Jieran Fang

Enhanced graphene photodetector with fractal metasurface

We designed and fabricated a broadband, polarization-independent photodetector by integrating graphene with a fractal Cayley tree metasurface. Our measurements show an almost uniform, tenfold enhancement in photocurrent generation due to the fractal metasurface structure.

Time-domain modeling of silver nanowires-graphene transparent conducting electrodes

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.

Optical characteristics of vertically aligned arrays of branched silver nanowires

Vertically aligned branched silver nanowires (Ag BNWs) were synthesized by electrodeposition. Typically, branched NWs seen in literature do not have vertical alignment of the branches, as opposed to the results shown here. A wavelength dependent absorption of these Ag BNWs was demonstrated in this work. The strong wavelength dependence of the reflectance and transmittance of these Ag BNWs may be attributed to the wave-guiding effect of the nanostructures.

Studies of plasmonic hot-spot translation by a metal-dielectric layered superlens

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.

Fractal metasurface enhanced graphene photodetector on glass substrate (Conference Presentation)

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.

Lasing boosted with plasmonic nanostructures

Plasmonics, a technique to tightly concentrate light down to the nanoscaleby coupling photons to surface plasmons,has been employed to downscale lasers to sub-wavelength dimensions. The resultant device is called a spaser, short for surface plasmon amplification by stimulated emission of radiation, or more generally plasmonic nanolaser, which is being explored for applications in areas such as sensing and biomedical imaging. In this talk we will first make an overview of the major advances achieved thus far in this emerging field. Then, we will present our work addressing two major challenges regarding wavelength tunablility and lasing directionality of plasmonic nanolasers. To this date, single plasmonic nanoparticles, two-dimensional arrays of nanoapertures in plasmonic metasurfaces, and bulk plasmonic metamaterials have been designed and utilized as plasmonic nanocavities. Lasing wavelength tunability and directionality have been achieved with these nanocavities. Interesting effects have been also observed in other geometries. For instance, the 3-D random nanostructures bring in a possibility to control the lasing resonance by simply tuning the polarization of the pump laser, which is out of reach of conventional dielectric-cavity based lasers. By engineering the absorption and scattering properties, the laser can operate without degradation of lasing performance in the presence of densely packed metal nanostructures; consequently, spatial confinement of lasing modes at micron-scales has been obtained. We also present our two approaches to numerically model realistic spasers and nanolaser arrays. In the frequency domain, the spasers that utilize plasmonic modes of metallic nano-particles are modeled within the classical electrodynamics scattering framework using an intensity-dependent Lorentzian dielectric function, which rigorously accounts for quantum-mechanical saturation effects. In the time domain, we have developed a systematic approach to study lasing in plasmonic nanostructures using a finite difference model coupled to the rate equations of a multi-level gain system (4- and 6-level system). The modeling results show good agreement with experimental data.

Broadband enhanced graphene photodetector with fractal metasurface (Conference Presentation)

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.

Optically pumped plasmonic nanolasers: Experimentally fitted time-resolved multiphysics modeling of lasing dynamics (Invited)

As an example of the light-matter interaction between a plasmonic system with gain media, we develop and analyze a multi-physics time domain model of an optically pumped plasmonic nanolaser. We utilize a classical finite difference time-domain (FDTD) model coupled to the rate equations of a generic 4-level gain system. First, we develop an online tool for the time domain simulation of multi-level gain systems, which is freely available at nanoHUB.org. The tool simulates the local (0-dimension) response of a 4-level gain system interacting with one or two sequential incident light pulses. This tool represents a convenient toy model for retrieving kinetic energy parameters of gain media for further numerical analysis. With the help of parameter tuning using our tool and feedback from experiments, we can further improve our understanding of the time-resolved physics of plasmonic nanostructures with gain. As an example, we study lasing behavior in silver nanohole arrays coated with Rhodamine-101 (R101) dye. The experimentally-fitted time-resolved 3D model is in good agreement with the measured data. The simulated emission intensity shows lasing effect matching with the experimental measurements.

Time-domain model of 4-level gain system fitted to nanohole array lasing experiment

We developed an accurate three dimensional time domain model of a 4-level gain system fitted to lasing experiment with a silver nanohole array. The simulated emission intensity showed clear lasing effects confirmed by optical experiments.

Time-resolved lasing dynamics for plasmonic system with gain (Presentation Recording)

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.