Narrow-linewidth, continuous-wave, intracavity terahertz lasers based on stimulated polarition scattering
thesisposted on 28.03.2022, 16:09 authored by Yameng Zheng
Terahertz radiation has attracted strong interest in non-destructive testing, spectroscopy, and imaging due to its absorption by many organic and inorganic materials, and its transmission through many substances such as textiles. Terahertz laser sources based on the stimulated polariton scattering process have been proven to be an effective method to generate terahertz radiation, and this is a rapidly developing field of research. Intracavity configurations based on this technology offer an opportunity to achieve continuous wave terahertz generation, an approach which has not been extensively investigated. Continuous wave terahertz sources are highly desirable for a wide range of applications due to the characteristics of being simply interfaced with detectors such as Golay cells and pyroelectric detectors, and portable spectrometers. What is of particular significance is that the linewidth of the emission of these sources can be made very narrow, as required for high-resolution terahertz spectroscopy. This thesis focuses on developing an intracavity continuous wave terahertz laser source based on the stimulated polariton scattering process. The thesis starts by describing the design and optimisation of a continuous wave, intracavity terahertz laser source. Studies have been made with regards to the analysis of the state-of-art in this laser source, identifying its shortcomings and enhancing its power scaling capacity and stability. The main factors influencing the SPS-driven continuous wave terahertz characteristics have been addressed, leading to a maximum terahertz power of 23.1 μW being generated, a ten times increase over the previously reported work. Beam distortion due to photorefractive and thermal effects has been overcome by optimising the pump scheme. Development of this intracavity THz source with regards to linewidth narrowing is also studied. The core technology to achieve linewidth narrowing is the implementation of an etalon in the fundamental laser resonator. The thesis offers insight into using an etalon in two different ways in these THz sources. The first examined case is where an etalon is used as an intracavity band-pass filter, and its tuning characteristic as a function of tilt angle is investigated. A key finding here is that losses induced by etalon tilt (and hence walk-off) play a critical role. The second examined case is where an etalon is used at normal incidence, to deliberately form a coupled cavity with unique pass-band structure. In both cases, highly effective linewidth narrowing is observed. A surprising and important finding is that narrowing of the linewidth in the SPS laser leads to an improvement in the THz output power, and this is supported through investigation of phase matching efficiency. Single longitudinal mode operation is successfully achieved in the system, taking advantage of coupled cavity effects formed by an etalon aligned at normal incidence in the cavity. This method brings the terahertz linewidth down to 70 MHz (compared to the linewidth of 100 GHz in the "free running" case) and delivers an output THz power of 18.2 μW.