Toshiki Yamada

Recoverable degradation and internal field forming process accompanied by the orientation of dipoles in organic light emitting diodes

Recoverable degradation of a polyurethane (PU) derivative with large dipoles on the side chain was investigated. Spontaneous recovery and reverse bias-assisted recovery were observed. The observed spontaneous recovery and reverse bias-assisted recovery are explained by the formation of an internal electronic field due to the re-orientation of dipoles. The re-orientation of dipoles was evaluated by second harmonic generation measurement. The relatively large estimated value of x zyy (2) , approximately 7 X 10 -7 esu (2.93 X 10 -10 m/V) at 10 V, indicates a high degree of orientation of dipoles, which generates a relatively large internal electronic field.

ELECTRIC QUADRUPOLE SECOND-HARMONIC GENERATION SPECTRA IN EPITAXIAL VANADYL AND TITANYL PHTHALOCYANINE FILMS GROWN BY MOLECULAR-BEAM EPITAXY

For epitaxial vanadyl and titanyl phthalocyanine films with Laue symmetry 4/mmm, second-harmonic generation (SHG) spectra due to only electric quadrupole and electric quadrupole coupling mechanisms were obtained under a suitable geometry, while the contributions of electric dipole, magnetic dipole, and magnetic dipole coupling mechanisms were eliminated. In addition to a resonant peak previously reported, two peaks were observed at higher energies. An oriented gas model analysis showed that in-plane components of molecular hyperquadrupole polarizabilities cause the resonances. On the basis of the molecular electronic structure calculated by the density functional theory, the resonant processes are attributed to electric quadrupole transitions at SH frequencies.

Helicoidal distributed-feedback cavity action in a ferroelectric liquid crystal

Second-Harmonic (SH) generation was observed in homeotropically aligned ferroelectric liquid-crystal cells using fundamental light propagating along the helicoidal axis. Relatively strong SH light was observed in the helicoidal structure when the optical pitch was nearly the same as the SH-light wavelength, though, otherwise, the cancellation of the generated SH light by the helicoid results in negligibly weak SH intensity. Because the observed SH light is independent of the cell thickness, the SH light is attributed to the one generated from the surface region and the SH light generated inside the cell is confined and lost by the helicoid. This phenomenon indicates the helicoidal Distributed-FeedBack (DFB) cavity action and suggests the possibility of the helicoidal DFB laser using dye-doped ferroelectric liquid-crystal cells.

Optical Second Harmonic Generation Spectra in Vacuum-deposited Thin Films of Various Phthalocyanines

We have observed fairly strong second-harmonic generation (SHG) in vacuum-deposited thin films of phthalocyanines (Pc) having central metal atoms; Co, Cu, Zn, VO, TiO, Pb, Sn and a metal-free Pc. Based on the remarkable spectral dependences, the SHG was concluded to be due to higher-order mechanisms: in all the films, a resonance peak due to an electric quadrupole transition was observed at the SH wavelength corresponding to the higher-energy side of the Q band. The spectral structures in the molecules with a similar shape are similar to each other, indicating that the electronic states responsible for the spectra are ascribed to Pc macrocycles. The χ(2) values corresponding to in C∞v symmetry and the conventional electric dipole mechanism are in the range of 1.4 ∼ 10 × 10−9 esu for these Pc films.

Observation of the lowest Ag excited state lying in the optical gap in polydiacetylene by means of second-harmonic generation spectroscopy

Abstract We have observed second-harmonic generation (SHG) spectra in polydiacetylene (poly [5,7-dodecadiyne-1,12-diyl-bis-ethylurethane], PDA-4U2 for short) films of red and blue forms. In addition to resonant peaks located at the high-energy side of the main absorption peak, a new resonant peak was observed at the lower-energy side of the absorption peak in blue-form PDA-4U2. In contrast, in red-form PDA-4U2, the SHG resonance peak position in the lower-energy side coincides with the absorption peak position. The resonance enhancement of SHG originates from the electric-quadrupole transition to the dipole-forbidden state. The lowest resonance peak observed in the blue-form PDA-4U2 strongly suggests the existence of the lowest A g excited state lying below the lowest B u excited state.

Surface structure and orientation of polyamic acid alkylamine salt Langmuir-Blodgett films having an azobenzene pendant unit

The structure of polymeric Langmuir-Blodgett (LB) films was studied by scanning force microscopy (SFM), Fourier transform infrared spectroscopy (FTIR) spectroscopy, and second harmonic generation (SHG) measurements. Polyamic acid alkylamine salt LB films, having an azobenzene pendant unit, were prepared by mixing the polyamic acids with single-chain alkylamines or multichain alkylamines. When the single-chain alkylamine was used, the SFM image of the LB film showed an island-like structure. The formation of the islands was attributed to a self-aggregation of the alkylamines in the LB film. FTIR and SHG measurements suggested that the single-chain alkylamines in the LB film had a poor orientation, and the azobenzene units had a tilt angle of 45.3° from the surface normal. On the other hand, the LB films having multichain alkylamines exhibited a well-ordered orientation. The surface structure was observed uniform and there existed few defects in the SFM images. The multichain alkylamines were oriented approximately perpendicular to the surface plane, and the azobenzene unit had a tilt angle of 39.8°.

In-Situ Observation of SHG and its Origin in Vacuum Deposited Copper Phthalocyanine Film

Abstract In situ observation system of second harmonic generation (SHG) during vacuum deposition has been constructed. It allows us to observe SHG from transmitted and reflected directions. The advantages of the system is summarized in the following two points: (1) Detailed dependence of SHG on film-thickness can be easily obtained. (2) The measurement can be achieved without exposing films to the atmosphere. This system was applied to vacuum evaporated films of copper phthalocyanine (CuPc) to clarify the origin of SH activity in such a centrosymmetric molecule. We suggest the importance of quadrupolar contribution on the basis of the thickness dependence of SHG and the comparison with the theoretical analysis.

Evaluation of thermal stability of electro-optic polymer by thermally stimulated depolarization current measurement

The thermal relaxation process of electro-optic (EO) polymers, Disperse Red 1 (DR1)/poly(methyl methacrylate) (PMMA) guest/host polymer and side-chain copolymer DR1–PMMA (PMMA-co-DR1), was studied using thermally stimulated depolarization current (TSDC) measurement. For the guest/host polymer, two TSDC peaks were identified at temperatures of −50 °C (β-peak) and around 100 °C (α-peak) due to dipolar depolarization. It was also suggested that the α-peak is ascribed to disordering of guest DR1 molecules together with randomization of the main body of the PMMA. For the side-chain copolymer, a simple relaxation process of the side-chain DR1 was observed in contrast with the guest/host polymer. Analyzing the TSDC, we estimated the activation energy and relaxation times, and discussed the relaxation mechanisms. Results showed that the TSDC method is available for evaluating the thermal stability of EO polymer.

Selective dissociation of merocyanine J-aggregates with linearly polarized laser light studied by optical second harmonic generation

Abstract We have studied the dissociation of merocyanine J-aggregates in Langmuir-Blodgett (LB) films upon irradiation with polarized pulsed laser light. Optical second harmonic generation (SHG) as well as optical absorption measurements indicate the selective dissociation of the domains with the transition dipole moment parallel to the polarization direction of the bleaching beam. The LB film with no J-aggregates after irradiation with circularly polarized light maintains its second harmonic activity, though it is weaker, indicating that the in-plane non-centrosymmetric structure of each domain remains to some extent after dissociation to the monomers. Conversely, thermal treatment brings about centrosymmetric distribution of the monomers, resulting in no SHG.

One-dimensional exciton system in trans-polyacetylene

Abstract Recently, we clarified that a sharp absorption/reflection peak at 1.5 eV of directly synthesized trans-polyacetylene films comes not from square-root singularity of joint density of states between one-dimensional bands, but from one-dimensional 1B u exciion state. In this letter, we found a peak at about 0.96 eV in second-harmonic-generation spectra, and attributed this peak to resonance enhancement at the energy corresponding to the fundamental wavelength. The resonance state is not electric-dipole-forbidden but an electric-quadrupole- or magnetic-dipole-allowed state, most probably a 2A g exciton state.