High-resolution THz gain measurements in optically pumped ammonia

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 μm. 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 dB×m^-1 at 26 μbar with a pumping power of 40 mW. The (4,4) line showed lower gain of 6.4 dB×m^-1 at 34 μbar with a pumping power of 62 mW. To our knowledge these THz gains are the highest measured in a continuous-wave MIR pumped gas.