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Soviet Journal of Quantum Electronics

Influence of spatial coherence of a laser field on the efficiency of two-photon absorption

Tatiana N. Smirnova, Eugene A. Tikhonov

Published Jan 31, 1976• 1 citation
Publisher

Abstract

An investigation was made of the dependence of the two-photon absorption of ruby laser radiation in alcohol solutions of rhodamine 6 G on the intensity of the exciting field in the case of single-frequency emission and emission of a large number of transverse modes. The efficiency of the two-photon absorption in the case of multimode excitation was 1.8 times (determined with a relative error of 25 % ) greater than the efficiency of the absorption as a result of single-frequency excitation. The measured absorption coefficient was close to that calculated on the assumption that the single-frequency field was spatially coherent and the spatial fluctuations of the multimode field were described by a Gaussian distribution.

DOI
10.1070/qe1976v006n01abeh010798

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An investigation was made of the dependence of the two-photon absorption of ruby laser radiation in alcohol solutions of rhodamine 6 G on the intensity of the exciting field in the case of single-frequency emission and emission of a large number of transverse modes. The efficiency of the two-photon absorption in the case of multimode excitation was 1.8 times (determined with a relative error of 25 % ) greater than the efficiency of the absorption as a result of single-frequency excitation. The measured absorption coefficient was close to that calculated on the assumption that the single-frequency field was spatially coherent and the spatial fluctuations of the multimode field were described by a Gaussian distribution.
The possibility of a practically total reduction of quantum photon fluctuations in cw laser radiation is discussed. This effect appears as a result of two-photon radiation absorption using a resonance transition in atoms excited from the ground state to the upper level, relaxing to the other states. For this case the possibility of induced two-photon emission is excluded, that is, the source of additional photon number fluctuations in a field mode.1 In this process the Fock states with the photon number n ≥ 2 are continuously reduced from the coherent state of the excited field. For the large time values of interaction the Fock states |n = 0 > and |n = 1> remain in the field. The effect of spin orientations of photons of the excited field on the quantum noise reduction process for the two-photon absorption by atoms on the transitions between the S → S and S → D terms is analyzed. The possibility of transformation of coherent and thermal fields (λ ≃ 6022 Å and λ = 5787 Å) into quasi-Fock states, as a result of its two-photon resonance absorption by Na homogeneous vapor, is considered.
Measurements of two-photon absorption spectra have been made for the case where the exciting laser has a random-telegraph phase. The resulting spectral shapes are compared to theoretical predictions and to previous data taken with a phase-diffusing laser field [Elliott et al., Phys. Rev. A 32, 887 (1985)]. A striking dependence of the absorption spectrum on the second-order coherence of the field was observed. Using the theory for the propagation of second-order spatial coherence, we draw an analogy between diffraction and two-photon absorption which we use to interpret the two-photon absorption spectra.
Paraxial equations for the intensity and eikonal of a light wave, and also a kinetic equation for the difference between the level populations are used to analyze deformation of the space–time structure of a laser pulse occurring in a medium exhibiting two-photon absorption under noncoherent interaction conditions. The analysis is made using time and space compression functions, and the results of a numerical solution of two-dimensional equations for the propagation of radiation. A determination is made of the principal laws governing the changes in the time and space distributions of the intensity of radiation under quasisteady and transient interaction conditions, and also in the case of a small signal. It is shown that two-photon absorption results in a significant inhomogeneity of the space–time structure of a pulse. The results are given of experimental investigations of changes in the time, space, and energy characteristics of a neodymium laser pulse undergoing two-photon absorption in gallium arsenide.
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.