Quantum Cascade Laser-Pumped Molecular Lasers as a Widely Tunable Room-Temperature Terahertz Source

Abstract

In this work, we will study the development of quantum cascade laser-pumped molecular lasers (QPMLs). This compact, room-temperature, and widely tunable across the entire terahertz spectrum source represents a significant addition to the field of terahertz (THz) radiation generation. This coherent, narrow bandwidth source fills the gap between 1 THz and 3 THz, where current solutions face significant limitations in output power and operational practicality. QPML is based on traditional optically pumped far infrared (OPFIR) sources, with Quantum Cascade Lasers (QCLs) replacing gas discharge lasers as pump sources. QCLs' continuous tunability across the infrared region enables the pumping of any roto-vibrational transitions, overcoming the narrow spectral overlap constraints inherent to conventional gas-discharge pump lasers. In this dissertation, QPMLs are experimentally and numerically studied using various molecular gases, including nitrous oxide (N2O), fluoromethane (CH3F), and ammonia (NH3). We will systematically study the unique rotational-vibrational energy levels, transition selection rules, and lasing performance of each molecule, achieving wide tunability from 0.2 THz to 3.5 THz and producing more than 1 mW of power on some transitions. To further improve the performance and stability of our source, we will examine multiple resonator configurations, evaluate their performance, and optimise their design for QCL pumping. This comprehensive evaluation reveals the impact of resonator geometry on pumping efficiency and pump laser stability, both of which have a significant effect on the QPML performance. Numerical simulations, complemented with experimental data, study critical performance factors such as gas absorption efficiency, pump power distribution, and resonator back-reflection dynamics. Further, this work introduces a novel rotational spectroscopy enhancement technique, leveraging the continuous tunability of QCLs to pump molecules to excited vibrational states, and enhance the detection sensitivity of rotational transitions at these excited states. Using this method, rotational spectra of excited vibrational states in N2O were accurately measured, achieving better resolution and sensitivity with a simpler experimental setup compared to alternative methods.