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Journal of Experimental and Theoretical Physics Letters

Relaxation of excited states of an atom in the vicinity of a nanoparticle

G. N. Nikolaev

Published Jun 1, 2013• 0 citations
Publisher

Abstract

Relaxation of an excited state of an atom in the vicinity of a nanoparticle has been described taking into account the rotational degrees of freedom of the atom. The description has been compared with the widely used generalized (vector) two-level model of an atom and the classical oscillator model. Relaxation dynamics of real atoms (excluding the atoms with the angular momenta J m = 1 and J n = 0 of the upper and lower levels, respectively) has been found to differ significantly from the predictions of those models.

DOI
10.1134/s0021364013070084

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Relaxation of an excited state of an atom in the vicinity of a nanoparticle has been described taking into account the rotational degrees of freedom of the atom. The description has been compared with the widely used generalized (vector) two-level model of an atom and the classical oscillator model. Relaxation dynamics of real atoms (excluding the atoms with the angular momenta J m = 1 and J n = 0 of the upper and lower levels, respectively) has been found to differ significantly from the predictions of those models.
Numerical simulations of the excitation spectra of resonant fluorescence bound by the dynamic interaction of two different two-level nanoparticles have been performed. We considered the excitation of one of the particles by monochromatic light with a frequency close to the transition frequency of this particle, at an irradiation intensity leading to a small population of its excited state. The formalism for the description of resonance fluorescence by the Heitler--Ma method was applied to a complete compound system of the marked particle pair and quantized emission field. The resulting expressions for the excitation spectrum reflect consideration of the absorption of the fluorescence photon without attributing it to the emission of any one particle. Expressions for the excitation spectra when taking into account such absorption of the fluorescence photon that allows us to attribute it to the radiation of one or another of the particles were obtained by solving a modified system of equations of the Heitler--Ma method, which reflects the addition of non-interfering amplitudes of the emission field states of each particle. It was found that the shape of the excitation spectra of the stationary resonance fluorescence of the particles in question is determined by the shape of the corresponding intensity spectra of the spontaneous fluorescence of these particles. Keywords: mathematical modeling, interacting two-level nanoparticles, resonant fluorescence, excitation spectrum.
We report the results of numerical modelling of emission of a two-level atom near a metal nanoparticle under resonant interaction of light with plasmon modes of the particle. Calculations have been performed for different polarisations of light by a dipole approximation method and a complex multipole method. Depending on the distance between a particle and an atom, the contribution of the nonradiative process of electron tunnelling from a two-level atom into a particle, which is calculated using the quasi-classical approximation, has been taken into account and assessed. We have studied spherical gold and silver particles of different diameters . The rates of electron tunnelling and of spontaneous decay of the excited atomic state are found. The results can be used to develop nanoscale plasmonic emitters, lasers and photodetectors.
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Optical properties of ensembles of realistic quantum emitters coupled to plasmonic systems are studied by using adequate models that can take into account full atomic geometry. In particular, the coherent effects such as forming ``dark states,'' optical pumping, coherent Raman scattering, and the stimulated Raman adiabatic passage (STIRAP) are revisited in the presence of metallic nanoparticles. It is shown that the dark states are still formed but they have more complicated structure, and the optical pumping and the STIRAP cannot be employed in the vicinity of plasmonic nanostructures. Also, there is a huge difference in the behavior of the local atomic polarization and the atomic polarization averaged over an ensemble of atoms homogeneously spread near nanoparticles. The average polarization is strictly related to the polarization induced by the external field, while the local polarization can be very different from the one induced by the external field. This is important for the excitation of single molecules, e.g., different components of scattering from single molecules can be used for their efficient detection.
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We give a density-matrix account of an atom in a dilute vapor which is subject to a magnetic field and through which is propagating plane-polarized monochromatic light with frequency in the vicinity of a transition between levels of total angular momenta J=0J=0J=0 and 1, where the degeneracy may be in either the upper or lower level. The master equation approach allows saturation effects to be treated in a nonperturbative manner, and collisions are included within the context of the impact approximation. The particular case of the Voigt effect in a $(\stackrel{\ensuremath{\rightarrow}}{1}0)$ transition (lower level J=1J=1J=1) is considered in detail, and contrasted with that in $(\stackrel{\ensuremath{\rightarrow}}{0}1),$ already published; in the absence of collisions, the $(\stackrel{\ensuremath{\rightarrow}}{1}0)$ rotation spectrum is of Lorentzian form at all intensities, while if collisional relaxation is much faster than radiative decay, it takes the same form as for the $(\stackrel{\ensuremath{\rightarrow}}{0}1)$ case. The rotation produced by a magnetic field of arbitrary direction and the effects of atomic motion are also discussed.