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

Line competition and line selection in off-resonantly pumped systems

M.‐A. Dupertuis, S.A. Salito, Rainer Salomaa, M. R. Siegrist

Published Dec 1, 1985• 0 citations

Abstract

For some time already it has been known that off-resonantly pumped far infrared (FIR) lasers oscillate on the Raman line [1]. This feature has led to important applications such as the realisation of widely tunable CH3F lasers (150–1200 μm) [2]. Only recently it has been shown [3] that mode competition is responsible for the fact that Raman oscillation (R) dominates line centre oscillation (LC) in a three-level system. The same considerations are valid in the equivalent situation of a single refilling transition (|0 > → |1> in Fig. 1). In certain experiments [4–7] Λ and V transitions (Fig. 1) appear in combination. For this case LC and R oscillations have been observed to exist simultaneously in one of the FIR transitions [5].

DOI: 10.1109/irmm.1985.9126682

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For some time already it has been known that off-resonantly pumped far infrared (FIR) lasers oscillate on the Raman line [1]. This feature has led to important applications such as the realisation of widely tunable CH3F lasers (150–1200 μm) [2]. Only recently it has been shown [3] that mode competition is responsible for the fact that Raman oscillation (R) dominates line centre oscillation (LC) in a three-level system. The same considerations are valid in the equivalent situation of a single refilling transition (|0 > → |1> in Fig. 1). In certain experiments [4–7] Λ and V transitions (Fig. 1) appear in combination. For this case LC and R oscillations have been observed to exist simultaneously in one of the FIR transitions [5].

In weakly, off-resonantly pumped multilevel systems gain is usually observed at the transition frequencies (line-center lines) and at the pump-offset-dependent frequencies (Raman lines). Modes developing at these frequencies are subject to strong mutual competition due to several coupling mechanisms. A general six-level system is investigated in this paper. Line competition effects are studied in particular in the simplified basic four-level cases comprising parallel, cascade, and V-\ensuremath{\Lambda} subsystems. We predict the type of transitions which survive and ultimately remain active. A comparison with experimental results using far-infrared lasers is made.

We examine the problem of line competition in an off-resonantly pumped three-level loop configuration that is encountered in four-wave mixing. Population and coherence pulsations play a key role during the competition. Analytical results in the small-signal-gain region provide some insight into the start-up of the loop oscillation. The general problem with saturating fields and strong line competition is solved numerically. We show that in the asymptotic regime of oscillation all the lines may survive, although some of them remain quite weak. We show also that our results are compatible with recent experiments.

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.