Victor Weiss

Holographic recording and all-optical modulation in photochromic polymers

Polymers doped with photochromic spirooxazine and spiropyran dyes have been investigated for real-time holographic recording. The temporal holographic response was found to depend strongly on optical recording configurations and on the recording beam intensities. The exposure sensitivities that were required for maximal diffraction efficiency were 250 mJ/cm2 for the polymers doped with spirooxazine and 650 mJ/cm2 for those doped with spiropyran. All-optical modulation of the holographic gratings with 2000 lines/mm is possible by modulation of a separate excitation beam of incoherent UV radiation. The modulated gratings can be stored for relatively long times or erased immediately.

Planar configuration for image projection.

A flat panel, compact virtual image projection display is presented. It is based on a light- guided optical configuration that includes three linear holographic gratings recorded on one planar transparent substrate so as to obtain a magnified virtual image for a small input display. The principles of the projection display, unique design, and procedures for experimentally recording an actual planar configuration are presented, along with evaluation results. The results reveal that a field of view of +/-8 degrees can be readily achieved at a distance of 36 cm, making such planar configurations attractive for head-up displays.

Designing holographic lenses with different recording and readout wavelengths

A method for designing and recording transmission holographic lenses, having low aberrations and high diffraction efficiencies, in the presence of a recording–readout wavelength shift, is presented. The method is based on a recursive design technique, in which the final hologram is recorded with complex wave fronts that are derived from intermediate holograms. The design is illustrated for a lens with an f-number of 2.5 and a large offset angle, recorded at 488 nm and read out at 633 nm. A nearly diffraction-limited spot size and an efficiency of >80% are measured.

Holographic recording by photodeposition techniques

Photodeposition is an emerging new thin-film material deposition and optical image patterning technique with photographic recording characteristics. In the past, lasers and incoherent light sources were exploited for photodeposition of various materials in micropatterns. Here we report that photodeposition can also be used for recording holographic relief gratings of 2000 lines/mm and periodically modulated depths of 1 to 20 nm.

Photopolymeric holographic recording media: in-situ and real-time characterization

Photoactive polymer matrices are investigated by and for holographic recording at 514 nm. The photopolymers are based on acrylamide monomers, which are dissolved together with xanthine dyes and other additives in polyvinylalcohol. Dry plastic coatings are obtained by casting the aqueous polymer solutions on glass substrates. Photorecording occurs in real-time and in-situ, without any post-exposure processing. This paper describes the influences of chemical additives on the photorecording process. Specifically, the addition of diphenyl iodonium chloride (DPI-Cl), in conjunction with triethanolamine (TEA), significantly increases the exposure sensitivities, by a factor of over three (to about 15 mJ/cm2). This sensitizing effect is shown to originate from a superadditive effect between TEA and DPI-Cl. The mechanism of the superadditive effect is discussed by a proposed reaction model. The exposure sensitivities are also significantly influenced by the PVA binder parameters, such as average molecular weights and degree of hydrolization. The present formulations produce large enough refractive index modulations, so that very high diffraction efficiencies (DEs > 90%) are obtained. The dynamic range of refractive index modulations was increased from 0.014 to 0.018 by the addition of glutaraldehyde crosslinking, which also improves the dimensional stability of the holograms.

Grating microstructures in photoactive polymers by laser interference patterns

Abstract The formation of grating microstructures in new photopolymer formulations are investigated by holographic recording. Holographic recording in cast polymer layers occurs in real-time and in-situ, meaning that no wet-chemical or post-thermal/photochemical processing are required. The hydrophilic photopolymers are based on acrylamide monomers, which are dissolved together with photoactive components, such as the sensitizing dye eosin and the new charge transfer agent diphenyl iodonium (DPI) salt with triethanolamine, in polyvinyl alcohol. The addition of DPI significantly increases the exposure sensitivity from about 35 to 12 mJ/cm2, i.e. by a factor of about three, without affecting the maximum obtainable diffraction efficiency (DE). The influences on the storage mechanism by other additives, such as cross-linking agents and plasticizers are also investigated. The introduction of the cross-linking agent bisacrylamide stabilizes the formed structures against thermal and environmental relaxations. The introduction of plasticizers have a significant effect on the spatial modulation of the refractive index in the recorded grating patterns. Large enough refractive index modulations can be obtained, so as to yield very high DEs of about 90%. The angular-DE and thickness-DE responses closely follow the theoretical predictions, according to the coupled-wave theory. These photopolymer layers have potential in photonics applications, such as holographic optical elements and waveguide structures.

Photodeposition of thin cadmium films from CdS colloid solutions

Abstract A new liquid-phase photodeposition process of cadmium thin films from aqueous cadmium sulphide colloid solutions has been developed. In this process, the photoexcitation of CdS colloid particles causes the formation of elementary cadmium which is partly precipitated in the bulk of the photoreactor and partly adsorbed on the reactor optical window, creating thin films. The initial photodeposition growth rates are small, indicating an adsorption-controlled process. The continuation stage growth is much faster and also thermally activated. The quantum yield increases by several orders of magnitude from visible (514 nm) to UV (366 nm) radiation. The adsorption affinity of photo-precipitated cadmium particles is highest for cadmium material sites on the substrate. Onto UV pre-deposits, ultrathin films could efficiently be grown with visible argon laser lines. Preliminary image recordings indicate the potential use for high resolution cadmium thin film pattern applications.

Storage mechanism of volume phase holograms recorded in silver halide emulsions

A relatively simple model for elucidating the holographic storage mechanism in bleached silver halide recording materials is developed. In this model the storage of holographic gratings includes three separate contributions that are due to silver halide grains, a gelatin matrix, and voids in the gelatin matrix. Linear refractive-index-modulation-versus-exposure responses are assumed for the individual contributions, which are summed to produce the overall diffraction-efficiency (DE)-versus-exposure response. Calculated and experimental results reveal that, through reduction of one or more of the contributions to the holographic storage in a composite emulsion system through chemical means, the maximum achievable DE’s increase and may reach 100% in the case of a homogeneous and lossless phase grating. Specifically, the addition of sulfite or triethanolamine to the processing formulations in a reversal bleach processing procedure or in a silver halide sensitized gelatin procedure had the effect of increasing the experimental DE’s. The corresponding calculated DE-versus-exposure responses indicate that these additives suppressed the gelatin tanning or matrix void contributions.

Design and experiments of planar optical light guides for virtual image displays

Planar optical light guides that are suitable for compact, relatively large virtual image projection displays, of either see-through or non-see-through capabilities, are presented. Such light guides are comprised of three diffractive elements that are recorded on a single substrate. The basic principles, design methods, experimental procedures, calculated as well as experimental results are presented. The results reveal that a relatively large field of view and uniform luminance over the entire output image can be obtained, even when the distance from the light guide to the viewer is relatively large.

Spatial-frequency response of holographic gratings photodeposited from inorganic colloids

Surface photodeposition is a photon-assisted process by which thin films are formed on substrates immersed in colloid solutions. We experimentally evaluate the resolution capabilities of the photodeposition process with amorphous selenium colloids by recording holographic gratings at different spatial frequencies, up to 2200 lines/mm. The experimental diffraction efficiencies are analyzed in terms of a theoretical model, which relates the spatial-frequency response to optical recording parameters and colloid particle sizes. The maximal experimental diffraction efficiency reaches 13% with a spatial frequency of f = 1100 lines/mm. The diffraction efficiencies decrease monotonically with spatial frequency, and drop to half of the maximal diffraction efficiency at f ≈ 1500 lines/mm. These resolution capabilities are achieved with colloid particle sizes extending up to 80 nm. The theoretical derivation indicates that to obtain spatial frequencies above 3000 lines/mm, one should restrict the colloid particle size to a(max) ≤ 30 nm.