In this paper, we report the theoretical and experimental possibility of achieving a quarter-wave plate regime
by using high-contrast gratings, which are binary, vertical, periodic, near-wavelength, and two-dimensional high
refractive index gratings. Here, we investigate the characteristics of two distinct designs, the first one being composed
of silicon-dioxide and silicon, and the second one being composed of silicon and sapphire. The suggested quarter-wave plate regime is achieved by the simultaneous optimization of the transverse electric and transverse magnetic transmission coefficients, TTE and TTM, respectively, and the phase difference between these transmission coefficients, such that |TTM| ≃ |TTE| and ∠TTM − ∠TTE ≃ π/2. As a result, a unity circular polarization conversion efficiency is achieved at λ0 = 1.55 μm for both designs. For the first design, we show the obtaining of unity conversion efficiency by using a theoretical approach, which is inspired by the periodic waveguide interpretation, and rigorous coupled-wave analysis (RCWA). For the second design, we demonstrate the unity conversion efficiency by using the results of finite-difference time-domain (FDTD) simulations. Furthermore, the FDTD simulations, where material dispersion is taken into account, suggest that an operation percent bandwidth of 51% can be achieved for the first design, where the experimental results for the second design yield a bandwidth of 33%. In this context, we define the operation regime as the wavelength band for which the circular conversion efficiency is larger than 0.9.
We investigate the absorption characteristics of InGaN solar cells with high indium (0.8) content and a one-dimensional periodic nano-scale pattern (implemented) in the InGaN layer theoretically. The short-circuit current of our InGaN-based solar cell structure is calculated for different lattice constant, etch depth, and fill factor values. A substantial increase in the absorption (17.5% increase in short-circuit current) is achieved when the photonic crystal pattern is thoroughly optimized.
We designed, fabricated and characterized AlxGa1- xAs/GaAs p-i-n resonant cavity enhanced (RCE) photodetectors with near-unity quantum efficiency. The peak wavelength is in the 780 - 830 nm region and post-process adjustable by recessing the top surface. Transit time limited bandwidth for these devices is in excess of 50 GHz. Possible applications of these detectors include conventional measurements of low light levels, quantum optical experiments that use pulsed sources and short-haul high speed communications.
In this paper, we review our research efforts on RCE high- speed high-efficiency p-i-n and Schottky photodiodes. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs based RCE photodiodes. For RCE Schottky photodiodes, we have achieved a peak quantum efficiency of 50% along with a 3-dB bandwidth of 100 GHz. The tunability of the detectors via a recess etch is also demonstrated. For p-i-n type photodiodes, we have fabricated and tested widely tunable devices with near 100% quantum efficiencies, along with a 3-dB bandwidth of 50 GHz. Both of these results correspond to the fastest RCE photodetectors published in scientific literature.
Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is very desirable. In RCE structures, the electrical function of the photodiode is largely unchanged, but optically it is subject to the effects of the cavity, mainly wavelength selectivity and a large enhancement of the resonant optical field. The increased optical field allows photodetectors to be made thinner and therefor faster in the transit-time limited operation, while simultaneously maintaining a high quantum efficiency at the resonant wavelengths. The combination of RCE detection scheme with Schottky photodiodes allows for fabrication of high-performance photodetectors with relatively simple material structure and fabrication process. In RCE Schottky photodiodes, a semi-transparent metalization can be used simultaneously as the electrical contact and the top reflector for the resonant cavity. Device performance is optimized by varying the thickness of the Schottky metalization and utilizing a dielectric matching layer. We present theoretical and experimental results on spectral and high-speed properties. We have demonstrated RCE Schottky photodiodes in (Al, In)GaAs/GaAs material system with temporal response of 10 ps full-width-at-half-maximum. These results were measurement setup limited and a conservative estimation of the bandwidth corresponds to more than 100 GHz. The photodiodes were designed and fabricated for 900 nm and 840 nm resonant wavelengths. The best measured quantum efficiency is around 50% which is slightly less than the theoretical prediction for these devices.