Satoshi Enomoto

Comparative study on light-induced absorption between MgO:LiNbO3 and MgO:LiTaO3

A comparative study on the light-induced absorption was performed for lithium niobate and lithium tantalate crystals in which the antisite defects were eliminated by MgO doping. The induced absorption was measured in a spectral window ranging from visible to near infrared with a time resolution of 10 ns. Upon pulsed-light irradiation, a broad absorption peaking near 1250 nm was induced in MgO-doped lithium niobate. It was attributed to the small free polaron, an electron trapped at the regular Nb ion site. In contrast, such absorption in the near infrared originating from the free polaron was absent in MgO-doped lithium tantalates. The findings are highly relevant to the frequency conversion performances of high-power and pulsed lasers based on the current periodically-poled devices. The results also suggest guidelines for improving the crystal qualities.

Analysis of polymerization with photo-Fries rearrangement in liquid crystal displays

Polymer-sustained-alignment liquid crystal display has high potential for low power consumption, fast response, and low level of image sticking. In this study, we clarified the mechanism for a formation of a polymer layer with polymerization of the monomer 2,6-dimethacryloyl-oxy-naphthalene (2,6-DMANaph) without initiators under UV light exposure. Experimental results revealed that the polymerization of 2,6-DMANaph occurred with the generation of radicals from the monomer as reactive intermediates of a photo-Fries rearrangement. It took about 20 min for 0.5 wt. % of the monomer in the LC to convert to the polymer, indicating that the generation of the radicals derived from the reactive intermediates of the photo-Fries rearrangement is effective for the formation of the polymer layer. Voltage holding ratio of the LC cell having the polymer layer formed from the monomer 2,6-DMANaph was over 99%, which was comparably high value.

Generation mechanism of residual direct current voltage for liquid crystal cells with polymer layers produced from monomers

ABSTRACT Liquid crystal cells having polymer layers produced from copolymers of 4,4′-dimethacryloyl-oxy-biphenyl (4,4′-DMABiph) and anthracene-carrying monomers, 2,7-dimethacryloyl-oxy-anthracene (2,7-DMAAnth) and 2-methacryloyl-oxy-anthracene (2-MAAnth), were fabricated, and generation mechanism of residual direct current voltage (VrDC) was clarified. The VrDC was improved with the LC cells having the polymer layers produced from the copolymers of 4,4′-DMABiph and 2,7-DMAAnth due to large rate constant of polymerisation. In contrast, the VrDC was increased with the LC cells having the polymer layers produced from the copolymer of 4,4′-DMABiph and 2-MAAnth. The increase in the VrDC was derived from the increase in the concentration of polymerised- or unpolymerised-terminally radical and ion generated from 2-MAAnth. The results suggest that the monomers should show large rate constant of polymerisation with plural number of polymerised groups for improving the VrDC. GRAPHICAL ABSTRACT

Evaluation of image sticking property on liquid crystal displays with polymer layers produced from phenanthrene-carrying monomers

ABSTRACT As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the liquid crystal (LC) cells having the polymer layers produced from monomers of phenanthrene (Phen)-carrying monomers, 2,7-, 3,8-, 1,8- and 3,6-dimethacryloyl-oxy-phenanthrene (DMAPhen), were prepared and investigated image sticking phenomenon with evaluations of residual direct current voltage (VrDC) and difference in pretilt angle before and after application of alternate current voltage (Δtilt). The VrDC values were small for the LC cells with the polymer layers produced from the Phen-carrying monomers used in this study. On the other hand, Δtilt depends on the molecular structure of the Phen-carrying monomer. The Δtilt values became small with increasing the linearity of the monomer structure for the Phen-carrying monomers. The findings would be useful for obtaining the small level of image sticking in the PSA-LCDs produced from the Phen-carrying monomers. GRAPHICAL ABSTRACT

UV-induced homeotropic orientation of nematic liquid crystals dissolving two species of monomers carrying liquid-crystalline side-chain group and anthracene group

ABSTRACT We propose the technique for formation of the polymer orientation film (POF) by polymerisation of the monomers being dissolved in the liquid crystal (LC) material for controlling the orientation of the LC molecules. For obtaining the homeotropic orientation, combination of the two monomers, 4-(4ʹ-octyloxy-biphenyl-4-yloxy)-butyl ester (AOBBE) and 2,7-dimethacryloyl-oxy-anthracene (DMAAnth), was found to be useful. The monomer DMAAnth initiates the polymerisation under ultraviolet (UV) light exposure, and the AOBBE unit induces the homeotropic orientation. The monomer DMAAnth is useful for maintaining the high voltage holding ratio and low residual direct-current voltage after UV light exposure because the molecules of DMAAnth do not remain in the LC layer. Graphical Abstract

UV-induced vertical alignment liquid crystal cell with two species of monomers having vertical-alignment-induced side-chain group and phenanthrene group

ABSTRACT We propose the method for formation of the vertical alignment polymer film by polymerisation of the monomers being dissolved in the liquid crystal (LC) material. For obtaining the vertical alignment, combination of the two monomers, 4-(4ʹ-octyloxy-biphenyl-4-yloxy)-butyl ester (AOBBE) and 2,7-dimethacryloyl-oxy-phenanthrene (DMAPhen), was found to be useful. The monomer DMAPhen initiates the polymerisation under UV light exposure, and the AOBBE unit induces the vertical alignment without generating any alignment defects. The monomer DMAPhen is useful for maintaining the high voltage holding ratio and low residual direct current voltage after UV light exposure because the molecules of DMAPhen do not remain in the LC layer. Graphical Abstract

Fabrication of vertically aligned liquid crystal cell without using a conventional alignment layer

ABSTRACT We propose a novel method to fabricate a vertical alignment (VA) of liquid crystal (LC) molecules without using a conventional alignment layer such as polyimide film. The method produces the vertical alignment polymer layer (VAPL) by polymerisation of a monomer or mixed monomers including in the LC layer above TNI of the LC material. The VA mode LC cell with the VAPL (VAPL-LC cell) produced from the mixed monomers of acrylic acid 4-(4ʹ-octyloxy-biphenyl-4-yloxy)-butyl ester and 1,2-bis-(4-methacryloxy-phenyl)-2,2-dimethoxy-ethane-1-one exhibited enough level of alignment state and electro-optical property with high voltage holding ratio. We can expect that the VAPL-LC cell is useful for next-generation displays such as flexible liquid crystal displays because the method does not need the process including high temperature over 200°C. GRAPHICAL ABSTRACT

Effect of water adsorption onto the polymer layer produced from the monomer carrying amide group in liquid crystal cells

ABSTRACT The voltage holding ratio (VHR) of an active-matrix liquid crystal display (AMLCD) is a significant parameter because the image quality of the AMLCD decreases in case of low VHR. In some cases, the VHR of the LC cell, which is a prototype of the AMLCD, is decreased under an environment of high humidity and temperature. In order to restrict the significant decrease in the VHR, we proposed an LC cell with the polymer layer produced from the monomer 1-acryloyl-oxy-5-acryloyl-amino-naphthalene (1-AO-5-AANp). The LC cell exhibited comparably high VHR after being placed in an environment of high humidity and temperature. We presume that water molecules would be effectively adsorbed onto the polymer layer carrying the amide group, leading to a restriction of the significant decrease in VHR. GRAPHICAL ABSTRACT

Pulsed-light-induced absorption in LiNbO 3 and LiTaO 3 crystals with MgO doping

LiNbO3 (LN) and LiTaO3 (LT) are key materials for optical frequency conversions. The major issues of these materials were light-induced absorption (LIA) and photorefractive (PR) damage. It was revealed that the small bound polaron (an electron is trapped at the antisite defect) causes LIA and PR damage in congruent LN crystals [1,2]. The MgO doping eliminated the antisite defects and suppressed LIA and PR damage [3,4]. Nowadays, the application of these materials has been extended to frequency conversion of pulsed lasers. Here peculiar LIA issues come up. Even if the photon energy is less than the energy bandgap, the pulsed light causes two-photon absorption (TPA) and parasitic harmonic generations, that lead to carrier excitations. The carriers are trapped at polaronic states, inducing optical absorption. Recently, it has been revealed that a large absorption in the near infrared is optically induced in MgO doped LNs [5–7].