Simulation of Electric Quadrupole and Magnetic Dipole Transition Efficiencies in Optical Near Fields Generated by a Subwavelength Slit Array

We calculate optical near fields generated by a periodically structured 50-nm-wide slit array of a 50-nm-thick gold film with a finite difference time domain method. Efficiencies of the electric quadrupole (E2) and magnetic dipole (M1) transitions, which are associated with the space derivative of the excitation light field, are evaluated for the calculated Optical fields. The E2 transition efficiency is largely enhanced in the case where the polarization of the incident light is perpendicular to the slit direction, while such an enhancement is not expected for the M1 transition for this polarization. The large enhancement of the E2 transition efficiency originates not only from the field enhancement but also from its localization near the slit edge. The slit period dependence of the enhancement of the E2 transition efficiency suggests the enhancement to be affected by surface plasmon resonance. In the case where the polarization of the incident light is parallel to the slit, a small enhancement is obtained for the E2 and M1 transitions, the value of which corresponds to (1/k(0)d)(2), where k(0) is the wave number of the homogenously propagating light in vacuum and d is the slit width.