Novel methods of terahertz generation with semiconductor microcavities and beam steering

Abstract

The terahertz (THz) region of the electromagnetic spectrum is considered to be technologically under-developed in comparison to its neighbouring wavelength regions owing in part to the lack of compact, tuneable, and low-power sources. Conversely, the field of polaritonics has experienced rapid growth in recent years and has thus been cited as a promising new candidate for both generation and detection of THz. This is because a polariton system can provide emission frequency selection, lasing without inversion of population, emitters a few mm thick, and - depending on the material - room temperature operation. This thesis aims to investigate potential new sources of THz radiation capable of being used in novel optoelectronic devices and sensors based on semiconductor microcavities with multiple embedded GaAs quantum wells, operating in both the strong and weak coupling regime. The thesis focuses on non-linear phenomena as a method of THz generation, beginning with THz emission via two photon absorption [105] (Chapter 2 ), emission within a parabolic quantum well due to the cascade effect [182] (Chapter 3 ), and finally THz guiding using emitters based on the lateral photo-Dember effect (Chapter 4 ). Theory and experimental findings conclude that whilst polariton emitters remain an exciting test-bed for the study of bosonic interactions, and a promising source of THz radiation, direct detection of such emission needs to be provided, therefore the realisation of a THz-polaritonic device still requires development. It was found that cutting-edge parabolic cavities [102] remain the best candidate for microcavity THz due to the amplification supplied by the unique excitation scheme similar to that of the well-known quantum cascade laser. This work further concludes that, using simple lens geometry, lateral photo-Dember emitters are capable of being focused in free space and can provide THz frequency selection providing an alternative to cumbersome parabolic mirrors used for sample interrogation in a THz time-domain spectroscopy set-up. This implies that THz can be easily guided or launched on to a waveguide combatting another fundamental problem with the THz region - in vitro or in situ analysis.

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