Design and fabrication technologies for single-mode quantum cascade lasers.

Quantum Cascade Lasers (QCL's) are unipolar semiconductor lasers offering the potential for low cost, high power, laser sources emitting in the mid- and far- infrared. Single-mode devices are required for spectroscopic and imaging applications because of their narrow emission linewidth and are achieved through distributed feedback (DFB) designs where gratings are incorporated in or close to the active region. This thesis describes design and fabrication technologies for single-mode yield improvements and improved wavelength targeting of QC lasers. Mid-infrared, single-mode laser designs are developed utilising novel fabrication processes and designs in the indium phosphide (InP) based material system. Lasers without facet coatings, with single-mode yields up to 80% and side mode suppression ratios (SMSR) greater than 25dB are observed. Metalised surface gratings, buried (overgrown) gratings and lateral gratings are defined using inductively coupled plasma (rCP) etching to produce DFB lasers operating above room temperature for wavelengths near to lOllm. A deep etched lateral grating quantum cascade DFB is demonstrated for the first time in the InP material system. Waveguide modelling demonstrates accurate methods to predict the grating coupling strength of the DFB lasers with good agreement to experimental results. The emission wavelength (A. -IOllm) is found to be highly dependent on the laser ridge width with the experimental shift in wavelength found to be much greater than that predicted. A ridge width and temperature dependence on emission wavelength is utilised in an array device where a continuous tuning range in excess of 230nm is observed. ii Finally, an increase in thermal conductance and a symmetric far-field profile are observed for lasers (A. - 4J.lm) with narrow ridge widths. The lasers were designed for improved heat extraction from the active region and a beam quality factor of M2 ::::: 1. Almost identical lateral and vertical far-field profiles with high duty cycle operation at thermoelectric temperatures are observed for a ridge waveguide laser that is approximately 5 J.lm wide.