optical path to linearly scale up the sensitivity.
Graphene has been demonstrated to be a promising photo-detection material because of its ultra-broadband absorption, compatibility with CMOS technology, and dynamic tunability. There are multiple known photo-detection mechanisms in graphene, among which the photovoltaic effect has the fastest response time thus is the prioritized candidate for ultrafast photodetector. There have been numerous efforts to enhance the intrinsically low sensitivity in graphene photovoltaic detectors using metallic plasmonic structures, but such plasmonic enhancements are mostly narrowband and polarization dependent. In this work, we propose a gold Cayley-tree fractal metasurface design that has a multi-band resonance, to realize broadband and polarization-insensitive plasmonic enhancement in graphene photovoltaic detectors. When illuminated with visible light, the fractal metasurface exhibits multiple hotspots at the metal-graphene interface, where the electric field of the incident electromagnetic wave is enhanced and contributes to generating excess electron-hole pairs in graphene. The large metal-graphene interface length in the fractal metasurface also helps to harvest at a higher efficiency the electron-hole pairs by built-in electric field due to metal-graphene potential gradient. To demonstrate the concept, we carried out experiment using Ar-Kr CW laser, an optical chopper, and lock-in amplifier. We obtained experimentally an almost constant ten-fold enhancement of photocurrent generated on the fractal metasurface compared to that generated on the plain gold-graphene edge, at all tested wavelengths (488 nm, 514 nm, 568 nm, and 647 nm). We also observed an unchanged photoresponse with respect to incident light polarization angles, which is a result of the highly symmetric geometry of the fractal metasurface.
To study the light-matter interaction between plasmonic systems and gain media, numerous theoretical and numerical methods have been proposed. Among them, because of its accurate treatment of the quantum property of gain media, the time domain (TD) multi-physics approach is viewed as the most powerful method, especially for analysis of transient dynamics. Even though the finite difference, finite-volume and finite element TD methods can be readily coupled to a multi-level atomic system through auxiliary differential equations, for each of them however there is limited information on accurate TD kinetic parameters fitted with experimental measurements. In this work, we develop a multi-physics time domain model to inspect our most recent lasing experiment with a silver nanohole array. We use a classical finite difference time-domain (FDTD) model coupled to the rate equations of a 4-level gain system. To retrieve kinetic energy parameters for accurate modeling, we first fit 1-D simulations with pump-probe experiments studying Rhodamine-101 (R-101) dye embedded in epoxy on an indium tin oxide silica substrate. The retrieved parameters are then fed into a 3-D model to study the lasing behavior of the R-101-coated nanohole array. The simulated emission intensity shows a clear lasing effect, which is in good agreement with the experimental measurements. By tracing the population inversion and polarization dynamics, the amplification and lasing regimes inside the nanohole cavity can be clearly distinguished. With the help of our systematic approach, we can further improve understanding of the time-resolved physics of active plasmonic nanostructures with gain.
Transparent conducting electrodes (TCE) consisting of silver nanowires (SNW) with a single-layer graphene (SLG)
cover demonstrate higher optical transparency and lower sheet resistance than indium tin oxide (ITO) and are
comparable to the best reported results in TCEs. SNW layer is simulated using the spectral averaging of the FDTD
transmittance data from indiscriminately selected frames. Simulations are done for a number of frames until a
convergent set of averaged spectra is obtained. SLG layer is simulated separately and contributes to the total
transmittance as a multiplicative constant.
We have studied the ability of a lamellar near-field superlens to transfer an enhanced electromagnetic field to the far side
of the lens. In this work, we have experimentally and numerically investigated superlensing in the visible range. By
using the resonant hot-spot field enhancements from optical nanoantennas as sources, we investigated the translation of
these sources to the far side of a layered silver-silica superlens operating in the canalization regime. Using near-field
scanning optical microscopy (NSOM), we have observed evidence of superlens-enabled enhanced-field translation at a
wavelength of about 680 nm. Specifically, we discuss our recent experimental and simulation results on the translation of
hot spots using a silver-silica layered superlens design. We compare the experimental results with our numerical
simulations and discuss the perspectives and limitations of our approach.
Historically, the methods used to describe the electromagnetic response of random, three-dimensional (3D), metal-dielectric composites (MDCs) have been limited to approximations such as effective-medium theories that employ easily-obtained, macroscopic parameters. Full-wave numerical simulations such as finite-difference time domain (FDTD) calculations are difficult for random MDCs due to the fact that the nanoscale geometry of a random composite is generally difficult to ascertain after fabrication. We have developed a fabrication method for creating semicontinuous metal films with arbitrary thicknesses and a modeling technique for such films using realistic geometries. We extended our two-dimensional simulation method to obtain realistic geometries of 3D MDC samples, and we obtained the detailed near- and far-field electromagnetic responses of such composites using FDTD calculations. Our simulation results agree quantitatively well with the experimentally measured far-field spectra of the real samples.