Nuclear “anti-stokes” transitions induced by laser radiation

Boris M. Antipenko, S. P. Voronin, T. A. Privalova

Лев Абрамович Ривлин

Recoil in radiative gamma transitions leads to a discrete line spectrum of absorption and emission resonances in an ensemble of free nuclei. This opens up the possibility of efficient pumping of such nuclei and of hidden inversion in an nonstandard 'optical' scheme without participation of the third auxiliary level, as well as in Stokes and anti-Stokes schemes. Quantitative estimates are given for several nuclides.

Wayne T. Whitney, Nicholas Djeu

Dimitrios Charalambidis, Natalia E. Karapanagioti, O. Faucher +1 more

H. A. Haus, Paul L. Kelley, Herbert J. Zeiger

A three-dimensional analysis of stimulated Raman emission is presented for a medium through which a cylindrical laser beam passes. The buildup of the Stokes radiation from the spontaneous fluctuations in the Raman medium is considered. A generalization of the fluctuation-dissipation theorem is derived relating the current-current fluctuations at the Stokes frequency to the laser-induced (negative) conductance at this frequency. The anti-Stokes radiation is treated as arising from the interaction of the laser field and the molecular vibrations accompanying the Stokes radiation. The anti-Stokes radiation emerges in a narrow cone around the familiar phase-matching direction given by ${\mathbf{\ensuremath{\beta}}}_{\ensuremath{-}}=2{\mathbf{k}}_{L}\ensuremath{-}{\mathbf{\ensuremath{\beta}}}_{+}$, where ${\mathbf{\ensuremath{\beta}}}_{\ensuremath{-}}$ and ${\mathbf{\ensuremath{\beta}}}_{+}$ are the (real parts of the) propagation constants of Stokes and anti-Stokes waves in the medium, and ${\mathbf{k}}_{L}$ is the laser propagation vector. The angular width of the anti-Stokes radiation cone is evaluated.

A. David Buckingham

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.