Light-induced Gratings Research Articles

Photoempfindliche Koplanarleitungen auf CdS-CdSe-Al2O3-Basis für optisch gesteuerte Mikro- und Millimeterwellenkomponenten

Article Photoempfindliche Koplanarleitungen auf CdS-CdSe-Al2O3-Basis für optisch gesteuerte Mikro- und Millimeterwellenkomponenten was published on March 1, 1999 in the journal Frequenz (volume 53, issue 3-4).

Photoinduced processes in solid polymer solutions of dyes in an interference field of laser radiation

An investigation was made of the relationships governing the photochemical mechanism of formation of light-induced gratings in solid polymer solutions of a dye with a high quantum yield of the triplet states. The combined analysis of the results of real and numerical experiments was made for a solution of eosin K in gelatin. The protonation rate constant of the dye was measured and the dependence of the diffraction efficiency on the duration of irradiation was explained taking diffusion of the dye into account. A method was proposed for determination of the duffusion coefficient in a spatially modified interference field of the laser radiation. The diffusion coefficients were found.

Influence of Growth Defects on Carrier Lifetime and Transport in Semiinsulating GaAs Studied by Transient Light-Induced Grating Technique

Influence of Growth Defects on Carrier Lifetime and Transport in Semiinsulating GaAs Studied by Transient Light-Induced Grating Technique

Mechanism of energy migration due to charge transfer vibronic excitons: role of ferroelectric phase transition in solid solutions KTa 1− xNb xO 3

Charge transfer vibronic excitons (CTVE) induced by two-photon absorption within degenerate four-wave mixing (DFWM) of the ferroelectric solid solution KTa 1− x Nb x O 3 are considered as an origin of the energy migration effects which control the life time of transient light-induced gratings. The CTVE are created by charge transfer between O and Ta ions, or O and Nb ions with the appearance of O −Ta 4+ or O −Nb 4+ pairs accompanied by vibronic origin lattice distortion. CTVE corresponds to an electron-hole bipolaron. The proposed model of energy transfer is based on the resonance cross-relaxation between CTVE of the same type due to indirect CTVE-CTVE interaction via acoustic phonon modes mixed with soft TO-phonon modes. The latter lead to a critical increase of the diffusion coefficient as a function of the Nb ion concentration. Such a tendency was detected in the experiment. The model explains the experiments quantitatively.

Nondestructive Resolution of Higher Harmonics of Light-Induced Volume Gratings in PMMA with Cold Neutrons

Illumination of photosensitized (PMMA) with light triggers nonlinear polymerization processes. In this way periodical density structures with nearly ${10}^{4}$ lines/mm can be generated which are beyond the resolution limit of light-optical investigation methods. We report on the first direct experimental observation of such higher harmonics in deuterated (PMMA) by the nondestructive method of diffraction of neutrons at a wavelength of 1.2 nm. Diffraction efficiency of the second harmonic was found to be 200--500 times weaker than that of the first order. The origin of higher harmonics is attributed to the nonlinearities of the processes of chain growth and of termination.

Optical storage in an azobenzene-polyimide film with high glass transition temperature

A polyimide polymer containing azobenzene side groups was used to demonstrate the ability of optical storage. Writing and reading processes in the polyimide film were performed using degenerate four-wave mixing. The time response of the light-induced gratings was characterized by three components, a fastest, a faster and a slower, which were attributed to the excited-state population, the trans–cis isomerization and the reorientation of azobenzene molecules, respectively. The dependence of this dynamic behavior on the sample temperature was investigated, and was found that with the increase of the sample temperature the contribution of the orientation component to the light-induced gratings became more and more dominant.

Scattering Parameter Analysis of Light-Induced Periodic Microwave Structures

Based on the scattering parameter concept, an exact analysis of the reflection and transfer properties of light-induced waveguide gratings applied to microwave or millimetre wave frequencies, with special regard to internal return loss and multiple reflections, is reported. After a short presentation of the foundations of periodically photoexcited semiconductor waveguide structures, the S-parameters of a multi-stage optoelectronic grating configuration under optical CW-excitation are calculated. Moreover, the influence of an impedance mismatch between waveguide grating and measuring system is outlined quantitatively in the form of a modified scattering matrix.

Bistable Centers in CdMnTeSe:In and CdMnTe:Ga Crystals Studied by Light-Induced Gratings

Bistable Centers in CdMnTeSe:In and CdMnTe:Ga Crystals Studied by Light-Induced Gratings

Light-Induced Gratings in CdMnTeSe:In Crystals

We show tlat DX-like centers in Cd1-xMnxΤe1-ySex:In crystal can be used in writing volume holographic gratings. Tle scattering efficiency is higher than 10% which proves the dispersive character of the light-induced gratings. Two different metastable centers with different 1attice relaxation were found in the crystal. Measurements of the power dependence of the degenerate four-wave mixing scattering efficiency testify that both of these centers have negative U properties. PACS numbers: 74.62.Dh, 61.72.Ji, 42.65.Hw

Transient light induced grating study of excitation diffusion in Y 3Sc 2Ga 3O 12 laser crystals

Excitation migration over the lower energy levels in Y 3Sc 2Ga 3O 12 (YSGG) laser crystals doped with Cr, Er, Tm, and Ho ions was studied using the two-wave mixing light induced grating (LIG) technique. The results of the LIG measurements were compared to the data obtained from the luminescence kinetics and spectra overlap studies of the migration. The comparison of the three different spectroscopic techniques allowed more detailed analysis of the complicated pattern of the energy transfer processes in the crystals than any one method alone. The migration was found to be very efficient, especially in highly doped crystals. The diffusion coefficients determined from direct LIG measurements were much higher than those calculated in dipole-dipole approximation from absorption and emission spectral measurements. This implies stronger than dipole-dipole coupling between interacting ions. In Tm doped crystals a surprisingly strong increase of the migration efficiency between Tm concentrations 8 × 10 20 cm −3 and 1.5 × 10 21 cm −3 was observed both in LIG diffusion and luminescence kinetics experiments. This too cannot be explained in terms of low multipolar energy transfer. Exchange energy transfer and the percolation model are discussed as possible explanations for the experimental results.

Theoretical studies on laser-induced gratings in organic photoisomers

A theoretical model for formation of light-induced gratings in organic photoisomers is presented. Using this model, dynamic behaviors of the light-induced gratings are studied theoretically for two different cases: in the first case, in addition to a CW laser generating the writing beams and the reading beam, a pulsed laser is used as an optical pump, in the second case, the writing beams are provided by a pulsed laser, and the reading beam comes from a CW laser. Agreement between experiment and theory is also demonstrated.

Second Sound and Nonlinear Optical Phenomena in Smectic A Liquid Crystals

Abstract We investigated theoretically the nonlinear phenomena (a self-focusing, a self-trapping, stimulated light scattering and four-wave mixing) in smectic A liquid crystals (SA) caused by a new physical mechanism of cubic nonlinearity determined by the propagating mode of a layer normal displacement, or the so-called second sound. A nonlinear part of a refractive index caused by a layer normal deformation which is responsible for a self-focusing effect, and a gain coefficient in the case of a stimulated light scattering (SLS) on the second sound are one or two orders of magnitude greater than the similar quantities for isotropic organic liquids. The light-induced dynamic grating of layer deformations generates a high-frequency longitudinal electric field due to the flexoelectric effect. The light-induced dynamic grating of layer normal displacement ultimately excites a rotational hydrodynamic flow with a considerable velocity. Using the slow varying amplitude approximation we calculated the light wave...

Light-Induced Dynamic Grating Investigations of Glassy Liquid Crystals for Optical Storage Applications

Abstract The dynamics of light-induced gratings in weakly absorbing glasses possessing liquid crystalline orientational order have been investigated using a gated cw-laser. It is shown that structural relaxation processes are substantially important for the formation of reversible holographic storage effects. The resulting complex dynamics of grating formation have been analyzed by applying a time-domain master-plot construction for the first time. Intensity and temperature dependence of the obtained relaxation time exhibits a Vogel-Fulcher behavior and the relaxation process can be described by a stretched exponential law of the Kohlrausch-Williams-Watt type.

Study of migration in Cr:Er:YSGG using an upconversion light induced grating technique

An excitation migration over the level 4 I 1 1 2 was studied in Cr:Er:YSGG laser crystal using an upconversion nonholographic light induced grating (LIG) technique A long distance of migration (≃ μm) was found. The diffusion coefficient was determined to be equal to 7 × 10 −7 cm 2/ s. This value was compared to that calculated in dipole-dipole approximation from the emission and absorption spectra overlap. The analysis shows, that excitation migration in Er:Cr:YSGG cannot be described in terms of dipole-dipole energy transfer, other possibilities are discussed.

Best-Case Reflection Analysis of Low-Loss Excited Plasma Gratings

The microwave reflection characteristics of light-induced plasma waveguide gratings under the best-case assumption of neglecting all the dark-state waveguide losses are reported. As a result, the best-case maximum attainable major-lobe peak reflection coefficient of a symmetric grating structure amounts to 31,8 %. In the case of asymmetric excitation, up to 50% can be achieved.

The Distributed Feedback of Counter-propagating Waves in a Periodically Modulated Medium with Relaxing Cubic Nonlinearity

Abstract An interaction of two counter-propagating waves in a periodically modulated medium with relaxing cubic nonlinearity is considered. On the basis of analytical solutions of the equations under the weak distributed feedback (DFB) approximation, we show that an increase in the reflectivity of ‘nonlinear’ DFB-structures takes place due to phase mismatching between the radiation and the light-induced grating. We carry out a numerical simulation of the obtained equations for arbitrary values of the coupling coefficient, Bragg detuning and incident pulse intensity. We also analysed the dynamics of the regular temporal modulation of initially continuous wave radiation caused by the ‘nonlinear’ grating.

Transient grating optical nonlinearities in nematic, cholesteric and smectic liquid crystals

The transients of Kerr-like optical nonlinearities in various liquid crystals observed with picosecond light-induced dynamic gratings are discussed. It is shown that relatively high intensities and strong optical fields of short laser pulses lead to rapid molecular reorientation, ultrasound generation, multiphoton absorption, cholesteric helix deformation and other new phenomena which are not only of interest for basic liquid-crystal research but also determine photonic switching times in liquid crystalline all-optical devices.

Photorefractive optical fuzzy-logic processor based on grating degeneracy

A novel optical fuzzy-logic processor using light-induced gratings in photorefractive crystals is proposed and demonstrated. By exploiting grating degeneracy, one can easily implement parallel fuzzy-logic functions in disjunctive normal form.

Optimierung von lichtinduzierten Wellenleitergittern hinsichtlich Substrateigenschaften und optischer Anregung

Based on earlier published results from theoretical and experimental investigations on light-induced distributed Bragg reflection waveguide gratings, a significant increase of the major-lobe peak reflection coefficient at microwave and millimetre wave centre frequencies can be achieved by optimizing the optical excitation wavelength as well as the surface recombination velocity and the carrier lifetime of semiconductor substrate. Further improvement of the reflection of the single-crystal full-semiconductor substrate formerly used by a ceramic one with a polycrystalline semiconductor film evaporated

Light-induced thermal gratings in LiNbO3: Fe

Holographic gratings are recorded in LiNbO3: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelengthλ = 532 nm, pulse durationtp = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe2+ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO3: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.