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A new method for upconversion of monochromatic radiation

Howard C. Volkin

Published Aug 1, 1973• 0 citations
OpenAlex

Abstract

Frequency upconversion by means of stimulated Raman scattering is examined. A monochromatic beam is scattered into the anti-Stokes wave in a medium with an inverted population of the pair of energy levels involved. A method is proposed in which the beam from a laser tuned to the desired anti-Stokes frequency provides an initial stimulating wave of sufficient intensity to ensure the desired Raman scattering dominates competitive processes.

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The anti-Stokes Raman laser process is an interesting method for frequency upconversion and generation of tunable UV radiation. This method requires no index matching and allows high-conversion efficiencies and output energies and large frequency shifts in a single step. A basic requirement is a population inversion between a metastable level and a suitable lower-lying level. So far anti- Stokes laser emission has been demonstrated using the metastable TI (6P3/2). l(5P1/2). and Br (4P1/2) levels, where a population inversion with respect to the corresponding atomic ground states has been generated by photodissociation of suitable compounds.1-3 For the Tl system efficiencies of 10 % and output energies of ~1 mJ (at 377 nm) have been reported,1 and with Br a shortest wavelength of 149 nm was possible by upconversion of 157.5-nm radiation of a F2 laser.3

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Journal of Physics B Atomic and Molecular Physics•Oct 14, 1985•0 citations
Frequency upconversion by means of stimulated Raman scattering is examined. A monochromatic beam is scattered into the anti-Stokes wave in a medium with an inverted population of the pair of energy levels involved. A method is proposed in which the beam from a laser tuned to the desired anti-Stokes frequency provides an initial stimulating wave of sufficient intensity to ensure the desired Raman scattering dominates competitive processes.
A new process capable of scattering intense anti-Stokes waves that uses stimulated electronic Raman scattering in metal vapor is proposed. The coherent material excitation produced by a prepump pulse couples with the forthcoming main pulse exclusively to create polarization that radiates the anti-Stokes wave. The selection rule governed by angular momentum conservation is the key to cause such strong coupling to induce unidirectional upconversion without any population inversion. The power conversion efficiency from the pump wave to the anti-Stokes wave is theoretically expected to reach 100% in this process.
The anti-Stokes Raman laser process is an interesting method for frequency upconversion and generation of tunable UV radiation. This method requires no index matching and allows high-conversion efficiencies and output energies and large frequency shifts in a single step. A basic requirement is a population inversion between a metastable level and a suitable lower-lying level. So far anti- Stokes laser emission has been demonstrated using the metastable TI (6P3/2). l(5P1/2). and Br (4P1/2) levels, where a population inversion with respect to the corresponding atomic ground states has been generated by photodissociation of suitable compounds.1-3 For the Tl system efficiencies of 10 % and output energies of ~1 mJ (at 377 nm) have been reported,1 and with Br a shortest wavelength of 149 nm was possible by upconversion of 157.5-nm radiation of a F2 laser.3
Raman oscillation, frequency upconversion, and Raman amplification can be achieved in a second-order nonlinear medium at the phonon-polariton resonance. By beating two optical fields, a second-order nonlinear polarization is generated inside the medium. Such a polarization induces a spatially uniform nonpropagating electric field at the beat frequency, which in turn mixes with the input optical field at the lower frequency to generate or amplify the anti-Stokes optical field. Raman oscillation can be efficiently reached for the copropagating configuration. In comparison, efficient frequency upconversion and large amplifications are achievable for the counterpropagating configuration. These Raman processes can be used to effectively remove transverse-optical phonons before decaying to lower-frequency phonons, achieve laser cooling, and significantly enhance coherent anti-Stokes Raman scattering. The counterpropagating configuration offers advantages for amplifying extremely weak signals.
A simple description of the stimulated Raman emission of Stokes radiation, and of the induced absorption found by Stoicheff at the anti-Stokes frequency, is based on the semiclassical theory of radiation. This theory gives the well-known formulas for absorption and stimulated emission when carried to the first order in the time-dependent perturbation due to the interaction of the light beam with the molecules. If there are two light beams of frequencies ωL and ω, the second-order perturbation may be associated with stimulated emission at the Stokes frequency ω=ω−1=ωL—ωR and with induced absorption at the anti-Stokes frequency ω=ω1=ωL+ωR, where ωL is the laser frequency and ωR a Raman-active vibrational frequency; in both cases, there is a molecular transition to the excited vibrational state. The highly directional and very sharp anti-Stokes emission at ω1 arises from the conversion by molecules in intense beams at ωL and ω−1 of two ωL photons into an ω−1, ω1 pair. Like spontaneous emission from excited states, the production of this anti-Stokes radiation requires the full quantum theory of radiation for an adequate description. The emission of ω1 and the observed directional absorption of ω−1 due to the reverse conversion ω−1+ω1→2ωL are explained. The anti-Stokes emission is proportional to the Stokes intensity and to the square of that of the laser, and to the square of the molecular number density; its sharpness is due to the absence of a molecular transition. The generation of photon pairs ω−n+ωn, where ωn=ωL+nωR, can occur for all values of n, but it is most efficient for n=1, due to resonance in the appropriate molecular hyperpolarizability and to the intensity of the Stokes beam at ω−1.
A calculation is made of the intensity of the Stokes radiation in the stimulated Raman scattering of ultrashort light pulses in a dispersive medium employing the asymptotic approximation for arbitrary modulation of the amplitude and phase of the pump pulse. Particular attention was paid to the effect of phase modulation on the stimulated Raman scattering. Calculations were also made of the intensity of the anti-Stokes component in the scattering of an ultrashort pulse in a dispersive medium and of the optimal angle for the observation of the anti-Stokes rings.
The passage of strong monochromatic laser radiation through a resonance medium consisting of three-level atoms is investigated. It is shown that if collisions are taken into account in the scattered waves the resonances at combination frequencies occur to first order in the laser field intensity. A numerical analysis of the dependence of the stimulated electronic Raman scattering (SERS) components' gain factors on the pump frequency is carried out. The dispersion curves of the SERS components' gain (absorption) factors for fixed values of pump frequency and intensity are plotted.