Koichi Sakaguchi

Third-order nonlinear optical properties of bismuth-borate glasses measured by conventional and thermally managed eclipse Z scan

Third-order nonlinearity one order of magnitude larger than silica is measured in bismuth-borate glasses presenting a fast response (<200 fs). The results for the sign and magnitude of the nonlinearity were obtained using a combination of the eclipse Z scan with thermal nonlinearity managed Z scan, whereas the Kerr shutter technique was employed to obtain the electronic time response of the nonlinearity, all performed with 76 MHz repetition rate 150 fs pulses at 800 nm. Conventional Z scans in the picosecond regime at 532 and 1064 nm were also independently performed, yielding the values of the third-order nonlinear susceptibilities at those wavelengths. The results obtained for the femtosecond response, enhanced third-order nonlinearity of this glass (with respect to silica), place this glass system as an important tool in the development of photonics devices. Electro-optical modulators, optical switches, and frequency converters are some of the applications using second-order nonlinear properties of the Bi-glass based on the rectification model.

Low loss photonic components in high index bismuth borate glass by femtosecond laser direct writing

Single mode, low loss waveguides were fabricated in high index bismuth borate glass by femtosecond laser direct writing. A specific set of writing parameters leading to waveguides perfectly mode matched to standard single-mode fibers at 1.55 microm with an overall insertion loss of approximately 1 dB and with propagation loss below 0.2 dB/cm was identified. Photonic components such as Y-splitters and directional couplers were also demonstrated. A close agreement between their performances and theoretical predictions based upon the characterization of the waveguide properties is shown. Finally, the nonlinear refractive index of the waveguides has been measured to be 6.6 x 10(-15) cm(2)/W by analyzing self-phase modulation of the propagating femtosecond laser pulse at the wavelength of 1.46 microm. Broadening of the transmitted light source as large as 500 nm was demonstrated through a waveguide with the length of 1.8 cm.

Enhanced stability of the second-order optical nonlinearity in poled glasses

Glasses in a wide range of chemical composition were thermally poled and the stability of the second-order optical nonlinearity induced was studied through isothermal annealing experiments. Enhanced stability was found in aluminosilicate and aluminoborosilicate with respect to silica (at least a five order of magnitude increase in lifetime). Such dramatic enhancement was explained by the strong decrease of the alkali ion mobility due to specific changes in glass composition.

Very thin photoalignment films for liquid crystalline conjugated polymers : Application to polarized light-emitting diodes

Photoaligned polyimide films with different film thicknesses were prepared on quartz substrates, and uniaxially aligned glassy poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layers were formed on top. The photoluminescence polarization ratio of the PFO layer rapidly increased with increasing polyimide film thickness, and beyond a thickness of 1.6nm, it was saturated at ∼11. This result shows that the 1.6-nm-thick photoaligned polyimide film works as a good alignment layer for PFO. We succeeded in fabricating a polarized light-emitting diode with a polarization ratio of 29 at 459nm and a brightness of 700cd∕m2 by using a 2.8-nm-thick polyimide photoalignment layer.

Highly polarized polymer-based light-emitting diodes fabricated by using very thin photoaligned polyimide layers

Polarized polymer-based light-emitting diodes (PLEDs) have been fabricated by inserting a very thin photoaligned polyimide film into the device structure. The photoaligned polyimide film was used to form a highly oriented layer of light-emitting polymer, poly (9,9-dioctylfluorenyl-2,7-diyl) (PFO). The polyimide contains azobenzene in the backbone structure, allowing us to control the alignment of its backbone structure by optical treatment. Since the photoalignment treatment is scratch-free, the thickness of the photoaligned film can be reduced below 4 nm without decreasing its alignment ability for PFO. Even though polyimide is a good insulator, such a very thin photoaligned film can be inserted into PLED structures. We examined the influence of the thickness of the photoaligned polyimide film and the light-emitting layer on the polarization ratio and the current efficiency of the polarized PLEDs. Using a 2.9 nm-thick polyimide photoalignment layer and a 101 nm-thick PFO light-emitting layer, we have suc...

Optical limiting behavior of bismuth oxide-based glass in the visible range

The authors report experimental results on the optical limiting behavior of a bismuth oxide-based glass by exciting the samples with nanosecond laser pulses at 532 and 598 nm. The results show that two-photon and free-carrier absorption processes contribute for the nonlinear absorption. Values for beta, the two-photon absorption coefficient, and sigma_e, the absorption cross section due to free carriers, were determined. The values for beta and sigma_e are dependent on the amount of bismuth oxide in the glass composition.

Bleaching of sol-gel glass film with embedded gold nanoparticles by thermal poling

Gold clusters embedded in glass are expected to be hard to dissolve in the form of ions since gold is essentially a nonreactive metal. In spite of that, bleaching of Au-doped nanocomposite sol-gel glass film on a soda-lime glass substrate is demonstrated in which electric-field thermal poling is employed to effectively dissolve randomly distributed gold nanoparticles (15nm in diameter) embedded in a low conductivity sol-gel glass film with a volume filling factor as small as 2.3%. The surface plasmon absorption band at 520nm is suppressed in the region covered by the anodic electrode. The phenomenon is explained by the ionization of the gold nanoparticles and the redistribution of gold ions in the glass matrix due to the action of the extremely high electrostatic field locally developed during poling.

High-Performance Solution-Processed n-Channel Organic Thin-Film Transistors Based on a Long Chain Alkyl-Substituted C60 Derivative

We report on high-performance solution-processed n-channel organic thin-film transistors based on a long-chain alkyl-substituted fullerene derivative, C60-fused N-methylpyrrolidine-meta-dodecyl phenyl (C60MC12), by surface modification of an insulator. C60MC12 films were fabricated on self-assembled monolayer (SAM)-treated gate insulators by spin-coating, which was facilitated by hydrophilic patterning of the edge of the substrate. X-ray diffraction revealed that the crystallinity of C60MC12 films was improved by SAM treatment. The octadecyltrichlorosilane-treated device demonstrated a high mobility of 0.4–0.5 cm2 V-1 s-1, which is comparable with those of amorphous silicon thin-film transistors. An improvement in bias stress stability by the SAM treatment was also observed.

Color Control and White Emission of Organic Light-Emitting Device by External Light

Color control and white emission of organic light-emitting device by external light irradiation was achieved by employing an opto-electronic gate out of titanyl phthalocyanine and two emission layers that emit complimentary color of each other. The recombination site in the device can be tuned by intensity of external light. Without laser irradiation, holes and electrons recombine mainly at the blue emission layer, thus only a blue emission is observed. On the other hand, electroluminescent peak around 560 nm increases by laser irradiation: the emission color is continuously and reversibly varied from blue to orange via white.

Photoconductive Characteristics of Hydro-Oxygenated Amorphous Titanium Oxide Films Prepared by Remote Plasma-Enhanced Chemical Vapor Deposition

The photoconductive characteristics of amorphous titanium oxide films (a-TiOx:OH) prepared by remote plasma-enhanced (RPE) chemical vapor deposition, in inert Ar atmosphere with/without a small amount of oxygen, have been studied. In inert Ar atmosphere without oxygen, the residual decay of photocurrent (long decay time) is observed after removing UV irradiation. On the other hand, in inert Ar atmosphere with a small amount of oxygen, the photocurrent under UV irradiation is influenced strongly by an environmental gas with oxygen gas. The maximum of photocurrent (photosensitivity) is nearly inversely proportional to oxygen partial pressure, suggesting that the present films are useful for sensors of environmental oxygen gases. The decay of photocurrent, after removing UV irradiation, follows a stretched exponential function. The effective decay time τ is nearly inversely proportional to oxygen partial pressure. The results will be discussed in terms of the surface reaction of oxygen gas on the present films.