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

This STTR Phase I project aims at developing a lab-on-chip optical absorption spectrometer spanning 3-15 micron molecular fingerprint region, with monolithic integration of quantum cascade laser (QCL), quantum cascade detector (QCD) and slotted photonic crystal waveguides (PCWs) in a mid-infrared photonic waveguiding platform. Monolithic integration is achieved with a single epitaxially grown heterostructure that simultaneously integrates an active region functioning as both QCL and QCD with slotted PCW devices in an underlying passive waveguide. Multiple slotted PCWs are self-aligned with QCLs and QCDs, QCLs being tuned across the entire wavelength range by distributed feedback gratings and temperature tuning. The device achieves 100 times group velocity reduction of light and 10 times electric field intensity enhancement in low-index slots in a chip-integrated gas cell within a slotted PCW. Compared to free space, our device thus provides 1000 times geometrical interaction length reduction of light with the analyte leading to enhanced optical absorption by analytes in the optical path. Experimentally demonstrated sensitivities indicate that our device will detect most gases in sub-100ppb range with size, weight and power (SWaP) advantages better than current spectroscopy instruments. System cost is significantly reduced by eliminating bulky gas cells and high finesse mirrors and bulk optics.