GaAs-based quantum well and quantum dot compact microlasers

Abstract

The capability to confine and manipulate photons at micro and nano scale opens enormous opportunities to integrate optoelectronic devices, and develop quantum information processing technologies. The most critical issue is to develop new device concepts and technologies that will reliably confine light in a small region via basic physical mechanisms such as photonic bandgap effect, total internal reflection (TIR) and plasmonic propagation. The contents in this thesis are presented along these directions.

First of all, a large triangular or square resonator based on TIR can resonate at many distinct wavelengths. It will be shown theoretically and experimentally that the introduction of air trenches to certain positions inside resonator can lead to a single-mode operation. Such trenches will considerably increase the radiation losses of most resonant modes, except for the mode with weak magnetic fields at the trench positions. Since most of the modes are not able to overcome their radiation losses, this high quality (Q) factor mode will be the only mode to reach lasing in the modified structure.

On the other hand, microdisks with circular shape and good light confinement can be another candidate for waveguide coupling. Therefore the theoretical coupling of light from a GaAs-based microdisk laser into an in-plane waveguide is also analysed, from air cladding to photonic crystal and metallic claddings. Among them, a properly designed plasmonic layer, introduced beside the waveguide can improve the coupling efficiency close to 80%.

However the drawback of using these claddings is either generating additional resonances or requiring the metal deposition, neither of which is very practical. Due to these reasons, a spiral waveguide working both as a waveguide and circular Bragg reflector is designed to be non-evanescently coupled with the microdisk. And after optimisation, 90% coupling efficiency can be reached. In addition such structure is compact and without any additional metal deposition. By adding a taper to this structure, coupling efficiency up to 80% into a slot waveguide can be achieved.

These microdisks normally generate many whispering gallery modes (WGMs) with high Q factor. However the Q factors decrease dramatically with the disk size and it is difficult to realise lasing operation in the WGM mode of a small microdisk.

In contrast, a well designed square lattice single-defect photonic crystal (PhC) cavity structure with much smaller cavity size can guarantee both WGM modes and a high Q factor (16500) by using the photonic bandgap effect. But one drawback is that it can only operate at a single peak with extremely low output power. In contrast, pseudo-noise sequenced single-defect PhC cavity arrays can produce multi-wavelength lasing with nearly uniform channel spacing and similar Q factors. Moreover due to the higher Q factors and increment in both mode and active volumes, such arrays are able to produce higher output power, but with very little increase in the pumping threshold.

Finally fabrication and characterisation of a quantum dot single-defect PhC cavity shows a single peak in the whole gain spectrum which provides strong evidence for future demonstration of multi-wavelength lasing.