Spectral Control and Topological Beam Synthesis in Second-order Downconversion Processes

by Yaqun Liu (Applied Physics, KTH)

Friday 26 Sept 2025, 13:00 → 16:00 Europe/Stockholm

Pärlan (Albano Building 1)

Opponent: Jean-Louis Coutaz, University Savoie Mont Blanc

Supervisor: Valdas Pasiskevicius, Ljus och materiens fysik; Fredrik Laurell, Ljus och materiens fysik

Abstract

The terahertz (THz) spectral region is gaining increasing relevance for applications in high-capacity wireless communications, advanced spectroscopy, security screening, and next-generation charged particle accelerators. Nonlinear optical techniques offer a versatile and powerful route to generating THz radiation, either through direct generation by optical rectification (OR) or via difference frequency generation (DFG) from mid-infrared (mid-IR) sources. This thesis addresses key challenges in accessing and controlling radiation with two primary nonlinear optical mechanisms: the development of narrowband, tunable mid-IR sources using optical parametric oscillation (OPO), and the generation and spatiotemporal characterization of structured THz wave packets using OR. 

A power-scalable OPO based on type-II quasi-phase matched Periodically Poled Potassium Titanyl Phosphate (PPKTP), pumped at 1.06 $\upmu$m is investigated. The system achieves precision tunability in the 2 $\upmu$m region with narrow linewidth, strong thermal robustness, and excellent spectral control, that is suitable for applications such as greenhouse gas (GHG) sensing and quantum photonics. The material properties and crystal design parameters are presented to enable accurate targeting of molecular absorption lines. Importantly, due to the narrowband and scalable nature, this mid-IR source is also promising for driving efficient nonlinear frequency conversion into the THz regime via DFG.

The generation of complex few-cycle THz wave packets with vector and scalar field singularities were also explored. A novel approach based on OR along the three-fold symmetry axis of zinc blende crystals enables the direct generation of THz vortex beams carrying orbital angular momentum (OAM). This scheme further allows full control over vector THz beams through a dual-pump configuration, enabling access to the entire higher order Poincaré (HOP) sphere. A comprehensive theoretical framework is developed to describe this nonlinear electric field synthesis. Furthermore, a polarization-resolved time-domain electro-optic sampling technique is experimentally implemented for synthesis and full (2+1)D mapping of the THz electric field amplitude and phase, enabling detailed analysis of structured light propagation. The technique paves the way for THz hyperspectral and vector field imaging. 

This work highlights the versatility of nonlinear optics for spectral control and structured light generation in mid-IR and THz regions and lays a foundation for compact, tunable, and high-performance photonic sources tailored for advanced photonic applications.