Hongyue Gao

Investigation of multiple holographic recording in azo-dye-doped nematic liquid-crystal film

We report multiple holographic recording and optical address recognition schemes in Methyl Red-doped liquid-crystal film without an applied electric field. Ten gratings are recorded at a single location of a 5 mm2 area by using optical multiplexing methods, and the diffraction efficiencies are studied for multiplexed gratings. Diffraction behavior of angular and peristrophic multiplexed gratings is discussed, and two methods that involve recording angles and peristrophic rotation angles are presented for optical recognition.

Electrically switchable multiple volume hologram recording in polymer-dispersed liquid-crystal films

Multiple Bragg hologram recording was investigated in trimethylolpropane triacrylate based polymer-dispersed liquid-crystal thin films. Ten volume holograms were stored at a single location of less than 0.8u2002mm2 area with the diffraction efficiencies of 6%–15%, by a simplified method of peristrophic multiplexing, and the formation process of multiple volume holograms was presented. Furthermore, volume hologram capacity was studied both experimentally and theoretically in this film. Based on the maximum of Δn, ∼2×10−2, the theoretical capacity of a location could be improved to 25 Bragg holograms with the relatively high diffraction efficiency of 1%, and the holographic storage density of more than 20u2002Gbits/in.2 was achieved just by peristrophic multiplexing. Finally, electrically switchability for multiple holograms with a low threshold voltage of ∼3u2002V/μm was realized, which demonstrates that this material is a potential medium for electrically controlled holographic storage or diffraction device.

Dynamic characterizations of high diffraction efficiency in volume Bragg grating formed by holographic photopolymerization

Volume Bragg grating with 96% diffraction efficiency (DE) was efficiently formed by holographic photopolymerization in blend syrup of photocurable trimethylolpropane triacrylate monomer and nematic liquid crystal. The formation dynamics of the composite gratings was quantitatively characterized under the frame of one-dimensional reaction-diffusion model with a revision of individual decay constants for monomer diffusion and reaction. Initial parameters of diffusion and reaction were analytically determined from the measured first order DE at the beginning stage. Evolutions of the DE, both in curing and postcuring periods, were excellently simulated, especially with postcuring reaction been taken into account.

Analytical rates determinations and simulations on diffusion and reaction processes in holographic photopolymerization

An analytical method is proposed to determine the initial diffusion and reaction coefficients for monomer molecules in holographic photopolymerization processes. Those parameters can be obtained directly from the measured first order diffraction efficiency at the onset stage in the formation of volume Bragg gratings. Simulations, according to these parameters, on the evolution of the diffraction efficiency, based on one-dimensional reaction-diffusion model, were well consistent with experimental data in our trimethylolpropane triacrylate based monomer and liquid crystal composite material. It is shown that diffusion and reaction both play important roles in postcuring process.

Reactive radical facilitated reaction-diffusion modeling for holographic photopolymerization

A phenomenological concentration of reactive radical is proposed to take the role of curing light intensity in explicit proportion to the reaction rate for the conventional reaction-diffusion model. This revision rationally eliminates the theoretical defect of null reaction rate in modeling of the postcuring process, and facilitates the applicability of the model in the whole process of holographic photopolymerizations in photocurable monomer and nematic liquid crystal blend system. Excellent consistencies are obtained in both curing and postcuring processes between simulated and experimentally measured evolutions of the first order diffraction efficiency of the formed composite Bragg gratings.

Gap thickness retrieval on air etalon by using a focused incoherent white-light beam

In a transmittance spectrum of air etalon composed of ITO-coated glass plates, stronger spectral modulation is observed by using a focused white-light incident beam compared with that by using a conventionally collimated beam. The light source is a single commercial light-emitting diode. An angular integrated multiple-beam Fabry-Perot model is proposed that is well fitted to the measured transmittance spectra. The gap thickness of the air etalon is sensitively retrieved and mapped at a lateral surface resolution of about 1 mm, and the unparallelness of the etalon angled as low as several microradians is thereafter estimated.