Rare-gas precipitates in metals as quantum dots for polaritons

A study of the optical spectra of metals containing inclusions of rare-gas precipitates of spherical shape is presented. A unique feature of this system is that rare-gas atoms inside the precipitates form a crystal lattice even at temperatures much higher than room temperature. Energy spectra of the size quantization of excitons in Xe, Kr, Ar, and Ne precipitates in Al and their manifestation in reflection spectra are calculated, taking into account polariton effects (dipole-dipole exciton interactions), spatial dispersion, and mixing of the electronic excitations of the inclusion with collective excitations of the surrounding metal (plasmons). It was shown that (i) the proximity of energies of the exciton levels and the plasmons localized on the inclusion (surface plasmons) results in a gigantic shift (up to 1 eV) of the levels of coupled excitations, (ii) the transfer of the oscillator strength from the plasmon level to the exciton levels leads to the amplification of optical transitions in bubbles by several orders of magnitude. If the energy of the surface plasmon is smaller than the energy of the surface polariton (Ar, Ne), the latter is situated inside the discrete exciton spectrum and is coupled to the discrete exciton levels. In this case the surface polariton line in the reflection spectra is broadened by a set of dips representing quantized exciton levels. In the case of Xe and Kr in Al, the energy of the surface plasmon level is larger than the energy of the surface polariton. As a result, the surface polariton is pushed out of the exciton band and manifests itself in the reflection spectrum by a very narrow dip.