High-resolution THz gain measurements in optically pumped ammonia

Abstract

This study is aimed at the evaluation of THz gain properties in an optically pumped NH3 gas. NH3 molecules undergo rotational-vibrational excitation by mid-infrared (MIR) optical pumping provided by a MIR quantum cascade laser (QCL) which enables precise tuning to the NH3 infrared transition around 10.3 mu m. Pure inversion transitions, (J = 3, K = 3) at 1.073 THz and (J = 4, K = 4) at 1.083 THz were selected. The THz measurements were performed using a THz frequency multiplier chain. The results show line profiles with and without optical pumping at different NH3 pressures, and with different MIR tuning. The highest gain at room temperature under the best conditions obtained during single pass on the (3,3) line was 10.1 dBxm(-1) at 26 mu bar with a pumping power of 40 mW. The (4,4) line showed lower gain of 6.4 dBxm(-1) at 34 mu bar with a pumping power of 62 mW. To our knowledge these THz gains are the highest measured in a continuous-wave MIR pumped gas.