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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

New phenomena in optically pumped far-infrared superradiant and Raman emissions

Damien P. Scherrer

Published Dec 7, 2017• 0 citations
Publisher

Abstract

We report on new effects achieved by optical pumping of far-infrared superradiance and Raman emissions in CH3F, CH3CN, D20, NH3 by rapidly truncated 101.1m CO2 laser pulses. These phenomena include drastic reduction of far-infrared-pulse duration, competition of forward superradiance and Raman emission, stability of backward superradiance, periodic back-and-forth fluctuation of the A-like three-level molecular system.

DOI
10.1117/12.2298203

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Physical Review A•Aug 1, 1981•46 citations
We report on new effects achieved by optical pumping of far-infrared superradiance and Raman emissions in CH3F, CH3CN, D20, NH3 by rapidly truncated 101.1m CO2 laser pulses. These phenomena include drastic reduction of far-infrared-pulse duration, competition of forward superradiance and Raman emission, stability of backward superradiance, periodic back-and-forth fluctuation of the A-like three-level molecular system.
In order to produce ultrashort far-infrared (FIR) laser pulses we have investigated the superradiance and Raman emission of various molecular transitions optically pumped with the pulses of 10 micrometers -hybrid- or high-pressure single-mode-CO2 lasers truncated within 10 ps. Furthermore we have truncated directly the FIR laser pulses with the first FIR plasma shutter. In our experiments we have observed new phenomena which include e.g. new interrelations between superradiance, swept-gain superradiance and Raman emission, anticorrelated fluctuations of pump radiation versus FIR emission, first optical free induction decay of 10 micrometers -CO2-laser pulses with FIR emitting molecular gases. Finally, we have proved the first spontaneous infrared solitons in stimulated far-infrared Raman scattering on NH3. We have observed solitons in the depletion region of the infrared pump radiation with corresponding dark solitons in the Stokes pulses and detected the laser phase change connected with the soliton generation.
Several polar molecules possessing strong far-infrared (FIR) transitions have been optically pumped by a mode-locked CO2TEA laser in a single-pass configuration. All the resulting FIR pulses are shorter than the sub-T2pump pulses; subnanosecond FIR pulses are observed. These short FIR pulses exhibit lethargic gain, stimulated Raman emission, and multiple-pulse superradiance. The experimental results are modeled by numerical solution of the Maxwell-density matrix equations for two fields interacting with a three-level system. Level degeneracy is included, and the pump is described by a multimode power spectrum with a parabolic envelope, resulting in a periodic train of pulses. Variation of the detuning of the pump from resonance allows simulation of the superradiant or Raman cases. This fully coherent model, which is the major contribution of this paper, explains key experimental observations such as the noncoincidence of the Raman peak with the pump modulation minimum and the enhancement of multiple-pulse superradiance due to remanent polarization.
Superradiance has been observed in the far infrared on a pure-rotational transition in optically pumped methyl fluoride. Observed behavior included single pulses, shot-to-shot fluctuation, dependence on the pump pulse length, preferential polarization, and longitudinally asymmetric emission (the onset of swept-gain superradiance). A numerical solution of the Maxwell-Bloch equations including degeneracy, Doppler broadening, and population and polarization source terms, and taking into account all the transitions which might possibly contribute, produces results which agree well with the observations. To do this, only a single free parameter is used; this is a factor of about 10 multiplying the expected initial tipping angle (which is of the order of ${N}^{\frac{\ensuremath{-}1}{2}}$).