A generalized theoretical description of a light-matter interaction beyond a dipole approximation is developed on the basis of the multipolar Hamiltonian with the aim of understanding the near-field excitation of molecules at the 1 nm scale. The theory is formulated for a system consisting of a molecule and a near field, where a nonuniform electric field plays a crucial role. The nonuniform light-matter interaction is expressed in terms of a spatial integral of the inner product of the total polarization of a molecule and an electric field so that the polarization is treated rigorously without invoking the conventional dipole approximation. A nonuniform electronic excitation of a molecule is demonstrated by solving a time-dependent Kohn-Sham equation in real space and real time with an implementation of the nonuniform light-matter interaction. The computations are performed to a linear chain molecule of dicyanodiacetylene $\left({\text{NC}}_6\text{N}\right)$. The nonuniform electronic excitation clearly shows inhomogeneous electron dynamics in sharp contrast to the dynamics induced by a uniform electronic excitation under the dipole approximation. Despite the inversion symmetry of ${\text{NC}}_6\text{N}$, the nonuniform excitation generates even harmonics in addition to the odd ones. Higher-order nonlinear optical response and quadrupole excitation are also observed.

• Received 15 May 2009

DOI:https://doi.org/10.1103/PhysRevA.80.043409