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Journal of Physics B Atomic and Molecular Physics

The time development of adiabatic two-photon absorption. I. Low intensity regime

L. Allen, K A Eagles, Carlos R. Stroud

Published Jun 14, 1982• 12 citations
Publisher

Abstract

The time dependence of the two-photon absorption process is experimentally investigated in the low intensity regime as a function of laser linewidth, Doppler width, polarisation and delay between pulses, using 5 ns pulses at two different frequencies. Excellent agreement is found between experiment and the theoretical predictions of Allen and Stroud. The experiment arguments and expands earlier work on the dependence of linewidth in the two-photon process. The lineshape of the Hansch-type laser producing the pulses is shown to be best described by a Gaussian rather than a Lorentzian or as the result of an Ornstein-Uhlenbeck process.

DOI
10.1088/0022-3700/15/11/010

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We have measured absorption spectra for Doppler-free two-photon absorption from a laser field characterized by strong amplitude fluctuations. This field, known as the real Gaussian field, is generated by randomly modulating the amplitude of the output of a cw stabilized dye laser. The absorption rate from this field is enhanced over that from a field whose intensity is nonvarying and of the same average value. When the laser bandwidth is very large, the absorption spectrum is limited, not by the laser width, but by the width of the atomic resonance. Our measurements are in excellent agreement with calculated spectra.

Annual Meeting Optical Society of America•Jan 1, 1992•0 citations
The time dependence of the two-photon absorption process is experimentally investigated in the low intensity regime as a function of laser linewidth, Doppler width, polarisation and delay between pulses, using 5 ns pulses at two different frequencies. Excellent agreement is found between experiment and the theoretical predictions of Allen and Stroud. The experiment arguments and expands earlier work on the dependence of linewidth in the two-photon process. The lineshape of the Hansch-type laser producing the pulses is shown to be best described by a Gaussian rather than a Lorentzian or as the result of an Ornstein-Uhlenbeck process.
Experimental evidence of field-correlation effects on weak-field two-photon absorption in atomic sodium is presented. In the case of a nearly Lorentzian laser power spectrum the absorption profile has four times the spectral width of the exciting field, in agreement with predictions by Mollow. The measurement is carried out with counterpropagating laser beams to cancel Doppler broadening. The width of the two-photon absorption spectrum is decreased by partially decorrelating the counterpropagating laser beams. Increasing the correlation time of the frequency fluctuations, resulting in a nearly Gaussian laser power spectrum, has also been observed to decrease the width of the absorption spectrum. An extension of the time-dependent second-order perturbation theory to these additional cases yields good agreement.
An unconventional perturbation treatment of the theory of two-photon absorption in a four-level atomic system close to two-photon resonance is developed on the basis of the optical Bloch equations. The results for the longtime populations of the intermediate and upper states are valid in low- and medium-intensity regimes. In both regimes the upper state population shows a lorentzian resonance centred on zero two-photon detuning with a width equal to the natural linewidth of the upper state. The intermediate state population shows a similar behaviour in the intermediate regime only, with no significant dependence on two-photon detuning in the low-intensity regime. However, in both regimes the population of the virtual intermediate state is at least comparable to that of the resonant upper state. The perturbation theory results have been checked by direct numerical calculations based on the Bloch equations. Numerical calculations confirm the presence of a high-intensity regime characterized by power shifts, broadening and saturation effects.
For pt.I see ibid., vol.8, p.1795 (1975) where the theory of Doppler-free two-photon transitions was discussed for the case of one single laser frequency. Here, the effects of two different frequencies are considered. When the residual Doppler width of the two-photon transition is less than the natural linewidth, the previous results are essentially regained. For large residual Doppler shifts, the absorption profile is a power-broadened Lorentzian if the frequency of the probe field is larger than the frequency of the strong field. In the opposite case, the absorption peak splits because of the AC Stark effect. The high-intensity distortions are minimized when equal frequencies can be used. Power-dependent shifts do not ordinarily occur with a resonantly tuned intermediate level. Some modifications of the spectra are introduced by phase-disturbing collisions.
We have measured absorption spectra for Doppler-free two-photon absorption from a laser field characterized by strong amplitude fluctuations. This field, known as the real Gaussian field, is generated by randomly modulating the amplitude of the output of a cw stabilized dye laser. The absorption rate from this field is enhanced over that from a field whose intensity is nonvarying and of the same average value. When the laser bandwidth is very large, the absorption spectrum is limited, not by the laser width, but by the width of the atomic resonance. Our measurements are in excellent agreement with calculated spectra.