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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Two-photon light absorption by atoms near the body surface with spectral features

Gennadiy Nikolaev, Oleg Uuemaa

Published Oct 21, 1998• 0 citations
Publisher

Abstract

Two-photon light absorption by an atom situated near to surface of a solid with spectral features in the vicinity of resonance frequencies of transitions of atoms is theoretically investigated. These features can be caused, for example, polariton or exiton resonance of a solid. It is shown, that the two-photon light absorption an atom situated near to surface can essentially differ from those in a free space. The spectrum of a two-photon absorption becomes asymmetric over the difference between exciting light frequency and resonant transition frequency of the atom. It is established, that even small spectral features of solid can give rise to significant changes of a spectrum of a two-photon absorption of light. It is established also, that such changes of a spectrum of a two- photon absorption of light are nonlinear increased with increase of the intensity of the exciting light and with approach of the atom to the surface of solid.

DOI
10.1117/12.328247

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Two-photon light absorption by an atom situated near to surface of a solid with spectral features in the vicinity of resonance frequencies of transitions of atoms is theoretically investigated. These features can be caused, for example, polariton or exiton resonance of a solid. It is shown, that the two-photon light absorption an atom situated near to surface can essentially differ from those in a free space. The spectrum of a two-photon absorption becomes asymmetric over the difference between exciting light frequency and resonant transition frequency of the atom. It is established, that even small spectral features of solid can give rise to significant changes of a spectrum of a two-photon absorption of light. It is established also, that such changes of a spectrum of a two- photon absorption of light are nonlinear increased with increase of the intensity of the exciting light and with approach of the atom to the surface of solid.
The line shape and strength of two-photon absorption in an atomic vapor are studied both theoretically and experimentally for the case of absorption of unequal-frequency photons. Special attention is given to the situation in which the intermediate state is resonant or nearly resonant. It is shown that for the resonant case, maximum absorption rates and Doppler-free spectra are simultaneously obtained. Either counterpropagating or copropagating beams can be used. With counterpropagating beams the two-photon linewidths are quite insensitive to power broadening of the intermediate state. With copropagating beams the two-photon spectra can also reveal the intermediate-state structure with a resolution determined mainly by the intermediate-state lifetime (nearly as well as can be done using any spectroscopic technique). The theoretical analysis provides a closed-form solution for the line shape and strength of the two-photon spectrum. In general the line shape is composed of two features. One is Doppler broadened and predominates when the intermediate state is nonresonant. The second feature is Doppler free, and it predominates with a resonant intermediate state. Detailed comparisons of experimental measurements of the line shape and strength of two-photon absorption in Na vapor are made with the theory and found to be in excellent agreement. Signal strengths for the onresonance case are 6 \ifmmode\times\else\texttimes\fi{} 109{10}^{9}109 larger than the case of equal-frequency photons with no loss in resolution.
Understanding the processes underlying the optical excitation of atomic vapor in the vicinity of solid surfaces is of interest both from physical and technical points of view. Selective reflection spectra of laser light from an interface of a transparent dielectric material and resonant vapor are known to be Doppler-free and to provide important spectroscopic information about the atom-wall interaction. On the other hand, atomic vapors have such a large and fast nonlinear response on or near resonances that they are capable of processing signals and images with milliwatt lasers at a time scale of submicroseconds. In the present paper we investigate linear and nonlinear optical properties of a thin film of atomic vapor confined between two solid surfaces. Assuming two-level atoms, we solve the Maxwell- Bloch equations for light reflection from resonant atoms. The interaction between the resonant atoms and the wall is accounted for through the boundary conditions for all elements of density matrix. The transient polarization is shown to make an important contribution not only to the field structure in the vicinity of the wall but also to the interference pattern of the reflected field. It is found that the spectrum of reflection depends remarkably on the film thickness. Strong enhancement of signal is also found from this study and possible devices for the detection are discussed.
Two-photon absorption of a continuous train of ultrashort pulses in a gas gives rise to resonances when the sum of the frequencies of two locked modes coincides with the transition frequency in the gas. It is shown that the width of these resonances is not governed by the pulse duration but by the width of the twophoton transition. Resonance intensities are of the same order of magnitude as in the single-frequency regime at the same power level. It is worth noting an important feature of the use of ultrashort pulses in studies of two-photon resonances due to the 1S-2S transition in the hydrogen atom: there is a strong increase in the efficiency of conversion of dye laser radiation to the ultraviolet range.
It is well known that irradiation with intense laser light can lead to nonlinear absorption processes by individual molecules. However, the mutual interaction between two molecules can lead to cooperative nonlinear processes which result in entirely new features in the absorption spectrum. In this paper, the theory of cooperative two-photon absorption is developed using the principles of quantum electrodynamics, and expressions are derived for the rate of cooperative absorption for transitions which are forbidden in the absence of any such interaction. Several different cases are examined in detail, including crystalline or matrix-isolated species, gaseous and liquid mixtures, and van der Waals molecules. It is shown that the cooperative absorption rate can be substantially increased by choosing materials with suitable energy levels where one of several resonance enhancement mechanisms can be exploited. Finally, methods of observation are discussed, and a specific photochemical example is given for the case of a mixture of formaldehyde and deuterioformaldehyde.
The absorption spectrum for weak radiation of a two-state atom near the surface of a phase-conjugating four-wave mixer has been evaluated. Also included is the option that the atom is illuminated by a near-resonant laser field. In the secular limit with respect to the laser-dressed atomic states the absorption spectrum has two lines, just as for a free atom. The strengths and the widths of these lines depend on the phase-conjugate reflectivity, as well as on the optical parameters. It is shown that increasing the reflectivity drives the atom towards saturation through three-photon processes. Also the driving laser tends to saturate the transition, but it is shown that both mechanisms have a different effect on the absorption profile, and can hence be distinguished experimentally.