Planar integrated plasmonic mid-IR spectrometer

The convergence of silicon photonics and infrared plasmonics allows compact, chip-scale spectral sensors. We report on the development of a compact mid-IR spectrometer based on a broad-band IR source, dielectric waveguides, a transformer to convert between waveguide modes and surface plasmon polaritons (SPP), an interaction region where analyte molecules are interrogated by SPPs, an array of ring resonators to disperse the light into spectral components, and photodetectors. The mid-IR light source emits into a dielectric waveguide, leading to a region that allows coupling of the incident photons into SPPs. The SPPs propagate along a functionalized metal surface within an interaction region, where molecular analytes cause loss at wavelengths corresponding to their characteristic vibrational modes. After a suitable propagation length, the SPP will be coupled back into a dielectric waveguide, where specific wavelength components will be out-coupled to detectors by an array of ring resonators. We have selected a 3.4 micron LED as the IR source, based on both cost and performance. Initial coupling experiments with circular waveguides formed from GLSO glass include measurement of the loss per mm. Electrodynamic simulations have been performed to inform the photonic/plasmonic transformer design. The SPP propagation length necessary for a discernible change in the signal due to absorption in the interaction region has been estimated to be on the order of 1 mm, well within the bounds of calculated propagation lengths for infrared SPPs on Au.