Comparison of Cross-section Profile Designs for Integrated Polarization Mode Controllers

Many laser applications in quantum technology require circularly polarized light at wavelengths accessible only through the GaAs-AlGaAs material system. Two proven designs of polarization mode convertor (PMC) were compared through simulation for a modified commercial 830 nm GaAs-AlGaAs structure. Structure type 1 used a deep-etched ridge waveguide, with a shallow-etched slot placed asymmetrically within the ridge. Structure type 2 also used a deep-etched ridge, but with sides of the ridge etched to different depths. Both structures support zero-order TE and TM modes, with the effective index difference between the modes determining the length of the PMC device. Although a range of dimensions and compositions was investigated, it was found the geometry of the waveguides did not significantly affect the effective index difference. The refractive index of the waveguide core was swept from 3.39 to 3.43, corresponding to Al contents from 0.45 to 0.3 respectively. In both structures, a higher index leads to the mode being concentrated in the core, which may lead to lower losses. As the structures give similar results, the main decider as to which one should be chosen should be the relative simplicity of fabrication

Quantum cascade laser gain medium modeling using a second-nearest-neighbor sp3s∗ tight-binding model

A ten-band sp3s∗ second-nearest-neighbor tight-binding model has been used to model the electronic structure of various AlxGa1−xAs quantum cascade laser gain media. The results of the calculations have been compared with experimental emission wavelength data, and it has been shown that the model predicts the photon energies at the peaks in the gain coefficient spectra agreeing, on average, to within 4 meV of the experimental values. Comparison of the results of the calculations with results from a two-band k · p model shows that the tight-binding model is able to find the X-like states simultaneously with the Γ-like states. These X-like states were found to be strongly localized within the barriers. Finally, the model has also been applied to InAs/AlSb and InAs/AlSb/GaSb QCLs

Voltage recovery in intermediate band solar cells

The intermediate band solar cell (IBSC) is based on a novel photovoltaic concept and has a limiting efficiency of 63.2%, which compares favorably with the 40.7% efficiency of a conventional, single junction solar cell. It is characterized by a material hosting a collection of energy levels within its bandgap, allowing the cell to exploit photons with sub-bandgap energies in a two-step absorption process, thus improving the utilization of the solar spectrum. However, these intermediate levels are often regarded as an inherent source of supplementary recombination, although this harmful effect can in theory be counteracted by the use of concentrated light. We present here a novel, low-temperature characterization technique using concentrated light that reveals how the initially enhanced recombination in the IBSC is reduced so that its open-circuit voltage is completely recovered and reaches that of a conventional solar cell