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Acta Physica Sinica

Single-photon Raman and super-Raman processes in a two-photon resonantly pumped four-level system

Changxin Wang, Ensheng Fu, Runwen Wang

Published Jan 1, 1987• 0 citations
OpenAlex

Abstract

In this paper, the dressed-atom approach is successfully used to derive the analytic solution of FIB gain foeffieient for two-photon resonantly pumped four-level system. The gain characteristics and AC stark effect of the system are discussed in detail. It is theoretically proved that the stimulated super-Raman process can lead to high power and frequency stable FIR lasers.

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Driven two-level atoms as gain media for two or more photon amplification

Y. Zhu, S. Morin, Qilin Wu +3 more

It is well known that driven two-level atoms can exhibit optical amplification.1 This fact has led to the recent demonstration of single-photon lasing in cell and atomic-beam type ensembles of two-level atoms.2 We show that the same simple two-level system can also be utilized as a two- or more-photon amplifying medium. The underlying physics can be understood in terms of the atom field dressed-state picture. When driven by a field detuned from the atomic resonance, significant population inversion between the dressed-state doublets can be created. In the dressed picture, single-photon lasing occurs between inverted sublevels of adjacent dressed-state doublets. Transitions between nonadjacent doublets are still inverted, but require two or more photons. In the degenerate case, the multiphoton emission frequencies differ from the single-photon emission frequency and can hence be selectively enhanced using an optical cavity. Due to the peculiar structure of dressed levels, the multiphoton emissions are all connected through near resonant intermediate states. The multiphoton emissions can also be viewed as a stimulated hyper-Raman scattering of successively higher order.

Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
In this paper, the dressed-atom approach is successfully used to derive the analytic solution of FIB gain foeffieient for two-photon resonantly pumped four-level system. The gain characteristics and AC stark effect of the system are discussed in detail. It is theoretically proved that the stimulated super-Raman process can lead to high power and frequency stable FIR lasers.
Driven two-level atoms are shown to exhibit strong, resonantly enhanced, two-photon gain. This gain, unlike that observed in nonlinear mixing processes, is not constrained by a phase-matching condition involving the pump beam. Physically, the gain can be associated with inverted two-photon transitions between dressed states or multiphoton stimulated Raman scattering, and it may be useful in the realization of a two-photon laser. Comparison with the well-known single-photon gain found in the same system is made.
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 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.
A formalism suited for the study of two-photon resonant processes in atomic vapors is developed. Applications to the study of hyper-Raman scattering, four-photon parametric conversion and second-harmonic generation were carried out. Hyper-Raman scattering and four-photon parametric conversion were analyzed at steady state in the small-gain and saturation regimes. For the small-gain region, the results of perturbation theory are reproduced. The behavior of Stokes and idler fields in the saturation regime is shown to be in agreement with experimental results. Several characteristics of the line shapes experimentally observed are also qualitatively explained. A time-frequency analysis of the transients of the Stokes gain provides a method of studying the evolution of Stark shifts and dynamical Stark splittings, as well as constituting a valuable tool to distinguish between Raman-type and population-type lines.
It is well known that driven two-level atoms can exhibit optical amplification.1 This fact has led to the recent demonstration of single-photon lasing in cell and atomic-beam type ensembles of two-level atoms.2 We show that the same simple two-level system can also be utilized as a two- or more-photon amplifying medium. The underlying physics can be understood in terms of the atom field dressed-state picture. When driven by a field detuned from the atomic resonance, significant population inversion between the dressed-state doublets can be created. In the dressed picture, single-photon lasing occurs between inverted sublevels of adjacent dressed-state doublets. Transitions between nonadjacent doublets are still inverted, but require two or more photons. In the degenerate case, the multiphoton emission frequencies differ from the single-photon emission frequency and can hence be selectively enhanced using an optical cavity. Due to the peculiar structure of dressed levels, the multiphoton emissions are all connected through near resonant intermediate states. The multiphoton emissions can also be viewed as a stimulated hyper-Raman scattering of successively higher order.