Terahertz Sensing Based on Photonic Crystal Cavity and Resonant Tunneling Diode

We report terahertz sensing applications of photonic crystal cavity with a high quality factor (similar to 9300). The frequency response of the cavity was measured using a compact terahertz source based on a resonant tunneling diode (RTD), and the results were compared with the results obtained using a bulky, precise frequency multiplier. The oscillation frequency of the RTD was tuned by changing the applied bias voltage resulting from the voltage-dependent capacitance of the diode. We successfully observed the resonant spectrum of the cavity at 318 GHz using a spectroscopic system. As a preliminary experiment for the sensing application, we measured a clear frequency shift of the resonant spectrum when a thin dielectric tape with a thickness of 5 mu m, which is smaller than 1/100 the wavelength, is attached to the surface of the cavity.