All-optical parallel processing by photoinduced complex refractive index changes in guided wave geometry containing organic dye

In order to construct molecular photonics system we have been studying ultrafast (picosecond, femtosecond), colour changes and highly sensitive detection of steady and transient absorption changes by the optical waveguide method. We recently proposed and demonstrated a novel all-optical light modulation and optical switching by the use of reversible changes of a complex refractive index upon photoexcitation in a guided mode geometry composed of silver and polymer thin films containing organic dyes. In addition to the surface plasmon resonance, we can observe very sharp dips depending on the thickness of a polymer film in an incident angle dependence of reflectance. An incident angle of a probe beam was set at a value corresponding to the minimum reflection due to the guided wave mode. Pulsed laser excitation causes changes of absorption spectrum of the polymer films due to the excited state formation, photochromism, and other photoreaction. The increase of the imaginary part results in the increase of reflection. According to the Kramers-Kronig relationship the real part also changes, which results in the shift of the minimum. So we can select appropriate wavelength for all-optical light modulation or self-holding optical switch in this geometry. The response time and spatial resolution of this system are expected to be much superior than practically used SLMs based on liquid crystals, since it depends on photoexcitation or photoreaction of molecules.