A dense array of vertical ZnO nanowires on a-plane sapphire substrate was synthesized by a simple chemical vapor
deposition method. The electrolyte-based Schottky contact of the ZnO nanowires was investigated by electrochemical
impedance spectroscopy. An n-type semiconductor behavior and a flat-band potential of about 0 V (0.05 V) versus
Ag/AgCl electrode were obtained for the synthesized ZnO nanowires. The ~ 0 V flat-band potential is suggested to be a
balanced result of (1) the Femi-level difference induced by the Schottky contact at the ZnO/electrolyte interface and (2)
the oxygen vacancy induced surface adsorption effect at the ZnO nanowire surface. Hydrogen plasma treatment was
carried out to passivate the oxygen vacancies in the ZnO nanowires. An obvious shift of the flat-band potential to about -
0.6 V was obtained for the same ZnO nanowire array sample after the hydrogen plasma treatment. The negative flatband
potential, indicating an electron depletion region at the surface of ZnO nanowires, is observed owing to the Fermilevel
difference between the n-type ZnO nanowires and the electrolyte, without a strong influence of the oxygen
vacancy-related surface adsorption effect. Moreover, the carrier density in the ZnO nanowires was increased by almost
four orders of magnitude after the hydrogen plasma treatment. The increase in carrier density confirms existing reports
of hydrogen atoms occupying interstitial sites in the ZnO nanowires in addition to the oxygen vacancies after the
hydrogen plasma treatment.
We have investigated properties of poly(9-vinylcarbazole) (PVK) doped with 30% wt 4-dibutylamino-4'-nitrostilbene
(DBANS), depending on orientation of the polar DBANS molecules. Appearance of the orientation-induced built-in
electric field was proven optically by the solid electric field induced second harmonic generation and electrically by the
current-voltage characterization. Modification of optical properties was evidenced by the spectral dependencies of the
absorption coefficient. The thermally stimulated current spectra had demonstrated that carrier transport and trapping are
Many organic compounds in solid state have nonlinear optical properties due to the orientation of the molecules in a
polymer matrix. In this work, all-optical poling and second harmonic generation in a composition consisting of 1 mass%
of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) compound in poly(methyl methacrylate) (PMMA) matrix
were studied. Thin films were prepared by solvent casting. The 1.064-μm fundamental and 532-nm second harmonic
wavelengths of a Nd:YAG laser were used. It is shown that DMABI molecules can be oriented by the method of all-
optical poling, and that the process is related to the photoinduced switching between two equally stable states of the
We demonstrate the possibility of optically inducing the resolution of a racemic mixture of molecules. We thus measured an optical induction of optical rotation. For this, we used an all optical method based on a pump (Ar-488 nm) - probe (He-Ne-632 nm) experiment exciting a thin layer of a new chiral photoisomerizable chromophore in a PMMA matrix.
We measure third-order optical response of two organometallic compounds using the degenerate four wave mixing method. From measurements of DFWM efficiency, we deduce the values of third-order susceptibilities χ<3>. From measurements of χ<3>, we deduce the values of the second-order hyperpolarisabilities γ. The merit factor for each compound is given and the value obtained for the most efficient compound in terms of γ(second order hyperpolarizability) is 104 larger than the value of CS2, which is a reference material. The obtained optical nonlinearities are compared to those of other compounds previously studied. A preliminary correlation between structure and third-order optical properties is proposed.
All optical poling technique allows orientation of dye molecules in a polymer matrix by purely optical means. The coherent superposition of two beams at fundamental (FUND)and second harmonic (SH)frequencies results in the presence of the polar field E(t) inside the material, able to break the centrosymmetry of the medium. The temporal average cube 3>t of this field E(t) = Eω(t)+E2ω(t), is non-zero, which leads to an orientation-selective excitation of molecules and the second order χ(2)-susceptibility grating is encoded, with a period satisfying the phase matching condition for SH generation. The physical origin of the effect lies in the orientational hole-burning in the initially isotropic distribution of dye molecules (via trans-cis trans photoisomerisation). We investigate other phenomena responsible for the SHG in polymer films like the electron transfer between a polymer matrix (donor of electrons) and a dye molecule (electron acceptor) and processes which accompany polar orientation mechanism like the one-direction photoinduced molecular migration. The stability of the induced polar order after orientation has been also studied and the crosslinkable polymer system has been developed D side chain molecules with ends can thermally react with epoxy groups were randomly inserted in the side groups of the polymer backbone, which leads to the reticulation and enhancement of the rotational stability. After poling the second order nonlinear response exhibits a much longer relaxation time. In the set-up configuration used we monitor non-perturbatively the all optical poling and there is no necessity of taking into account the phase difference between writing beams (fundamental and SH). The periodical modulation of the relative phase provides the desired control on the polar order at the molecular level as well as the possibility of encoding surface relief gratings with a period equal to the half of the period of the χ(2) grating.
All-optical poling permits the polar orientation of molecules. FOr an efficient poling of thin films, relative phases, amplitudes and polarizations of the two interfering beams must be controlled. We present an original stable one-arm interfermeter which is specific to the recording of two-color interferences. It relies on teh index dispersion of optical glasses. This particular interference technique permits true real-time non-perturbative monitoring of the polar orientation process and a fast all optical poling of thin film materials, without need for phase control.
All-optical poling technique permits purely optical orientation of dye molecules in a polymer film. The experiment includes two phases: the writing (seeding) period and the readout one. In seeding phase two beams, the fundamental (omega) and its second harmonic (SH, 2(omega) ) irradiate the sample and as a result of the coherent interference between them the second order (chi) (2)-susceptibility grating is encoded, with a period satisfying the phase matching condition for SH generation. During the readout step only the fundamental beam is incident onto the material and the second harmonic beam generated by the medium is observed at the back side of the sample. The coherent superposition of two beams at (omega) and 2(omega) frequencies results in a presence of a polar field E(t) inside the material, which can break the centrosymmetry of the medium. The physical origin of the effect lies in the orientational hole-burning in the initially isotropic distribution of dye molecules. It has been demonstrated that efficient all-optical poling requires optimization of relative intensities and relative phase of the seeding beams. An original technique of non-perturbative monitoring of the all-optical poling process without any necessity of taking care of the phase difference between seeding beams is presented. This new technique was applied to several new dye-polymer systems.
All-optical poling technique permits purely optical orientation of molecules. The experiment consists of two steps: the writing (so-called seeding) period and the readout one. In the seeding phase two beams, the fundamental one ((omega) ) and its second harmonic (SH, 2(omega) ) illuminate the sample and print in the medium the second order (chi) (2) susceptibility grating with a periodicity satisfying the condition of phase matching for SH generation. In the readout period only the fundamental beam irradiates the sample, and the second harmonic generation is observed at the sample output. Efficient all-optical poling requires optimisation of the seeding beam intensities and their relative phase difference. We propose a novel technique for non-perturbative monitoring of the all-optical poling process and the easy method of orienting molecules without any necessity of taking into account the phase difference between seeding beams.