Abstract

If we can control optical response of a molecular dark state, there are possibilities of novel photochemical reactions and molecular spectroscopy. However, we cannot directly observe the dark state by one photon excitation when the spatial symmetry of the light wave is different from that of the molecular dark states, for example, with non-dipole type spatial pattetrn. The spatial variation of excitation light is usually negligible for a molecule smaller than a few nanometers and the long wavelength approximation (LWA) is well satisfied, where the light wavelength is much longer than the spatial extension of an electronic wavefunction. On the other hand, for semiconductor nanostructures with several tens nm size, we have previously revealed that the non-dipole type optical transition can be dominant beyond LWA under a resonant optical excitation of excitons by using our developed theory to treat spatial structures of the light field and the matter wavefunction on an equal footing [1-3]. Furthermore, due to the progress of the nanofabrication techniques, one can design the localized light field with extremely steep spatial variation comparable to the size of molecules in the vicinity of a metal nanogap (MNG) between closely spaced metal nano blocks [4]. Paying attention to such a spatial variation of localized light field, we may realize the control of a molecular dark state beyond LWA. Therefore, in this contribution, we theoretically study the linear optical response of a molecule near a MNG.

© 2009 IEEE