Holographic Polymer-dispersed Liquid Crystal Gratings 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.

Accurate, Efficient and Rigorous Numerical Analysis of 3D H-PDLC Gratings.

This work presents recent results derived from the rigorous modelling of holographic polymer-dispersed liquid crystal (H-PDLC) gratings. More precisely, the diffractive properties of transmission gratings are the focus of this research. This work extends previous analysis performed by the authors but includes new features and approaches. More precisely, full 3D numerical modelling was carried out in all analyses. Each H-PDLC sample was generated randomly by a set of ellipsoid geometry-based LC droplets. The liquid crystal (LC) director inside each droplet was computed by the minimisation of the Frank elastic free energy as a function of the applied electric field. The analysis carried out considered the effects of Frank elastic constants , and ; the anchoring strength ; and even the saddle-splay constant . The external electric field induced an orientation of the LC director, modifying the optical anisotropy of the optical media. This effect was analysed using the 3D split-field finite-difference time-domain (SF-FDTD) method. In order to reduce the computational costs due to a full 3D tensorial analysis, a highly optimised method for high-performance computing solutions (HPC) was developed. The influences of the anchoring and voltage on the diffraction efficiencies were investigated, showing the potential of this approach.

Electrically tunable dual-wavelength organic laser based on holographic polymer dispersed liquid crystal grating

Abstract An organic dual-wavelength surface-emitting distributed feedback (DFB) laser with low threshold and high conversion efficiency based on holographic polymer dispersed liquid crystal (HPDLC) grating is reported. In such a laser configuration, the dye blend 4-(dicyanomethylene)-2-methyl-6-( p -dimethylaminostyryl)-4H-pyran (DCM)/1,3,5,7,8-pen- tamethyl-2,6-diethylpyrromethene-difluoroborate (PM567) -doped HPDLC grating is formed on the top of an organic semiconductor poly(2-methoxy-5-(20 -ethyl-hexyloxy)-p- phenylenevinylene) (MEH-PPV) layer. The dual-wavelength laser located at 605.2 nm and 629.8 nm are obtained in a single beam. The dual-wavelength laser performance under an external electric field are investigated and illustrated for the first time. This study provides some new ideas for the improvement of tunable dual-wavelength laser and extends more applications in laser displays and integrated photonic circuits.

Electrically tunable holographic waveguide display based on holographic polymer dispersed liquid crystal grating

In this Letter, we present an electrically tunable holographic waveguide display (HWD) based on two slanted holographic polymer dispersed liquid crystal (HPDLC) gratings. Experimental results show that a see-through effect is obtained in the HWD that both the display light from HWD and the ambient light can be clearly seen simultaneously. By applying an external electric field, the output intensity of the display light can be modulated, which is attributed to the field-induced rotation of the liquid crystal molecules in the two HPDLC gratings. We also show that this electrically tunable performance enables the HWD to adapt to different ambient light conditions. This study provides some ideas towards the development of HWD and its application in augmented reality.

Self-orientation effect of liquid crystals on holographic polymer-dispersed liquid crystal and distributed feedback lasers

The average orientation of a liquid crystal (LC) director to the grating formation, morphology, and switching properties of a holographic polymer-dispersed liquid crystal (HPDLC) grating was systematically investigated in this study. The grating possessed high diffraction efficiency and low scattering with the LC director being parallel to the grating vector. The scanning electron microscope confirmed the well-defined morphology with the LC director being parallel to the grating vector. The grating was easily switched when the LC director was perpendicular to the grating vector. Moreover, polarization excitation was performed to investigate the polarization dependence behavior of the HPDLC-distributed feedback (DFB) laser. The results confirmed that the HPDLC grating is suitable as a laser oscillation when the LC director is parallel to the grating vector. Finally, the tuning range was obtained for the HPDLC DFB laser by applying an external electric field. The tunability, ease of fabrication, and mass production make the HPDLC DFB lasers suitable as smart laser sources for spectroscopy and communication.

Tolerance of holographic polymer-dispersed liquid crystal memory for gamma-ray irradiation.

The radiation-hardened characteristics of holographic polymer-dispersed liquid crystal (HPDLC) memory are discussed in the application for an optically reconfigurable gate array. The radiation experiments are conducted using a cobalt 60 gamma radiation source to examine the tolerance of a 100Mrad total ionizing dose for the HPDLC memory. The optical properties are compared in the conditions before and after gamma-ray irradiation for the fabricated HPDLC gratings. The effects of gamma-ray irradiation on the internal grating structure are also investigated by polarization optical microscopy and scanning electron microscopy observations. The HPDLC memory irradiated by a 100Mrad total ionizing dose demonstrates the implementation of the optical reconfiguration in a gate-array VLSI.

Electrically tunable two-dimensional holographic polymer-dispersed liquid crystal grating with variable period

An electrically tunable two-dimensional (2D) holographic polymer-dispersed liquid crystal (H-PDLC) grating with variable period was fabricated by inserting a cylindrical lens in a conventional holographic interference beam. The interference between the plane wave and cylindrical wave resulting in varying intersection angles on the sample, combined with dual exposure along directions perpendicular to each other, generates a 2D H-PDLC grating with varied period. We have identified periods varying from 3.109 to 5.158µm across a 16mm width, with supporting theoretical equations for the period. The period exhibits a symmetrical square lattice in a diagonal direction, with an asymmetrical rectangular lattice in off-diagonal locations. With the first exposure at 2s and the second exposure at 60s, the phase separation between the prepolymer and liquid crystal was most evident. The diffraction properties and optic-electric characteristics were also studied. The diffraction efficiency of first-order light was observed to be 13.5% without external voltage, and the transmission efficiency of non-diffracted light was 78% with an applied voltage of 100V. The proposed method provides the capability of generating period variation to the conventional holographic interference path, with potential application in diffractive optics such as tunable multi-wavelength organic lasing from a dye-doped 2D H-PDLC grating.

Wide-viewing-angle switchable image splitter obtained using varied-line-spacing H-PDLC Bragg grating

ABSTRACTWe propose the use of a varied-line-spacing (VLS) holographic polymer-dispersed liquid crystal (H-PDLC) Bragg grating as a switchable image splitter to generate a compatible three-dimensional (3D) stereogram and to increase the viewing angle of the observer. To fabricate the VLS grating, a cylindrical lens is adopted to form a cylindrical wave, which interferes with a plane wave, generating an H-PDLC grating with a continuously varying period. The proposed holographic optical element (HOE)-based image splitter comprises two VLS H-PDLC gratings. It can be attached on a designed pattern, with two rectangles taking the place of conventional liquid crystal display panel pixels, and can separate them into right and left viewing fields with a wider range. Experimental results show that the movement of the eyes of the observer can reach 37.6 mm. A theoretical simulation indicates that a shorter focal length of the cylindrical lens can yield a larger movement range. Switching between two-dimensional and 3D modes can be performed by applying an external alternating-current voltage at ~80 V. The contrast ratio of the diffracted images induced by crosstalk is greater than 60%, which indicates the feasibility of the proposed HOE for 3D image splitting.

Enhancement of conversion efficiency for an organic semiconductor laser based on a holographic polymer dispersed liquid crystal

In this paper, we report a high-conversion-efficiency organic semiconductor distributed feedback laser. The gain layer of the laser device is made from poly (2-methoxy-5-(20-ethylhexyloxy) p-phenyl-enevinylene) (MEH-PPV), and the holographic polymer dispersed liquid crystal (HPDLC) grating is used as the external light feedback layer. Thus the parameters of the laser device can be modulated independently. The solution of MEH-PPV in xylene (6 mg·mL-1) is deposited on the bottom glass substrate by spin-coating (2000 r/min). The MEH-PPV layer thickness is controlled at (80±2) nm by the spin-coating rate and confirmed by the Dektak profilometer. The HPDLC is made by the photo-induced phase separation method. To determine the orientations of LC molecules, the diffraction efficiency of each sample is measured by a He-Ne laser. The diffraction efficiency is defined as the diffracted light intensity in the first order divided by the incident light intensity. If p light diffraction efficiency (ηp) is much larger (smaller) than s light diffraction efficiency (ηs), it can be thought of as a symbol of a fairly good alignment of LC along the grating vector (grating grooves). When the period of HPDLC grating is larger than 450 nm, ηp is greater than ηs, and the averaged orientation of liquid crystal molecules is aligned along the grating vector direction, i.e., orthogonal to the holographic plane. For feedback light propagating along the grating vector, the refractive index modulation is dependent on the difference between the polymer refractive index np and the ordinary refractive index no of phase-separated LC. These two values are very close to each other, thus the effective light feedback for lasing output is not high. However, when the period of HPDLC grating is smaller than 450 nm, ηs is greater than ηp, and the orientation of phase-separated LC is altered. The refractive index modulation of feedback light originates from the difference between the polymer refractive index np and the extraordinary refractive index ne of phase-separated LC, thus the refractive index modulation can be improved and the HPDLC layer can provide better light feedback. The lasing threshold is 0.70 μJ/pulse, and the conversion efficiency is 2.5% for the sample with a grating period of 593 nm. However, the lasing threshold is lowered to 0.18 μJ/pulse, and the conversion efficiency increases to 6.4% for the sample with a grating period of 395 nm. These results show that the output lasing performance can be improved by using small period grating, since it has bigger refractive index in the grating vector direction (the lasing feedback direction). The laser performance of sample with small grating period is improved in some aspects such as threshold energy, conversion efficiency to some extent compared with those reported previously.

Effects of monomer functionality on performances of scaffolding morphologic transmission gratings recorded in polymer dispersed liquid crystals

The effects of monomer functionality on performances of holographic polymer dispersed liquid crystal (HPDLC) transmission gratings are systematically investigated. Acrylate monomers with an average functionality ranging from 2.0 to 5.0 are used to prepare these samples. We find scaffolding morphologic transmission gratings (characterized by a high phase separation degree, a well alignment of LCs and low scattering loss) can be obtained irrespective of the monomer functionality, although the exact optimal curing intensity varies. The temporal evolution of the grating formation is studied and the onset time of the LC phase separation decreases significantly with the increase in average monomer functionality. It is also shown that the gratings prepared from low average functionality monomers require a comparatively low switch-off electric field (~8 V μm−1) whilst suffering from mechanical fragility and long-term instability. Our results not only provide a complete understanding of scaffolding morphologic gratings in terms of the material composition effect, but also provide insight into the formation mechanisms of non-droplet morphologic HPDLC gratings.

Improvement on the Performance of Holographic Polymer-Dispersed Liquid Crystal Gratings with Surface Plasmon Resonance of Ag and Au Nanoparticles

An investigation and comparison of the characteristics of holographic polymer-dispersed liquid crystal (H-PDLC) gratings doped with Ag and Au nanoparticles (NPs) is presented. The experimental results show that the diffraction efficiency and electro-optical properties of holographic gratings are improved through the addition of Ag and Au NPs. It is notable that Ag NP-doped gratings have far superior performance than those containing Au NPs. Furthermore, we find that the diffraction efficiency improvement can be explained as being due to the surface plasmon resonance of the metal NPs embedded in the PDLC. We theoretically simulate the surface plasmon resonance (SPR) wavelengths of the Ag and Au NPs within the PDLC and find that the laser wavelength used to fabricate the holographic gratings is much closer to the SPR wavelength of Ag NPs than that of Au NPs. Atomic force microscopy images show that the phase separation of the Ag-doped grating is improved, and the grating structure is much smoother than those of the undoped samples and the Au NP-doped samples, because of the SPR properties of the Ag NPs in the PDLC microstructure during the holographic recording process.

Highly diffractive, reversibly fast responsive gratings formulated through holography

Versatile and reversibly rapid responsive one-dimensional photonic crystals with a diffraction efficiency of 97%, which consisted of uniformly 273 ± 48 nm wide liquid crystal belts within transmission holographic polymer dispersed liquid crystal (HPDLC) gratings, were formulated by a facile single step holography based on a hyperbranched monomer. The effect of the monomer average functionality on the photopolymerization kinetics and the electro-optical performances as well as the grating morphologies was investigated. The results show that the low intrinsic viscosity of the hyperbranched monomer accounts for a well-structured morphology in terms of providing a prolonged gelation time for the liquid crystals to diffuse from the light illumination region during the holographic polymerization induced phase separation. Another intriguing observation is that with an increase in the hyperbranched monomer loading, the diffraction efficiency of the HPDLC gratings gradually increases from zero to an average of 94% and then levels off. This is quite different from previous results that gave less than a 50% diffraction efficiency when the monomer average functionality was larger than 4.

Electro-optical characteristics of holographic polymer dispersed liquid crystal gratings doped with nanosilver

We report on the synthesis and characteristics of a holographic polymer dispersed liquid crystal (H-PDLC) switchable grating based on nano-Ag particles. The influence of doping different concentrations of nano-Ag on the diffraction efficiency, driving voltage, and response time of the H-PDLC grating is investigated. The best grating characteristics were achieved with 0.05% nano-Ag doping. Calculated and experimental results reveal that the improvement of the characteristics is likely due to the surface plasmon effect of nano-Ag.

Holographically Formed, Acoustically Switchable Gratings Based on Polymer-Dispersed Liquid Crystals

We report holographic polymer-dispersed liquid crystal (H-PDLC) gratings driven by surface acoustic waves (SAWs). Our experiments show that upon applying SAWs, the H-PDLC grating exhibited switchable properties: The diffraction of the H-PDLC grating decreased, whereas the transmission increased. This acoustically switchable behavior is due to the acoustic streaming-induced realignment of liquid crystals as well as absorption-resulted thermal diffusion. Such SAW-driven H-PDLC gratings are potentially useful in many photonic applications, such as optical switches, spatial light modulators, and switchable add/drop filters.

Holographic polymer-dispersed liquid crystal grating with low scattering losses

A simple method to fabricate low-scattering holographic polymer-dispersed liquid crystal grating is proposed. The optical efficiencies of the grating are tested. The results show that the scattering losses are almost totally suppressed, at less than 0.25%, and the diffraction efficiency of the grating reaches as high as 99%, indicating a great improvement compared with grating fabricated in the conventional way. In addition, the electro-optical performance maintains good behaviour, and thermo-stability testing shows that the good optical properties of the grating can be maintained in a relatively wide temperature range, satisfying the application requirements of displays and other optical instruments.

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.

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.

Improvement of Electro-Optical Properties of HPDLC Gratings by in situ Shearing During Holographic Recording

We report the results of the recording of holographic polymer-dispersed liquid crystal (HPDLC) reflection gratings while applying a shear stress parallel to the film plane. The shear is transmitted through the film by moving one glass window with respect to a fixed glass window during the holographic recording in a single beam, total internal reflection geometry. The optical properties of the resulting Bragg grating are related to timing and magnitude of the stress. High diffraction efficiency for light polarized in a direction parallel to the stress is obtained with nearly zero diffraction efficiency (DE) for the perpendicular polarization. Contrary to post-recording stress-induced polarization sensitization, the in situ process results in permanently polarized gratings. The polarization sensitivity is related to stress-induced morphology changes of liquid crystal droplets that are frozen at gelation of the thio-ene polymerization process.

Diffraction Properties of Anisotropic Volume Gratings Formed in Polymer-Dispersed Liquid Crystal

The diffraction properties of highly polarized holographic polymer-dispersed liquid crystal (HPDLC) gratings fabricated by various grating formations are investigated as functions of the incidence angle of light and polarization state. Dominant anisotropic diffraction characteristics are obtained in HPDLC gratings by controlling the grating formation. Phase separation by photopolymerization to form the gratings is studied by evaluating the anisotropic diffraction and configuration of the separated phase of the gratings. The diffraction efficiency and the characteristic ratio of diffraction efficiency in polarization parallel to the grating vector to that in polarization perpendicular to the grating vector are markedly affected by liquid crystal (LC) contents. Structural analysis by optical polarization microscopy and scanning electron microscopy (SEM) is carried out to investigate the effect of the morphology of HPDLC gratings on diffraction properties. The grating morphologies are composed of fine droplets. The resultant gratings show novel diffractive performance consisting of high diffraction efficiency and polarization dependence over a wide angular range of incident light.

Huge retardation of grating formation in holographic polymer-dispersed liquid crystals

Phototriggered kinetics is investigated in the dark for holographic polymer-dispersed liquid crystal gratings. Subsequently to holographic recording, the diffraction efficiency first slightly increases, then decreases, and afterwards starts to grow again. The remarkable growth continues for several days until the diffraction efficiency stabilizes at values several times larger than the initial ones. The initial increase is attributed to a dark polymerization reaction as confirmed by electron paramagnetic resonance spectroscopy. The slow process is associated with the kinetics of a phase-separation mechanism. The non-monotonic behaviour of the diffraction efficiency is explained by superposition of refractive-index and extinction modulations.