Exploiting second-order nonlinear phenomena for the generation and detection of coherent  terahertz electric fields
Creators: Carnio, Brett N
Publisher: name: University of Alberta Library
Year: 2022

<p>This thesis explores terahertz radiation sources and detectors, where the driving physical mechanism for generation and detection is second-order nonlinear phenomena. The heart of this work considers novel crystals and waveguiding arrangements for terahertz radiation generation and/or detection to advance the field of nonlinear optics.</p>

<p>Experimental investigations are conducted using emerging pnictide and chalcogenide ternary crystals for both the generation and detection of terahertz radiation. While a CdSiP2 crystal is shown to provide appreciable optical rectification phase-matching (surpassing that of a ZnGeP2 crystal), an AgGaSe2 crystal exhibits unprecedented optical rectification phase-matching (coherence length of ~800 µm for frequencies between 0.5-2.9 THz). A BaGa4Se7 crystal is revealed as a highly-efficient terahertz radiation emitter (i.e. the terahertz radiation power produced by BaGa4Se7 is better than ZnTe in select terahertz spectral bands). Subsequently, a ZnGeP2 crystal is shown to exhibit exceptional electro-optic phase-matching, allowing it to surpass the bandwidth of ZnTe for terahertz radiation detection.</p>

<p>Numerical and experimental techniques are utilized to investigate waveguiding arrangements for terahertz radiation generation. However, due to the lack of numerical methods capable of incorporating all 18 dispersive second-order nonlinear tensor elements, two separate formalisms are developed to integrate second-order nonlinear effects into finite-difference time-domain simulations. Using the developed methods and experimental terahertz time-domains spectroscopy techniques, LiNbO3 planar waveguides are investigated for producing terahertz radiation. Key observations include ultra-broadband terahertz radiation generation spanning 0.18-106 THz, terahertz radiation generation enhancement near the phonon resonances of LiNbO3, phase-matched terahertz radiation produced in the backward direction (i.e. the direction opposite to the propagation direction of the excitation electric field), and high experimentally-realized optical-to-terahertz conversion efficiencies (i.e. &gt;10-5).</p>

<p>Interestingly, the aforementioned nonlinear finite-difference time-domain formalisms are not restricted to the terahertz frequency regime, but can accurately model second-order nonlinear processes within various other spectral regions. To make use of such an exciting outcome, this thesis briefly extends beyond the terahertz spectral regime to examine second-order nonlinear effects for the generation of radiation in the infrared and visible spectral regimes. Accordingly, an entirely new class of multi-band photonic sources is proposed, in which a single waveguiding structure concurrently satisfies phase-matching for several second-order nonlinear processes. A multi-band waveguide is experimentally-realized, which simultaneously produces phase-matched THz radiation and phase-matched radiation in the visible spectral regime.</p>

<p>The findings in this thesis are invaluable to the continuing development of terahertz radiation sources and detectors being driven by the physical mechanism of second-order nonlinear phenomena.</p>