An azo-carbazole dye, 3-[(4-Nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzEtOH), and its relatives doped in polyacrylate
films have been known to show ‘photorefractive’ effect without external electric field even in symmetric optical
alignment. We have already observed strong energy transfer due to phase-shifted grating by two-beam coupling
experiments made for NACzEtOH doped PMMA films. Although the operation mechanism is still unknown, the high
efficiency of diffraction is very promising for the application to real-time holography. For the convenience of analysis,
we modified Kogelnik’s theory for thick grating by correcting the constraint condition and applied it to evaluate
refractive index modulation and other parameters quantitatively. In order to clarify the operation mechanism, we
conducted the writing and erasing of gratings by using red and green laser beams and analyzed the diffraction magnitude
and response dynamics, showing that the response time strongly depended on the writing wavelength, and that the
grating formation and its phase shift were not always synchronized.
We demonstrate very low threshold laser oscillation (Eth= 0.78 ±0.5 μJ/cm2) from an organic semiconductor thin film employing 2,5-bis(p-(N-phenyl-N-(m-tolyl)amino) styryl)benzene (BSB-Me) as an active gain medium equipped with a second order distributed feedback resonator. Further, we demonstrate laser oscillation under optical excitation from an organic light emitting device equipped with transparent ITO hole and electron injection electrodes. In addition, we demonstrate injection and transport of high current density over J > 500 A/cm2 in an organic light emitting diode using highly thermal conductive substrates and a small electrode under short pulse voltage excitation.
Topologically well defined molecules are called 'hyper-structured molecules (HSM)'. We have developed rigid dendrimers as a typical example of HSM which shows the intramolecular energy transfer from the outer shell to the core. These molecules are applicable for the light harvesting such as antenna molecules.
The polarization and depolarization behavior of electric field-induced polar alignment in a stable photorefractive polymer containing carbazole was measured by thermally stimulated current (TSC) and electro-optic (EO) modulation experiments. A relaxation peak was observed around the glass transition temperature (Tg) for the poled photorefractive polymer in a TSC experiment. The apparent TSC was not observed at room temperature. The polarization calculated from the relaxation peak linearly increased with the strength of the poling electric field. This result was consistent with the EO behavior which showed the linear dependence of the EO coefficients on the poling electric field. The polar alignment induced by the electric field at an elevated temperature was stable at room temperature, which could lead to the long-term stability of the photorefractive responses without an external applied electric field.
As a new class of multi-functional compounds, three kinds of calix[4]arene molecules were newly synthesized: calix[4]arenes containing two 3-nitro-N-allyl carbazoles, two N- allyl carbazoles and one of each at the lower rim. These compounds take cone conformation in solution because of the 1,3-di-O-substitution at the lower rim. Those compounds take amorphous state as proved by differential scanning calorimetry and powder X-ray diffraction and they can be made into films without supporting the polymer matrix. Second- harmonic generation signals were observed in the films of the two calix[4]arenes containing nitro groups after electric poling at a temperature above glass transition point. Enhancement of d33 in the calix[4]arene with nitrocarbazole derivatives compared with the calix[4]arene with both carbazole and nitrocarbazole derivatives was observed, which might be attributed partially to some extent to the conformational regulation between the two carbazole chromophores. Using two beam couple experiment, photorefractive effect was observed in the film of the calix[4]arene with both carbazole and nitrocarbazole derivatives doped with 2,4,7-trinitro-9-fluorenone. This is the first report on the photorefractive property of calix[4]arene compound.
Anisotropy of transient absorption change and electroabsorption response in metallophthalocyanine (MPc) aggregates were investigated in terms of the influence of intermolecular interaction to the 2D character of the molecule. In an MPc aggregate intermolecular interaction with less symmetry might result in appearance of 1D character, depending on the nature of interaction and arrangement of molecules. In experiment, polarization- dependent electroabsorption response in various MPcs doped in PMMA presented a 1D character at the red-shifted absorption band due to coplanar intermolecular interaction and a 2D one at the blue-shifted band due to cofacial interaction. Polarization-dependent subpicosecond transient absorption change in various aggregate vanadylphthalocyanine, evaporated and doped-polymer films in phase-I and phase-II aggregation, presented a variety of anisotropy: in phase-I aggregate doped in polystyrene, a wavelength-dependent anisotropy similar to that of electroabsorption response was obtained; in phase-I aggregates doped in PMMA or evaporated, a 2D character was observed in almost whole wavelength region; in phase-II aggregates both 1D and 2D characters were in the red-shifted absorption band. These result are partly elucidated by the exciton coupling model in 2D molecules. It is also suggested that pump wavelength dependence of anisotropy and existence of two or more aggregate species might play important rolls.
Anisotropy of third-order nonlinear optical properties in di-n-butyl fluorescein derivatives was investigated by electroabsorption spectroscopy. In a series of fluoresceins, the derivatives with Cl substitution at phenyl ring presented a new intense absorption band at the wavelength range from 550 to 620 nm, which is at longer wavelength side of the main absorption band in other derivatives. This new absorption band was attributed to intramolecular charge transfer transition due to Cl substitution at the phenyl ring, according to the spectral profile of electroabsorption which was proportional to the second derivative of linear absorption spectrum. Polarization dependence of normalized absorption coefficient was about 2:1 at the main absorption band in all derivatives. However, in the charge transfer transition band, the polarization dependence was about 1:1,2, suggesting that the charge transfer direction is nearly perpendicular to the otpical transition. This result is consistent with structural aspect that the charge transfer direction is located along to the bond between the Cl-substituted phenyl ring and condensed ring, while the optical transition is along to the longitudinal axis of the condensed ring.
We report a series of polymers with linear, macrocyclic and hyperbranched structures for electroluminescent (EL) applications. The polymers are polycarbazoles (PC) containing different substituents. The polymers are amorphous and are soluble in common organic solvents such as chloroform, and tetrahydrofuran (THF). High optical quality films were obtained by spin-coating from the polymer solutions of chloroform or THF. All these polymers show strong photoluminescence under a UV-lamp illumination. Single and bilayer EL devices consisting of anode/hole transfer layer/electron transfer layer/cathode have been fabricated and characterized. The effects of polymer structures on the energy levels and EL properties are discussed. The results indicated that macrocyclic oligomers and hyperbranched polymers are new candidates for EL devices.
The Langmuir-Blodgett (LB) films of tolan derivatives were studied by a new second-harmonic generation (SHG) phase measurement in terms of the directional sense of the net dipole moment and molecular structures. 4-Docosylamino-tolan derivatives with 4'-nitro group (ANT-22) or 4'-dicyano-vinyl (DCNANT-22) group were transferred by horizontal lifting. In rotational Maker-fringe, the envelope of ANT-22 LB film shows an in-plane polar anisotropy, and that of DCNANT-22 LB film shows a polar anisotropy perpendicular to the substrate. Nonlinear optical chromophore in ANT-22 is bound to lie almost flat upon the plane of the substrate, while DCNANT-22 molecules in thick multilayers tend to align perpendicular to the substrate. It was found by SHG phase measurement that the directional sense of the net dipole moment of ANT-22 was perpendicular to the compression direction.
Two-beam coupling gain obtained from an orientational index grating was observed without applying an external field in a photorefractive polymeric material with ow glass transition temperature. A novel carbazole derivative with a photorefractive function was used as the material doped with a sensitizer for photocarrier generation. The mechanism to form the grating and to give the gain was investigated by two-beam coupling measurements.
Photocarrier generation properties and an electro-optic response were studied in acceptor-substituted carbazole materials. Two kinds of carbazole systems were investigated: molecularly doped polymer and main-chain polymer. The photoconductive sensitivity of 1.2x1012(S cm1)I(W cm2) was obtained with a field strength of 5x1O5V/cm at a
wavelength of 534nm. The electro-optic coefficient 33 of l.lpmJV was obtained with a field strength of 5x1O5V/cm at a wavelength of 532nm. These results show the monolithic photorefractive polymer could be developed by using acceptor-substituted carbazoles.
We have developed novel multifunctional materials for photorefractive applications. Two kinds of molecular systems were studied: acceptor-substituted carbazole main-chain polymers and oligomers. The photocarrier generation properties and second-order nonlinear optical responses in acceptor-substituted carbazoles were examined by xerographic discharge, second-harmonic generation and electro-optic measurements. The multifunctionalities of the main-chain polymers and oligomers with carbazole moieties were confirmed, which fulfill all the requirements for photorefraction. Photorefractive effects were confirmed by two-beam coupling and four-wave mixing measurements. These multifunctional carbazole main-chain polymers and oligomers enable us the development of monolithic photorefractive materials.
The transient absorption spectrum of a nonaggregated vanadylphthalocyanine solution has been measured by a subpicosecond pump-probe spectroscopy technique. A biexponential response with time constants of 4 ps and 800 ps has been observed. The dependence of the polarizations of pump and probe light has been investigated. The parallel-to- perpendicular ratio, i.e. the ratio of transient absorption change between configurations where pump and probe polarizations are parallel and perpendicular to each other, respectively, is 4:3. This is different from the ratio in ordinary 1D molecules, reflecting the 2D character of the vanadylphthalocyanine molecule.
Novel two-dimensional conjugated systems have been developed for second- and third-order nonlinear optics. Four types of asymmetrically substituted macrocycles and non-aggregated phthalocyanines were synthesized and their second- and third-order nonlinear optical properties were characterized: meso-monosubstituted octaethyl metalloporphyrins, (beta) - monosubstituted tetraphenyl metalloporphyrins, mono (tert-butyl) vanadyl phthalocyanine, tris (tert-butyl) mononitro metallophthalocyanines, and hexadeca (trifluoroethoxy) metallophthalocyanines. Control of molecular packing in asymmetric vanadyl phthalocyanine was described in terms of the enhancement of the third-order nonlinear optical susceptibility. No evidence of molecular aggregation was observed in optical absorption and electroabsorption spectra for hexadeca (trifluoroethoxy) metal-free and vanadyl phthalocyanines. The effects of the substituents and metallation on the dipole moment for asymmetric porphyrins is discussed. A preliminary investigation of the second-order nonlinear optical responses for Z-type Langmuir-Blodgett films and poled molecularly doped polymer is presented.
Electroabsorption spectra have been measured on poly(methyl methacrylate) thin films doped with three kinds of metallophthalocyanines: tetrakis (tert-butyl) metal-free and lead phthalocyanines, and hexadeca (trifluoroethoxy) metal-free phthalocyanine. The third-order nonlinear optical susceptibility, (Chi) (3) (-(omega) ; (omega) , O, O), was determined from the electric field-induced absorption change. The considerable differences in absorption, electroabsorption, and complex (Chi) (3) (-(omega) ; (omega) , O, O) spectral profiles of these metallophthalocyanines are attributable to the difference in molecular aggregation states.
Optical Kerr effect were applied to an all-optical switching device in the form of nonlinear waveguide directional couplers. The nonlinear directional coupler had a nonsymmetrical waveguide whose structure consists of a quartz thin gap between two Corning 7059 guided layers on the pyrex substrate with ion-milled grating and organic thin film as a top layer. The vacuum-deposited polydioacetylene film was used as an organic nonlinear material. Switching phenomena in this nonlinear directional coupler were confirmed for 10 ns pulse Nd:YAG laser and 150 fs pulse duration of mode-locked Ti:Sapphire laser.
The nonlinear optical channel waveguide consisting of the lead phthalocyanine derivative doped polymer layer over the ion-exchanged glass substrate was fabricated. The single-mode propagation was observed in this polymeric channel waveguides. Propagation loss was estimated to be 2 dB/cm at 830 nm. Nonlinear optical properties are investigated by measuring the intensity-dependent refractive index change using a femtosecond interferometric method.
Second- and third-order nonlinear optical responses of photoconductive materials were studied by optical second and third harmonic generation and femtosecond time-resolved spectroscopy. Four types of photoconductive molecular systems were investigated: vanadylphthalocyanine thin films, polymeric charge-transfer complex, single crystals of 9-hydroxyethylcarbazole derivatives, and carbazole head-to-tail polymers.
The molecular design and assembly of metallophthalocyanine systems that have the enhanced macroscopic third-order nonlinear susceptibility (chi) (3)ijkl(-(omega) 4; (omega) 1, (omega) 2, (omega) 3) and show ultrafast responses are described. Enhancement of the third harmonic susceptibility (chi) (3)1111(-3(omega) ; (omega) , (omega) , (omega) ) was observed in vanadyl phthalocyanine vacuum-deposited film with the staggered stacking arrangement induced by thermal treatment. Processable polymeric system based on tert-butyl mono-substituted rich vanadyl phthalocyanine was developed and the favorable staggered stacking arrangement was induced in a polymer matrix to enhance (chi) (3)1111(- 3(omega) ;(omega) ,(omega) ,(omega) ). Femtosecond-time-resolved spectroscopy was performed on vanadyl phthalocyanine thin films with different morphological forms in order to elucidate the exciton dynamics. The exciton decay kinetics consists of three processes: exciton-exciton annihilation, exciton-phonon coupling, and much slower triplet state formation. The decay rate of the exciton population via exciton-exciton annihilation was found to have a time dependence of t-1/2. The vacuum-deposited film and spin-coated film of vanadyl phthalocyanine derivatives showed similar decay behavior, whereas molecular beam epitaxy film showed a faster decay with a time constant of several hundreds of femtoseconds.
The third order optical susceptibilities XIfli(-3w ; co, a, )
for PMMA films doped with a soluble metallophthalocyanine derivative
are determined by optical third harmonic generation measurements at
wavelengths of 1543nm and 1907nm. The x3 values are 6-7 times
enhanced at resonant and nonresonant wavelengths, concurrent with an
absorption change induced by solvent vapor treatment. The enhancement
of third order optical nonlinear susceptibilities in phthalocyanine
thin films is discussed in terms of molecular stacking.
Nonlinear optically active compounds newly designed and synthesized in Frontier
Research Program were reviewed: single crystal of dicyanovinyl anisole (DIVA) for
blue light generation, one- or two-dimensional r -conjugated systems such as polythiophenes,
polydiynes, soluble phthalocyanines and annulenes, for third order
optical nonhinearities and waveguide application. As a promising technique to control
the molecular orientation and assembly, the organic molecular beam epitaxy (OMBE) is
proposed.
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