Martinez, Anna (2025) Terahertz photonics: from generation and control to spectroscopy and advanced imaging. [Tesi di dottorato]

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Abstract

Terahertz (THz) science has rapidly evolved into a multidisciplinary field with applications ranging from materials characterization to advanced imaging and ultrafast spectroscopy. Despite this progress, generating intense, broadband, and controllable THz radiation at high repetition rates remains a key challenge for enabling real-world technologies such as nonlinear THz spectroscopy, THz-driven electron emission, and high-throughput imaging. This thesis investigates complementary approaches for THz generation, control, and application, combining theoretical modelling, experimental implementations, and data-driven analysis. A first research line focuses on two-color laser-induced plasmas, developing a rigorous photocurrent model and demonstrating advanced control of ultrabroadband THz emission. By tuning chirp, phase, and crystal geometry, the work achieves temporal waveform shaping, energy optimization, and—most notably—the generation of fully circularly polarized THz pulses with ellipticity up to 0.99, maintained across a bandwidth exceeding 30 THz. These results establish plasma-based sources as powerful and flexible platforms for vectorial THz field engineering. A second line examines optical rectification in nonlinear crystals, exploring both complex tilted-pulse-front schemes in lithium niobate and highly efficient collinear generation in organic crystals such as DAST and DSTMS. Operating at repetition rates up to several hundred kilohertz, these systems deliver record average powers and efficiencies, providing practical benchmarks for next-generation high-repetition-rate THz sources. Finally, the thesis applies THz time-domain spectroscopy and THz hyperspectral imaging to real-world problems. Applications include characterization of electrical conductivity properties of nanostructures, chestnut quality assessment, microplastic detection in soil, and protein differentiation in silica matrices, illustrating the increasing maturity and societal relevance of THz-based diagnostics. Overall, this thesis advances THz photonics by bridging fundamental physical mechanisms, high-performance source development, and data-driven spectroscopic applications, contributing to the transition of THz technologies from laboratory research to practical use in science and industry.

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