Katsuyuki Nobusada (Department of Theoretical and Computational Molecular Science, IMS) and his collaborator (Takeshi Iwasa) have developed a nonuniform light-matter interaction theory for near-field-induced electron dynamics in nanostructures and revealed unusual optical excitation processes.
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 NC6N. 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 NC6N, the nonuniform excitation generates even harmonics in addition to the odd ones. Higher-order nonlinear optical response and quadrupole excitation are also observed. We further calculated optical forces induced by a near field for a 1-nm-sized metal particle mimicked by a jellium model and for C60. A highly localized near field nonuniformly polarizes these molecules. The locally induced polarization charges in the molecules are partly canceled by the screening charges. The polarization and screening charges generally contribute to the attractive and repulsive forces, respectively, and a sensible balance between these charges results in several peaks in the optical force as a function of the frequency of the near field. The resonance excitation does not necessarily maximally induce the net force, and the force exerted on the molecules strongly depends on the details of their electronic structures. The optical force is larger in the metal particle than in C60. We also found that the optical force depends linearly on the intensity of the near field.
Paper Information
Phys. Rev. A 80, 043409 (2009)
"Nonuniform light-matter interaction theory for near-field-induced electron dynamics"
T. Iwasa and K. Nobusada
Phys. Rev. A 82, 043411 (2010)
"Near-field-induced optical force on a metal particle and C60: Real-time and real-space electron dynamics simulation"
T. Iwasa and K. Nobusada
