Photoinduced Volume Research Articles

Methacrylic azopolymers for holographic storage: A comparison among different polymer types

The photoinduced anisotropy and volume holographic storage in a series of polymers with different architectures and azo contents of 7% and 20% in weight have been investigated. Measurements of the birefringence (Δ n) induced with linearly polarised 488 nm light show that for polymers with an azo content of 20%, the highest and more stable Δ n values are achieved in copolymers with a di-block architecture. The Δ n induced in polymers with 7 wt% azo content is not stable. Volume holographic polarisation gratings have been stored in the polymers. Only in the case of the di-block copolymer with an azo content of 20% in weight, the stored gratings are stable. A diffraction efficiency higher than 90% and an angular selectivity of 4° have been achieved in gratings with a period of 2 μm. It can be concluded that the di-block copolymer with 20 wt% azo content is the most suitable, among the studied copolymers, as volume holographic storage material.

Modelling of photoinduced changes in chalcogenide glasses: a-Se and a-As2Se3

The microscopic processes in chalcogenide glasses caused by illumination have been investigated using tight-binding molecular dynamics computer simulation. After the photon absorption we have treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. The interplay between photo-induced bond breaking and inter-chain bond formation leads to either volume expansion or shrinkage. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. The model is supported by the results of recent measurements carried out by Y. Ikeda and K. Shimakawa.

In-situ real-time simultaneous measurements of photoinduced bandgap and volume changes in amorphous chalcogenide films

To understand the dynamics and correlations between photodarkening (PD) and photoinduced volume changes (PVC) in amorphous chalcogenides, a simultaneous PD and PVC measuring system based on computer analyzed spectroscope and phase-shifting interferometer (PSI) has been developed. This new system provides us the records of both PD and PVC in amorphous chalcogenides during illumination with common time axis. The time evolutions of PD and PVC are represented by a stretched exponential function and a just exponential function, respectively, suggesting that the dynamics of the PD is different from the PVC in a-As2Se3 films.

Modelling of photo-induced changes in chalcogenide glasses: a-Se and a-AsSe

The microscopic processes in amorphous selenium caused by light-illumination have been investigated using molecular dynamics simulation. After photon absorption we treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. The interplay between photo-induced bond breaking and inter-chain bond formation leads to either volume expansion or shrinkage. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. In the non-ideal case, a part of the microscopic processes is metastable and the total expansion includes the transient and metastable changes. We can describe this macroscopic effect, too. Our rate-equation models for a-Se and a-AsSe are supported by the results of recent measurements carried out by Y. Ikeda and K. Shimakawa.

Photoinduced volume change in arsenic chalcogenides by band-gap light

Photoinduced structural changes in molecular semicrystalline ${\mathrm{As}}_{4}{\mathrm{Se}}_{3}$ films have been characterized by extended x-ray absorption fine structure and x-ray diffraction under in situ band-gap laser illumination. Transient and permanent changes in the local and long-range structure have been determined quantitatively. In contrast to fully amorphous arsenic selenide films, a photoinduced contraction has been observed in semicrystalline ${\mathrm{As}}_{4}{\mathrm{Se}}_{3}$ films. A comprehensive phenomenological model is proposed for the photoinduced volume change in As-based chalcogenides. This model is based on two competing processes viz. intermolecular interaction through Coulombic repulsive force that results in photoexpansion, and cross-linking of molecular units through bond switching that causes photocontraction. A guideline is presented for selecting chalcogenide glass composition with specific photoinduced volume change characteristics.

In situ simultaneous measurements of photodarkening and photoinduced volume changes in chalcogenide glasses

To understand the dynamics and correlations between photodarkening (PD) and photoinduced volume changes (PVC) in chalcogenide glasses, we built up an in situ simultaneous PD and PVC measuring system based on a computer controlled spectroscope and a phase-shifting interferometer (PSI). This new system provides us the records of both PD and PVC in chalcogenide glasses during illumination with a common time axis. We have observed that the PVC reaches equilibrium faster than the PD in a-As2Se3 films.

Universal feature of photo-induced volume changes in chalcogenide glasses

We have investigated in chalcogenide glasses the microscopic processes caused by illumination using tight-binding molecular dynamics computer simulation. After photon absorption we treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. This model is supported by the results of recent measurements.

Combination of microstructuring and laser-light irradiation for the reversible wettability of photosensitised polymer surfaces

The wetting characteristics of surfaces of polymers doped with photochromic spiropyran molecules can be tuned when irradiated with laser beams of properly chosen photon energy. In particular, UV laser pulses at 308 nm are responsible for the enhancement of the hydrophilicity of the surfaces, since the embedded non-polar spiropyran molecules convert to their polar merocyanine isomers upon UV absorption. The process is reversed upon irradiation with green laser pulses at 532 nm. When the photochromic-polymeric surfaces are micropatterned using soft lithography or photo-polymerisation techniques, they can change their wettability towards a more hydrophobic or more hydrophilic behaviour depending on the dimensions of the patterned features and on the hydrophilicity-hydrophobicity of the flat surface. Furthermore, the light-induced wettability variations on these structured surfaces are greatly enhanced compared to the flat surfaces. This significant increase to the wettability changes is attributed to the combination of the photochromic interconversions upon laser irradiation together with the photoinduced reversible volume changes of the patterned features.

Photocontrolled Variations in the Wetting Capability of Photochromic Polymers Enhanced by Surface Nanostructuring

The wetting characteristics of surfaces of polymers doped with photochromic spiropyran molecules can be tuned when irradiated with laser beams of properly chosen photon energy. The hydrophilicity is enhanced upon UV laser irradiation since the embedded nonpolar spiropyran molecules convert to their polar merocyanine isomers. The process is reversed upon green laser irradiation. Structuring of the photochromic polymeric surfaces with soft lithography enhances significantly the hydrophobicity of the system, indicating that the water droplets on the patterned features interact with air that is trapped in the microcavities, thus creating superhydrophobic air-water contact areas. Furthermore, the light-induced wettability variations of the structured surfaces are enhanced by a factor of 3 compared to those on the flat surfaces. This significant enhancement is attributed to the photoinduced reversible volume changes to the imprinted gratings, which additionally contribute to the wettability changes due to the light-induced photochromic interconversions.

Photoinduced Volume Changes in Amorphous Selenium

We have modeled the photoinduced volume change in amorphous selenium. After photon absorption, we treated the excited electron and hole independently within the framework of the tight-binding formalism. We found covalent bond breaking in amorphous networks with photoinduced excited electrons, whereas excited holes contribute to the formation of interchain bonds. We also observed a correlated volume change of the amorphous samples. Our results provide a new and universal description, which can simultaneously explain the photoinduced volume expansion and shrinkage. This model is supported by very recent in situ surface height measurements for amorphous selenium.

Characterization of a highly photorefractive RF-sputtered SiO2-GeO2 waveguide

We present the characterization of highly photorefractive Er3+/Yb3+-doped 75SiO2-25GeO2 planar waveguides, single mode at 1550 nm, deposited by radio-frequency-magnetron-sputtering (RFMS) technique. Details of the deposition process are reported. The material presents an intense absorption band (alpha approximately 10;3/10;4 cm;-1) in the UV region. Irradiations by a KrF excimer laser source at lambda = 248 nm have produced large positive (up to 310-3) refractive index changes, without the need of particular sensitization procedures. Direct measurements of UV photo-induced volume densification demonstrates that glass compaction accounts for large part of the refractive index change. Highly efficient photo-induced phase gratings have thus been fabricated in the waveguide.

Holographic recording in [Sb(PO3)3]n–Sb2O3 glassy films by photoinduced volume and refraction index changes

Glassy films of 0.2[Sb(PO3)3]n–0.8Sb2O3 with 0.8μm-thickness were deposited on quartz substrates by electron beam evaporation. A contraction in the film thickness (photoinduced decrease in volume) and photobleaching effect associated with a decrease of up to 25% in the index of refraction has been observed in the films after irradiation near the bandgap (3.89eV), using the 350.7nm (3.54eV) Kr+ ion laser line with 2.5W/cm2 for 30min. A loss of 30% in the phosphorus concentration was measured by wavelength dispersive X-ray microanalysis in the film after laser irradiation with 5.0W/cm2 for 1.0h. These photoinduced changes in the samples are dependent on the power density and intensity profile of the laser beam. Using a Lloyd’s mirror setup for continuous wave holography it was possible to record holographic gratings with period from 500nm up to 20μm and depth profile of ∼50nm in the films after laser irradiation with 5.0W/cm2 for 1h. Real-time diffraction efficiency measurements have shown that ultraviolet irradiation induces first a refractive index grating formation, and after this, the photocontraction effect takes place generating an irreversible relief grating. Diffraction efficiency up to 10% was achieved for the recorded gratings. 3D-refraction index measurements and atomic force microscopy images are presented.

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se 8 ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyze the photo-induced instability in amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photo-induced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination.

Real-time in situ measurements of photoinduced volume changes in chalcogenide glasses

To understand the dynamics of photoinduced volume changes in chalcogenide glasses, we developed the real-time in situ surface height measuring system based on optoelectronic and image processing technologies. This new method provides us a record of in situ volume changes in chalcogenide glasses during illumination.

Recording of Blue Light-Induced Energy and Volume Changes within the Wild-Type and Mutated Phot-LOV1 Domain from Chlamydomonas reinhardtii

The time-resolved thermodynamics of the flavin mononucleotide (FMN)-binding LOV1 domain of Chlamydomonas reinhardtii phot (phototropin homolog) was studied by means of laser-induced optoacoustic spectroscopy. In the wild-type protein the early red-shifted intermediate LOV 715 exhibits a small volume contraction, Δ V 715 = −1.50 ml/mol, with respect to the parent state. LOV 715 decays within few μs into the covalent FMN-Cys-57 adduct LOV 390, that shows a larger contraction, Δ V 390 = −8.8 ml/mol, suggesting a loss of entropy and conformational flexibility. The high energy content of LOV 390, E 390 = 180 kJ/mol, ensures the driving force for the completion of the photocycle and points to a strained photoreceptor conformation. In the LOV-C57S mutated protein the photoadduct is not formed and Δ V 390 is undetected. Large effects on the measured Δ Vs are observed in the photochemically competent R58K and R58K/D31Q mutated proteins, with Δ V 390 = −2.0 and −1.9 ml/mol, respectively, and Δ V 715 ≈ 0. The D31Q and D31N substitutions exhibit smaller but well-detectable effects. These results show that the photo-induced volume changes involve the protein region comprising Arg-58, which tightly interacts with the FMN phosphate group.

The role of photogenerated charges in photoinduced expansion of chalcogenide glasses

A recently formulated electrical model of photoinduced volume expansion caused by the Coulomb interaction between photogenerated carriers in chalcogenide glasses is examined quantitatively. It is shown that, depending upon photoexcitation conditions, the model predicts either expansion or contraction. However, the calculated expansion and contraction are both far below experimentally observed values. This implies an important role of charge-density fluctuations in photoinduced structural phenomena observed in chalcogenide glasses.

Intramolecular structural model for photoinduced plasticity in chalcogenide glasses

Selected spectral features of Raman spectra of glassy ${\mathrm{As}}_{2}{\mathrm{S}}_{3}$ subjected to elongation stress and sub-band-gap light illumination are analyzed and compared with polarization-dependent information obtained from the bulk glass at room temperature and near the glass transition temperature. The data are suggestive of specific structural changes which involve the transformation of atomic arrangements from realgarlike ${\mathrm{As}}_{4}{\mathrm{S}}_{4}$ molecules, originally present in virgin (untreated) fibers, into planar orpimentlike clusters. Implications of these atomic rearrangements to the incipient photoinduced fluidity---the onset of plastic deformation---in ${\mathrm{As}}_{2}{\mathrm{S}}_{3}$ glass are discussed. Kinetics of photoinduced plastic changes is compared to that of Raman spectra changes, revealing a qualitative similar behavior. An approximate estimation of the relative contribution of intermolecular rearrangements and the intramolecular structural mechanism proposed in this paper has revealed that the latter is responsible for almost 30% of the photoinduced elongation of the fiber's length at room temperature. The proposed mechanism can as well serve as rationale for understanding the photoinduced volume expansion observed in chalcogenide glasses.

Photoinduced volume changes related to photoinduced anisotropy in chalcogenide glasses

We report metastable photoinduced volume expansion (hyperexpansion) of up to 10 µm in height and several millimetres in diameter for the chalcogenide glass As 2 S 3 . This is an order of magnitude larger than reported to date in any material, including chalcogenide glasses. We suggest a model for the effect and propose that its origin is similar to the origin of photoinduced anisotropy in glassy chalcogenides proposed earlier. The model is based on the anisotropic chemical bonding found in most chalcogenide glasses; the anisotropy arises from the fact that chalcogen atoms exhibit both strong covalent bonding and weak van der Waals bonding. The origin of the photoinduced volume changes (expansion and contraction) is in a thermally induced diffusion, or athermal tunnelling, of photoexcited chalcogen atoms, with possible subsequent sublimation from the sample surface. The primary role of hyperpolarizable van der Waals bonds is supported by the absence of photoinduced volume changes in the chalcogenide glass Ga 1.4 La 0.6 S 3, in which chalcogen atoms do not bond via van der Waals bonds.

The influence of a photoinduced volume charge on the weak-radiation-induced interband photoproduction of carriers in semiconductors with impurity recombination centers

Analytical expressions for the average concentrations of photogenerated electrons and holes are obtained outside the framework of the quasineutrality approximation in the case of a strong surface recombination in the presence of a single-level impurity recombination centers. It is shown that the quasineutrality approximation becomes incorrect with decreasing sample size in the direction of irradiation.

Photoinduced volume expansion and contraction in a-Si:H films

Photoinduced volume expansion and contraction in hydrogenated amorphous silicon (a-Si:H) films have been studied. Photoinduced volume contraction seems to occur in a-Si:H films having hydrogen contents less than ∼5 at.%. The wavelength dependence of the photoinduced volume expansion indicates that one of driving forces of this phenomenon may be relaxation energy of photoexcited carriers. The reciprocity relation between light intensity and illumination time does not hold both in photoinduced volume expansion and photodegradation. The mechanism of the photoinduced volume changes is also discussed.