Optical tuning of the whispering gallery modes of a silicon microsphere

Silicon being a choice for semiconductor industry for past many decades has shown great importance in integrating photonics with complementary metal oxide semiconductor (CMOS) technology. In recent years, whispering gallery mode (WGM) resonators have been studied extensively for many photonics applications like optical filters, modulators, resonators, amplifiers, and phase shifters. Thus WGM resonators can be used towards realization of all optical functional devices. Here, in this work, we have performed the optical tuning of WGMs of a silicon microsphere of radius 500 µm at 1427 nm. The optical tuning of the WGMS is achieved by modulating the refractive index of the silicon with a pump laser at 405 nm, causing a red shift of the resonances. WGMs with a mode spacing of ∆λ = 0.23 nm and quality factors of ~60000 have been observed. For an unfocused continuous wave (CW) pump illumination at 405 nm, a red shift of 13.5 pm and with focused pump laser beam, a red shift of 24.5 pm is observed. Further, a red shift of 40 pm is observed, when an unfocused 8 Hz 80% duty cycle pulsed pump laser is applied to the silicon microsphere. The observed red shift in the WGMs is due to the thermo-optical effect in silicon in response to slow modulation of the refractive index of silicon by pump laser.