Infrared‐to‐THz Detection and Spectroscopy with Whispering‐Gallery‐Mode Microresonators

A novel room-temperature photodetector based on a silica-microsphere optical resonator operating from the mid-infrared to the terahertz spectral region is reported. The sensor relies on a free-space visible laser whose frequency is locked to the center of a whispering-gallery mode (WGM). The devised scheme automatically tracks any small change in the microresonator's size and refractive index that occurs upon illumination with radiation absorbed by silica. In this way, direct detection of quantum-cascade laser beams around 4.3 and 111 μm wavelengths is shown, down to minimum noise-equivalent power levels of ≈1 pW √Hz−1 with a dynamic range >100 dB. The microsphere resonator is then exploited, for the first time, as a photodetector in absorption spectroscopy of the 12CO2 fundamental ro-vibrational band and of very-weak 13CH3OH rotational lines.