AuAg Bimetallic Non-Alloyed Nanoparticles on SiO2 Spacer Layer for Improved Light Absorption in Thin-Film c-Si Solar Cells.

We present a light trapping structure consisting of gold and silver (AuAg) bimetallic non-alloyed nanoparticles (BNNPs) on a silicon dioxide (SiO2) spacer layer over crystalline silicon (c-Si) film, designed to improve the absorption of thin-film c-Si solar cells. Prior to fabrication of the AuAg BNNPs on the SiO2 spacer layer, numerical investigations were carried out using electromagnetic field simulation following the finite-difference time-domain method. The hemispherical Au8Ag8 BNNPs were fabricated and deposited on a 15 nm-thick SiO2 spacer layer, which enhanced light trapping in the c-Si film over a broad wavelength range (450-1100 nm). Specifically, more than 85% of the incident light was absorbed in the c-Si film at 620 nm wavelengths due to the strong scattering of the Au8Ag8 BNNPs. To the best of our knowledge this is the first case presenting such a theoretical calculation and experimental study of the efficient light trapping by AuAg BNNPs on space layer for increasing the absorption in thin-film c-Si solar cells.

III-nitride core-shell nanorod array on quartz substrates.

We report the fabrication of near-vertically elongated GaN nanorods on quartz substrates. To control the preferred orientation and length of individual GaN nanorods, we combined molecular beam epitaxy (MBE) with pulsed-mode metal-organic chemical vapor deposition (MOCVD). The MBE-grown buffer layer was composed of GaN nanograins exhibiting an ordered surface and preferred orientation along the surface normal direction. Position-controlled growth of the GaN nanorods was achieved by selective-area growth using MOCVD. Simultaneously, the GaN nanorods were elongated by the pulsed-mode growth. The microstructural and optical properties of both GaN nanorods and InGaN/GaN core-shell nanorods were then investigated. The nanorods were highly crystalline and the core-shell structures exhibited optical emission properties, indicating the feasibility of fabricating III-nitride nano-optoelectronic devices on amorphous substrates.

Light trapping enhancement induced by bimetallic non-alloyed nanoparticles on a disordered subwavelength flexible thin film crystalline silicon substrate using metal-assisted chemical etching.

We report the application of gold and silver (AuAg) bimetallic non-alloyed nanoparticles (BNNPs) on disordered subwavelength structures (d-SWSs). The combined advantages of the plasmonic structures and d-SWSs improved the light trapping performance of flexible thin film crystalline silicon (c-Si) solar cells. Antireflective d-SWSs were fabricated using spin-coated Ag ink and subsequent metal-assisted chemical etching, which reduced the ion-induced surface damage produced by the dry etching process. The dimensions of the d-SWSs were finely tuned by adjusting the Ag ink ratio. Au8Ag8 BNNPs were employed on optimized d-SWSs to achieve low reflectance at broadband wavelengths. The Au8Ag8 BNNPs on the d-SWSs showed 180% and 145% enhanced absorption compared to bare c-Si film and Au8Ag8 BNNPs on c-Si film, respectively, in the wavelength range of 300-1100 nm. After 200 cycles of bending the antireflection of the structures remained similar to the original level. This study introduces new approaches for light management in flexible thin film c-Si solar cells over the broadband wavelength range.

Supersymmetrical bounding of asymmetric states and quantum phase transitions by anti-crossing of symmetric states.

Von Neumann and Wigner theorized the bounding and anti-crossing of eigenstates. Experiments have demonstrated that owing to anti-crossing and similar radiation rates, the graphene-like resonance of inhomogeneously strained photonic eigenstates can generate a pseudomagnetic field, bandgaps and Landau levels, whereas exponential or dissimilar rates induce non-Hermicity. Here, we experimentally demonstrate higher-order supersymmetry and quantum phase transitions by resonance between similar one-dimensional lattices. The lattices consisted of inhomogeneous strain-like phases of triangular solitons. The resonance created two-dimensional, inhomogeneously deformed photonic graphene. All parent eigenstates were annihilated. Eigenstates of mildly strained solitons were annihilated at similar rates through one tail and generated Hermitian bounded eigenstates. The strongly strained solitons with positive phase defects were annihilated at exponential rates through one tail, which bounded eigenstates through non-Hermitianally generated exceptional points. Supersymmetry was evident, with preservation of the shapes and relative phase differences of the parent solitons. Localizations of energies generated from annihilations of mildly and strongly strained soliton eigenstates were responsible for geometrical (Berry) and topological phase transitions, respectively. Both contributed to generating a quantum Zeno phase, whereas only strong twists generated topological (Anderson) localization. Anti-bunching-like condensation was also observed.

Shape-controllable, bottom-up fabrication of microlens using oblique angle deposition.

This Letter reports a novel method for the simple fabrication of microlens arrays with a controlled shape and diameter on glass substrates. Multilayer stacks of silicon dioxide deposited by oblique angle deposition with hole mask patterns enable microlens formation. Precise control of mask height and distance, as well as oblique angle steps between deposited layers, supports the controllability of microlens geometry. The fabricated microlens arrays with designed geometry exhibit uniform optical properties.

4 channel × 10 Gb/s bidirectional optical subassembly using silicon optical bench with precise passive optical alignment.

We demonstrate an advanced structure for optical interconnect consisting of 4 channel × 10 Gb/s bidirectional optical subassembly (BOSA) formed using silicon optical bench (SiOB) with tapered fiber guiding holes (TFGHs) for precise and passive optical alignment of vertical-cavity surface-emitting laser (VCSEL)-to-multi mode fiber (MMF) and MMF-to-photodiode (PD). The co-planar waveguide (CPW) transmission line (Tline) was formed on the backside of silicon substrate to reduce the insertion loss of electrical data signal. The 4 channel VCSEL and PD array are attached at the end of CPW Tline using a flip-chip bonder and solder pad. The 12-channel ribbon fiber is simply inserted into the TFGHs of SiOB and is passively aligned to the VCSEL and PD in which no additional coupling optics are required. The fabricated BOSA shows high coupling efficiency and good performance with the clearly open eye patterns and a very low bit error rate of less than 10-12 order at a data rate of 10 Gb/s with a PRBS pattern of 231-1.

AuAg bimetallic nonalloyed nanoparticles on a periodically nanostructured GaAs substrate for enhancing light trapping.

We present a light trapping structure consisting of AuAg bimetallic nonalloyed nanoparticles (BNNPs) on cone-shaped GaAs subwavelength structures (SWSs), combining the advantages of plasmonic structures and SWSs for GaAs-based solar cell applications. To obtain efficient light trapping in solar cells, the optical properties' dependence on the size and composition of the Ag and Au metal nanoparticles was systematically investigated. Cone-shaped GaAs SWSs with AuAg BNNPs formed from an Au film of 12 nm and an Ag film of 10 nm exhibited the extremely low average reflectance (R(avg)) of 2.43% and the solar-weighted reflectance (SWR) of 2.38%, compared to that of a bare GaAs substrate (R(avg), 37.50%; SWR, 36.72%) in the wavelength range of 300 to 870 nm.

Directional Terahertz Radiation from GalnP Lateral Superlattice.

We devised directionally controllable THz emission sources based on lateral composition modulation (LCM) structures. LCM structures were composed of In-rich Ga0.47In0.53P and Ga-rich Ga0.51In0.49P layers whose period was in quantum scale of ~`5 nm. The inherent type II band alignment in these structures leads to electron-hole (e-h) separation and plays a key role in generating later- ally polarized dipole ensembles, thus concomitantly emitting enhanced transmissive THz waves as compared to bulk sample. On the other hand, in lateral geometry, changes in THz fields between LCM and bulk structures turned out to negligible since the vertical electronic diffusion was allowed in both samples.

Electro-absorption modulator with an asymmetric coupled triple quantum well for low-voltage operation.

We present an electro-absorption modulator based on the enhanced electro-optic effect of an asymmetric coupled triple quantum well (ACTQW) to achieve a large transmittance difference at a low driving voltage for high-definition (HD) three-dimensional (3D) imaging applications. Our numerical calculations show that an ACTQW structure can provide a significantly lower-voltage operation without degrading the absorption coefficient change at the operating wavelength of 850 nm. The fabricated electro-absorption modulator (EAM) based on an ACTQW shows that the operating voltage can be reduced by nearly 50% compared with an EAM based on a conventional rectangular quantum well while also achieving a large transmittance difference in excess of 50%, which is in good agreement with the numerical calculation results. These results suggest that using an EAM with an ACTQW is a promising approach for the realization of a high-resolution 3D imaging system.

Bi-SERS sensing and enhancement by Au-Ag bimetallic non-alloyed nanoparticles on amorphous and crystalline silicon substrate.

We have demonstrated Au-Ag bimetallic non-alloy nanoparticles (BNNPs) on thin a-Si film and c-Si substrate for high SERS enhancement, low cost, high sensitivity and reproducible SERS substrate with bi-SERS sensing properties where two different SERS peak for Au NPs and Ag NPs are observed on single SERS substrate. The isolated Au-Ag bimetallic NPs, with uniform size and spacing distribution, are suitable for uniform high density hotspot SERS enhancement. The SERS enhancement factor of Au-Ag BNNPs is 2.9 times higher compared to Ag NPs on similar substrates due to the increase of the localized surface plasmon resonance effect. However there is a decrement of SERS peak intensity at specific wavenumbers when the surrounding refractive index increases due to out-phase hybridization of Au NPs. The distinct changes of the two different SERS peaks on single Au-Ag BNNPs SERS substrate due to Au and Ag NPs independently show possible application for bi-molecular sensing.

Observation and tunability of room temperature photoluminescence of GaAs/GaInAs core-multiple-quantum-well shell nanowire structure grown on Si (100) by molecular beam epitaxy.

We report the observation of room temperature photoluminescence (PL) emission from GaAs/GaInAs core-multiple-quantum-well (MQW) shell nanowires (NWs) surrounded by AlGaAs grown by molecular beam epitaxy (MBE) using a self-catalyzed technique. PL spectra of the sample show two PL peaks, originating from the GaAs core NWs and the GaInAs MQW shells. The PL peak from the shell structure red-shifts with increasing well width, and the peak position can be tuned by adjusting the width of the MQW shell. The GaAs/GaInAs core-MQW shell NW surrounded by AlGaAs also shows an enhanced PL intensity due to the improved carrier confinement owing to the presence of an AlGaAs clad layer. The inclined growth of the GaAs NWs produces a core-MQW shell structure having a different PL peak position than that of planar QWs. The PL emission by MQW shell and the ability to tune the PL peak position by varying the shell width make such core-shell NWs highly attractive for realizing next generation ultrasmall light sources and other optoelectronics devices. PACS: 81.07.Gf; 81.15.Hi; 78.55.Cr.

Bimetallic non-alloyed NPs for improving the broadband optical absorption of thin amorphous silicon substrates.

We propose the use of bimetallic non-alloyed nanoparticles (BNNPs) to improve the broadband optical absorption of thin amorphous silicon substrates. Isolated bimetallic NPs with uniform size distribution on glass and silicon are obtained by depositing a 10-nm Au film and annealing it at 600°C; this is followed by an 8-nm Ag film annealed at 400°C. We experimentally demonstrate that the deposition of gold (Au)-silver (Ag) bimetallic non-alloyed NPs (BNNPs) on a thin amorphous silicon (a-Si) film increases the film's average absorption and forward scattering over a broad spectrum, thus significantly reducing its total reflection performance. Experimental results show that Au-Ag BNNPs fabricated on a glass substrate exhibit resonant peaks at 437 and 540 nm and a 14-fold increase in average forward scattering over the wavelength range of 300 to 1,100 nm in comparison with bare glass. When deposited on a 100-nm-thin a-Si film, Au-Ag BNNPs increase the average absorption and forward scattering by 19.6% and 95.9% compared to those values for Au NPs on thin a-Si and plain a-Si without MNPs, respectively, over the 300- to 1,100-nm range.

Broadband antireflective silicon nanostructures produced by spin-coated Ag nanoparticles.

We report the fabrication of broadband antireflective silicon (Si) nanostructures fabricated using spin-coated silver (Ag) nanoparticles as an etch mask followed by inductively coupled plasma (ICP) etching process. This fabrication technique is a simple, fast, cost-effective, and high-throughput method, making it highly suitable for mass production. Prior to the fabrication of Si nanostructures, theoretical investigations were carried out using a rigorous coupled-wave analysis method in order to determine the effects of variations in the geometrical features of Si nanostructures to obtain antireflection over a broad wavelength range. The Ag ink ratio and ICP etching conditions, which can affect the distribution, distance between the adjacent nanostructures, and height of the resulting Si nanostructures, were carefully adjusted to determine the optimal experimental conditions for obtaining desirable Si nanostructures for practical applications. The Si nanostructures fabricated using the optimal experimental conditions showed a very low average reflectance of 8.3%, which is much lower than that of bulk Si (36.8%), as well as a very low reflectance for a wide range of incident angles and different polarizations over a broad wavelength range of 300 to 1,100 nm. These results indicate that the fabrication technique is highly beneficial to produce antireflective structures for Si-based device applications requiring low light reflection.

Antireflective grassy surface on glass substrates with self-masked dry etching.

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.

High-efficiency light-trapping effect using silver nanoparticles on thin amorphous silicon subwavelength structure.

In this Letter, we experimentally demonstrate a hybrid structure consisting of metal nanoparticles deposited onto a subwavelength structure (SWS), which further increases the absorption of thin amorphous silicon (a-Si) and can possibly lead to a reduction in the minimum required thickness of the a-Si layer. Experimental results show that backscattering of the silver nanoparticles (Ag NPs) deposited on the top surface can be suppressed dramatically (by 85.5%) by the Ag NPs deposited on the SWS. We also experimentally prove that the thin a-Si SWS only lowers the surface reflectivity and does not increase the absorption rate of the material. The absorption of the thin a-Si layer can be increased by depositing Ag NPs onto a thin a-Si SWS, which not only reduces the backscattering of the metal NPs but also increases the light-trapping effect within thin a-Si through localized surface plasmon resonance properties. This decrease of reflection and increase in the light-trapping effect of Ag NPs on cone-shaped thin a-Si SWSs leads to extremely high average absorption (86.14%) within a 400 nm thick a-Si layer.

Antireflective silicon nanostructures with hydrophobicity by metal-assisted chemical etching for solar cell applications.

We present broadband antireflective silicon (Si) nanostructures with hydrophobicity using a spin-coated Ag ink and by subsequent metal-assisted chemical etching (MaCE). Improved understanding of MaCE, by conducting parametric studies on optical properties, reveals a design guideline to achieve considerably low solar-weighted reflectance (SWR) in the desired wavelength ranges. The resulting Si nanostructures show extremely low SWR (1.96%) and angle-dependent SWR (<4.0% in the range of 0° to 60°) compared to that of bulk Si (SWR, 35.91%; angle-dependent SWR, 37.11%) in the wavelength range of 300 to 1,100 nm. Relatively large contact angle (approximately 102°) provides a self-cleaning capability on the solar cell surface.

Optical absorption enhancement of hybrid-plasmonic-based metal-semiconductor-metal photodetector incorporating metal nanogratings and embedded metal nanoparticles.

We propose and numerically demonstrate a high absorption hybrid-plasmonic-based metal semiconductor metal photodetector (MSM-PD) comprising metal nanogratings, a subwavelength slit and amorphous silicon or germanium embedded metal nanoparticles (NPs). Simulation results show that by optimizing the metal nanograting parameters, the subwavelength slit and the embedded metal NPs, a 1.3 order of magnitude increase in electric field is attained, leading to 28-fold absorption enhancement, in comparison with conventional MSM-PD structures. This is 3.5 times better than the absorption of surface plasmon polariton (SPP) based MSM-PD structures employing metal nanogratings and a subwavelength slit. This absorption enhancement is due to the ability of the embedded metal NPs to enhance their optical absorption and scattering properties through light-stimulated resonance aided by the conduction electrons of the NPs.

Coupled tandem cavities based electro-absorption modulator with asymmetric tandem quantum well for high modulation performance at low driving voltage.

We propose and demonstrate a new electro-absorption modulator (EAM) based on coupled tandem cavities (CTC) having asymmetric tandem quantum well (ATQW) structure with separated electrode configuration to achieve large transmittance change over a broad spectral range at low driving voltage for high definition (HD) 3D imaging applications. Our theoretical calculations show that CTC with ATQW structure can provide large transmittance change over a wide spectral range at low driving voltage. By introducing separated electrode configuration, the fabricated EAM having CTC with ATQW structure shows a large transmittance change over 50%, almost three times larger spectral bandwidth compared to that of EAM having single cavity with a single thickness quantum well without significantly increasing the applied voltage. In addition, the CTC with ATQW structure also shows high speed modulation up to 28 MHz for the device having a large area of 2 mm x 0.5 mm. This high transmittance change, large spectral bandwidth and low voltage operation over a large device area for the EAM having CTC with ATQW demonstrates their huge potential as an optical image modulator for HD 3D imaging applications.

Enhanced power generation in concentrated photovoltaics using broadband antireflective coverglasses with moth eye structures.

We present the effect of broadband antireflective coverglasses (BARCs) with moth eye structures on the power generation capability of a sub-receiver module for concentrated photovoltaics. The period and height of the moth eye structures were designed by a rigorous coupled-wave analysis method in order to cover the full solar spectral ranges without transmission band shrinkage. The BARCs with moth eye structures were prepared by the dry etching of silver (Ag) nanomasks, and the fabricated moth eye structures on coverglass showed strongly enhanced transmittance compared to the bare glass with a flat surface, at wavelengths of 300 - 1800 nm. The BARCs were mounted on InGaP/GaAs/Ge triple-junction solar cells and the power conversion efficiency of this sub-receiver module reached 42.16% for 196 suns, which is a 7.41% boosted value compared to that of a module with bare coverglass, without any detrimental changes of the open circuit voltages (Voc) and fill factor (FF).

Wide spectral bandwidth electro-absorption modulator using coupled micro-cavity with asymmetric tandem quantum well.

For reliable three dimensional (3D) imaging system, it is necessary for the optical shutter to have a wide spectral bandwidth operation and enhanced modulation depth. We propose an electro-absorption modulator (EAM) based on coupled Fabry-Perot cavities with micro-cavity (CCMC) which uses asymmetric tandem quantum wells (ATQWs) to obtain improved spectral bandwidth and enhanced modulation depth. Several modulator designs are investigated to obtain improved modulation performance such as wider spectral bandwidth and enhanced modulation depth. It was found that among all the studied modulator geometries, CCMC structure with ATQWs provides the widest spectral bandwidth of 9.6nm and high modulation depth in excess of 50% at -24V, which is good agreement with theoretical calculations. These results suggest that EAM has excellent potential as optical shutter for 3D imaging application.