Photonic crystal and quasi photonic crystal Ge-on-Si lenses for the combination of QCL array outputs

The mid-infrared (MIR) molecular fingerprint region has gained great interest in the last past years thanks to development of laser source like Quantum Cascade Lasers (QCL). There are a lot of efficient technique to achieve solid and liquid spectroscopy detection. However, to probe several or complex molecules in this optical region, it could be necessary to use broadly tunable MIR source. A QCL array coupled to a specific lens array able to shape and combine beams into a single spot, could be a suitable source. This work is focused on the design and fabrication of integrated lenses (Photonic Crystal Lens & Quasi Photonic Crystal Lens) made with Germanium on Silicon Germanium platform. A Photonic Crystal Lens (PCL) is composed of a 2D holes lattice inside of a slab waveguide filed with Si0.6Ge0.4. The holes lattice is hexagonal with a constant parameter. The radius of those holes continuously varying, in the direction perpendicular to the light propagation, to induce a variation in optical path length. Then, the design of this gradient is the key to perform the desired lens function. A Quasi Photonic Crystal Lens (QPCL) is based on the same principle as the PCL, but instead of having a 2D lattice there is a linear (1D) lattice. So a QPCL is composed of 1 row of a fixed number of macroscopic holes in diamond pattern inside of a slab waveguide. Like the PCL, there is a hole size gradient to shape the optical path of the light. This works shows simulation results and the first design of integrated lens working at the wavelengths of approximate to 9 mu m with a focal of approximate to 200 mu m and with a side size (in the array direction) of 200 mu m.