Monolithic Slow Light Enhanced Chip-Integrated Absorption Spectrometer from 3-15 microns

We will develop a lab-on-chip optical absorption spectrometer spanning 3-15micron molecular fingerprint region, in a single epitaxially grown heterostructure that monolithically integrates thermally tuned distributed feedback quantum cascade laser (QCL) and quantum cascade detector (QCD) arrays with slotted photonic crystal waveguides (SPCWs) in a mid-infrared InGaAs passive photonic waveguide. In Phase 1, we showed proof-of-concept feasibility of all photonic components in the integrated structure. We designed and experimentally demonstrated high efficiency adiabatic taper coupling of QCL emission into underlying low-loss 3dB/cm InGaAs waveguides, suspended membrane SPCWs and subwavelength waveguides (SWWs), and polarization rotators to interface QCLs with SPCWs. Compact 50micron SWW bends and subwavelength grating couplers in suspended membrane InGaAs, and 230micron long polarization rotators with 95% conversion efficiency were designed at 6150nm wavelength. Demonstrated fabrication feasibility showed integration feasibility across entire 3-15micron range. We experimentally detected 3ppm carbon monoxide and 5ppm ammonia, showing sub-100ppb detection feasibility for most gases with design modifications. 100x group velocity reduction and 10x electric field intensity enhancement in low-index slot-cum-gas cell within SPCW reduces geometrical analyte interaction length of light by 1000x. The combination of QCL/QCD and SPCWs provides us a handheld platform with highest sensitivity and smallest form factor, critical for US Army.