Solid-state terahertz lase material bsed on molecular rystals

Abstract

The progress in terahertz (THz) technology in the last years created a demand for new sources of terahertz radiation, which are required for utilization of its full potential in everyday applications. This thesis deals with the subject of terahertz sources and specifically the development of novel terahertz lasers which would provide more efficiency and better tunability in a more compact package. To carry out this objective we chose two approaches. The first is the optimization of existing terahertz optically pumped molecular gas laser design. This type of lasers typically utilizes a mid-infrared CO2 discharge laser as optical pump to excite roto-vibrational states in molecular gases. A CO2 optical pump has multiple drawbacks especially its size and a limited number of lines that can be optically pumped. Both drawbacks can be solved by the replacement of the CO2 laser by a mid-infrared quantum cascade laser, which is small, more efficient and continuously tunable, which allows almost perfect resonant pumping of a terahertz active molecular gas. With this in mind, we made a series of experiments evaluating lasing properties of molecular gases, mostly NH3, during optical pumping by a quantum cascade laser. New lasing lines were tested for terahertz gain with success. The highest single pass gain was observed on the NH3 line saQ(3,3) (1.073 THz) with the value of 10.1 dB×m-1 (63 mW pumping power), which is the highest reported value for this type of laser. Moreover, additional experiments were performed using a new prototype of terahertz optically pumped molecular gas laser. The second approach is the development of a completely new type of terahertz lasers based on optically pumped solid state media. The idea is to optically pump vibrational modes in molecular crystals and to obtain a change of population on low frequency phonons. Molecular crystal materials were selected as a potential lasing material due to their vibrational modes in mid-infrared and terahertz spectral range. Fourier transform infrared spectroscopy and terahertz time domain spectroscopy were applied to analyze vibrational spectrum of the chosen molecular crystals (sugars, organic acids). Single crystals of L-tartaric acid were prepared, further analysed, and optical functions obtained. At the end of this work, the experiment for the evaluation of a terahertz gain in crystalline media during the optical pumping is proposed.