Study of InGaAs Quantum-Dot Microdisk Lasers
Date Issued
2012
Date
2012
Author(s)
Chien, Hao-Che
Abstract
Microdisk cavities have been exploited as high quality factor resonators due to the excellent confinement of whispering gallery modes (WGMs) and may be applied as low threshold laser cavities. Microdisks with embedded quantum dots (QDs) offer the potential of ultralow threshold semiconductor lasers due to the combination of the small mode volume and delta-like electron density of states. They can also be used to fabricate GaAs-based long-wavelength diode lasers for fiber communication systems. Both of the optically-pumped and current-injection microdisk lasers embedded with InGaAs QDs are studied in this dissertation. The fabrication techniques and measurement results of optical properties are discussed for these two kinds of devices.
In the study of optically-pumped microdisk lasers, we measured a 10-μm-diameter microdisk lasers by micro-photoluminescence (micro-PL) experiments. The corresponding quality factor can be larger than 14000. The radial modes can be greatly suppressed by introducing a small hole at the disk center. We also tried to fabricate microdisks with cavity size near 3 μm by two-step wet-etching in order to reduce the lasing modes inside the cavities. The numbers of lasing modes are reduced in these smaller microdisks. In temperature-dependent experiments, we have shown that the alignment of QD and whispering gallery mode energy may affect the lasing threshold.
In the study of current-injection microdisk lasers, we have fabricated our devices with Benzocyclobutene (BCB) polymer claddings. The first room temperature lasing phenomenon of QD injection microdisk lasers is demonstrated. The lowest threshold current at room temperature is 0.45 mA, which is measured from a 6.5-μm-diameter microdisk. In temperature dependent experiments, the negative characteristic temperature (T0) of QD microdisk lasers is observed. The transition temperature from negative T0 to positive T0 is around 150 K. From our rate-equation simulations, the experimental results indicate that the quality factor of our microdisk cavity is high.
Finally, the dynamic behaviors of both optically-pumped and current-injection microdisk lasers are studied. By the time-resolved micro-PL experiments, the QD carrier lifetimes corresponding to on-resonance and off-resonance wavelengths of a microdisk are determined. The average spontaneous emission rate is enhanced by a factor of 4.3, which is owing to the Purcell effect. The transient behaviors of current-injection QD microdisk lasers are studied at room temperature. Suppressed relaxation oscillations and fast turn-on behaviors are observed. The suppressed relaxation oscillation can be attributed to the enhanced spontaneous emission factor in microdisk lasers and the short turn-on time is due to the reduced carrier lifetime. A large-signal direct modulation at 1 Gbps is demonstrated. In temperature-dependent dynamic measurements, both of the transient lasing and steady-state lasing from side modes are suppressed at temperatures higher than 250 K. Therefore, the quantum-dot microdisk lasers have the potential to realize the single-mode operation under high-speed modulation at room temperature in highly integrated optical transceiver modules.
Subjects
semiconductor laser
microcavity
microdisk
quantum dot
Type
thesis
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