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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

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

Fritz Kurt Kneubühl, Damien P. Scherrer, Joachim Knittel, Dominique B. Moix, M. Baumgartner

Published May 10, 1996• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.239456

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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.
Stimulated Raman scattering is a nonlinear process, that has been studied extensively by many groups. Nevertheless, the presence of soliton-like pulses in the depletion region of the pump pulse is not yet completely understood. The origin of such spontaneous solitons is the rapid phase variation of the Stokes field induced by the quantum fluctuations of the electromagnetic field in vacuum. With the aid of a pulsed CO/sub 2/ laser system including a fast plasma shutter we have observed the first spontaneous Raman solitons in the forward stimulated Raman scattering (SRS) by the far-infrared-active laser gas NH/sub 3/.
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 ${N}^{\frac{\ensuremath{-}1}{2}}$).
We report on an investigation of simultaneous generation of several narrow-bandwidth laser-like coherent emissions from nitrogen molecular ions ( ) produced in intense mid-infrared laser fields. With systematic examinations on the dependences of coherent emissions on gas pressure as well as several laser parameters including laser intensity, polarization and wavelength of the pump laser pulses, we reveal that the multiple coherent emission lines generated in originate from a series of nonlinear processes beginning with four-wave mixing, followed with stimulated Raman scattering. Our analyses further show that the observed nonlinear processes are greatly enhanced at the resonant wavelengths, giving rise to high conversion efficiencies from the infrared pump laser pulses to the coherent emission lines near the transition wavelengths between the different vibrational energy levels of ground X and that of the excited B states.
We find that intense short pulses of XUV radiation can be produced by cooperative spontaneous emission from visible or IR laser pulses driving atoms or ions. The process depends on the generation and utilization of atomic coherence as is the case in lasing without inversion. However, the radiation process is not stimulated emission, but is rather cooperative spontaneous emission in the sense of Dicke. More precisely, the many atom mathematics of superradiance has much in common with coherent anti-Stokes Raman scattering. The generation of Raman coherence is considered from the perspectives of multi-photon excitation and breaking of adiabaticity. In particular, we find that the advent of an ultrashort (e.g. single cycle) pulse rewards rethinking of conventional (nanosecond) nonlinear optics with new insights.