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.

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.

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.

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.

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.

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.

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.

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.

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.

Localized surface plasmon resonance with broadband ultralow reflectivity from metal nanoparticles on glass and silicon subwavelength structures.

Metal nanoparticles (NPs) are well known to increase the efficiency of photovoltaic devices by reducing reflection and increasing light trapping within device. However, metal NPs on top flat surface suffer from high reflectivity losses due to the backscattering of the NPs itself. In this paper, we experimentally demonstrate a novel structure that exhibits localized surface plasmon resonance (LSPR) along with broadband ultralow reflectivity over a wide range of wavelength. Experimental results show that by depositing Ag NPs and Au NPs onto glass subwavelength structures (SWS) the backscattering effect of NPs can be suppressed, and the reflections can be considerably reduced by up to 87.5% and 66.7% respectively, compared to NPs fabricated on a flat glass substrate. Broadband ultralow reflection (< 2%) is also observed in the case of Ag NPs and Au NPs fabricated on cone shaped SWS silicon substrate over a wavelength range from 200 nm to 800 nm. This broadband ultralow reflectivity of Ag NPs and Au NPs on silicon SWS structure leads to a substantial enhancement of average absorption by 66.53% and 66.94%, respectively, over a broad wavelength range (200-2000 nm). This allows light absorption by NPs on SWS silicon structure close to 100% over a wavelength range from 300 nm to 1000 nm. The mechanism responsible for the increased light absorption is also explained.

GaAs based long-wavelength microring resonator optical switches utilising bias assisted carrier-injection induced refractive index change.

We propose and analyse a GaAs-based optical switch having a ring resonator configuration which can switch optical telecommunication signals over the 1300 nm and 1500 nm bands, using bias assisted carrier injection as the switching mechanism. The switching is achieved through variation in the refractive index of the ring resonator produced by changing the injected carrier density through the application of bias voltage. Detail analysis of the switching characteristics reveals that the amount of switching depends on the refractive index change, which indeed is a strong function of injected carrier density and applied bias voltage. An isolation of 25 dB can be achieved during the ON state, while more than 40 dB isolation is realised during the OFF state. More importantly, our analysis shows that the proposed GaAs-based switch can operate over the 1300 nm and 1500 nm optical telecommunication bands, that are much farther from the bandgap of the GaAs material, without the need for "conventional" Indium based ternary and quaternary semiconductor materials. It therefore extends the usable wavelength of GaAs based optoelectronic devices. Furthermore, we have presented detail calculations to quantify power-delay metric of the proposed device. The proposed optical switch maintains a smaller footprint as when compared to Mach-Zehnder Interferometer or Directional Coupler based switches therefore, making it suitable for large scale integration and implementing next generation optical interconnects, optical communication and computing.

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 (V(oc)) and fill factor (FF).

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).

Antireflective disordered subwavelength structure on GaAs using spin-coated Ag ink mask.

We present a simple, cost-effective, large scale fabrication technique for antireflective disordered subwavelength structures (d-SWSs) on GaAs substrate by Ag etch masks formed using spin-coated Ag ink and subsequent inductively coupled plasma (ICP) etching process. The antireflection characteristics of GaAs d-SWSs rely on their geometric profiles, which were controlled by adjusting the distribution of Ag etch masks via changing the concentration of Ag atoms and the sintering temperature of Ag ink as well as the ICP etching conditions. The fabricated GaAs d-SWSs drastically reduced the reflection loss compared to that of bulk GaAs (>30%) in the wavelength range of 300-870 nm. The most desirable GaAs d-SWSs for practical solar cell applications exhibited a solar-weighted reflectance (SWR) of 2.12%, which is much lower than that of bulk GaAs (38.6%), and its incident angle-dependent SWR was also investigated.

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.

Large aperture asymmetric Fabry Perot modulator based on asymmetric tandem quantum well for low voltage operation.

Large aperture image modulators used as demodulator in receiver path are an important component for the use in three dimensional (3D) image sensing. For practical applications, low voltage operation and high modulation performance are the key requirements for modulators. Here, we propose an asymmetric Fabry-Perot modulator (AFPM) with asymmetric tandem quantum wells (ATQWs) for 3D image sensing. By using ATQWs for the AFPM design, the device operated at -4.25V, and the operating voltage was significantly lower by about 23% compared to -5.5V of a conventional AFPM with 8nm thick multiple QW with a single QW thickness (SQWs), while achieving high reflectivity modulation in excess of 50%. The performance of the fabricated devices is in good agreement with theoretical calculations. The pixelated device shows a high modulation speed of 21.8 MHz over a large aperture and good uniformity. These results show that AFPM with ATQWs is a good candidate as an optical image modulator for 3D image sensing applications.

Size-dependent optical behavior of disordered nanostructures on glass substrates.

We demonstrate the distinctive optical properties of disordered nanostructures on glass substrates in accordance with changes in the average size of the nanostructures. Dissimilar sizes of nanostructures were fabricated by using different thicknesses of thermally dewetted Ag nanoparticles as etch masks. Unlike a flat glass substrate, the nanostructured glasses (NSGs) show a changed optical characteristic. By increasing the size of the nanostructures, the wavelength of the peak transmittance of about 99% gradually moved from 730 to 2000 nm. To clearly discern the effect of the different sizes of nanostructures, the normalized angle-dependent transmittance spectra of the NSGs were analyzed. Only if the size becomes relatively larger than the wavelength of the incident light are the transmittance spectra more strongly affected by the incident angle as well as by the relative size, rather than by the Fresnel reflection.

Wafer-scale broadband antireflective silicon fabricated by metal-assisted chemical etching using spin-coating Ag ink.

We report broadband antireflective disordered subwavelength structures (d-SWSs), which were fabricated on 4-inch silicon wafers by spin-coating Ag ink and metal-assisted chemical etching. The antireflection properties of the d-SWSs depend on its dimensions and heights, which were changed by the sintering temperature of the spin-coated Ag ink and etching time. The fabricated d-SWSs drastically reduced surface reflection over a wide range of wavelengths and incident angles, providing good surface uniformity. The d-SWSs with the most appropriate geometry for practical solar cell applications exhibit only 1.23% solar-weighted reflectance in the wavelength range of 300-1100 nm and average reflectance <5% up to an incident angle of 55° in the wavelength range of 300-2500 nm. This simple and low-cost nanofabrication method for antireflection could be of great importance in optical device applications because it allows mass production without any lithography processes or sophisticated equipment.

Antireflective property of thin film a-Si solar cell structures with graded refractive index structure.

We report the antireflective property of thin film amorphous silicon (a-Si) solar cell structures based on graded refractive index structure together with theoretical analysis. Optimizations of the index profile are performed using the rigorous coupled-wave analysis method. The graded refractive index structure fabricated by oblique angle deposition suppresses optical reflection over a wide range of wavelength and incident angle, compared to the conventional structure. The average reflectance of thin film a-Si solar cell structure with the graded refractive index structure is suppressed by 54% at normal incidence due to the effective refractive index matching between ITO and a-Si, indicating a reasonable agreement with calculated results.