UC Irvine

Orientational averaging of response tensors is an essential operation in calculating optical signals in randomly oriented liquid or gas phase molecular ensembles. When expanded in multipoles, rotational averaging can be performed simply through contraction with isotropic tensors. Alternatively, the multipolar expansion can be avoided and all multipoles may be naturally and implicitly incorporated via nonlocal response tensors which are fields and may not be rotationally averaged in a simple manner. The nonlocal response is useful when the light beam variation across the system is non negligible. Examples are nano-sculpted or short wavelength x-ray field. We derive exact expressions which recast optical signals as a convolution of a nonlocal response function of matter with a nonlocal intrinsic property of the electromagnetic field. The approach involves a gauge invariant calculation based on the minimal coupling Hamiltonian.