Infrared-to-THz Sensors Based on Whispering Gallery Mode Microresonators

Infrared (IR) and far-infrared detectors are crucial components in diverse research fields, such as optical metrology, nonlinear optics and spectroscopy, as well as civil and military applications. They are usually distinguished as photoconductive or photovoltaic devices and thermal devices [1]–​[3]. In particular, thermal detectors are the prominent type adopted for long-wavelength infrared radiation (THz) and rely on changes in physical properties of the sensor material caused by photon absorption. However, the frequency window from 0.1 to 10 THz is a largely underexploited section of the electromagnetic spectrum due the current lack of room-temperature detectors. Whispering-gallery mode (WGM) micro-resonators [4] have become popular in photonic systems thanks to ease of fabrication, high optical Q-factor and ultra-small mode volume [5], [6]. Their inherent sensitivity to temperature and refractive index variations makes them ideal tools for various applications, such as biosensing and spectroscopy [7]. We report a novel room-temperature radiation sensor based on a free-space visible laser that actively tracks the optical resonance of a silica microsphere fabricated on the tip of a single-mode optical fiber. Excitation of WGMs in the micro-resonator is obtained via free-space light scattering without using any prism or fiber guide [8]. Thanks to silica strong absorption in the IR, we show that the microsphere element enables detection of electromagnetic radiation from the mid-IR (MIR) up to the THz spectral range, and that the WGM-tracking system yields a real-time voltage signal proportional to the incident radiation power. Furthermore, the sensor exhibits NEP levels ~ 1 pW/√Hz, while no special insulation and cooling are required for operation.