Photonic crystal slot waveguide spectrometer for the detection of methane

Defect engineered photonic crystals, with sub-micron dimensions have demonstrated high sensitivity to trace volumes of analytes; however exact identification of analyte through spectroscopic signatures had not been demonstrated. We demonstrate a 300micron long photonic crystal slot waveguide device which combines slow light phenomenon in photonic crystal waveguides with large optical field intensity in a low index narrow slot at the center of the photonic crystal waveguide for highly sensitive spectroscopic detection of methane on-chip at 100 parts per million (ppm) or 0.2% permissible exposure limit. Photonic crystal slot waveguide provides a factor of 1000 reduction in interaction length compared to free-space infrared spectroscopy leading to enhanced optical absorption by analytes in the optical path. By measuring absorption differences in presence and absence of methane, near-infrared absorption spectrum of methane is determined.