Evanescent coupling of a center-cleaved mid-infrared quantum cascade laser to a suspended silicon-on-insulator waveguide

Two-dimensional finite-element analysis of the evanescent coupling of a center-cleaved quantum cascade laser to a suspended silicon waveguide in Silicon-on-insulator shows peak efficiency at a separation of 1.0 µm for a wavelength of 4 µm. OCIS codes: (130.0130) Integrated optics devices; (140.5965) Semiconductor lasers, quantum cascade; (130.3060) Infrared While the benefits of integrated silicon photonics are broadly accepted and have received significant interest in the near-infrared (near-IR), currently, there are only a few groups around the world investigating the role of group IV photonics in the mid-IR range. Among the attractions of silicon based technology for the mid-IR is the chip-level integration of passive and active components, as well as utilizing silicon's strong nonlinear optical effects. Also, given the already established role of silicon in the electronics industry, an integrated approach may drastically reduce the cost of manufacturing and miniaturization of such systems, enabling potentially disposable lab-on-chip CMOS compatible optoelectronic systems for use in spectroscopy, chemical and biological sensing, and free space communication (1). Despite the many advantages, a major challenge for mid-IR group IV photonics is the absence of monolithic lasers, amplifiers and detectors, while a wide variety of active photonic components already exist in III-V based semiconductor devices. Hybrid integration using direct and adhesive-assisted wafer bonding of GaAs and InP, the primary substrate materials of III-V photonics, with silicon require tackling the large lattice constant and thermal expansion coefficient mismatch between the substrates and have been extensively studied (2). Here we present a hybrid non-contact approach, where Quantum Cascade (QC) lasers, which are a very promising source of mid-IR radiation currently covering the 3-25 µm and the THz regions (3), are evanescently side-coupled to suspended silicon waveguides in order to relax the stringent wafer surface requirements. 2. Waveguide design and modeling Figure 1(a) shows the schematic of a center-cleaved QC laser ridge, where the red region signifies the active core. The waveguide, on the right, is fabricated in the silicon-on-insulator (SOI) material system. At telecom wavelengths the oxide layer provides the needed index contrast for optical confinement, however, silicon dioxide becomes optically lossy at λ > 3.6 µm (4) and hence is not suitable for the longer wavelengths. As a solution, the oxide layer underneath the guiding region is etched away, resulting in a suspended air-clad structure to minimize loss while increasing optical confinement. The dynamics of the coupled system show a strong dependence on the size of the gap between the center- cleaved QC laser and the waveguide, the width of the waveguide, the waveguide's radius of curvature, the length of the coupling region, as well as the wavelength of the propagating mode. In this work we have used the commercial software COMSOL Multiphysics for a 2D finite element analysis of this system which offers a variety of advantages compared to empirical or analytical descriptions and allows for the possibility of a more accurate prediction and optimization of aforementioned parameters. Figure 1(b) and 1(c) show a finite element simulation of such a setup, where the air gap between a single transverse-mode QC laser and an SOI waveguide is varied between 0.05 µm and 0.95 µm. The widths of the laser and the waveguide are chosen such that the propagation constant in the two regions is matched and the bending radius of the waveguide is 100 µm. The simulations show that coupling efficiency peaks when the separation of the QC laser and the waveguide is about 1.0 µm at a wavelength of 4.0 µm and that the gap size corresponding to the peak efficiency is larger at longer wavelengths, making the structure easier to assemble for longer wavelengths.