New phenomena related to pulsed far-infrared superradiant and Raman emissions

In order to produce ultrashort far-infrared (FIR) laser pulses we have investigated the superradiance and Raman emission of various transitions, e.g. 496 μm-CH3F, 385 μm-D2O, 373 μm-CH3CN, 291 μm- and 151 μm-NH3, optically pumped by rapidly truncated 10 μm-CO2 laser pulses produced by an optical-free-induction decay (OFID) 10 μm-CO2 laser system. Forward and backward superradiance, i.e. emission parallel and anti-parallel to the propagation direction of CO2 pump pulses, has been confirmed by the measurement of the pressure dependence of the pulse intensity as well as duration and delay versus the pump pulses. We have found that the durations of the superradiant FIR pulses are considerably shorter than the conventional pulsewidth limit given by the inverse linewidth of the transition. Thus, we have demonstrated that the fast truncation of about 10 ps of the plasma shutter used in our OFID 10 μm-CO2 laser system is essential for the reduction of the duration of FIR superradiant pulses. We have found that the backward FIR emissions are superradiant over the whole range of our experimental investigations and show high quality with respect to stability and reproducibility. This is important for applications. For the forward emissions we have observed as expected theoretically the onset of swept-gain superradiance and the appearance of Raman emission with increasing pressure in the FIR cell. This implies line competition between the superradiant emission and the Raman emission and results therefore in different emission regimes which we have systematically investigated.Thus, we have found emission regimes ideal to produce well-defined nano- and subnanosecond FIR single pulses which are among the shortest produced hitherto. (author) figs., tabs., 129 refs