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Date of Award

9-1978

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Supervisor

Professor J. Shewchun

Co-Supervisor

Professor B.K. Garside

Abstract

Most semiconductor laser configurations have an active region width comparable to the wavelength of the radiation giving rise to potentially important diffraction effects. In this work, optically confining mechanisms which can affect the magnitude of those diffraction effects are investigated in electron-beam-pumped GaAs bulk material and GaInAs hetero-epitaxial layers of various thicknesses.

One major aspect of the laser behaviour is the time delay between the start of the excitation and the onset of lasing. It has been reported that various semiconductor lasers can exhibit time delays to threshold which exceed by as much as two orders of magnitude the time required for the gain or the radiation to build up within the resonator. In the first part of this thesis, the dependence of the time delays on the electron-beam voltage and current is studied experimentally and theoretically in GaAs bulk material. A model is proposed which accounts for the long time delays observed on the basis of transient waveguiding. According to this model, there is a growing refractive index difference between the active region and the underlying passive region which progressively confines the radiation thereby reducing the cavity losses and eventually bringing about lasing threshold. The spatial and temporal variations of the refractive index are attributed to corresponding variations in the temperature and the gain. As an integral part of this model, minority carrier concentration and temperature profiles resulting from the differential energy loss profile of the electron beam are derived.

To test the above hypothesis, the measurements were repeated on GaInAs hetero-epitaxial layers. Incorporated in such structure is a permanent refractive index difference between the epitaxial layer and the substrate which confines the radiation within the epitaxial layer. As suggested by the model, for layer thicknesses of the order of the electron-beam penetration depth, only short time delays to threshold are observed. This demonstrates that a confining structure can affect dramatically the lasing behaviour.

In the second part of this thesis, the transverse distribution of the radiation within hetero-epitaxial layers is studied in more detail. An electromagnetic model for such a cavity is introduced in which the spatial variation of the refractive index within the epitaxial layer is approximated by a step profile. The importance of the permanent confining surface is shown to depend on the cavity parameters such as refractive index difference between active and passive region, absorption in the passive region and thickness of the epitaxial layer compared to the electron penetration depth. The specific conditions under which transient waveguide effects take preponderance over the permanent optical confinement are derived explicitly. It is found that the spatial distribution can be very sensitive to the permanent waveguide effects. Accordingly, the far-field diffraction patterns of the lasing radiation were measured on hetero-epitaxial layers of various thicknesses and on bulk material under various pumping conditions. The theoretical fit to the experimental data shows that multimode lasing is a common occurence in epitaxial layers. Moreover, the time-resolved spatial measurements on the bulk material confirm that long time delays to lasing threshold are effectively associated with optical confinement of the radiation.

There is evidence that waveguiding mechanisms as described here are also important factors in the behaviour of other semiconductor laser configurations. Time-resolved measurements of the far-field patterns could be used to verify this proposition.

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