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Physical Review A

Four-wave mixing in strongly driven two-level systems

Witold Chal upczak, Wojciech Gawlik, Jerzy Zachorowski

Published Jun 1, 1994• 28 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.49.4895

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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.
We have observed conical emission from a dense, strongly driven ensemble of two-level atoms without noticeable self-focusing. We present the arguments that conical emission, unaffected by autocollimation, is due to cooperative, \ifmmode \check{C}\else \v{C}\fi{}erenkov-like, spontaneous emission of collisionally perturbed, optically dressed atoms rather than to parametric wave mixing. Our results indicate the importance of many-atom correlations which must be taken into account when strong light interacts with dense media.
We study the interaction of a two-level atom with two lasers of different frequencies and amplitudes: a strong laser of Rabi frequency $2{\ensuremath{\Omega}}_{1}$ on resonance with the atomic transition, and a weaker laser detuned by subharmonics $(2{\ensuremath{\Omega}}_{1}/n)$ of the Rabi frequency of the first. We find that under these conditions the second laser couples the dressed states created by the first in an nnn-photon process, resulting in ``doubly dressed'' states and in a ``multiphoton ac Stark'' effect. We calculate the eigenstates of the doubly dressed atom and their energies, and illustrate the role of this multiphoton ac Stark effect in its fluorescence, absorption, and Autler-Townes spectra.