  Absorption spectroscopy is widely used in sensing and astronomy to understand
molecular compositions on microscopic to cosmological scales. However, typical
dispersive spectroscopic techniques require multichannel detection,
fundamentally limiting the ability to detect extremely weak signals when
compared to direct photometric methods. We report the realization of direct
spectral molecular detection using a silicon nanophotonic waveguide resonator,
obviating dispersive spectral acquisition. We use a thermally tunable silicon
ring resonator with a transmission spectrum matched and cross-correlated to the
quasi-periodic vibronic absorption lines of hydrogen cyanide. We show that the
correlation peak amplitude is proportional to the number of overlapping ring
resonances and gas lines, and that molecular specificity is obtained from the
phase of the correlation signal in a single detection channel. Our results
demonstrate on-chip correlation spectroscopy that is less restricted by the
signal-to-noise penalty of other spectroscopic approaches, enabling the
detection of faint spectral signatures.
