The aim of this research is to study photochemical reactions in single molecules induced by single photons. Even the organic photochromic molecules having the largest absorption cross-section known to date, would require 10 million molecules in order to completely absorb a single photon. This reaction has an extremely low efficiency in photon excitation. If we utilize a metallic nano-structure that confines photons into nanospace, it would be possible for a single molecule to be excited by a single photon, which corresponds to the extension of absorption cross-section of molecules.
For instance, surface plasmons localized on metallic nanoparticles and metallic nano-structures can have strength of their electric field enhanced by 105-106 times compared with that of the incident light. In photonic crystals, enhanced light-matter infractions may be achieved by the engineering of photonic band dispersion properties.
In these circumstances, it is highly possible that the selection rules in photon excitation can be changed for the molecules are exposed to extremely strong electric field gradient in case that wave function of molecule is nearly overlapping with the distribution of electromagnetic field in such nanospace.
By utilizing metal nano-structures and photonic crystals non-linear optical phenomena like two-photon absorption can be induced using with weak light, although observation of two-photon absorption conventionally repairs irradiation by intense ultra short pulse lasers.