PHOTOINDUCED BIREFRINGENCE AND NUMERICAL SOLUTION OF A NEW DYNAMIC MODEL IN AN AMORPHOUS COPOLYMER CONTAINING AZOBENZENE GROUPS

Photoinduced birefringence is investigated in a new amorphous copolymer containing azobenzene groups. The levels of birefringence signal are found to depend an the polarization angle between the pump beam and the probe beam, and on the ellipticity of the pump beam. Both the growth and decay processes of the birefringence signal can be described by known biexponential equations. The rate constants and the amplitudes associated with the growth process of the photoinduced birefringence are observed to display a linear dependence with the pump beam intensity. A new dynamic model of the photoinduced birefringence is presented taking into account the contributions of both the bans and cis isomers of azobenzene groups and the local polymer segments. The numerical treatment of this model shows good agreement with the experimental data in the whole writing-erasing processes of the photoinduced birefringence conducted in our polymer samples.

Geometry Control of Photo-induced Microstructures in an Azobenzene

The mechanisms of photoinduced microstructures in an azobenzene polymer film are presented.They are based on the spatial periodic modulation of optical intensity and the photoisomerization of azobenzene molecules with the movement of main chains.Experiment and theory jointly point out the possibility of photoinducing desired spatial microstructures in azobenzene organic polymer via adequate optical lattices and adequately polarized‘writing’beams.

Femtosecond Reflections of Bulk GaAs and GaAs/AlGaAs Multiple Quantum Wells

We present room temperature femtosecond rejection studies on the photocarrier dynamics of GaAs and GaAs/AlGaAs multiple quantum wells(MQW).A rising wing with a time constant of about.5 ps and an increased amplitude per carrier is observed for the MQW sample at carrier densities lower than 5×10^(11) cm^(-2).This phenomenon is explained as the saturation of the excitonic transitions.

Ultrafast Heavy Hole-Phonon Scattering in Highly n-Doped GaAs

Ultrafast heavy hole-phonon interactions in highly n-doped GaAs have been selectively studied with femtosecond absorption saturation measurements by tuning the pump-probe photon energy just above the doped electron Fermi edge.A heavy hole-phonon scattering time of~300fs has been measured,which is consistent with that calculated by a numerical model.Accordingly,an optical deformation potential of about 31 eV has been estimated for this highly n-doped sample.

Mutual Quenching of Self-pumped Phase Conjugation by Incoherent ps Laser Pulses

When two beams of incoherent cw mode-locked ps laser pulses were incident upon opposite faces perpendicular to the a-axis of a Cu-doped KNSBN crystal,no mutually pumped phase conjugation as that generated by continuous waves was observed,however,the stronger laser beam generated its self-pumped phase conjugation which was weakened by the weaker one.When the intensities of the two beams were equal,self-pumped phase conjugation was completely quenched.

Fluorescence-Detected Ultrafast Free-Induction Decay in Powdered Rare Earth Solids

Fluorescence interferometry is developed and applied to study ultrafast amplitude and phase dynamics for fleeinduction decay in powdered rare earth solids. The time-resolved phase dynamics of free-induction decay throughout the decaying process is accurately determined by using a novel dual-channel correlation technique and subpicosecond dephasing time is measured for Nd3+ solids at room temperature. The phase dynamics is well simulated with linear coherent polarization theory.

Pattern Formation and Quasicrystal Structure in Azobenzene Polymer Film

Pattern formation in azobenzene polymer film by degenerate four-wave mixing is reported.Island arrays with specific patterns are analyzed with scanning electron microscopy and polarizing optical microscopy.It is demonstrated that the control of photo-induced nanostructure sized micropattern in the nonlinear organic film is possible by using properly polarized writing beams with the total incident power exceeding a certain threshold.

A Vector Theory on Spatial Solitons in Kerr Media

A new vector theory which concerns the necessary and sufficient conditions of the self-trapping of a linearly polarized light beam in Kerr media is presented after a generalization of Fermat's principle. It is rigidly proven under these conditions that 1+1-dimensional spatial solitons can exist in Kerr media but 2+1-dimensional solitons cannot. A new form of the general equation for 1+1-dimensional solitons in Kerr media is put forward and some of its solutions have been obtained.

Large Stable Photoinduced Anisotropy and Optical Information Storage in Poly [4-bis (N, N-oxyethylene) imino-4- nitroasobenzene Succinyl] Film by Two-Wave Mixing

Optical information storage caused by photoinduced anisotropy in poly[4'-bis(N,N-oxyethylene)imino-4-nitroazobenzene succinyl]film is studied by two-wave mixing.Using Nd:YAG second-harmonic pulses(100ps),grating of orientation of azobenzene molecules is formed in the film by photoinduced anisotropy.Up to third-order diffraction was observed.After turning off one of the two pump lights,the signals remained.When the weaker one of two pump lights was turned off,the intensities of diffraction beams of another pump light almost did not decrease.The information recorded in the film can be 100%maintained for a long time(monitored over 100 days without obvious decrease).

Two-Photon Absorption and Optical Power Limiting Based on New Organic Dyes

Two new organic dye samples J and L with a large two-photon absorption(TPA)cross section have been reported.The linear absorption spectra show that there is no linear absorption at the wavelength from 650 to 1200 nm.The molecular TPA cross section was measured to be as high as 2.59 × 10^(-47)cm^(4)·s and 2.98 × 10^(-47)cm^(4)·s at 1064 nm for samples J and L,respectively.The input-output curves indicate that there is a clear optical power limiting behaviour when the input intensity is higher than 0.4 GW/cm^(2).Furthermore,basic theorety of TPA process has been discussed.

Giant two-photon absorption of anatase TiO_(2) in Au/TiO_(2) core-shell nanoparticles

We report on deep-to-near-UV transient absorption spectra of core-shell Au∕SiO_(2) and Au∕TiO_(2) nanoparticles(NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase TiO_(2) NPs. The bleaching of the first excitonic transition of anatase TiO_(2) at ~3.8 e V is a signature of the presence of electrons/holes in the conduction band(CB)/valence band(VB) of the material. We find that while in bare anatase TiO_(2) NPs, two-photon excitation does not occur up to the highest used fluences(1.34 mJ∕cm^(2)), it takes place in the TiO_(2) shell at moderate fluences(0.18 mJ∕cm^(2)) in Au∕TiO_(2) core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of ~10~8–10~9. Remarkably, we observe that the bleach of the 3.8 eV band of TiO_(2) lives significantly longer than in bare TiO_(2), suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.

Transient Photoluminescence Studies of Tunneling in Asymmetric Double Quantum Wells

1 Introduction Carriers in semiconductor quantum wells and superlattices can move in both paral-lel and perpendicular directions to interfaces of layers. Carriers must penetrate poten-tial barriers in their perpendicular movement and therefore, this kind of transport iscalled the carrier tunneling. Since Esaki and Tsu’s initiative research in 1973, especiallyafter the quantum well and superlattice materials came into the world, the carrier tun-neling in semiconductor heterostructures has received comiderable attention: (i) In

Optical-field-induced Freeder-icksz transition of a nematic liquid crystal doped with dia-zophenyl

4[4’-(phenylazo)phenylazo]phenol was synthesized and characterized. Optical-induced Freedericksz transition of 5CB liquid crystal doped with trace diazophenyl was studied. It was observed that the doped 5CB dramatically decreased the optical field threshold, and the Freedericksz transition threshold occurred at optical intensity level of 0.3 mW/mm2 for the 12.5 μm thickness planar alignment doped 5CB, which is three orders of magnitude lower than that for the pure 5CB. This new phenomenon indicated that trans-cis isomerization of diazophenyl plays an important role in the decrease of the Freedericksz transition threshold.