Study of Carrier Dynamics in Microdisk Resonators by a Pump-Probe Technique
Date Issued
2015
Date
2015
Author(s)
Lin, Yen-Chih
Abstract
Microdisk resonators exhibit features of high quality factors and compact sizes due to strong light confinement of whispering gallery modes, which makes them suitable for very large scale integration in photonic integrated circuits. In this dissertation, GaAs-based microdisk resonators were fabricated by using two-step wet etching method to obtain smooth sidewalls. Furthermore, to measure and study the optical properties of the passive and active microdisk resonators, tapered fibers were made to couple these devices evanescently. First, ultrafast all-optical switching in GaAs microdisk resonators was demonstrated using a femtosecond pump-probe technique through tapered-fiber coupling. The temporal tuning of the resonant modes resulted from the refractive index change due to photoexcited carrier density variation inside the GaAs micoridisk resonator. Transmission through the GaAs microdisk resonator can be modulated by more than 10 dB with a switching time window of 8 ps in the switch-off operation using pumping pulses with energies as low as 17.5 pJ. The carrier lifetime was fitted to be 42 ps, much shorter than that of the bulk GaAs, typically of the order of nanoseconds. The above observation indicates that the surface recombination plays an important role in increasing the switching speed. Finally, carrier dynamics in InAs/GaAs quantum-dot microdisk resonators were also characterized by femtosecond pump-probe measurements through tapered-fiber coupling. A momentary blue shift of the resonant modes caused by carrier induced refractive index change in the active region of the microdisk resonator was observed. The slower blue shift of the resonant modes results from the slower processes of carrier capture into the quantum dots as well as carrier relaxation inside the quantum dots. This is attributed to the discrete energy levels in quantum dots, which suppresses the rate of phonon-mediated scattering processes. In the recombination process, the carrier lifetime in the microdisk resonator is faster than that of the bulk material. The surface recombination in the microdisk resonator is suggested to be the crucial factor.
Subjects
pump-probe method
microdisk resonator
carrier dynamics
Type
thesis
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