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 led 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 ≈ 9µm with a focal of ≈ 200µm and with a side size (in the array direction) of 200µm.
The mid-infrared (MIR) molecular fingerprint region has gained great interest in the last past years thanks to development of semiconductor laser source like Quantum Cascade Lasers (QCL). Nevertheless, because of the small size of the waveguide of such devices (≈ 10μm), the beam at the output of such source has an extreme divergence (could be < 45 deg) which makes it difficult to use without specific optics. Several solutions, such as classical lens in chalcogenide glass or parabolic mirror, have been used to shape the laser beam. However, this kind of solution remain expensive and not always usable for small component. This paper present a new kind of lens for the collimation of MIR laser beam, very compact and with a focal length highly adjustable. The fabrication of this dielectric flat lens has the advantages of the semiconductor fabrication techniques and a single etch step on a wafer is sufficient to perform the lens. The main principle is to structure the wafer surface with sub wavelength pattern to induce a local variation of the refractive index. Then the mapping of this local index is the key to control the phase of an input beam and to perform the desired lens function. This works shows simulation results and demonstrate the first prototype of this device for wavelength close to 9μm with a focal length and numerical aperture of almost 150μm and 0, 5. This prototype is disc-shaped with a diameter of 100μm made on InP wafer.