Hongpeng Liu

Diffusional enhancement of volume gratings as an optimized strategy for holographic memory in PQ-PMMA photopolymer

The dark enhancements of diffraction efficiency in single and multiple gratings are investigated theoretically and experimentally in phenanthrenequinone doped poly-(methyl methacrylate) materials. It is demonstrated a possibility to improve holographic characteristics of the material via the enhancement. Nearly 17-fold increment of diffraction efficiency is observed after exposure. The dependences of PQs concentration on the rate and increment of dark enhancement are achieved quantitatively. And the enhancement in multiplexing is presented as a simple and efficient method to improve response of the material and homogeneity of diffraction efficiency. PQs diffusion and enhancement process of refractive index modulation are simulated by a diffusion model for describing enhancement dynamics qualitatively and quantitatively. This study provides a significant foundation for the application of dark enhancement in holographic storage.

Mutual diffusion dynamics with nonlocal response in SiO 2 nanoparticles dispersed PQ-PMMA bulk photopolymer

Mutual diffusion dynamic model with nonlocal response was proposed to describe the grating formation in SiO2 nanopraticles dispersed PQ-PMMA photopolymer. The mutual-diffusion physical mechanism between PQ and SiO2 nanoparticles is analyzed. The grating formation kinetics and dynamic redistribution of components is simulated by introducing the nonlocal effect. In experiment the dark enhancement of grating after short exposure and the photopolymerization under consecutive exposure are measured. The improvement of SiO2 nanoparticles for the holographic properties is achieved quantitatively. Finally the comparison of theoretical and experimental results is presented for understanding the mutual-diffusion characteristics.

Study of effective optical thickness in photopolymer for application

The impact of the effective optical thickness of a photopolymer on its holographic performance was studied. To overcome the attenuation of gratings for a better uniformity, a multilayer approach was introduced, by adjusting concentrations of dye along the depth of photopolymer to compensate the attenuation of recording light due to absorption. Multilayer photopolymers with thicknesses over 500 μm were designed, fabricated, and characterized experimentally, exhibiting better Bragg selectivity. More holograms were stored in multilayer material by angular multiplexing, and the cumulative grating strength was enhanced, leading towards larger holographic storage capacity.

Holographic humidity response of slanted gratings in moisture-absorbing acrylamide photopolymer

Holographic humidity response is characterized in detail using transmission and reflection geometry in moisture-absorbing acrylamide photopolymer. The diffraction spectrum and its temporal evolution at various relative humidity are measured and analyzed. The quantitative relations between relative humidity and holographic properties of slanted gratings are determined. The responsibility of holographic gratings for various relative humidity is observed by the spectrum response of gratings. The extracted humidity constants reflect the applicability of reflection and transmission gratings at different humidity regions. The humidity reversibility experiment is achieved for confirming repeatability of the sensor. These experiments provide a probability for improving the applicability of a holographic humidity sensor. Finally, the extended diffusion model is derived by introducing the expansion coefficient to describe the dynamic swelling process. This work can accelerate development of the holographic sensor and provide a novel strategy for exploring the swelling mechanism of photopolymer.

Enhancement of spectrum strength in holographic sensing in nanozeolites dispersed acrylamide photopolymer.

Holographic sensing of organic vapor is characterized at transmission and reflection geometries in ZSM-5 nanozeolites dispersed acrylamide photopolymer. Nano-zeolites as absorption medium are dispersed into the polymer to enhance the absorptivity to organic vapor. Obvious enhancements of spectrum strength are observed during the sensing process. Two primary factors causing the enhancement, absorption of nanozeolites and photopolymerization induced by broadband white light, are analyzed experimentally. Significant increment provides a quick and intuitive identification strategy for holographic sensing. Accompanying with the wavelength blue-shift, the shrinkage of sample is measured quantitatively under homogeneous white light. It is further demonstrated that the significance of nanozeolites absorption. Finally a theoretical model with mutual diffusion is used to simulate the swelling process. This study provides significant foundation for the application of holographic sensor.

Improvement of holographic thermal stability in phenanthrenequinone-doped poly(methyl methacrylate-co-methacrylic acid) photopolymer

Experimental studies of holographic thermal stability in phenanthrenequinone (PQ)-doped poly(methyl methacrylate-co-methacrylic acid) [P(MMA-co-MAA)] photopolymers are presented. A possibility to improve the thermal stability of holograms is demonstrated by doping methacrylic acid (MAA) into the poly(methyl methacrylate) (PMMA) polymer matrix. MAA as a copolymerization monomer can form a more stable polymer matrix with methyl methacrylate (MMA) monomer and increase average molecular weight of photoproducts, which finally depress the diffusion of photoproduct. The optimized MAA concentration copolymerized into P(MMA-co-MAA) polymer matrix can bring nearly a months lifetime of gratings, which is obviously an improvement over the usual PQ-PMMA material under thermal treatment.

Two-way shift of wavelength in holographic sensing of organic vapor in nanozeolites dispersed acrylamide photopolymer.

Holographic sensing of alcohol organic vapor is characterized in detail at transmission and reflection geometries in Y nanozeolites dispersed acrylamide photopolymer. The two-way shift of the diffraction spectrum and its temporal evolution with various vapor concentrations are measured. Obvious blueshifts of diffraction spectrum peaks are observed and analyzed in two recording geometries. The competition mechanism between decreasing the average refractive index and swelling the grating fringe space is proposed for exploring the wavelength shift mechanism. In the reflection grating, as organic vapor increases, the redshift after the blueshift of the wavelength peaks are observed clearly. We further demonstrate the significance of this competition mechanism. In the low concentration region, at transmission <700  ppm and reflection <400  ppm in nanozeolites dispersed polymer, the blueshift of the wavelength is a significant factor in identifying an organic vapor with a low refractive index. These experimental results provide a probability for improving the applicability of a holographic sensor. This work can accelerate the development of the holographic sensing strategy and provide a novel identification method for organic vapor.

Photorefractive properties of doubly doped lithium niobate crystals with Zr and Fe

Zr:Fe:LiNbO3 crystals with various of [Li]/[Nb] ratios were grown by Czochralski technique from melts having compositions varying between 46-58 mol% Li2O. The compositions of the growth crystals were determined by the inductively coupled plasma technique (ICP-AES). The defect structures were analyzed by UV-vis spectroscopy. The photorefractive properties of the Zr:Fe:LiNbO3 were investigated by using two-beam coupling experiment. The results showed that as the [Li]/[Nb] ratio increased, the diffraction efficiency decreased, but photorefractive sensitivity increased , and the response time decreased.

Nonvolatile hologram storage properties in In:Fe:Cu:LiNbO3 crystals

We studied some factors affecting the properties relevant to nonvolatile hologram storage by using two-color recording scheme in In(0,1mol.%):Fe:Cu:LiNbO3 crystals. Two-color recording is achieved by use of 488nm blue light acts as the sensitized light and 633nm red light acts as the recording light. First, the effect of In3+ doping on the nonvolatile photorefractive properties was investigated experimentally. Next, the influences of the pre-illumination processing, the red-to-blue intensity ratio as well as the oxidation-reduction treatments on the nonvolatile photorefractive properties of In(1mol.%):Fe:Cu:LiNbO3 crystal were studied in detail.

Improvement of temperature-induced spectrum characterization in a holographic sensor based on N-isopropylacrylamide photopolymer hydrogel

A novel thermo-sensitive N-isopropylacrylamide photopolymer was developed for improving the temperature and humidity responses of holographic sensors. Diffraction spectra of holographic volume gratings recorded in the materials were characterized to explore the sensing response capacity. A dependence of peak wavelength on the temperature was observed and provided a quantitative strategy for holographic sensing applications. Expansion of the humidity range induced a strong extension of wavelength shift. Finally, the temperature response reversibility was demonstrated experimentally. Our sensing results were completely different from the reported typical acrylamide polymer system. Compared with the former, we obtained a more sensitive temperature response and an evident shift expansion (>200  nm) at a relative humidity of 70% or higher. These results can obviously improve the thermo-sensitivity of a holographic sensor and expand the practical application area of the holographic sensing strategy.