Holographic Polymer Dispersed Liquid Crystal Research Articles

P‐38: See‐through Optical Configuration with Transmitted H‐PDLC

AbstractA near‐to‐eye see‐through optical configuration is proposed based on dual transmitted grating fabricated by interference in Holographic Polymer Dispersed Liquid Crystal (H‐PDLC), which provides compact, light, and low cost solution for see‐through display. The transmitted gratings attached at both the edge and center of the glass substrate acted as in‐coupling and out‐coupling elements respectively. A prototype with thick glass substrate for see‐through configuration was demonstrated, which could couple the incident successfully into substrate and then coupled the beam outward by the other grating.

Lasing from colloidal InP/ZnS quantum dots

High-quality InP/ZnS core-shell nanocrystal quantum dots (NQDs) were synthesized as a heavy-metal-free alternative to the gain media of cadmium-based colloidal nanoparticles. Upon UV excitation, amplified spontaneous emission (ASE) and optical gain were observed, for the first time, in close-packed InP/ZnS core-shell NQDs. The ASE wavelength can be selected by tailoring the nanocrystal size over a broad range of the spectrum. Moreover, the optical gain profile of InP/ZnS NQDs was matched to the second order feedback of holographic polymer-dispersed liquid crystal gratings, leading to the very first demonstration of an optically-pumped, nanocrystal laser based on InP/ZnS core-shell NQDs.

Biodegradable Holographic Polymer-Dispersed Liquid Crystal

The a-D-glucose was chemically modified with an allyl isocyanate (MG) and introduced into the polymer matrix for holographic polymer-dispersed liquid crystal (HPDLC), and the effects were studied in terms of morphology, grating formulation dynamics and electro-optical and biodegradable properties. Phase separation and diffraction efficiency increased at low content of (MG ≤ 4 wt%), while a rapid increase in crosslink density entrapped the LC droplets within the polymer to give poor phase separation, small droplet size, and low diffraction efficiency at high content. The HPDLC film was driven only with the addition of MG due to the increased droplet size with a minimum driving voltage of 18 V at 6.0 wt% MG. With the addition and an increasing amount of MG, the biodegradation of the composite film in a buffer solution was significantly increased in proportion to its amount.

H-PDLC based waveform controllable optical choppers for FDMF microscopy

An electrically waveform controllable optical chopper based on holographic polymer dispersed liquid crystal grating (H-PDLC) is presented in this paper. The theoretical analyses and experimental results show that the proposed optical chopper has following merits: (1) controllable waveform, (2) no mechanical motion induced vibrational noise, and (3) multiple-channel integration capability. The application of this unique electrically controllable optical chopper to frequency division multiplexed fluorescent microscopy is also addressed in this paper, which has the potential to increase the channel capacity, the stability and the reliability. This will be beneficial to the parallel detection, especially for dynamic studies of living biological samples.

Structural investigations of multiple gratings recorded in polymer-dispersed liquid crystals film by holography

Angular multiple transmission gratings were recorded simultaneously in acrylate-based polymer-dispersed liquid crystals film by holography. The micro-structure of the gratings was investigated through atomic force microscopy and scanning electron microscopy, and we found that with the exception of the holographic volume structure, each grating has a surface relief with the depth of tens of nanometres. The effects of surface relief on the optical and electrical performances of the multiple gratings were tested, and the results indicate that the diffraction efficiency of multiple gratings is mainly determined by the refractive index modulation of the volume structure; however, the diffraction energy of surface relief is less than 5% of the total. The results show no evident influence of the surface relief on Bragg angle and drive electric field. Through experimental analysis, it is also shown that volume shrinkage of about 2% for the gratings occurs after exposure. In addition, multiple images were recorded in this work, the readout and erasing performances were tested, and the transmission of the sample in visible spectrum was detected; our results indicate a prospective application for holographic polymer-dispersed liquid crystal film in displays or data storage.

Effect of surface rubbing alignment on electro-optical properties of holographic polymer dispersed liquid-crystal gratings

In order to obtain holographic polymer dispersed liquid crystal( HPDLC) grating with high diffraction efficiency and low threshold, the effect of parallel rubbing alignment on electro-optical properties of HPDLC grating is investigated. According to the theoretical analysis, the improvement of phase separation structure and the lower order difference of the liquid crystal droplets are essential for perfect electro-optical properties of the grating. The phase separation is significantly improved due to the diffusion match between monomers and liquid crystals in the case of parallel alignment treatment. So the diffraction capability is enhanced while the driven voltage is reduced obviously. Moreover, the order difference of liquid crystal droplets is reduced because of the uniform liquid crystal alignments and consequently the scattering loss is reduced greatly. Experimental results indicate that the diffraction efficiency of grating with rubbing alignment increases from 75.6% (traditional HPDLC grating) to 98.1% and the driving voltage is lowered from 9.2 V/μm to 4.8 V/μm. And also, only in the case of certain surface rubbing strength, the HPDLC with high diffraction efficiency and low threshold will be realized.

Low driving voltage holographic polymer dispersed liquid crystals with chemically incorporated graphene oxide

A minute amount of allyl isocynate functionalized graphene oxide (MGO, 0.05–0.25%) has been covalently incorporated into the polyurethane acrylate based matrix of holographic polymer dispersed liquid crystals (HPDLC), and the effects were studied in terms of compound viscosity, grating kinetics and morphology, diffraction efficiency (DE), and electro-optical properties of the HPDLC films at two levels of laser writing power. MGO at very low content (0.05%, 0.10%) significantly reduced the compound viscosity, increased the polymerization rate, limited droplet coalescence, and increased the diffraction efficiencies (74%, 64%) over the MGO-free compound (48%). Polymer conductivity and local electrical field increased with increasing MGO content to give a large decrease in operating voltage in accordance with a theoretical prediction. With high laser intensity, well defined grating structure with small droplet size was seen from the SEM morphology to give a fast grating formation with high saturation diffraction.

Distributed-feedback lasing from dye-doped holographic polymer dispersed liquid crystal transmission grating

In this paper,we report the fabrication of dye-doped holographic polymer dispersed liquid crystal(HPDLC) transmission grating and the property of distributed-feedback lasing from dye-doped HPDLC transmission grating. Under the excitation of a frequency-doubled Nd:yttrium-aluminum-garnet laser operating at a wavelength of 532 nm, optically pumped lasing with narrow linewidth and low threshold was observed from a DCM dye-doped HPDLC transmission grating with the grating period of 586 nm. The results showed that the emitted lasing peak centered at about 603 nm with a full width at half maximum (FWHM) of only 1.4 nm, and the threshold pumping intensity was about 17.3 μJ, which is evidently lower than the previously reported values.

All-organic two-dimensional photonic crystal laser based on holographic polymer dispersed liquid crystals

In this paper, we report a kind of dye-doped two-dimensional photonic crystal based on holographic polymer dispersed liquid crystals (HPDLC) with a lattice constant of 582nm, which is prepared conveniently with a single step holographic exposure. Under the excitation of a frequency-doubled Nd:yttrium-aluminum-garnet laser operating at a wavelength of 532nm, optically pumped lasing with narrow bandwidth and low threshold is observed from a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dye-doped two-dimensional photonic crystal. The results show that the emitted lasing peak is centered at about 603nm with a full width at half maximum of only 0.4nm, and the threshold energy is about 22.7 J, which is evidently lower than the reported previously. The laser bandwidth decreases by a factor of three from 1.4nm to 0.4nm compared with that of the dye-doped HPDLC transmission grating. This result exhibits a bright prospect in application of tunable photonic crystal laser.

Transflective multiplexing of holographic polymer dispersed liquid crystal using Si additives

Morphology, grating formation dynamics and electro-optical performance of transflective multiplexing with holographic polymer dispersed liquid crystal (HPDLC) were investigated in the presence of silica nanoparticles (Aerosil R812 (RS) and modified Aerosil 200 (MS)) and silicon monomer (vinyltrimethoxy silane (VTMS)) by using three coherent laser beams. The addition of Si additive significantly augmented the diffraction efficiencies of transmission and reflection gratings due to the enhanced phase separation with large LC channels. The film was driven only with Si additives which are enriched at the polymer-LC interfaces. As the additive content increased, driving voltage decreased to a minimum of 30 V at 2.0 wt% VTMS. It was found that the interface modification and large droplet size are crucial to operate the film. Among the three types of Si additive, VTMS showed the highest electro-optical performance due to its low viscosity and high reac- tivity.

Sensor for monitoring the vibration of a laser beam based on holographic polymer dispersed liquid crystal films

A continuous multiple exposure diffraction grating (CMEDG) is fabricated holographically on polymer dispersed liquid crystal (PDLC) films using two-beam interference with multiple exposures. The grating is fabricated by exposing a PDLC film to 18 repeated exposure/non-exposure cycles with an angular step of ~10°/10° while it revolves a circle on a rotation stage. The structure of the sample thus formed is analyzed using a scanning electron microscope (SEM) and shows arc-ripples around the center. From the diffraction patterns of the formed grating obtained using a normally incident laser beam, some or all of the 18 recorded arc beams can be reconstructed, as determined by the probing location. Thus, it can be applied for use as a beam-vibration sensor for a laser.

Analysis of effects of oxidized multiwalled carbon nanotubes on electro-optic polymer/liquid crystal thin film gratings

This work focuses on experimentally demonstrating the modification in diffusion kinetics, formation of holographic polymer dispersed liquid crystal gratings and an improvement in its electro optic response by doping them with multi-walled carbon nanotubes. Results indicate a faster rise and fall times which is attributed to the reduction in size of the liquid crystal droplets formed and a reduction in switching voltage due to change in dielectric properties of the medium as manifested by a rise in capacitance. Real time diffraction efficiency measurements reveal a time delay in the appearance of the diffracted order due to non-participation of the nanotube in the polymerization induced phase separation process. An analysis of this effect is presented based on the Stoke-Einstein's diffusion equation incorporating shape anisotropy of the nanotubes.

Electrically tunable lasing from a dye-doped two-dimensional hexagonal photonic crystal made of holographic polymer-dispersed liquid crystals

Mode-dependent electrically tunable lasing of transverse electric (TE) and transverse magnetic (TM) modes is demonstrated in a dye-doped two-dimensional hexagonal photonic crystal made of holographic polymer-dispersed liquid crystals (LCs). Over 10 nm redshifting in lasing with multiple peaks in nonpolarizing spectra is obtained by applying external voltages up to 40 V. The splitting of lasing spectra between two perpendicular polarizations with the applied voltage is observed, which can be explained through the difference in effective refractive index of LC droplet change with the applied electric field for TE and TM modes.

Enhancement of electro-optical properties in holographic polymer-dispersed liquid crystal films by incorporation of multiwalled carbon nanotubes into a polyurethane acrylate matrix

Holographic polymer-dispersed liquid crystal (HPDLC) films were fabricated with varying amounts of multiwalled carbon nanotubes (MWCNTs) to optimize the electro-optical performance of the HPDLC films. The MWCNTs were well dispersed in the prepolymer mixture up to 0.5 wt%, implying that polyurethane acrylate (PUA) oligomer chains wrap the MWCNTs along their length, resulting in high diffraction efficiency and good phase separation. The hardness and elastic modulus of the polymer matrix were enhanced with increasing amounts of MWCNTs because of the reinforcement effect of the MWCNTs with intrinsically good mechanical properties. The increased elasticity of the PUA matrix and the immiscibility between the matrix and the liquid crystals (LCs) gradually increased the diffraction efficiency of the HPDLC films. However, the diffraction efficiency of HPDLC films with more than 0.05 wt% MWCNTs was reduced, caused by poor phase separation between the matrix and LCs because of the high viscosity of the reactive mixture. HPDLC films showing a low driving voltage (<3 V µm−1), a fast response time (<10 ms) and a high diffraction efficiency (>75%) could be obtained with 0.05 wt% MWCNTs at 40 wt% LCs. Copyright © 2010 Society of Chemical Industry

Formation of holographic memory for defect tolerance in optically reconfigurable gate arrays

Holographic polymer-dispersed liquid-crystal (HPDLC) memory using liquid-crystal composites is proposed for new optical information processing. Formation of HPDLC memory using a photomask is discussed for parallel programmability to realize fast reconfiguration of optically reconfigurable gate arrays (ORGAs). The defect tolerance of HPDLC memory is investigated to clarify the defect limitation of holographic configurations using ORGAs. Experimental results show that the noise ratio less than 15% applied to HPDLC memory rarely affects its diffraction pattern or a reconfiguration context.

Porous silicon and porous polymer substrates for optical chemical sensors

Mesoporous materials, such as porous silicon and porous polymer gratings (Bragg structures), offer an attractive platform for the encapsulation of chemical and biological recognition elements. These materials include the advantages of high surface to volume ratio, biocompatibility, functionality with various recognition elements, and the ability to modify the material surface/volume properties and porosity. Two porous structures were used for chemical and biological sensing: porous silicon and porous polymer photonic bandgap structures. Specifically, a new dry etching manufacturing technique employing xenon difluoride (XeF2) based etching was used to produce porous silicon Porous silicon continues to be extensively researched for various optical and electronic devices and applications in chemical and biological sensing are abundant. The dry etching technique to manufacture porous silicon offers a simple and efficient alternative to the traditional wet electrochemical etching using hydrofluoric acid. This new porous silicon material was characterized for its pore size and morphology using top and cross-sectional views from scanning electron microscopy. Its optical properties were determined by angular dependence of reflectance measurements. A new class of holographically ordered porous polymer gratings that are an extension of holographic polymer dispersed liquid crystal (H-PDLC) structures. As an alternative structure and fabrication process, porous polymer gratings that include a volatile solvent as the phase separation fluid was fabricated. Porous silicon and porous polymer materials were used as substrates to encapsulate gaseous oxygen (O2) responsive luminophores in their nanostructured pores. These substrate materials behave as optical interference filters that allow efficient and selective detection of the wavelengths of interest in optical sensors.

Temperature effect on the lasing from a dye-doped two-dimensional hexagonal photonic crystal made of holographic polymer-dispersed liquid crystals

Temperature dependent lasing was demonstrated in a dye-doped two-dimensional hexagonal photonic crystal made of holographic polymer-dispersed liquid crystals (LCs) along ΓM direction in TE polarization. A redshift in lasing peaks was observed as the temperature increased from 25 to 45 °C. The downward movement of photonic band of TE polarization, majorly caused by the decrease in the anisotropy of LC droplets with the increase in temperature, is responsible for the redshift in lasing peaks.

Thermal control of transmittance/diffraction states of holographic structures composed of polymer and liquid crystal phases

A microperiodic structure composed of polymer and liquid crystal (LC) phases, called holographic polymer dispersed liquid crystal (HPDLC), was fabricated by a photo-induced phase-separation technique. It is anticipated that the HPDLC can be used as a thermo-driven light controller, in particular, for an autonomous solar-ray control window, which utilizes the nematic-to-isotropic ( N– I) phase transition phenomenon in the LC phase of the periodic structure. In the present study, the transmittance/diffraction properties of samples with different thermal behaviors were examined, and it was found that 15% of the solar radiation was switched between the transmittance and diffraction states as a function of temperature at around 30 °C. Furthermore, a more practical solution for using the HPDLC for a solar-ray control window was demonstrated. From the viewpoint of structural analysis, scanning electron microscopy reveals that LC droplets, a few tens of nanometers in size are, distributed and coalesce to form periodic LC layers. By carrying out polarizing microscopy and performing transmittance/diffraction measurements as a function of temperature, it is observed that the LC molecules in the droplets are strongly oriented along the grating vector and their orientations decline with a rise in temperature. These observed structures are of importance for the generation of a large change in the transmittance/diffraction properties due to N– I transition.

Effects of multiwalled carbon nanotube on holographic polymer dispersed liquid crystal

The morphology and electro-optical performance of holographic polymer dispersed liquid crystal (HPDLC) were investigated with the addition of multiwalled carbon nanotube (CNT), both pristine and chemically modified one (CNT-CC). With the addition of CNT, the diffraction efficiency increased and showed a maximum at 0.6% while nucleation was delayed due to the increased mixture viscosity. Film was driven only with CNT due to the induced local electric field of polymer to overcome the threshold resistance. Among the two types of CNT, chemically modified one gave finer CNT dispersion, lower mixture viscosity, larger liquid crystal (LC) droplet, higher diffraction efficiency, and shorter response time while the pristine CNT decreased the driving voltage. Copyright © 2010 John Wiley & Sons, Ltd.

Reduction of Volume Shrinkage in Holographic Polymer Dispersed Liquid Crystal Based on Epoxy Containing Polymer Matrices

Abstract The grating formation dynamics, SEM images, diffraction efficiency, and electrooptical properties of transmission grating of holographic polymer dispersed liquid crystals (HPDLCs) have been investigated by incorporating three different types of monofunctional reactive diluents into the host polymer matrices. A gradual increase of real time diffraction efficiency, a decrease of LC droplet size, and an increase of diffraction efficiency of the composite film were obtained with the addition of epoxy acrylate monomers due to the increased viscosity of polymer/LC mixtures leading to decreased droplet coalescence. In addition, epoxy acrylate monomers induced decreased volume shrinkage according to their ring structure with bulky group. Driving voltage increased with the addition of epoxy monomers whereas rise time and decay time decreased as a result of the decreased LC droplet size. At an optimum composition of epoxy monomer, a minimum switching voltage of 6 V µm−1 and rise time of 0.15 ms and a decay time of 18.05 ms were respectively obtained.