Couplings in GaAs/AlGaAs/Metal Photonic Waveguides with Metallic Variations
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Couplings in GaAs/AlGaAs/Metal Photonic Waveguides with Metallic Variations Meng-Mu Shih University of Florida, Gainesville, FL 32611, U.S.A. ABSTRACT To have better light-emitting performance, semiconductor-metal periodic photonic waveguides can generate stable wavelengths. This work constructs a multi-parameter model to compute the backward-wave mode-coupling coefficients, which are important to the analysis and performance of photonic devices. For such a semiconductor-metal hybrid structure, a proper photonic technique needs to be utilized to solve this computational complexity. Numerical results demonstrate how the materials of metal gratings, the corrugation amplitudes of metal gratings, and the metallic aluminum mole fraction can affect the coupling coefficients. Further physical interpretation and discussion can support and explain the above results. The results can help engineers decide the values of parameters used in fabrication. Future work and applications will be proposed. INTRODUCTION A semiconductor laser has an optical waveguide structure with multiple semiconducting layers such as active layer, buffer layer and cladding layer. In a planar waveguide with the above layers, there is no optical feedback mechanism and such laser has poor performance. GaAs/AlGaAs lasers with embedded metal gratings obtaining distributed feedback (DFB) mechanism were discussed in the 1970s [1]. In a waveguide with the corrugated grating adjacent to the semiconductor structure, the DFB can improve laser performance and applications: low beam divergence [1] for targeting, narrow line-width [2] for sensing, and stable output wavelength for optical communications [2]. By using the metal as the grating material, the electrical, thermal and optical properties of lasers can be potentially improved. Such grating can provide the following advantages [3] by serving as: an electrical contact with small resistance, a heat sink with small thermal impedance, and a shiny metal with high diffraction efficiency. Figure 1 schematically shows the cross-section view of a waveguide structure for a typical GaAs/AlGaAs DFB semiconductor laser, operating at the wavelength less than the midinfrared range. This four–layer waveguide structure is composed of three semiconductor layers and one metallic layer. To compare with the results obtained by ray-optics method [3], these layers start with the following optical parameters of refractive indices: active layer or called active region (na= 3.6 for GaAs), buffer layer (nb= 3.4 for AlGaAs), cladding layer (nd= 3.4 for AlGaAs) and metal contact layer (nc). Since the metallic aluminum mole fraction (x) or the Al composition in AlGaAs can affect the optical properties for the semiconductor compounds, the refractive index of the buffer layer and cladding layer can be changed depending on the Al composition [4]. The corresponding Al composition for refractive index 3.4 is about x= 0.35 [4]. When the aluminum composition becomes larger, the semiconductor compound starts to become indirect band-gap material.
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