Molecular photonics, information processing based on strong interactions of photons and molecules, has attracted much attention, since it can fully utilize many superior properties of both photons and molecules. We have been developing materials showing ultrafast absorption changes in the visible to near infrared regions by photoinduced electron transfer and reverse reactions or by excited electronic state formation. Guided wave mode (GWM) geometry composed of a prism, a metal thin film, and a photoresponsive polymer film successfully achieved sensitive, all optical, and very fast control of reflectance by transient or persistent changes of GWM conditions by means of photoinduced complex refractive index changes upon nanosecond to femtosecond laser excitation. All-optical parallel control of reflected probe light by pump (writing) light was demonstrated by photochromic spiropyran or fulgide, and various metallophthalocyanines dispersed in appropriate polymers.
Key challenges in achieving higher-quality liquid crystal displays for future generations are developing new liquid crystal materials with faster electro-optical response and simplifying the fabrication process of devices. Blue phases1-4, kinds of liquid crystal phases, have two major advantages over commonly used nematic phases. First, the response is much faster5-8. Second, the zero-electric field state is optically isotropic unlike nematic phases, that is, no surface treatment (no rubbing) is necessary, leading to a simplification of the fabrication process. One of the problems of blue phases that the available temperature range is very narrow has been overcome by our recent achievement, the polymer-stabilized blue phases9,10. Here we show that the polymer-stabilized blue phases surmount another problem that the blue phase is very fragile to an electric field. We also demonstrate the sufficiently large electric field-induced birefringence and the micro-second response of the polymer-stabilized blue phases without any surface treatment.
Optical enhancement or decrease of the nonlinear optical (NLO) properties of organic compounds is expected at the excited state due to changes in dipole moments and oscillator strength caused by electron density distribution different from that at the ground state. It is necessary to evaluate NLO properties at the excited state in development and practical application of highly functional organic molecules. A novel excited state hyper-Rayleigh scattering (HRS) measurement system was constructed to evaluate hyperpolarizability (beta) at the excited state of molecules in solutions. The system is composed of a ns Nd:YAG probe laser of various intensities and a ns tunable pump laser for excitation. The HRS intensity at 2(omega) for p-nitoaniline in methanol solutions, proportional to the square of the incident laser power, increased upon excitation at 464nm and also depended on the delay time between a pump and a probe pulse. The (beta) value for p-nitroaniline at the excited triplet state was evaluated.
A novel all-optical parallel switching device was proposed based on photoinduced changes of a complex refractive index in a guided mode thin film composed of silver and polymer containing organic dyes. An incident angle of a probe beam was set at a value corresponding to the minimum reflection due to a guided wave mode. Pulsed laser excitation caused changes of absorption of the polymer films due to excited state formation or photochromism, which resulted in the increase of reflection because the guided mode condition was changed. According to the Kramers-Kronig relationship the real part also changes, which results in the shift of the minimum. We can select the wavelength for all-optical switching in this geometry. We used phthalocyanines and photochromic spiropyran dispersed in poly(vinyl alcohol) or polystyrene. For the former system, transient absorption due to the excited triplet gave a highly reversible very fast modulation of green light upon repeated excitation by ns laser. For the latter, self-held switching was achieved upon UV or visible laser excitation with response times less than 20 ns. The details of these systems will be presented.
Ultrafast dynamics of transient photobleaching and recovery of cadmium sulphide (CdS) nanoparticles of different size ranging from less than 3.5 nm to greater than 5.0 nm, dispersed in Nafion films, in self-assembled films, and in aqueous solutions were studied upon excitation at 400 nm with a femtosecond laser. All samples gave transient bleaching in less than 1 ps and a multi-exponential recovery in the wavelength region above about 410 nm. The peak wavelength of the transient bleaching and its time dependence together with the recovery behavior were affected by the particle size, surface treatment and the supporting medium. The peak of the transient bleaching spectrum of larger CdS nanoparticles showed a red-shift during the recovery process, while for the smaller particles no such change was observed. CdS nanoparticles in self-assembled films showed an extremely long-lived bleaching recovery process. Fluorescence-activated CdS nanoparticles in aqueous solutions, which showed very strong and sharp fluorescence near the exciton absorption, showed very fast recovery. These results were explained by the decay of excitons and the recombination of surface trapped electron-hole pairs with different energies depending on the particle size.
Transient species upon pulsed laser excitation of ultrathin polymer and Langmuir-Blodgett (LB) films was detected sensitively by the optical waveguide (OWG) method. Tetratolylporphyrin in poly(methylmethacrylate) films and amphiphilic tetraphenylporphyrin in mixed LB films with dioctadecyldimethylammonium were prepared on the surface of OWGs. These films were excited with the second harmonics of a ns Nd:YAG laser in controlled environment. The transient absorption of excited triplet porphyrins was detected sensitively by Ar+ laser, He-Ne laser, or diode lasers as a probe. Transient absorption in LB films with only two monolayers was observed by the present method. It was about hundred times increase of sensitivity as compared with a conventional normal incidence measurement. Different decay dynamics were observed for excited triplet state porphyrins in polymer films and in LB films, which was attributed to the different spatial distribution of chromophores.
Ultrafast photon-mode recording based on photoinduced electron transfer reaction was proposed using ion-pair charge-transfer (IPCT) complexes of 4,4-bipyridinium salts. Results with two kinds of counter anions, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and iodide, were reported. These anions made electronic interactions with 4,4-bipyridinium ions in solutions and in solid films to give characteristic absorption in the visible region. The dynamics of color changes from pale yellow or orange to blue upon excitation of IPCT bands of these complexes in solutions were studied by femtosecond (fs) pulsed laser. Transient absorption at about 600 nm appeared in about 0.3 ps in both samples, which was controlled by the time-resolution of our fs laser system. Such extremely fast color changes were due to the fact that the IPCT absorption band is associated with the electronic transition from a partially charge-transferred ground state to an alomost completely charge-separated excited state. The decay behavior was totally different between two salts. Tetrakis[3,5- bis(trifluoromethyl)phenyl]borate salts showed a decay curve composed of a fast component with 80 ps lifetime and of an extremely slow one corresponding to steady and reversible color changes.
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