Nonlocal current-response theory of structured-light dichroism

Abstract

We develop a microscopic theory of optical absorption and structured-light dichroism in a nonlocal minimal-coupling framework. Starting from the minimal-coupling Hamiltonian, we express absorption as a bilinear functional of the electromagnetic vector potential and the nonlocal current response, providing a general description of light--matter coupling in optical vortex beams and other inhomogeneous fields. Dichroic signals are identified as helicity-odd projections of the nonlocal response kernel, and the response is resolved into symmetry, tensorial, and mode-space sectors. For single helical modes, the theory yields diagonal OAM-resolved contributions together with the corresponding selection rules and symmetry constraints. For mixed modes, interference between distinct OAM components provides access to off-diagonal coherence of the nonlocal kernel, with a tensor structure determined by the polarization composition of the field. The theory also clarifies how local structures associated with symmetric spatial dispersion, optical chirality, and tensorial anisotropy emerge as gradient-level manifestations of the underlying nonlocal response.