This research program has been devoted to the exploration of a new class of light emitters in the violet/blue wavelength range using novel growth and microcavity fabrication techniques augmented with advanced optical methods. In addition to developing more efficient gallium nitride based blue light emitters, this project paved the way for a new type of optical gain medium and devices based on polaritons, quasiparticles resulting from a mixture of cavity photons and excitons. The most striking feature of the polariton lasers is that they can produce lasing at much lower current thresholds, i.e. much lower input power levels, than the conventional lasers. Their transformative applications include low-threshold optical logic elements with significant energy savings and reduced carbon emissions as well as integrated systems in optical communications and biomedical imaging.

The aim of this project was to develop a fundamental understanding of polaritons and polariton lasing in vertical cavities based on GaN family of semiconductors. Achieving the project goals required efforts in (1) enhancement of radiative recombination efficiency in InGaN active regions through improvement of both material quality and carrier injection (i.e. reduction of carrier overflow), with optimized active region and electron injector designs, (2) improvement of exciton-photon coupling through novel vertical cavity designs with high reflectivity distributed Bragg reflectors and efficient electrical injection schemes, and (3) providing a fertile educational environment for training students in the fundamentals of cutting-edge semiconductor optoelectronics and microcavity physics.

We have identified the carrier overflow as the dominant mechanisms responsible for efficiency loss at high injection, and developed and optimized stepwise and graded electron injectors and InGaN active regions to mitigate it. Moreover, by improving carrier injection symmetry with Mg delta doped barriers quantum efficiency in InGaN light emitters were enhanced by 20% and the efficiency rollover significantly reduced. In addition, active regions of different semipolar crystal orientations were demonstrated to be suitable platforms for efficient light emitters and the effects of strain on Indium incorporation to the active regions were quantified. The knowledge of polarization of emitted light and timescales corresponding to light emission at different temperatures led to identification of the origins of radiative processes, revealing for example a significant contribution of excitons (30% at 0.4 microJ/cm² excitation) to radiative recombination at room temperature in nonpolar m-plane GaN.

High quality optical cavities and large exciton binding energy in InGaN quantum wells were shown to result in a record Rabi splitting of ~75 meV, indicative of strong exciton-photon coupling needed to enhance polariton lasing. A novel laser design based on an innovative fabrication method employing two epitaxial lateral overgrowth steps was implemented to produce microcavities with all dielectric reflectors. This approach also led to light emission under electrical injection through only the naturally formed nearly defect-free active regions and current confinement without any oxidation steps. Such vertical cavities were shown to exhibit higher quality factors (1400) and an order of magnitude lower stimulated emission threshold densities than their hybrid counterparts with semiconductor bottom reflectors. It should be emphasized that the method developed here simplifies the fabrication process, eliminating the need to remove the substrate, while also providing improved material quality.

In addition to the advancement of microcavity device technologies together with the associated materials science for the development of a new type of laser, the broader impacts invo...

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