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Acta Optica Sinica

Two-photon processes in four levels system

Ong D

Published Jan 1, 1982• 1 citation

Abstract

Optically pumped FIR lasers give only discrete spectrums. This property restricts the practical use of lasers. Since 1977, many research works have been done to tune the output frequency of FIR laser, of which some are tried to find the possibility of tuning the laser lines by changing the pumping frequency continuously. But it is not easy to get an accurate result due to the complexity of theoretical calculation especially for quantum system of more than three energy levels. In this research work, two photon processes in four levels system have been studied by means of graph theory, which was developed by T. A. DeTemple et al. It was found that the output frequency of optically pumped FIR laser could be tuned with a frequency range equal to the frequency difference of two FIR transition frequencies. The tuning curves of FIR frequency and the curves of system gain coefficient vs. pump detuning frequencies were given.

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Optically pumped FIR lasers give only discrete spectrums. This property restricts the practical use of lasers. Since 1977, many research works have been done to tune the output frequency of FIR laser, of which some are tried to find the possibility of tuning the laser lines by changing the pumping frequency continuously. But it is not easy to get an accurate result due to the complexity of theoretical calculation especially for quantum system of more than three energy levels. In this research work, two photon processes in four levels system have been studied by means of graph theory, which was developed by T. A. DeTemple et al. It was found that the output frequency of optically pumped FIR laser could be tuned with a frequency range equal to the frequency difference of two FIR transition frequencies. The tuning curves of FIR frequency and the curves of system gain coefficient vs. pump detuning frequencies were given.

The semiclassical density matrix treatment is applied to serial four-level systems for the steady state. Phase and energy relaxations as well as Doppler effects andMdegeneracy are accounted for. The calculations are restricted to parallel polarized fields, avoiding problems of quantum interference. The formalism is used to calculate the gain spectra of different four-level far-infrared (FIR) laser schemes: double resonance pumped laser, Raman laser, two-photon pumped laser, and hyper-Raman-laser.

The interactions among R35, T34 and S54 processes in a six-level system were studied. Near the cross point of the resonant curves of these processes in the multiphoton resonance diagram of the six-level system, such interactions should be very strong. We found theoretically that in the case of off-resonant-pumpnig, the interactions among these multiphoton prooesseg should also lead tosplitting of the laser spectral line, which is called off-resonant-pumping AC Stark effect. Because the influences of saturation effect and electric dipole transition enhancement effect were not the same for different multiphoton processes, contributions of different multiphoton processes to the off-resonant AC Stark effect could be distinguished by the injection of FIR signal into the system.

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.