Mara Reinfelde

Holographic recording in amorphous chalcogenide semiconductor thin films

Abstract A detailed study of the amorphous As–S–Se and As2S3 films as recording media for optical holography and electron beam lithography is presented. The results of R&D on resist based on the amorphous As–S–Se thin films for manufacturing of embossed holographic labels are discussed. The holographic recording of transmission and Bragg gratings was studied.

Subwavelength-period gratings in amorphous chalcogenide thin films

Thin films of amorphous chalcogenide semiconductor As2S3 and As?S?Se systems were studied to record refractive-index and surface-relief modulated gratings with a period of 0.15?1??m. The period and profile of the surface-relief modulated gratings were measured by AFM. The polarization properties of refractive-index modulated gratings are shown.

Polarization holograms and diffraction anisotropy in amorphous chalcogenides

Polarization hologram recording based on the effect of photoinduced anisotropy (PA) is reviewed, focusing on amorphous chalcogenides (ACh). Possible PA mechanisms in ACh are considered. Polarization holographic grating recording in amorphous As?S?Se (a-As?S?Se) films is experimentally studied and analysed in comparison with scalar recording. It is holographically established that linearly polarized 632.8 nm light produces photoinduced anisotropy and the chalcogen related D+, D? centre reorientation and generation mechanism is proposed. It is used to explain the observed peculiarities of polarization (vector) recording in comparison with scalar recording based on photoinduced structural changes: much lower diffraction efficiency (4 ? 10?3?% versus?4%), much larger specific recording energy (6400?J/(cm2?%) versus 20?J/(cm2?%)), difference in spatial frequency response, instability (vector hologram lifetime of about two days versus practically permanent scalar holograms), the absence of hologram self-enhancement (present in scalar recording), and near perfect reversibility. It is also experimentally found that light diffraction from the polarization holographic gratings in a-As?S?Se films is indeed anisotropic since the readout wave polarization diffracted in the minus first order is changed in such a way that the linear signal wave polarization orthogonal to the reference wave polarization is reconstructed. The results obtained are discussed.

Hologram recording in azobenzene oligomers

Elementary hologram (holographic grating) recording and their coherent optical erasure have been experimentally studied in azobenzene oligomer (ABO) layers differing by their chemical composition, matrices and by the connection type of azobenzene chromophores to the matrix (dispersed or covalently bound). The best holographic parameters (7.9% diffraction efficiency and 86 J/cm2 specific recording energy) were achieved in the samples with covalent bonding to the matrix. Vector recording is also possible. Recording is unstable and reversible. The coherent optical erasure studies have shown its efficiency dependencies on the initial diffraction efficiency, erasing beam intensity and grating period which are different for three groups of ABO samples. The conclusion is made that recording is due to the photoinduced alignment of the azobenzene chromophores followed by refractive index changes. These are the first results and further studies are in progress.

Photoinduced anisotropy and holographic recording in amorphous chalcogenides

The effect of photoinduced anisotropy and its application to vector hologram recording is reviewed focusing on amorphous chalcogenides. Vector holographic grating recording in amorphous As-S-Se(a-As-S-Se) films is experimentally studied and analyzed in comparison with scalar recording. It is holographically established that a linearly polarized 632.8 nm light produces photoinduced anisotropy and the chalcogen related D+, D- center reorientation and generation mechanism is proposed. It is used to explain the observed peculiarities of vector recording in comparison with scalar recording based on photoinduced structural changes: much lower diffraction efficiency (4 X 10-3% versus 4%), much larger specific recording energy [6.4 kJ/(cm2%) versus 20 J/(cm2%)], difference in spatial frequency response, instability (vector hologram lifetime of about two days versus practically permanent scalar holograms), the absence of hologram self-enhancement (present in scalar recording), near perfect reversibility. It is also experimentally found that vector holograms in a-As-S-Se films indeed reconstruct the signal wave polarization but only in the minus first diffraction order. It is also shown that photoinduced anisotropy also contributes to the scalar hologram recording in amorphous chalcogenides stimulating it by means of subbandgap readout light and enabling a subbandgap recording.

Polarization holographic recording in Disperse Red1 doped polyurethane polymer film

In this report holographic recording of polarisation and surface relief gratings in Disperse Red 1 (DR1) doped polyurethane polymer films was studied. In this material DR1 is chemically bounded to polyurethane polymer main chain. Polarization holographic recording was performed by two orthogonal circularly polarized 532 nm laser beams. Photoinduced birefringence is a precondition for polarization holograms recording, therefore a detailed study of a photoinduced birefringence and changes of optical properties was performed. The lasers with wavelengths of 375nm, 448nm, 532 nm and 632.8 nm were used as pumping beam for sample excitation. The photoinduced birefringence Δn was measured at 532 nm and 632.8 nm wavelengths. The photoinduced birefringence dependence on the pumping beam wavelength and intensity was investigated. Surface relief grating (SRG) formation was observed during polarization holographic recording process. A profile of SRG was studied by AFM. A relationship between SRG formation and photoinduced birefringence has been discussed.

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.

Effects of light polarization and crystal orientation on the holographic recording efficiency in doped LiNbO 3 crystals

The holographic recording efficiency in doped LiNbO3 crystals has been studied both experimentally and theoretically depending on the type of dope, on the recording geometry and on the polarizations of the recording and readout light.. The studied crystals can be arranged in the following order by their efficiency: LiNbO3:Cu, LiNbO3:Fe (yet having a smaller thickness and donor concentration than LiNbO3:Cu), LiNbO3:Fe+Cu, LiNbO3:Fe+Ti, LiNbO3:Ti. It was found that the recording geometry with the holographic grating vector along the optical axis is much more efficient than in the perpendicular configuration.This fact is in accordance with the photorefraction theory based on photogalvanic and linear electro-optic effects (PGE-LEO theory). Other recording mechanisms are active, too, but much less eficient. The recording efficiency polarization dependence is mainly determined by dopes.It is different in the cases of Fe and Cu impurities. It also follows from our studies that photoconductivity along the optical axis is much larger than in the perpendicular direction.

Holographic recording of subwavelength structures in amorphous chalcogenide thin films

Holographic recording of surface-relief and refiactive-index-modulated gratings with a period ranging from 0.15 to 1 μm in thin amorphous chalcogenide films of As-S-Se and As2S3 is studied. The profile of the surface-relief gratings is measured by atomic force microscopy. By structuring the As-S-Se films with the subwavelength-period surface-relief gratings their reflectivity is decreased from 25-30% to less than 2%. In the refractive-index-modulated As2S3 films polarization effects are revealed. It is shown that the angular selectivity of the holographic recording in amorphous chalcogenide thin films can be substantially improved by decreasing the grating period.

Angular selectivity of thin gratings

The angular selectivity of thin gratings is studied both experimentally and theoretically. The concepts of thick and thin gratings are analyzed. Thin holographic gratings recorded in a-As-S-Se films have exhibited pronounced and oscillatory diffraction efficiency angular dependences. These results are explained by the obliquity factor in Fresnel-Kirchhof diffraction integral and by finite beam and grating sizes. It is also shown that oscillatory diffraction efficiency angular dependences, most probably, arise due to the interference of diffracted waves of different orders because dephasing can be significant for small grating strengths and large enough readout angles. Fabry-Perot resonator effect can contribute as well. Thick gratings in a-As2S3 and a-As-S-Se films are also studied for comparison. The conclusion is made that normally all gratings possess angular selectivity and existing criterions underestimate the angular selectivity of thin gratings because they neglect factors other than dephasing.