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AC Stark-effect-enhanced near-resonant wave mixing in a four-level system. 1: Experiment

Chen-Yu Tai, Charles C. Kim

Published Jun 9, 1986• 0 citations
OpenAlex

Abstract

Two laser beams at frequencies v(1) and v(2) are used to produce nonlinear scattering (wave mixing) in I2 vapor at frequency 2v(1) + v(2). The scattering results from near resonance allowed transitions in I2 molecules and is observed to be resonantly enhanced when neither the one-, two-, nor three-quantum transitions are in resonance.

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Two laser beams at frequencies ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{2}$ are used to produce nonlinear scattering (wave mixing) in I2{\mathrm{I}}_{2}I2​ vapor at frequency 2${\ensuremath{\nu}}_{1}$+${\ensuremath{\nu}}_{2}$. The scattering results from near-resonant allowed transitions in I2{\mathrm{I}}_{2}I2​ molecules and is observed to be anomalously strong when both laser beam frequencies are detuned in the same direction from frequencies corresponding to resonance in the free molecule. The strong scattering is interpreted in terms of a resonantly enhanced ac Stark shift caused by a configuration interaction between the ``dressed molecular levels.'' By tuning the two pulses simultaneously, a tuning range of approximately 2 A\r{} (50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) is achieved in the mixed wave. The scattering process should therefore have practical application in the efficient generation of tunable coherent radiation in the ultraviolet and vacuum-ultraviolet range of the spectrum. Some of the spectroscopic properties of the three-quantum state at E\ensuremath{\sim}53 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are studied.
We report on an ac-Stark-effect-enhanced wave-mixing experiment in I2{\mathrm{I}}_{2}I2​ vapor in which an output uv signal at frequency 2${\ensuremath{\omega}}_{1}$+${\ensuremath{\omega}}_{2}$ is maximized when the input frequencies ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ are both detuned in the same direction from the resonance frequencies of the free molecule. A theory based on a configuration interaction between the ``dressed molecular levels'' is presented to explain the strong enhancement of the uv signal under these conditions. The data presented demonstrate that tunable coherent radiation at ultraviolet and vacuum-ultraviolet frequencies can be generated by the wave-mixing process we describe.
Resonance-enhanced four-wave-mixing processes were studied in molecular iodine by detection of a wave generated at frequency ${\mathrm{\ensuremath{\omega}}}_{3}$=2${\mathrm{\ensuremath{\omega}}}_{1}$-${\mathrm{\ensuremath{\omega}}}_{2}$ for a wide range of Raman frequencies (${\mathrm{\ensuremath{\omega}}}_{1}$-${\mathrm{\ensuremath{\omega}}}_{2}$). On the basis of known spectroscopic constants of the two electronic states involved, the electronic ground state X1{\mathit{X}}^{1}X1${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}^{+}$ and the electronic excited state B3{\mathit{B}}^{3}B3${\mathrm{\ensuremath{\Pi}}}_{\mathit{u}0}^{+}$, an interpretation of the spectral features in terms of three essentially different four-wave-mixing processes is given. Resonance enhancement through bound states with discrete energy levels and the enhancement effect by dissociative continuum states are considered. In addition to resonance-enhanced coherent anti-Stokes Raman-scattering (CARS) processes, in which the Raman resonances are in the electronic ground state, two resonant schemes are identified in I2{\mathrm{I}}_{2}I2​, in which Raman resonances in an excited state are probed: excited-state parametric and nonparametric CARS. The latter process is an example of electronic population transfer in a four-wave-mixing cycle.
When an atomic vapor interacts with an intense nearly resonant laser field, the ac Stark effect causes a profound modification of the weak-field absorption spectrum. Two of the induced spectral features can lead to amplification of a weak probe wave. We have previously measured in atomic sodium vapor high probe-wave gain (G = 38) due to the three-photon effect and a somewhat lower gain (G = 4) when the probe wave is detuned from the pump laser frequency by the inverse of the atomic response time.1 We present the results of an experimental and theoretical study of the energy coupling between two intense laser fields of comparable intensity through their interaction with an atomic vapor. In our experiments, a tunable alexandrite laser provides microsecond laser pulses of the required intensity that are nearly resonant with the 4s → 4p transition in potassium. We observed large energy transfer (50%) between the two waves at the nearly degenerate resonance and measured the intensity dependence of the coupling efficiency. Theoretical modeling of the interaction is based on a continued-fraction solution of the density-matrix equations of motion.
We present experimental results of ac-Stark-effect-enhanced four-wave mixing in a dense sample of two-level atoms. The mixing efficiency exhibits strong maxima when the beams fulfill resonance and phase-matching conditions for the transitions in a dressed atom. The experimental arrangement enabled the verification of recent theories of parametric interactions in strongly driven systems. Good agreement of these theories with the experimental results allows one to use the wave mixing in the studies of properties and dynamics of strongly driven, dense, two-level systems. In particular, the possible role of parametric interaction in the phenomenon of conical emission has been analyzed in this manner.