Kyoichi Sasaki

Ultrafast wide range all-optical switch using complex refractive-index changes in a composite film of silver and polymer containing photochromic dye

An all-optical ultrafast switch which operates at a wide wavelength range has been demonstrated based on photoinduced changes of real and/or imaginary parts of complex refractive index in a composite thin film of silver and polymer containing photochromic spiropyran dye. Very fast switching ON and OFF of a reading beam within 20 ns in a 400–800 nm range was achieved at the incident angle of guided-wave mode using a photochromic spiropyran-doped polystyrene thin film and a ns pulsed UV or visible laser as a writing beam. These responses were caused by the changes of complex refractive index due to photochromism. No applied power was required to hold the switched state. An image was also written to the probe beam by irradiation through a pattern.

Ultrafast all-optical switch using complex refractive index changes of thin films containing photochromic dye

An all-optical switch has been proposed based on photoinduced changes of an imaginary part of complex refractive index in a composite thin film made by silver and polymer thin films containing photochromic dye. Very fast switching on and switching off of a reading beam was demonstrated at the incident angle of guided wave mode using a photochromic spiropyran-doped polystyrene thin film and a ns pulsed laser as a writing beam. These responses were caused by the changes of an extinction coefficient due to photochromism. No applied power was required to hold the switched state.

Ultrafast all-optical spatial light modulation based on complex refractive index changes of copper phthalocyanine-doped polymer film upon photoexcitation in guided mode geometry

Abstract Ultrafast all-optical reflectance control was achieved by photoinduced complex refractive index changes in thin poly(vinyl alcohol) (PVA) films containing water-soluble phthalocyanine at the incident angle corresponding to a sharp guided mode resonance upon excitation with a ns pulsed laser as a writing beam. The rise time of reflectance was less than 20 ns and the fall time was controlled by the lifetime of the excited triplet phthalocyanine, 31 ns for copper phthalocyanine (CuPcS). The spatial resolution was determined to be better than about 3 μm by using a photochromic dye. The present system based on photoexcitation of CuPcS will allow all-optical spatial light modulation upto about 30 MHz.

Reflection-Mode All-Optical Parallel Switching in Guided Wave Geometry Including Photochromic Compounds

Abstract A novel all-optical parallel switching device was demonstrated by using photoinduced changes of a complex refractive index in a guided mode thin film composed of silver and polymer containing photochromic dyes. An incident angle of a probe beam was set at a value corresponding to the minimum reflection due to the guided mode. Pulsed laser excitation caused changes of absorption of the polymer films due to 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. Appropriate wavelength can be selected for all-optical switching in this geometry. We used photochromic spiropyran dispersed in polystyrene. Self-held parallel switching was achieved upon UV or visible laser excitation with response times less than 20 ns.

Optical waveguide studies on photoinduced electrochromism in ultrathin films of ion-pair charge-transfer complexes of 4,4′-bipyridinium ions

Abstract Langmuir-Blodgett (LB) films and ultrathin polymer films containing 4,4′-bipyridinium tetrakis[3,5-bis(trifluoromethyl) phenyl]borate (TFPB − ) were deposited on the surface of an optical waveguide (OWG) glass. Colour changes due to photoinduced electron transfer reaction upon excitation of ion-pair charge-transfer complexes between 4,4′-bipyridinium and TFPB − in these ultrathin films, even in a single monolayer, were sensitively detected by the OWG technique in a degassed atmosphere. The LB and polymer films showed different time-dependent photoresponses.

Highly sensitive detection of transient absorption in dye-doped ultrathin polymer films by the TiO2/K+ composite optical waveguide method upon pulsed laser excitation

A composite optical waveguide (OWG) composed of a 10-18nm thick titanium dioxide (TiO(2)) film sputtered on a conventional K(+)-doped optical waveguide was first applied to detect transient absorption of organic dyes in ultrathin polymer films upon excitation with ns laser. The thickness of the TiO(2) film considerably affected the relative sensitivity of the composite OWG. The composite OWG with 10nm thick TiO(2) gave much stronger transient absorption for 30-415nm thick polymer films containing organic dyes than that with 18nm TiO(2). Transient absorption of phthalocyanine and spiropyran in 20-135nm thick polymer films was detected 3-20 times more sensitively by the composite OWG with 10nm TiO(2) than the conventional K(+)-doped OWG which showed a 150-fold sensitivity as compared with the usual normal incidence method. The relative sensitivity of the composite waveguide was also affected by the thickness and refractive index of polymers.

All-optical parallel switching by the use of guided-wave geometry composed of a thin silver film and a polymer thin film containing organic dye

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.

Extremely sensitive detection of transient species in laser flash photolysis of ultrathin organized molecular films by optical waveguide

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.

Evaluation of Anisotropic Photoresponses in Ultrathin Organic films by a Highly Sensitive Polarized Optical Waveguide Method

Abstract Highly sensitive polarized absorption measurement by the optical waveguide (OWG) detection system was made to analyze the orientation of photogenerated radicals in Langmuir-Blodgett (LB) films. The extinction coefficient changes upon photoexcitation of specific ion-pair charge-transfer complexes between two kinds of amphiphilic 4,4′-bipyridinium and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate were estimated by this system. Amphiphilic 4,4′-bipyridinium ions in LB films deposited directly on an OWG showed different anisotropy from those deposited on an OWG covered with three monolayers of cadmium arachidate. The structure of substituents also affected the orientation.

Highly sensitive optical waveguide detection of transient species in Langmuir-Blodgett films upon pulsed laser excitation

Abstract Transient species upon pulsed laser excitation of ultrathin Langmuir-Blodgett (LB) films was detected sensitively by the optical waveguide (OWG) method. Amphiphilic tetraphenylporphyrin in mixed LB films with dioctadecyldimethylammonium at 1:5 and 1:50 were prepared on the surface of OWGs. These films were excited with the second harmonics of a ns Nd: YAG laser in degassed environment. Transient absorption of excited triplet porphyrins in LB films with only two monolayers was detected 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 LB films depending on the molar ratio, which was attributed to inhomogeneous distribution of chromophores.