Whispering-Gallery-Mode Optical Microshell Resonator Infrared Detector

Abstract

We demonstrate a thermal infrared (IR) detector based on a high quality-factor (Q) whispering gallery mode (WGM) borosilicate glass microspherical shell resonator and investigate its performance to detect IR radiation in 1 – 20 ${\mu }\text{m}$ wavelength range. The resonator exhibits a temperature sensitivity of 1.17 GHz/K with a Q-factor of 3 million and can be configured as a high sensitivity infrared sensor. The microspherical shell IR sensor exhibited a responsivity of 7.88 kHz/nW and achieved a noise-equivalent-power (NEP) of 19 nW/ $\sqrt {Hz}$ experimentally. A laser Doppler vibrometer (LDV) is used to measure the physical expansion of the microspherical glass resonator when IR radiation is absorbed. Based on the experimentally measured diametric expansion of the shell per unit IR power absorbed, the NEP of 19 nW/ $\sqrt {Hz}$ corresponds to a dimensional change of 2 pm which can be resolved using the resonator. Using COMSOL modeling, thermal expansion analysis was performed for the absorbed IR Power. Using these values of dimension change of the microspherical shell, the dependence of resonance frequency shift on absorbed IR power was simulated. These models show that a NEP of 690 pW/ $\sqrt {Hz}$ can be achieved for the microspherical shell with a diameter of 2 mm and a thickness of 2 ${\mu }\text{m}$ .