Realization of two-photon absorption induced by electric field gradient of optical near-field

Our paper was published in Applied Physics Letters

Maiku Yamaguchi, Katsuyuki Nobusada, Tadashi Kawazoe, and Takashi Yatsui, “Two-photon absorption induced by electric field gradient of optical near-field and its application to photolithography,” Appl. Phys. Lett., Volume 106, Issue 19, May 2015, 191103

[Significance of this article]

In the present study, we have investigated the optical near-field (ONF) gradient effect on electron excitation in a quantum dot via model calculations combining a density matrix formalism and a classical Lorentz model. The electric field gradient of the ONF was found to cause two-photon absorption by an unusual mechanism. Furthermore, the absorption exhibits a nonmonotonic dependence on the spatial arrangement of the nanosystem, completely different from that of conventional two-photon absorption induced by an intense electric field. The new two-photon absorption process was verified in a previous experimental observation, by reinterpreting the results of ONF photolithography.

The present idea of the two-photon absorption due to the electric field gradient is not limited to the model system considered here. The idea provides a basis for fundamental optical responses in nanostructures and also for developing functional materials, paving the way for the ONF excitation dynamics that are completely different from those of conventional optical response processes.