Ryuji Matsumoto

Highly sensitive ultrafast all-optical light modulation by complex refractive-index changes in guided mode geometry composed of a photoresponsive polymer and a low-refractive-index polymer

Highly sensitive ultrafast all-optical light modulation is proposed on the basis of photoinduced complex refractive-index changes in composite guided wave mode (GWM) geometry composed of a low-refractive-index polymer and a photoresponsive polymer. Both calculated and observed results indicated that reflectance, which was unity at an extinction coefficient k=0, decreased rapidly with increasing k until a certain value (kc), and increased again after that. The kc-value and the GWM dip width decreased with increasing the thickness of both polymer layers. The real-part changes shifted the GWM dip very sensitively. The low-refractive polymer contributed great improvements, such as high sensitivity and durability against a repeated femtosecond laser. The present device can give highly enhanced changes of probe light as compared with simple transmission-type devices or the GWM geometry on a metal thin film. A photoelectrochromic polymer was employed in this geometry to demonstrate ultrafast reflectance control ...

Ultrafast Responses of Doubly N-Confused Hexaphyrin Derivatives

ABSTRACT Confused hexaphyrin derivatives, which are remarkable as the organic optical materials for information processing technology, have been synthesized. Hexaphyrin derivatives, free base type, substituted one and metal complex were studied in view of photophysical properties. The effects of π-conjugation have been studied by measuring of absorption and fluorescence spectra. They were excited by femtosecond pulsed laser to see the photoresponsibility and dynamics. Instantaneous changes, transient bleaching or transient absorption, will be discussed in the visible region.

Ultrafast all-optical light modulation in the near infrared region by phase sensitive polymer guided wave mode geometry containing porphyrin tapes

A phase sensitive detection was introduced into reflection type all-optical light modulation based on guided wave mode (GWM) geometry. A polymer film containing a copper porphyrin tape and a low refractive polymer film formed GWM at specific incident angle and wavelength. Sensitive changes of output intensity were observed upon femtosecond laser excitation depending on azimuthal angles of the analyzer, which was caused by phase changes at the GWM by Δk alone. The present system has a great advantage as compared with the conventional GWM or simple transmission methods for all-optical light modulation especially for materials showing the ground state absorption.

Ultrafast photoresponsive materials and molecular photonics applications by guided wave mode geometry

Abstract 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 developed polymeric materials showing ultrafast absorption changes in the visible to near-infrared regions including the telecommunication wavelength by photoinduced electron transfer and reverse reactions between substituted pyridinium and its counter anion. By selecting the counter anion we succeeded in making steady and ultrafast absorption changes. Time-resolved absorption and fluorescence spectroscopy upon femtosecond laser excitation revealed reaction mechanism and ultrafast dynamics. In order to fully utilize such photoresponses in molecular photonics, we proposed guided wave mode (GWM) device composed of a prism, a low refractive index polymer film, and a photoresponsive polymer film. The reflectance in this GWM is controlled by many factors such as thickness values and complex refractive indices of two polymer layers as well as an incident angle and wavelength. We successfully achieved sensitive, all optical, and very fast control of reflectance by means of photoinduced complex refractive index changes upon femtosecond laser excitation.

Highly sensitive and ultrafast light modulation with a vanadyloxophthalocyanine-doped composite polymer guided wave mode device

We propose a guided wave mode (GWM) device with prism coupling, which is composed of a low-refractive-index polymer layer and a vanadyloxophthalocyanine-doped polymer layer. Reflectance dips due to the GWM were observed at specific incident angles depending on the wavelength and the thickness of each layer. The reflectance at the GWM can be controlled by the real part (n) and/or the imaginary part (k) of the complex refractive index of the waveguide layer. The k dependence of reflectance showing a minimum of almost zero at a certain value defined as kc was confirmed in the visible to near infrared region. The kc value varies linearly with the intensity of the evanescent wave. Highly sensitive and ultrafast control of reflection in the GWM geometry was demonstrated by the photoinduced complex refractive-index change upon femtosecond laser excitation of vanadyloxophthalocyanine.

Effects of gold nanoparticles on photocurrents of zinc-porphyrin-viologen linked compound-gold nanoparticle composite films

The fabrication of a zinc-porphyrin(ZnP)-viologen(V) linked compound with six methylene group (ZnP(6)V)–gold nanoparticle (AuP) composite films on indium-tin-oxide (ITO) electrodes were performed by the electrostatic layer-by-layer adsorption technique. The photocurrents in ZnP(6)V–AuP composite films were larger than those in ZnP(6)V films, and were much larger than those in ZnP films as a reference. The large enhancement of the photocurrents in ZnP(6)V–AuP composite films are most likely attributable to the combination of localized surface plasmon resonance due to AuP and photoinduced intramolecular electron-transfer due to the V moiety in the linked compound.

Characterization of Multistep Electron Charging and Discharging of a Silicon Quantum Dots Floating Gate by Applying Pulsed Gate Biases

Electron charging and discharging operations for a silicon quantum dots (Si-QDs) floating gate with discrete charged states were characterized in n-metal–oxide–semiconductor field-effect-transistors (MOSFETs) by applying single-pulsed gate biases Vg with pulse widths ranging from 10 ns to 100 ms. The drain current levels for charged states increase stepwise with Vg pulse width, which indicates multistep electron charging in the Si-QDs floating gate due to quantization energy and electron charging. The gate voltages required to minimize the time for charging and discharging the Si-QDs floating gate are limited by the leakage current through the control oxide, which partially compensates the charging or discharging current through the tunnel oxide in the high-gate-voltage region. The transient variation of the drain current at zero gate bias, after the application of pulsed gate biases for the dot floating gate to be charged, shows that a critical transition from an unstable charged state to a stable one occurs, depending on the pulse height or width. This suggests the redistribution of electrons in the floating gate during the metastable state, which involves a reduction in the effective charging energy and the resultant elimination of the Coulomb blockade.

Materials and devices for ultrafast all-optical processing

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.

Photocurrent Enhancement in Porphyrin–Silver Nanoparticle Composite Films Using Nanostructures of Silver Nanoparticles

The fabrication of tetraphenyl porphyrin (TPP)–silver nanoparticle (AgP) composite films on indium–tin oxide (ITO) electrodes was carried out by the electrostatic layer-by-layer adsorption technique. Maximum enhancement in photocurrent action spectra as well as in fluorescence emission spectra was observed at an immersion time of 2 h. The effects of AgP on photocurrent and fluorescence suggested the effects of enhanced electric fields resulting from a localized surface plasmon resonance on the enhancement of photocurrent and fluorescence signals. The effect of AgP on the lifetime of the singlet excited state of TPP (1TPP*) indicated that the lifetime of 1TPP* decreases as compared with that in the absence of AgP substrate. The results on fluorescence lifetime suggested that the difference between the effects of AgP on photocurrent and fluorescence is most likely ascribed to the notion that the energy transfer from 1TPP* to surface plasmons due to AgP aggregates competes with photoinduced electron transfer from 1TPP* to O2 during photocurrent measurement.

Ultrafast all-optical light modulation with high wavelength resolution in a near-infrared region by composite polymer guided wave mode

A tricyclodecane polymer film with vanadyloxophthalocyanine was prepared on a low refractive index polymer film by spin coating. It showed a guided wave mode (GWM) dependent on the refractive index and thickness of each layer as minimum reflectance in near infrared spectrum. The width of GWMs was found to considerably decrease with increasing thickness of the low refractive index polymer. The reflectance at the GWM position was optically modulated by ultrafast complex refractive index changes upon femtosecond laser excitation. The present results will be used for ultrafast all-optical light modulation or switching with high wavelength resolution in photonic network systems.