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 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.
Highlights
Terahertz photonics is one of the newer fields in electromagnetic research
Thanks to the development of new mid infrared (MIR) quantum cascade lasers (MIR QCLs) we may overcome some disadvantages of the CO2 pump laser
First the THz spectra of the two lines were recorded without MIR pumping (QCL off)
Summary
Terahertz photonics is one of the newer fields in electromagnetic research. The terahertz band lies between microwave and infrared frequencies thanks to which it possesses unique properties. Terahertz quantum cascade lasers (THz QCLs) are relatively new sources demonstrated in 2002 [4] They can be small with reasonable output power and some tunability, they suffer from decreasing power with temperature and still require cryogenic cooling [5]. THz OPMLs were demonstrated in 1970 and the original design can still be found in commercial THz lasers [6, 7] They are powerful but they are bulky, not efficient and have limited tunability. Optical pumping in the mid infrared (MIR) region can be seen as a drawback if a CO2 (or N2O) discharge laser is used The tunability of these kind of lasers is limited due to the required coincidence of the line provided by the pumping laser with the MIR absorption line of the THz active molecular gas. Commercially available MIR QCLs are operated around room temperature
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