Holographic Experiments Research Articles

Influence of light intensity and crystal temperature on photorefractive charge-compensation processes in Al-doped Bi12TiO20 crystals

Photorefractive charge-compensation processes in bismuth–titanium-oxide crystals (Bi12TiO20) doped with Al are examined by holographic experiments. Our results clearly indicate that the compensating positive charges are thermally excited while the excitation of electrons is optically induced. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Description of photoinduced anisotropy in azobenzene side-chain polyesters

Photoinduced anisotropy in azobenzene side-chain polymers are characterized in two ways, by numerical solution of the proper rate-equations and by approximate analytical expression. Feasibility of these methods was investigated by comparison with results of polarization holographic experiments. Agreement with experimental results proves usefulness of both descriptions for different applications.

Optical and holographic properties of Bi4Ge3O12 crystals doped with ruthenium

Bi4Ge3O12 single crystals doped with different concentration of Ru were grown by the Czochralski technique. Optical absorption bands were induced by illumination with ultraviolet light at room temperature. The absorption coefficient values increased with increasing ruthenium concentration. The induced photochromic effect was completely reversible: all samples recovered their transparency after thermal treatment. Preliminary holographic recording experiments showed a fast response time and a relatively slow erase time during the reading process.

Photoinduced reversible optical gratings in photochromic diarylethene-doped polymeric thin films

We report on our investigation of the potential for all-optical reversible photostructuring of the photochromic molecule 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BFCP). The light-induced isomerization of BFCP was studied in solution as well as in polymeric thin films. The absorption change that is due to the photochromic reaction of BFCP was determined by conventional spectroscopic measurements. The absolute values of the refractive indices of doped polymer films were measured by the waveguide grating coupling technique for the two states of the photochromic molecule. The dynamics of the reactions and the light-induced refractive-index changes Δn of doped polymer samples were investigated in holographic grating experiments. Changes of as much as Δn=(3.5±0.5)×10-4 were obtained at 806 nm for 41.16-wt. % doping. Finally, a modified two-beam coupling experiment permitted the separation of absorption and refractive-index contributions to the grating buildup and the determination of the phase shifts of these gratings with respect to the intensity grating.

Effect of reflections on phase hologram recording in amorphous As-S-Se films

Experimental and theoretical studies of phase hologram recording in amorphous As-S-Se films with slightly wedge-shaped thickness profile are presented. It is shown that Fabry-Perot resonator effect caused by the interference of multiply reflected light beams inside the sample strongly changes the values of diffraction efficiency and its growth rate as well as the exposure time dependences of diffraction efficiency and transmissivity making them site-dependent and sample-dependent. Absorptivity and recording light intensity inside the sample are also significantly changed. The obtained results can be used to explain the holographic and optical experiments also in other materials with Fabry-Perot resonator effect such as other transparent thin films and photorefractive crystals.

Light deflection and convection in diffusion experiments usingholographic interferometry

A study of the effect of light deflection during diffusion studiesof ethanol into agarose gel using holographic laser interferometry ispresented. Furthermore it also demonstrates how a diffusive flux could giverise to a convective flux in holographic laser interferometry experiments. Theconvective and diffusive mass transfer is also theoretically compared in botha liquid phase and a gel phase for the ethanol-agarose system used.The current study shows that errors due to light deflection in holographiclaser interferometry are extremely small and can be neglected. It also showsthe importance of designing the diffusion experiments to avoid naturalconvection. In gels the convective flow is cancelled by the friction forcesbetween the liquid and the polymer network. However, in the liquid phase thenatural convection could occur even though the density differences in thephase are small.

Dual-use chromophores for photorefractive and irreversible photochromic applications

Holographic experiments are performed on a series of dual-use chromophore molecules wherein both irreversible photochromic and erasable photorefractive holographic gratings can be written in the same storage volume. At 675 nm, the chromophore undergoes a photochemical reaction leading to the creation of irreversible holographic gratings. Alternatively, at longer wavelengths, application of an electric field during grating formation allows the storage of erasable photorefractive holograms in the same location as previously stored permanent photochemical holograms. Photochemical gratings (eta > 60%) can be written in less than 1 min, whereas photorefractive gratings (eta > 50%) can be written in less than 1 s. The photochemical gratings have a diffusion-limited dark half-life of as long as two weeks depending on the glass transition temperature of the composite.

Surface relief gratings in photoaddressable polymers generated by cw holography

Surface relief gratings (SRGs) are a widely observed phenomenon in holographic experiments with photoaddressable polymers. This article proposes a model which is able to explain the observed effects in cw experiments. It is based on forces in an electrical field which are caused by the inhomogeneous spatial distribution of the refractive index and the permittivity in the illuminated sample.

Isotropic diffraction induced by concentration gratings in α-LiIO 3:Fe

Iron doped lithium iodate (α-LiIO 3:Fe) of hexagonal modification is investigated by holographic experiments in a geometry where an electrooptic contribution to the diagonal elements of the change of the dielectric permittivity tensor is forbidden by symmetry. Anisotropic diffraction (electrooptically allowed) which we expected to observe in that geometry did not occur. Instead, isotropic diffraction (electrooptically forbidden) from light-induced gratings was found. To clarify its origin the properties of these gratings are studied as well as photo-induced transparency occurring in the ultraviolet. The experiments show that two types of gratings exist, one of them being an absorption grating. The second grating is attributed to originate from concentration changes which arise from the transport of ions triggered by non-uniform illumination.

Surface relief gratings generated by pulsed holography: A simple way to polymer nanostructures without isomerizing side-chains

Photoaddressable polymers based on azobenzene side-chains have recently drawn a lot of industrial attention due to their possible application as data storage media. From the scientific point of view an additional effect, the occurrence of light induced surface relief gratings, is nowadays extensively studied. This article describes the behavior of side-chain and guest–host polymers (with and without azobenzene groups) in holographic experiments with pulse lasers. We observe surface relief gratings also for polymers, which only contain an absorbing, but nonisomerizing chromophore. Relief formation in pulsed holographic experiments is shown to arise from thermal effects, but not from trans–cis isomerization.

Photorefractive light scattering families in (111)-cutBi12TiO20crystals with an external electric ac field

We investigate here both theoretically and experimentally light-induced scattering in (111)-cut Bi12TiO20 crystals with an external ac field. Our simple analytic solution, which is nearly as precise as the numeric one, allows us to recognize the following otherwise hidden general features. Without the elasto-optic contribution, the scattering patterns are identical for the same value of xi=zeta(0)+(2/3)straight phi(p), where straight phi(p) is the initial polarization angle and zeta(0) is the angle of the external field. With the elasto-optic contribution, the scattering patterns for the same xi are still very similar. For xi not equal0, the scattering patterns depend differently on the elasto-optic coefficients p(12) and p(13) so that in principle p(12) and p(13) can be measured by purely holographic experiments. On the experimental side, we present scattering patterns for xi=0 and +/-30 degrees, showing thereby the similarity of the scattering patterns for equal values of xi. In all cases, we obtain good qualitative agreement of our analytic and numeric calculations with the experimental findings.

Self-enhanced nonvolatile holographic storage in LiNbO3:Fe:Mn crystals

The self-enhancement and self-depletion phenomena originated from the beam-coupling effect are observed during both the recording and fixing phases in LiNbO3:Fe:Mn nonvolatile holographic storage experiments. A numerical simulation and a steady-state analysis are developed to provide a mathematical evaluation to the phenomena. From the experimental and theoretical studies, it is seen that the highest diffraction efficiency available from the self-enhancement may be twice as large as the lowest one from the self-depletion, both during the recording and fixing phases. Therefore, the use of the self-enhancement is necessary for the practical applications.

Light-induced charge-transport properties of photorefractive barium-calcium-titanate crystals doped with rhodium

Nominally pure and rhodium-doped barium-calcium-titanate crystals of the congruently melting composition Ba0.77Ca0.23TiO3 (BCT) are grown by the Czochralski technique. These crystals are characterized by holographic and conventional electrical experiments. The doped samples have an absorption maximum at 630 nm. They are hole conductive, and for intensities between 0.1 and 1 W cm-2 all properties are explained within a one-center charge-transport model. Two-beam-coupling gain factors up to 8 cm-1 are obtained, and doping with rhodium increases the effective trap density. Although diffusion is the dominant charge-driving force, bulk photovoltaic currents also occur which are especially pronounced in the range between 460 and 640 nm.

Dielectric and Photoorientation Studies in Semicrystalline Melamine Monomers and Amorphous Melamine Networks Containing Azobenzene Chromophores

We demonstrate optical storage by means of trans−cis isomerization within melamine monomers as well as within the corresponding cross-linked melamine resins using holographic grating experiments. The melamine monomer is chemically linked with a photoaddressable azobenzene group. By thermal treatment the semicrystalline monomers can be converted into the amorphous cross-linked resin. Both the monomer material and the cross-linked resins display a photoinduced trans−cis isomerization of the azo groups, a stable photoorientation under irradiation with linear polarized light, and a stable storage of holographic gratings. The photokinetics in the different environments are found to be similiar but somewhat faster for the amorphous resins.

Characterization of charge generation and transport in a photorefractive organic glass: comparison between conventional and holographic time-of-flight experiments

Photorefractive organic systems presently show much longer response times than their inorganic counterparts. The origin of this limitation is not yet fully understood. We present a detailed investigation of the photoelectric processes involved in the photorefractive effect, namely charge generation and charge transport. A comparison between conventional (TOF) and holographic time-of-flight (HTOF) experiments, which were used to determine the charge carrier mobility, is presented. The mobility determined by TOF is shown to depend on the sample thickness. The results show that charge carrier generation and transport are not the limiting factors for the cw-response time of holographic experiments on this system.

Temperature variation of the photoinduced birefringence of an azo dye doped polymer

The influence of the temperature on the photoinduced birefringence on an azo dye doped polymer film has been studied. Sample is composed of a polymer matrix PVK (Poly(N-vinylcarbazole)), doped with 10 weight percent (wt%) of the birefringent molecule DMNPAA (2,5-dimethyl-4-(p-nitrophenylazo)anisole and 30 wt% of ECZ (N-ethylcarbazole) which acts as a plasticizer. Theoretical results can explain the behavior of the amplitude of the photoinduced birefringence versus temperature, Experimental and theoretical results are compared. Holographic recording experiments based on photoinduced birefringence have been carried out. Based on the effect of temperature, we have been able to significantly increase the diffraction efficiency of our sample.

Holographic and non-steady-state photocurrent characterization of photorefractive barium–calcium titanate

Measurements of non-steady-state photocurrents and of time-modulated conductivities for homogeneous illumination of varying intensity are performed with a photorefractive iron-doped barium–calcium titanate crystal of the congruently melting composition (Ba 0.77Ca 0.23TiO 3, BCT). The diffraction efficiency and the two-beam coupling gain are determined from conventional holographic experiments. Debye screening and diffusion lengths are found to be of the order of 0.1 μm. The response-time of the material is 2 s (light intensity 0.2 W/cm 2, wavelength 514 nm) and the dark storage time is 6 s. The measured refractive index changes are only one third of the values, which are expected from the known electrooptic coefficients and space-charge fields.

Photorefractive triphenylamine-based glass: a multifunctional low molecular weight compound with fast holographic response

A novel organic photorefractive composite system is presented consisting of the amorphous multifunctional low-molar mass compound DR1-DCTA, doped with C 60 as sensitizer and diisooctyl phthalate (DOP) as plasticizer. DR1-DCTA, a carbazole substituted triphenylamine with a covalently attached nonlinear optical azo chromophore unit has been synthesized in a 4 step reaction. The compound forms a stable glass. Its thermal, optical and electrochemical properties are discussed. Holographic experiments (2BC, DFWM) of the photorefractive composite exhibit a fast response time in the range of only 30 ms and a maximum gain coefficient of 90 cm –1 .

Reduction of light-induced refractive-index changes by decreased modulation of light patterns in photorefractive crystals

In holographic experiments many photorefractive crystals show refractive-index changes much smaller than one would expect from the known electro-optic coefficients and space-charge fields. We show that degradation of the interference pattern is the origin of this effect. The degree of modulation of a light grating in photorefractive crystals is measured by three methods and compared with the modulation of the grating of the incident light. All methods, measurement of the amplitudes of fundamental and second-order gratings, of the grating amplitudes in a crystal with and without another crystal in front of it, and of the drift currents through an inhomogeneously illuminated sample, yield consistent results: In lithium niobate there is almost no degradation of the light pattern. However, in our barium titanate and potassium tantalate–niobate samples the degree of modulation is smaller, and the reduction factor is 0.55 and 0.60, respectively. Inhomogeneities of refractive-index changes are shown to be the origin of the effect.

Investigation of the Photoinduced Optical Anisotropy of Azo Dye Mesophases

The photoinduced optical anisotropy (POA) of novel aqueous azo dye systems which form chromonic mesophases upon addition of methanol or sodium chloride was investigated. The phases were characterized between crossed polarizers. For further characterization rheological and holographic transient relaxation experiments were carried out. Distinctive differences in the hologram formation and decay were found for phases which exhibit the same texture in polarization microscopy. The generated holographic gratings could be assigned to either photoinduced phase transitions or an alignment of optical axes. In the present study we concentrate on a comparison of phases with the same macroscopic birefringence; results are discussed in comparison with models in the literature.