On the use of twisted photons for spectroscopy of impurity centers in crystals

Absorption of twisted photons for quadrupole ${}^{4}f\text{−}{}^{4}f$ transitions in $\mathrm{R}{\mathrm{e}}^{3+}$ impurity ions in micro- and macrocrystals is considered. Two types of twisted photon beams—the Laguerre-Gaussian and the Bessel-Gaussian beams—are considered and compared. Twisted photons have a nonzero transverse field gradient, which leads to selection rules for quadrupole transitions other than those in the case of the usual quadrupole transitions caused by a longitudinal field gradient. This allows one to observe new spectral lines which are absent in the usual absorption spectra. For microcrystals placed at the beam axis of twisted photons the otherwise dominating forced dipole ${}^{4}f\text{−}{}^{4}f$ transitions are absent, which simplifies the observation of new spectral lines.