Holographic Polymer-dispersed Liquid Crystal Film Research Articles

An overview of HPDLC films and their applications

ABSTRACT The fusion of holography to the LC-polymer composites give rise to holographic polymer dispersed liquid crystal (HPDLC) technology. The concentration of monomer unit in HPDLC films is quite high as compared to the other polymer-LC composite films. Difference in the rate of polymerisation in different regions of composite film gives rise to alternate bright and dark fringes. In this review article along with the underlying principle of HPDLC films, we have discussed some electro-optical (EO) parameters like response time, diffraction efficiency (DE) and driving voltages necessary for the evaluation of a film. The classification and types of HPDLC gratings, which actually depend upon writing set up, grating period and thickness, has also found good space in this article. Recent advances to improve the efficiency of HPDLC films by varying material mixture and fabrication techniques is also discussed. Different types of evolved morphologies typically, LC droplet, Policryps and Polymer Scaffolding is described. Some trending applications of HPDLC films such as sensor, photonic crystals, mirrorless lasing etc are also highlighted.

Fabrication of optical vortex lattices based on holographic polymer-dispersed liquid crystal films.

This Letter demonstrates optical vortex lattices based on holographic polymer-dispersed liquid crystal (HPDLC) films. The fabrication method uses a phase-only reflective spatial light modulator with numerically calculated phase profiles loaded on it to simplify multi-helical-wave interference. The beam profiles of the diffraction beams are simulated using the Fourier transform and compared with experimental results. The topological charges of the 1st-order diffraction beams reconstructed from the HPDLC films are examined using a Michelson interferometer.

Optical Properties of Electrically Tunable Two-Dimensional Photonic Lattice Structures Formed in a Holographic Polymer-Dispersed Liquid Crystal Film: Analysis and Experiment.

We report on theoretical and experimental investigations of optical wave propagations in two-dimensional photonic lattice structures formed in a holographic polymer-dispersed liquid crystal (HPDLC) film. In the theoretical analysis we employed the 2 × 2 matrix formulation and the statistical thermodynamics model to analyze the formation of anisotropic photonic lattice structures by holographic polymerization. The influence of multiple reflections inside an HPDLC film on the formed refractive index distribution was taken into account in the analysis. In the experiment we fabricated two-dimensional photonic lattice structures in an HPDLC film under three-beam interference holographic polymerization and performed optical measurements of spectral transmittances and wavelength dispersion. We also demonstrated the electrical control capability of the fabricated photonic lattice structure and its dependence on incident wave polarization. These measured results were compared with the calculated ones by means of photonic band and beam propagation calculations.

Transverse wave propagation in photonic crystal based on holographic polymer-dispersed liquid crystal

This study investigates the transversely propagating waves in a body-centered tetragonal photonic crystal based on a holographic polymer-dispersed liquid crystal film. Rotating the film reveals three different transverse propagating waves. Degeneracy of optical Bloch waves from reciprocal lattice vectors explains their symmetrical distribution.

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.

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.

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

Holographic grating formation in PVB doped polymer dispersed liquid crystal based on PUA

Different contents of poly(vinyl butyral) (PVB) have been incorporated into the conventional holographic polymer dispersed liquid crystal (HPDLC) composition based on liquid crystal mixture and polyurethane acrylate (PUA) with a particular composition. As the PVB content is increased, the hardness, elastic modulus and thermal stability of polymer matrix are improved because of the entanglement by PVB, which has a relatively high molecular weight compared with PUA oligomer. Diffraction efficiency is enhanced with the addition of PVB except for HPDLC film with 10wt.% PVB owing to augmentation of the phase separation between polymer and LC, caused by the increase of elasticity of the polymer matrix. However, the increase in viscosity on adding PVB produces a slow saturation time and coalescence of the LC droplet, showing a lower diffraction efficiency at the PVB content of 10wt.% than at 0wt.%.

Finite-difference time-domain method calculation of light propagation through H-PDLC

Presented is the study of the diffraction properties of transmission holographic polymer dispersed liquid crystal (H-PDLC) grating. We constructed a two-dimensional model of H-PDLC film with cylindrical LC droplets. Director distribution inside LC droplets was calculated using the Monte-Carlo method. To calculate light propagation through the film we have solved numerically the Maxwell equations using the finite- difference time-domain method (FDTD). The FDTD analysis of diffraction was performed for s- and p-polarized incident light. Externally applied electric field influence on the diffraction efficiency was studied.

Electrically Controllable Diffraction Efficiency of H-PDLC Film Composed of Ellipsoidal Liquid Crystal Droplets

In this work the electric field control of diffraction efficiency of holographic polymer dispersed liquid crystal (H-PDLC) film is investigated. It is supposed that H-PDLC film contains ellipsoidal bipolar liquid crystal (LC) droplets. We developed a theoretical model that combines anisotropic coupled-wave theory, Monte-Carlo simulations for director profile within droplet and statistical averaging with the orientational distribution function for droplet symmetry axes. Monte Carlo simulations are done in Lebwohl-Lasher (LL) lattice model supposing strong director tangential anchoring conditions at droplet surface. We calculated diffraction efficiency upon applied voltage for ellipsoidal droplets with different eccentricity and Gaussian orientational distribution function of droplets axes. We investigated diffraction properties of H-PDLC film for both s- and p- light polarizations.

Superprism phenomenon based on holographic polymer dispersed liquid crystal films

The polarization-dependent superprism phenomenon was demonstrated using holographic polymer-dispersed liquid crystal (HPDLC) films. The HPDLC film is designed and fabricated using three coplanar beams. The fabricated HPDLC film contained two-dimensional ordered nano-sized LC domains (∼150nm in diameter) embedded in a polymer matrix; its periodicity was estimated to be ∼350nm. The dispersion of white light from this HPDLC superprism was ∼50°, and the deflection of light output from it was consistent with the theoretical value obtained by the plane wave expansion method.

Diffraction measurement and analysis of slanted holographic polymer dispersed liquid crystal

In this article, slanted holographic polymer dispersed liquid-crystal films are prepared by using a single-prism interferometer. Their diffraction characteristics are measured at various incident angles. Based on the coupled-wave theory of diffraction efficiency and incident angles, the grating period, film thickness, and slanted angle of the volume grating are derived. A good agreement is demonstrated between the experimental and theoretical results, and supported by scanning electron microscopy observation. The electrical switching behavior of the grating is also described. The diffraction efficiency changes from 65% to a stable value of 10% at an electric field of 33V∕μm. On the other hand, based on the theoretical prediction of diffraction efficiency, the optimum film thickness and Bragg grating period are determined, which sheds light on the implementation of our experiments.

홀로그래픽 고분자 분산 액정을 이용한 홀로그램 이중 초점 렌즈 제작

다기능성 아크릴레이트 단량체와 네마틱 액정 등을 혼합한 홀로그래픽 고분자 분산 액정 박막을 제작하고 액정의 농도와 피부에서 인가한 교류 전기장 등에 대한 실시간 회절효율을 측정하였다. WPDLC 박막에 기록된 홀로그램은 적절한 교류 전기장의 on/off에 의해 가역적으로 소거와 재생이 가능함을 보였다. 이러한 전기광학 특성을 이용하여 30 cm와 40 cm의 이중초점을 갖는 홀로그램 렌즈를 제작하였다. We Have fabricated a holographic polymer-dispersed liquid crystal (HPDLC) thin film, which is composed of multifunctional acrylate monomer blended with the nematic liquid crystal mixture, and then investigated the real-time diffraction efficiencies for various amounts of liquid crystals and applied AC electric fields. It is experimentally shown that the holographic gratings recorded in the HPDLC film can be reversibly erased and reconstructed by switching on and off an appropriate applied AC electric field. By use of these electro-optic properties we have developed bifocal holographic lenses having two different focal lengths of 30cm and 40 cm.

Evolution of anisotropic reflection gratings formed in holographic polymer-dispersed liquid crystals

The temporal evolution of an anisotropic reflection grating produced in a holographic polymer-dispersed liquid crystal film is investigated. We find that this type of grating is preceded in time by an isotropic concentration grating, and that the development of the anisotropic grating can be delayed until several seconds after laser exposure. The formation of an anisotropic grating is nearly coincident with the onset of phase separation of liquid crystal and implies a macroscopic ordering of liquid crystal droplet directors. Detailed knowledge of grating evolution may allow in situ control over the polarization sensitivity of the hologram.

Expanded viewing-angle reflection from diffuse holographic-polymer dispersed liquid crystal films

A switchable diffuse reflective film with high color purity is demonstrated using holographic-polymer dispersed liquid crystals (HPDLC). By recording a diffuse hologram directly into the LC/polymer film, the diffuse mode HPDLC exhibit viewing angles an order of magnitude larger than the conventional mode. A simple phenomenological model based on coupled-wave theory is developed to describe our observations.

Droplet size effect on the memory-mode operating temperature of smectic-A holographic polymer dispersed liquid crystal

The memory-mode operation of a polymer dispersed liquid crystal (PDLC), which depends on the stiffness of the layer structure in the smectic-A phase, has been analysed by comparing ordinary PDLC and holographic PDLC (HPDLC). Although both PDLC films were fabricated from the same material, the temperature range of the memory-mode operation was lower in the HPDLC than in the ordinary PDLC. In calorimetric analysis, the differential scanning calorimeter curves of the ordinary and holographic PDLC films were the same. Therefore, the liquid-crystal phase does not depend on the structural difference. This suggests that the force exerted against the layer structure of the smectic phase is greater in HPDLC because the droplet size is smaller and the ratio of the surface to volume is higher.