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Optics and Spectroscopy

Nonlinear photoprocesses in bichromophores. II. Correlated population and fluorescence intensity fluctuations

Vadim A. Markel, M. I. Shtokman

Published Dec 1, 1988• 0 citations
OpenAlex

Abstract

We consider sationary fluctuations due to nonlinear excitation in a bichromophore, that is, in a molecule consisting of two chromophoric groups. We describe correlated fluctuations of the level populations and of the flux of flourescent photons emitted by a bichromophore. The population fluctuations are observable in experiments involving quasielastic particle scattering or Raman scattering of probe radiation. Experiments of this kind can yield information about the rate of nonradiative energy transfer. The photon correlation is characterized by antibunching and sub-Poissonian statistics and can yield information about collective excitation and nonlinear quenching.

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We consider sationary fluctuations due to nonlinear excitation in a bichromophore, that is, in a molecule consisting of two chromophoric groups. We describe correlated fluctuations of the level populations and of the flux of flourescent photons emitted by a bichromophore. The population fluctuations are observable in experiments involving quasielastic particle scattering or Raman scattering of probe radiation. Experiments of this kind can yield information about the rate of nonradiative energy transfer. The photon correlation is characterized by antibunching and sub-Poissonian statistics and can yield information about collective excitation and nonlinear quenching.
The fluorescence of individual pairs of perylenemonoimide chromophores coupled via a short rigid linker is investigated. Photon antibunching is reported, indicating collective effects in the fluorescence, which are further substantiated by the observation of collective triplet off times and triplet lifetime shortening. The experimental findings are analyzed in terms of singlet-singlet and singlet-triplet annihilation based on Förster type energy transfer. The results reported here demonstrate that the statistical properties of the emission light of isolated single quantum systems can serve as a hallmark of intermolecular interactions.
Significant fluctuations, above the shot-noise limit, have been observed in the intensity of fluorescent light scattered from atoms excited by an intense noisy laser, where a central role is played by the nonlinearity of the atom-field interaction. By considering the variance of the atomic populations, we show that ``noise spectroscopy'' is sensitive to the field statistics of the laser. We specifically consider the cases of resonance fluorescence (analogous in the weak-field limit to analysis by a Fabry-P\'erot interferometer), two-photon excitation, and double optical resonance for variable laser intensity and bandwidth. We show that large differences in the ``noise'' spectra can occur between lasers characterized by a phase-diffusion or a phase-jump model, although these models would give the same values for the mean atomic populations. We believe that observation of population fluctuations can become a useful method for characterizing laser noise.
The effects of amplitude and phase fluctuations of the pump laser on the resonance fluorescence under two-photon resonant excitation are studied. The averaging of the stochastic Bloch equations is carried out within the approximation scheme that has recently been applied to the case of one-photon resonant excitation. In the case of two-photon resonant excitation the stochastic pump field gives rise to multiplicative nonlinear Gaussian fluctuation effects. The phase fluctuations are treated in the phase-diffusion model, and the averaging over the amplitude fluctuations is performed by means of generating-function methods. Both the intensity and the intensity correlation of the fluorescent light are studied. When the amplitude fluctuations are weak, the cases of one- and two-photon resonant excitations are similar. The difference between both cases consists of a modification of the Rabi frequency and a faster damping of the Rabi oscillations in the two-photon case. For sufficiently fast correlation decay of the amplitude fluctuations their effect can simply be described by a rate. In the general case of n-photon resonant excitation this rate is larger by a factor of n2{n}^{2}n2 than that in the case of one-photon resonant excitation. When the amplitude fluctuations are sufficiently strong, the Rabi oscillations are ``washed out.'' In the case of one-photon resonant excitation both the intensity and the intensity correlation function of the scattered light develop into step functions over time. In the two-photon case the dynamics of both quantities is determined by a new time constant which acts as both the oscillation time and the damping time. Consequently an overshoot peak occurs in both the intensity and the intensity correlation instead of the tendency to become step functions.
The photon pair correlation in the laser-excited fluorescence of a single trapped and cooled Ba+{\mathrm{Ba}}^{+}Ba+ ion shows antibunching and, in addition, novel nonclassical phenomena absent in the fluorescence of two-level atoms. They include excessive transient values of the correlation caused by optical pumping, and temporally extended sub-Poissonian photon emission probability which arises from the transient excitation of nonabsorbing Raman coherence. The fluorescence also displays sub-Poissonian photon statistics.
Abstract : Temporal photon correlations and photon count statistics of fluorescence radiation, emitted by an atom in the vicinity of a phase conjugator, are considered. The fluorescent intensity and the two-photon correlation function are expressed in terms of photon emission operators. An explicit evaluation shows their dependence on the phase-conjugate reflectivity of the surface. The variance of the photon count distribution for long counting times is expressed in terms of the Q-factor, which is evaluated as a function of the reflectivity. It appears that for a reflectivity larger than unity, the photon correlation displays antibunching and the statistics is sub-poissonian.
The method previously used by the author for the measurement and calculation of the distribution function w N(T)of fluorescence photons from a single two-level atom that is continuously excited by laser light and has a unity fluorescence quantum yield is generalized to the case of a single molecule whose fluorescence quantum yield is smaller than unity and to the case of a three-level molecule whose fluorescence is blinking. The functions w N(T) calculated for these two cases demonstrate a sub-Poissonian distribution of fluorescence photons.