Properties of III-Nitride-Based Polariton and Spin Polariton Diode Lasers

Abstract

The cavity electrodynamic regime of strong coupling of emitter-photon interactions in a semiconductor microcavity gives rise to new light-matter entangled quasiparticles, also known as exciton-polaritons. The non-linear nature of the energy-momentum dispersions of these composite bosons has been suitably engineered and efficiently utilized to demonstrate inversionless coherent emission, or polariton lasing in submicron-scale optical cavities. Previous theoretical as well as experimental work on Gallium Arsenide and Cadmium Telluride-based systems operated at cryogenic temperatures, have shown the central importance of the nature of the output polarization of the emitted light originating from the radiative decomposition of these polaritons. Room-temperature operation of these lasers necessitates the use of wide-band gap semiconductors such as Gallium Nitride, because of their large free excitonic binding energies and oscillator strengths, which consequently lead to stronger and more robust exciton-photon strong coupling. Thus, the steady state output polarization characteristics of Gallium Nitride-based microcavity polariton lasers operated with unpolarized electrical injection, have been examined at room temperature. The output is essentially unpolarized below the nonlinear threshold injection current and is linearly polarized above it with a maximum degree of polarization of ∼ 22%. Besides other advantages, a spin-polarized laser offers inherent control of the output circular polarization. Electrical spin injection in a bulk Gallium Nitride-based microcavity polariton diode laser enables the realization of an electrically modulated low-energy circularly-polarized coherent light source. Successful electrical spin injection in bulk Gallium Nitride, which is the active layer of the polariton diode laser, has been independently confirmed from room-temperature four-terminal Hanlè spin precession measurements made on Gallium Nitride-based spin valves, and observation of hysteretic circular polarization in III-nitride-based light-emitting diodes. The optical selection rules governing the operation of the latter have also been elucidated. Electrical injection of spin polarized electrons is accomplished in all the above-mentioned devices via a n-type Cobalt Iron alloy/Magnesium Oxide spin injector contact. The output polarization characteristics of this polariton diode laser have been examined at room temperature. A degree of output circular (linear) polarization of ~ 25 (33) % is recorded under remanent magnetization. The helicity as well as the degree of the steady-state circular polarization is deterministically governed by the magnetizing field used to magnetize the ferromagnetic contacts. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed employing two distinct spin-dependent rate equation models, and there is good agreement between measured and calculated data in both cases. The present work also theoretically explores other optoelectronic properties of these spin polariton lasers. Optical effects arising from spin-induced gain anisotropy such as threshold reduction and emission intensity enhancement have been theoretically predicted for these diode lasers. An electrical excitation mechanism has also been formulated, which can potentially magnify the degree of a deterministic circular polarization of the output emission by an order of magnitude, compared to the injected electron spin polarization. The dissertation concludes with the discussion of the observation of a non-linear enhancement in the excitation-dependent photocurrent characteristics of the microcavity diodes with a threshold, which is consistent with the polariton lasing threshold. This is explained in the framework of an Auger-like process of excitonic dissociation into its constituent electron-hole pairs, which can be stimulated by the occupation of the polariton lasing states and the observed effect is therefore a unique manifestation of the bosonic final-state stimulation effect in polariton lasers.