Holographic Polymer Dispersed Liquid Crystal Research Articles

Quantitative analysis on self-focusing properties of H-PDLC flexible curved radius gratings.

In general, the shape of traditional holographic grating is fixed and immutable, the period will not change after fabrication, this means that the modulation effect on the light field is unalterable. However, traditional grating cannot satisfy all requirements of current optical systems. In order to increase the versatility of holographic grating, a flexible curved radius grating (FCRG) which consists of holographic polymer-dispersed liquid crystal (H-PDLC) was proposed. The FCRG has an important self-focusing property that it can be adjusted the focal length by changing its own radius of curvature correspondingly. In this paper, we use the scalar diffraction theory to analysis the interference and diffraction processes for FCRG under different conditions, then a relationship equation has been deduced to express quantitatively about FCRG between its radius and focal length. According to the relationship, a tunable holographic element is achieved for the function of mechanically-controlled self-focusing effect. Experiments show that the FCRG has two conjugated focusing effects on the positive first-order and negative first-order, both two effects can achieve focus-adjusted ability by changing their radius of curvature. The FCRG provides a way for the coupler of curved waveguide display system for augmented reality in the future.

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.

Tunable Waveguides Couplers Based on HPDLC for See-Through Applications.

Photopolymers have become an important recording material for many applications, mainly related to holography. Their flexibility to change the chemical composition together with the optical properties made them a versatile holographic recording material. The introduction of liquid crystal molecules in a photopolymer based on multifunctional monomer provides us the possibility to generate tunable holograms. The switchable holographic elements are a key point for see-through applications. In this work, we optimize the holographic polymer-dispersed liquid crystals composition to improve the performance of tunable waveguide couplers based on transmission gratings and specifically their response under an applied electric field. A variation around 60% in the transmission efficiency was achieved.

Electro-optical properties and frequency response of polymer-dispersed liquid crystal gratings doped with multi-walled carbon nanotubes

In this paper, a model developed for establishing the lowest possible driving voltage of Holographic polymer-dispersed liquid crystal (H-PDLC) based on the Maxwell–Wagner effect doped with specific nanotubes. The dependences of the turning point frequency on the doping concentration and the distribution of multi-walled carbon nanotubes (MWCNTs) are discussed. The MWCNTs can effectively adjust the relaxation frequency of grating, increase its LC droplet size, and decrease its dielectric permittivity. The diffraction efficiency and threshold voltage of the prepared switchable Bragg grating were found to reach 91% and 0.77 V μm−1, respectively, at 5 kHz. By comparing the results of experiments and simulations, we obtained the mass distribution ratio of MWCNTs in the LC and polymer regions. It was found that the MWCNTs diffuse into the LC regions during the polymerization process, and the use of a suitable driving frequency and doping concentration can minimize the driving voltage. This work provides a reference for studying the dynamic range optical tuning and polymerization kinetics of nanoparticles in switchable gratings.

Electrically switchable electro-optic filters for spectral line emission detection.

Remote detection of spectral line emission is an important capability in a number of areas, including defense and environmental science. In this paper, we report on a mechanism for spectral line emission detection that is not based on narrow bandpass filters or hyperspectral imagers, but is instead based on the use of switchable spectral filters. The use of a switchable filter enables a single sensor to perform remote sensing tasking in a broad passband, while also detecting emission in a particular spectral line. In this case, the switchable spectral filter studied is a holographic polymer dispersed liquid crystal (HPDLC) reflection grating. The concept is demonstrated through modeling a sensor with an integrated HPDLC filter and building a detection algorithm capable of detecting spectral line emission. The modeling framework is built upon four components: the background scene, the spectral line source, the HPDLC filter, and the sensor. Results from the model show probability of detection and probability of false alarm for spectral line sources of varying strength for a particular background scene.

High efficiency unidirectional optical transmission diode at 1550 nm based on H-PDLC Bragg grating-PC structure

In this paper, a method has been proposed for high efficiency unidirectional optical transmission diode at 1550 nm. We use a new composite structure, holographic polymer dispersed liquid crystal (H-PDLC) grating-photonic crystal (PC), to realize it. There exists the directional band gap for the PC structure in 1550 nm. Therefore, it is possible to use the H-PDLC Bragg grating to shift the incident angle to a particular value to satisfy the condition so that the PC energy band can be moved from the pseudo-band gap to the passband to achieve the purpose of unidirectional transmission optical diode with high-efficiency. So that when the 1550 nm beam diffracted from the H-PDLC grating to incident the PC structure, it will transmit the composite structure almost totally, but it will be completely none reflected from PC surface. The simulation results show that the forward transmittance is 94.1% and the backward transmittance is near 0. At the same time, H-PDLC grating provide a method to adjust the transmittance of the composite structure diode with its electrically-controlled characteristics. This composite structure design provides another efficient method for the realization of all-optical diodes in the field of optical communication.

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.

Analysis of the Imaging Characteristics of Holographic Waveguides Recorded in Photopolymers.

In this work, we study the imaging characteristics of an optical see-through display based on a holographic waveguide. To fabricate this device, two transmission holograms are recorded on a photopolymer material attached to a glass substrate. The role of the holograms is to couple the incident light between air and the glass substrate, accomplishing total internal reflection. The role of noise reflection gratings and shrinkage on the imaging characteristics of the device will be also explored. The holograms (slanted transmission gratings with a spatial frequency of 1690 lines/mm) were recorded on a polyvinyl alcohol acrylamide holographic polymer dispersed liquid crystal (HPDLC) material. We will show that sufficient refractive index modulation is achieved in the material, in order to obtain high diffraction efficiencies. We will demonstrate that the final device acts as an image formation system.

Reconfigurable Reflective Colors in Holographically Patterned Liquid Crystal Gels

Thin film holographic gratings have attracted significant attention due the tailorability of their photonic properties. The ability of holography to create materials with unique photonic properties is particularly promising when combined with stimuli-responsive liquid crystals. Holographic polymer-dispersed liquid crystals (H-PDLCs) are an example of such a system where nonuniform irradiation is used to template alternating polymer-rich and polymer-poor regions with a periodicity related to the illumination wavelength. These systems typically use nonmesogenic monomers at a concentration range where a significant volume fraction of the final system is polymer. Here, we present a reconfigurable material that was generated by holographically polymer stabilizing liquid crystals (H-PSLCs) using liquid crystalline monomers, thereby enabling both alternating regions to be liquid crystalline. The “stabilizing effect” of the liquid crystalline polymer leads to a material with regions of periodic transition temperatures. As written, the polymer-rich regions have a higher isotropic transition temperature than the polymer-poor regions. Upon heating these samples beyond the isotropic temperature of the polymer-poor regions, a vivid wavelength specific reflection appears due to the refractive index mismatch between the ordered liquid crystalline polymer-rich regions and the disordered isotropic polymer-poor regions. When these materials are further heated to the isotropic temperature of the polymer-rich regions, the samples again return to a transparent state because the refractive index modulation between the alternating isotropic regions is negligible. By tailoring the liquid crystalline isotropic transition temperature to be near-room temperature, modest AC-fields can be used as an Ohmic heat source to drive the thermal color change.

See-through 2D/3D compatible integral imaging display system using lens-array holographic optical element and polymer dispersed liquid crystal

A see-through 2D/3D compatible integral imaging display system including two projectors and a projection screen is proposed. The projection screen consists of a lens-array holographic optical element (LAHOE) and polymer dispersed liquid crystal (PDCL) film. The projection screen can operate at two different modes: the see-through and scattering display mode. With voltage, the projection screen works in the see-through display mode, 3D image for Bragg matched image can be presented, and 2D image can also be presented. Without voltage, the projection screen works in the scattering display mode, the 2D and 3D images can be presented. The prototype of the 2D/3D compatible integral imaging display system is developed. Experimental results confirm the feasibility of the proposed system.

Influence of structural parameters on organic semiconducting polymer lasers with an external holographic grating as feedback layer

We report on the fabrication and characterization of an organic distributed feedback laser excited from a semiconducting polymer gain layer, poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenyl-envinylene) (MEH-PPV). A holographic polymer dispersed liquid crystal grating is fabricated on top of the homogeneous MEH-PPV layer as an external cavity. We established a model to calculate the confinement factor Γ. With the change of laser structural parameters (the refractive index of the grating layer, the film thickness of the MEH-PPV layer, etc.), the confinement factor Γ changes accordingly. It was found that a single longitudinal lasing mode could be observed only when the confinement factor Γ was in a specific range. Choosing a reasonable thickness of gain film and prepolymer with higher refractive index can obtain a lower threshold emission laser.

Switchable pupil expansion propagation using orthogonal superposition varied-line-spacing H-PDLC gratings in a holographic waveguide system.

To solve the problem of small field of view in traditional holographic waveguides, this paper proposes a waveguide-coupling technique using orthogonal superposition varied-line-spacing (VLS) gratings. These gratings expand the x and y directions and orthogonally superimpose to achieve transmission in a waveguide and enlarge the pupil field of view. At the optical coupling input end and the coupling output end of a waveguide, one-dimensional VLS gratings are used to realize horizontal expansion of the image. Then a vertical VLS grating is orthogonally superimposed at the exit end to realize vertical expansion of the image. Finally, the waveguide transmission and expansion of the image are completed. In the experiment, holographic polymer dispersed liquid crystal one-dimensional VLS gratings in the x and y directions are fabricated and coupled with a waveguide. An image source with a diameter of 0.5cm is waveguide-transferred and coupled out, and then passed through the vertical grating. Amplification is performed to obtain an expanded image of a diameter of 2.28cm. In this study, the diffraction characteristics of the grating used to realize pupil expansion in the holographic waveguide system are analyzed and simulated. It is calculated that the diffraction efficiency of the VLS gratings can reach 80% or more in the 532nm band. Additionally, the characteristics of an electronically controlled switch are studied. Experimental results show that the method can be used for expanding the field of view and can be applied to waveguide systems for image transmission and expansion.

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.

Thermoplasmonic Activated Reverse-Mode Liquid Crystal Gratings

A new generation of reconfigurable optical components is conceived by bridging the photothermal properties of gold nanoparticles and the thermosensitivity of liquid crystalline materials. As such, gold nanorods (GNRs) heated using light are used to activate efficient hidden diffraction gratings realized in a blend made of a room temperature polymerizable liquid crystal (PLC) and nematic liquid crystal (NLC). Holographic liquid crystal polymer dispersed liquid crystal (HLCPDLC) gratings containing a small percentage of GNRs are fabricated with periodicity of 2.6 μm by means of a conventional UV holographic recording setup. The HLCPDLC structures are characterized using morphological, optical, and thermooptical techniques. Because of the initial refractive index match between the polymer-rich and LC-rich regions, the “grating” is hidden and the films appear transparent and nondiffractive. Illumination of these GNR-containing structures with a suitable light source (808 nm) induces a local heating due to the...

Diffusion kinetics investigations of Nano Ag-doped holographic polymer dispersed liquid crystal gratings

ABSTRACTIn this paper, we use multicomponent mutual diffusion method to derive a one-dimensional non-local diffusion dynamic model to describe the diffusion kinetics of a dynamic holographic polymer dispersed liquid crystal grating (H-PDLC) doped with nano-silver. The physical mechanism of diffusion between monomer and liquid crystal, monomer and nano-silver particles is analysed using this model. Using coupled-wave theory, the H-PDLC’s diffraction efficiency curve with the expose time are simulated due to the vivid changing of effective refractive index modulation caused by the movement of concentration of each component with the expose time. Correspondingly, in the experiment, the diffraction efficiency of the grating is measured in real time, which shows the improvement for the holographic properties because of nano-silver doped H-PDLC. The simulation results have a good agreement with experimental data by fitting the corresponding parameters of the model. In addition, through comparing with simulation and experimental results with doping different concentrations of nano-silver particles, the recipe and diffraction characteristics of H-PDLC grating can be improved. Thus, the diffusion Kinetics model can be used to optimise the phase separation of the PDLC grating, and finally to improve the opto-electrical properties of H-PDLC gratings.

Holographic Formation of Non-uniform Diffraction Structures by Arbitrary Polarized Recording Beams in Liquid Crystal-photopolymer Compositions.

In this work, the theoretical model of non-uniform diffraction structures’ holographic formation in liquid crystal-photopolymer (LC-PPM) composite materials with a dye-sensitizer is developed. The model takes into account the arbitrary character of amplitude and phase spatial distributions of recording light field, its arbitrary polarization state and also a non-linearity of the recording process. Two the most common types of liquid crystal-photopolymer composite are investigated: Holographic polymer-dispersed liquid crystals (H-PDLC) and polymer-stabilized liquid crystals (PSLC). Numerical simulations for the most common cases of holographic formation schemes are made. It is shown that due to the photo-induced Freedericksz transition, in the case of arbitrary polarization states of recording light beams, the non-uniform polarization diffraction grating (PDG) is formed in LC-PPM. Numerical simulations’ results show that PDG’s contribution to the change of the dielectric tensor of the media is comparable with the contribution of the photopolymerization-diffusion process.

Holographic Characteristics of Photopolymers Containing Different Mixtures of Nematic Liquid Crystals.

A holographic polymer dispersed liquid crystal (HPDLC) is used to record holographic diffraction gratings. Several mixtures of nematic liquid crystals (LC) are used as components of the HPDLC to evaluate their influence in static and dynamic basic properties. The diffraction efficiency obtained in the reconstruction of the holograms is evaluated to compare the influence of the different LC. Additionally, the samples are exposed to a variable electric field and the diffracted light intensity as a function of the applied voltage is measured to evaluate the influence of the LC. The results obtained show significant differences depending on the LC incorporated to the photopolymer.

Influence of Tert-Butylthiol and Tetrahydrofuran on the Holographic Characteristics of a Polymer Dispersed Liquid Crystal: A Research Line Toward a Specific Sensor for Natural Gas and Liquefied Petroleum Gas.

Tert-Butylthiol (TBT) and tetrahydrothiophene (THT) are odorant substances added to natural gas and liquefied petroleum gas to help their detection by the human smell. In this research, TBT and THT are incorporated into a holographic polymer-dispersed liquid crystal and their influence in the main holographic characteristics of the photopolymer are studied in order to open the way towards the design of a holographic sensor to detect natural gas and liquefied petroleum gas.

Integral imaging-based 2D/3D convertible display system by using holographic optical element and polymer dispersed liquid crystal.

An integral imaging-based 2D/3D convertible display system is proposed by using a lens-array holographic optical element (LAHOE), a polymer dispersed liquid crystal (PDLC) film, and a projector. The LAHOE is closely attached to the PDLC film to constitute a projection screen. The LAHOE is used to realize integral imaging 3D display. When the PDLC film with an applied voltage is in the transparent state, the projector projects a Bragg matched 3D image, and the display system works in 3D mode. When the PDLC film without an applied voltage is in the scattering state, the projector projects a 2D image, and the display system works in 2D mode. A prototype of the integral imaging-based 2D/3D convertible display is developed, and it provides 2D/3D convertible images properly.

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.