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Physical Review A

Far-infrared superradiance in methyl fluoride

Albert T. Rosenberger, Thomas A. DeTemple

Published Aug 1, 1981• 46 citations
Publisher

Abstract

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 ${N}^{\frac{\ensuremath{-}1}{2}}$).

DOI
10.1103/physreva.24.868

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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\ensuremath−12{N}^{\frac{\ensuremath{-}1}{2}}N2\ensuremath−1​).
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.
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.
We have found that room-temperature HF gas at millitorr pressures, optically pumped to produce a total inversion at a rotational transition in the v=1v=1v=1 vibrational state, emits a superradiant pulse with ringing after a considerable delay (\ensuremath{\sim} microseconds). A semiclassical analysis shows that for a high-gain system the pulse evolution is determined by a single parameter ${\ensuremath{\tau}}_{R}$, and that inhomogeneous broadening is unimportant. Close agreement between theory and experiment is obtained.