A broadband mid-infrared detector based on photonic integrated circuits and trampoline mechanical resonators

We report on the design, fabrication and characterization of a broadband mid-infrared thermal detector where the absorbed radiation is transduced onto the mechanical frequency of a low-stress temperature-sensitive silicon nitride trampoline resonator. To avoid large Johnson noise resulting from the inclusion of the absorber on the electrical readout circuit, we detect the mechanical motion optically by means of ultra-low-loss stoichiometric silicon nitride photonic integrated circuits 500 nm below the resonator. We study the mechanical frequency fluctuations with and without incident mid-IR and infer a noise-equivalent-power to absorbed power close to 1.5 pW/√Hz at a detection frequency of a 100 Hz, approaching the fundamental thermal fluctuation limit. In addition, we demonstrate that the proposed technology can be extended for readout of multiple frequency-multiplexed mechanical resonators with a single optical channel, setting the stage for large-scale arrays of resonators for focal plane array mid-IR cameras.