Paraexciton Excitation in Cu$_2$O under Laguerre--Gaussian Illumination

Abstract

In Cu$_2$O the lowest yellow exciton, the $\Gamma_2^+$ paraexciton, is optically inaccessible in conventional spectroscopy because transitions to this state are forbidden in both electric-dipole and electric-quadrupole approximations. We investigate whether optical fields carrying orbital angular momentum (OAM) can overcome this restriction. A microscopic symmetry analysis identifies the gradient-assisted $l=5$ and direct $l=6$ OAM channels as the leading contributions that couple to the paraexciton, independent of the detailed radial profile of the optical field. Calculations for finite-waist Laguerre--Gaussian beams, however, show that the corresponding matrix elements are strongly suppressed because the optical field varies only weakly over the exciton Bohr radius. Thus, satisfying the OAM selection rule alone is insufficient: efficient excitation requires not only the correct angular symmetry but also optical-field variations on the spatial scale of the exciton. This second condition is achieved by localized OAM fields. Expressing the coupling in terms of the physical intensity-ring radius provides a direct comparison between the optical and excitonic length scales and reveals that the optimal localization is determined primarily by the polynomial degree of the target cubic harmonic. For the degree-six $\Gamma_2^+$ paraexciton the strongest coupling occurs for an intensity-ring radius of approximately $6a_B$--$7a_B$. These results establish that paraexciton excitation is governed jointly by symmetry and spatial localization: cubic symmetry selects the allowed OAM channels, whereas the polynomial degree sets the characteristic radial scale for efficient coupling. This work provides both the symmetry framework and a practical design rule for engineering structured-light excitation of paraexcitons in Cu$_2$O.