Shiva Shahin

Ultrathin organic bulk heterojunction solar cells: Plasmon enhanced performance using Au nanoparticles

The plasmonic effect of gold nanoparticles (AuNPs) enhances light absorption and, thus, the efficiency of organic bulk heterojunction solar cells with poly (3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as active layer. We report optimization of this enhancement by varying the attachment density of the self-assembled AuNPs on silanized ITO using N1-(3-trimethoxysilylpropyl)diethylenetriamine. Using finite difference time domain simulations, the thicknesses of poly (3,4-ethylenedioxythiophene) (PEDOT): poly (styrenesulfonate) (PSS) and P3HT:PCBM layers were suitably varied to ensure broadband optical absorption enhancement and minimal exciton quenching within the active layer. Our experimental results demonstrate that for solar cell structures with 20% surface coverage, absorption is increased by 65% as predicted by simulations. Further, we show that AuNPs increase the efficiency by 30% and that silanization of ITO positively impacts device performance.

Luminescent hyperbolic metasurfaces

When engineered on scales much smaller than the operating wavelength, metal-semiconductor nanostructures exhibit properties unobtainable in nature. Namely, a uniaxial optical metamaterial described by a hyperbolic dispersion relation can simultaneously behave as a reflective metal and an absorptive or emissive semiconductor for electromagnetic waves with orthogonal linear polarization states. 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 controlled reduction of polarization anisotropy. We verify hyperbolic dispersion with numerical simulations that model the metasurface as a composite nanoscale structure and according to the effective medium approximation. Finally, we experimentally demonstrate >350% emission intensity enhancement relative to the bare semiconducting quantum wells.

Gain-enhanced high-k transmission through metal-semiconductor hyperbolic metamaterials

We analyze the steady-state transmission of high-momentum (high-k) electromagnetic waves through metal-semiconductor multilayer systems with loss and gain in the near-infrared (NIR). Using a semi-classical optical gain model in conjunction with the scattering matrix method (SMM), we study indium gallium arsenide phosphide (InGaAsP) quantum wells as the active semiconductor, in combination with the metals, aluminum-doped zinc oxide (AZO) and silver (Ag). Under moderate external pumping levels, we find that NIR transmission through Ag/InGaAsP systems may be enhanced by several orders of magnitude relative to the unpumped case, over a large angular and frequency bandwidth. Conversely, transmission enhancement through AZO/InGaAsP systems is orders of magnitude smaller, and has a strong frequency dependence. We discuss the relative importance of Purcell enhancement on our results and validate analytical calculations based on the SMM with numerical finite-difference time domain simulations.

Multiphoton microscopy as a detection tool for photobleaching of EO materials

Multi-photon microscopy operating at 1550 nm is employed as a rapid characterization tool for studying the photostability of three well-known electro-optical materials. Different nonlinear optical responses such as multi-photon excitation fluoresence, second-, and third-harmonic generation can be used as detection probes to reveal the degradation mechanisms. This technique is rapid, accurate, and can be used to study the photostability of a broad range of materials.

Ultra-thin organic photovoltaics with increased efficiency

Demand for cheap renewable energy sources is driving the development of organic photovoltaic devices (OPVs), which are generally lower in cost to develop than their silicon counterparts. In comparison with silicon, however, OPVs have a relatively low conversion efficiency. An OPV typically consists of an active material sandwiched between two electrodes (see Figure 1). When the system is illuminated, photons are absorbed inside the active layer, generating pairs of charge carriers, or excitons. To be an effective power source, these photogenerated excitons need to be dissociated into electrons and holes, to allow charge collection at the electrodes. This process must occur within the lifetime of the excitons, to avoid recombination.1 The active layer in organic solar cells is commonly around 100–300nm thick, because this results in practically complete light absorption in the spectral region near the solar maximum at 530nm. However, in organic materials, performance is then hampered by short exciton diffusion lengths (the distance an exciton can travel before the electron and hole recombine) and poor charge transport (especially electrons), leading to disappointing charge collection. As a result, an OPV’s efficiency is compromised by its relatively low photon absorption in ultrathin ( 50nm) active layer structures. One possible solution for capturing the light more efficiently is the application of metal nanoparticles (MNPs).2 MNPs enhance the absorption of solar radiation by near-field enhancement—when the large electric field (E-field) around the particles increases the probability of excitons dissociating into electrons and holes— and by increasing the forward scattering cross section (a measure of how much of the incoming light is scattered in the forward direction). The optical absorption spectra of MNPs depend on their size, so by tuning the size of the nanoparticles, we can achieve significant absorption in different parts of solar spectrum.3 We have devised a solar cell structure that takes advantage of E-field enhancement from 50nm-diameter gold nanoparticles Figure 1. The process of photon absorption to photocurrent generation in a bulk heterojunction organic solar cell. ITO: Indium tin oxide. PEDOT:PSS: Buffer layer. P3HT, PCBM: Electron donor and acceptor, respectively.

Plasmonic-enhanced organic solar cells

Organic bulk-heterojunction solar cells have several good characteristics, such as ease of fabrication, and low-cost materials. However, the bottleneck in their adoption is their much lower efficiency as compared with their silicon counterparts. In our previous work, we demonstrated that by appropriately inserting AuNPs in the OPV device, the efficiency can be increased by 30% and that silanization of ITO positively impacts device performance, where we identified the field enhancement due to AuNPs as the main reason for the increase in the efficiency of the device. In this work, we further investigate the impact of self-assembly of the gold nanoparticles on the efficiency by also considering two other factors which can possibly contribute to the improvement of our structure’s performance. One is the change in the substrate’s workfunction after silanization, and the other factor is the variations in PEDOT: PSS characteristics due to the AuNPs’ plasmonic resonance. We conclude that the AuNPs not only increase the photon absorption efficiency but also increase the conductivity of the surrounding medium (PEDOT: PSS) thereby facilitating charge transport through PEDOT: PSS.

Silicon nanoridge array waveguides for nonlinear and sensing applications.

We fabricate and characterize waveguides composed of closely spaced and longitudinally oriented silicon ridges etched into silicon-on-insulator wafers. Through both guided mode and bulk measurements, we demonstrate that the patterning of silicon waveguides on such a deeply subwavelength scale is desirable for nonlinear and sensing applications alike. The proposed waveguide geometry simultaneously exhibits comparable propagation losses to similar schemes proposed in literature, an enhanced effective third-order nonlinear susceptibility, and high sensitivity to perturbations in its environment.

SPM spectral broadening compensation using organic dyes with negative n 2

Spectral broadening caused by SPM degrades performance of light-wave, multichannel communication systems. We propose a novel method in LCOF to compensate the SPM spectral broadening in systems using different concentrations of negative n2 organic dyes.

Active hyperbolic metasurfaces at telecommunication frequencies

Summary form only given. Hyperbolic metasurfaces (HMS) combine the potential for chip-scale integration of optical metasurfaces with the properties of hyperbolic dispersion. In the ideal, lossless effective medium limit, HMS have an unbound optical density of states (DOS). Thelossless effective medium limit unbound DOS enables infinite mode densities in waveguides and cavities, and, in principal, an infinitely confined modal energy. The finite periodicity of real HMS, however, places an upper momentum limit on the optical DOS. Additionally, dissipation losses in real HMS limit the utility of high momentum states. To assess the possibility of reducing propagation losses in hyperbolic metamaterials, various studies have analyzed the effect of incorporating gain media. While early studies focused on using dye molecules for visible gain, more recently, indium gallium arsenide phosphide (InGaAsP) multiple quantum wells (MQW) has emerged as a viable candidate for gain at telecommunication frequencies. Herein, we report on the demonstration of active hyperbolic metasurfaces (HMS) at telecommunication frequencies. Built from nanostructured silver and InGaAsP MQW, the HMS exhibit hyperbolic dispersion across the near-infrared part of the spectrum. When driven by an external optical pump at 1064 nm, the HMS emit broadband radiation in the 1200 nm-1600nm spectral range. Extreme anisotropy of total emission with respect to the pump polarization is observed, with peak PL differing by more than an order of magnitude for orthogonal pump polarization states. Additionally, we measure degree-of-linear-polarization of the emission as high as 0.9. Experimental results are understood through analytical and numerical modeling, which illustrate the efficacy of the effective medium approximation. Finally, we report on device- and circuit-level applications of the active HMS.

Solving optimization problems with coupled dynamical elements

We propose a new approach to solve optimization problems using Generalized Lotka-Volterra with global inhibition. Stability criteria ensure the convergences of valid solutions are obtained and the solution is given by the switching time of the network elements. A network of coupled lasers is proposed.