Reflective Cholesteric Liquid Crystal Research Articles

Polarization conversion effect in cholesteric liquid crystal-based polarization volume gratings

Following the recent experimental discovery of a new polarization conversion phenomenon in polarization volume gratings (PVGs), in this paper, we investigate its underlying physical mechanisms in cholesteric liquid crystal (CLC) reflectors and PVGs. We examine the transition of eigenstates from circular to linear polarization as the incident angle deviates from the helical axis, which originates such an anomalous polarization conversion effect. While this transition enables PVGs to double the in-coupling efficiency and brightness uniformity in waveguide-based augmented reality (AR) displays, it also degrades the polarization selectivity and alters the transmitted polarization state in both CLC reflectors and PVGs, which in turn narrows down their spectral and angular bandwidth in the multi-layer design. These findings help not only deepen the understanding of polarization behaviors in CLC-based optical elements but also provide valuable insights to optimize their performances for emerging AR smart glasses.

Colorful multi-plane augmented reality display with dynamically tunable reflective Pancharatnam-Berry phase lens.

Reflective cholesteric liquid crystal (CLC) Pancharatnam-Berry phase lens (PBL) devices have attracted significant attention in augmented reality (AR) display due to their wide spectral and angular response bandwidths, high diffraction efficiency, and polarization selectivity. However, currently reported CLC reflective PBLs are either limited by monochrome display or suffers from complicated design for colorful display. Herein, we demonstrate a colorful multi-plane AR display system with dynamically tunable reflective PBL. The reflective PBL is fabricated by polymer-stabilized cholesteric liquid crystal (PSCLC) that provides dynamical and continuous tunability of color and focal length by direct current (DC) voltage. A proof-of-concept colorful multi-plane AR device is demonstrated, where over 90% diffraction efficiency at desired wavelength has been obtained. The proposed simple, compact, and light AR display system capable of color-imaging with multi-depth shows great application potential in the vehicle-mounted head-up display (HUD).

An l-isosorbide-based reactive chiral dopant with high helical twisting power for cholesteric liquid crystal polymers reflecting left-handed circularly polarized light.

For visible light reflective cholesteric liquid crystal polymers, reactive chiral dopant enantiomers with high helical twisting power are attractive. However, a chiral dopant for reflecting left-handed circularly polarized light has been missing so far. Here, we report the synthesis of a reactive, left-handed, l-isosorbide-based chiral dopant with a high helical twisting power of -48 μm-1 that can be used in visible light reflective cholesteric liquid crystal polymers. The right handed dopant enantiomer was also synthesized, showing a helical twisting power of +63 μm-1.

Near-Infrared Reflective Polymer Films Based on UV-327-Doped Zinc Oxide Nanoparticles.

We prepared cholesteric liquid crystal (CLC) films with broadband reflective properties by admixing organic dye UV-327 into inorganic zinc oxide nanoparticles (ZnO NPs), utilizing the principle of pitch distribution from a large to a small gradient along the film thickness direction, leading to broadband reflection. ZnO NPs are poorly dispersed and easy to gather, but they do not decompose easily. The addition of UV-327 makes up for the above shortcomings. UV-327 is an organic compound with good compatibility and dispersion with liquid crystal systems. Therefore, we used the method of mixing two UV-absorbing dyes (UV-327 and ZnO NPs) to obtain CLC films. UV-absorbing dyes (UV-327 and ZnO NPs) made the liquid crystal films form a UV intensity gradient in the direction of thickness, prompting the polymerizable monomers to polymerize faster on the stronger side of the light, leading to the relative diffusion of chiral molecules and polymerizable monomers, forming the concentration gradient of chiral molecules in the direction of thickness. The pitch has a gradient distribution as the chiral concentration varies. Then, anchored by the polymer network, the pitch gradient distribution no longer changes. Broadened reflective bandwidth can reach up to 881 nm. Furthermore, the film covers the near-infrared wavelength band well, which can be applied to future smart windows or laser shielding for medical and military applications. It is also believed that this achievement will optimize the preparation technology of broadband reflective CLC films in the future.

60‐2: Distinguished Student Paper: High‐Efficiency Folded Optics for Near‐Eye Displays

A new folded optical system with an enhanced efficiency using two polarization‐selective cholesteric liquid crystal reflectors is proposed. In conventional pancake optics, the optical path length is tripled but its efficiency drops to 25%. Our new configuration provides 50% efficiency, while the optical path length is doubled.

High‐efficiency folded optics for near‐eye displays

AbstractPancake lens structure offers a promising solution to compact formfactor for near‐eye displays. However, utilizing a half mirror to triple the optical path length causes a dramatic optical loss. The theoretical maximum optical efficiency is only 25%. To balance the tradeoff between compact formfactor and optical efficiency, here we propose a new folded optical structure with a doubled efficiency and doubled optical path length, using two polarization‐selective cholesteric liquid crystal reflectors. Simulation results agree with the experiment reasonably well. Two display configurations are established to evaluate the imaging performances of the proposed pancake lens structure.

Foveated imaging by polarization multiplexing for compact near‐eye displays

AbstractWe demonstrate a multi‐resolution foveated imaging system for virtual reality, which is enabled by a single display panel with polarization multiplexing to significantly increase the angular resolution without scarifying the frame rate. By using a polarization‐dependent cholesteric liquid crystal reflector, we have achieved a 4.4× higher resolution density while maintaining a compact form factor.

Preparation of cholesteric polymer networks with broadband reflection memory effect

ABSTRACT In this study, polymer network is prepared by broadband reflective cholesteric liquid crystal (CLC) film with washing out method. By the method of photoinduced molecular diffusion, the CLC film is prepared by adding different concentrations of ultraviolet (UV) absorption dyes into the LC system and irradiated with a series of UV light intensities for different times. The selective reflection property and broadband reflection memory effect of the polymer network are proved, and the response mechanism is given. By refilling the polymer network with different LCs, reflective bands with different central wavelengths and bandwidths can be obtained. It is confirmed that different fillers can recurrence the initial reflection bandwidth of CLCs in different degrees. These results may provide a new idea for total reflection CLC films.

Flat cholesteric liquid crystal polymeric lens with low f-number.

We use a simple photoalignment method to fabricate four reflective cholesteric liquid crystal (CLC) polymeric lenses with diameter D=2.45 cm and low f-numbers (f/2, f/0.9, f/0.45, f/0.33) at 550 nm. Such a flat CLC lens can be converging or diverging, depending on the handedness and direction of the incident light. Our CLC lenses can achieve ∼85% diffraction efficiency for a designated polarization state and manifest decent imaging ability.

Pitch Gradation by Ion-Dragging Effect in Polymer-Stabilized Cholesteric Liquid Crystal Reflector Device.

An IR reflector based on polymer-stabilized cholesteric liquid crystal (PSCLC) can selectively tune IR light reflection for smart window application. Broadening the reflection bandwidth to block more IR heat radiation requires the expansion of the pitch distribution in the PSCLC. Traditional attempts using ex situ direct current (DC) bias upon an already polymerized PSCLC reflector usually require a sustaining potential difference holding the pitch gradient of the reflector. Removing the DC bias will lead to a reflect bandwidth comeback. Here, we have developed an in situ DC curing strategy to realize an irreversible reflect bandwidth broadening. Briefly, a DC bias was used to drive the redistribution of impurity cations, which can be captured by the ester group of oligomers, during the photopolymerization. During the slow polymerization process, such trapped cations will drag the oligomers towards the cathode and compress the pitch length near the cathode before the oligomers form the long polymer chain. Consequently, a frozen pitch gradient by such an in-situ-electric-field-assisted dynamic ion-dragging effect leads to the formation of a pitch gradient along the electrical field direction. After removing the DC bias, the as-cured polymer is observed to have frozen such a gradient pitch feature without recoverable change. As a result, the PSCLC reflector exhibits steady bandwidth broadening of 480 nm in the IR region, which provides the potential for saving energy as a smart window.

Paintable temperature-responsive cholesteric liquid crystal reflectors encapsulated on a single flexible polymer substrate

Temperature-responsive photonic cholesteric liquid crystal reflectors deposited on flexible single substrates, encapsulated by protective polymer layers generated by photo-enforced stratification.

Facet-dependent electro-optical properties of cholesteric liquid crystals doped with Cu2O nanocrystals

Excellent electro-optical (E-O) performances are essential for high-quality reflective cholesteric liquid crystal (LC) displays, but are often limited by the high driving voltages required by these displays. Dispersing functional nanomaterials into the LCs has emerged as a promising approach to achieve outstanding E-O properties. In this work, we report the facet-controlled E-O properties of a chiral nematic LC (N*LC) doped with cubic, octahedral, and rhombic dodecahedral Cu2O. The outstanding E-O properties of the doped systems are related to the interaction between the liquid crystals and Cu2O dopants with different exposed crystal planes. Doping with octahedral and rhombic dodecahedral Cu2O reduces the stability of the planar state, as a result of both the surface abundance of active Cu atoms that interact with the polarized LC molecules, and the large amounts of vertexes and edges on the crystal surfaces, which accelerate the transition from the planar to the focal conic state under an applied electric field. Rhombic Cu2O is the most effective dopant for improving the E-O properties of the present LCs, resulting in a 65.31% reduction of the threshold voltage. The facet and morphology effects highlighted in this work provide a new pathway to develop excellent energy-saving meso-materials with exposed high-reactivity facets, improving their potential applications in electro-optical technologies and information displays.

Photoluminescence enhancement of amino-functionalized graphene quantum dots in two-dimensional optical resonators.

This paper reports on the emission characteristics of amino-functionalized graphene quantum dots (af-GQDs). We employed the variable stripe length method to measure the net optical gain of af-GQDs. Photoluminescence emission was enhanced through the efficient confinement of photons using an optical resonator. The two-dimensional resonator is made up of a cholesteric liquid crystal (CLC) reflector to enable the redistribution of spontaneous emission from the af-GQDs. The proposed method was shown to increase the intensity of peak emission to more than three times that of the reference sample without a CLC reflector. The peak emission intensity of af-GQDs in the optical resonator grows exponentially with an increase in excitation energy. These results demonstrate the feasibility of two-dimensional optical amplifiers based on CLC reflectors.

51-3: Polarization-Selective Reflective Liquid Crystal Lens with Wavelength-Tunability

We demonstrated a polarization-selective reflective lens with a wavelength-tunability based on a cholesteric liquid crystal (CLC). The reflective CLC lens in a plano-convex shape acts as a biconvex lens due to the Bragg reflection, and exhibits wavelength-tunability and polarization-selectivity like the conventional CLC cell.

Photoluminescent Dye Doped Cholesteric Liquid Crystal Reflector with Micro-Sized Patterns Using Reactive Mesogens

Photoluminescent Dye Doped Cholesteric Liquid Crystal Reflector with Micro-Sized Patterns Using Reactive Mesogens

Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector.

Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

18.2: Invited Paper: Flexible eWriter Technology and Applications

Reflective cholesteric liquid crystal display based flexible eWriters have remarkable writing feel, are rugged, high contrast, and require very little power to operate. These eWriters are redefining the writing space in this increasingly digital world. Flexible eWriters have been rapidly penetrating the consumer markets and now are poised to add new and unique applications in consumer, education, healthcare, and other fields. These applications and the eWriter technology will be discussed in detail.

콜레스테릭 액정 셀에서 염료를 이용한 Color Shift 제거

We propose a reflective cholesteric liquid crystal cell with reduced color shift, using dichroic dyes. Color-shifted light is absorbed by dye molecules in the proposed dye-doped cholesteric liquid crystal (DDCLC) cell. We have shown that the color shift can be reduced by increasing the concentration of dye molecules in the proposed DDCLC cell.

Bichromatic tuning of reflection bands in integrated CLC reflectors for optical switches, gates, and logic

This study investigates the bichromatic tuning of cholesteric liquid crystal (CLC) reflection bands from reflectors containing chiral azo dopants. Because the chiral azo molecules change their helical twist power in reversible photoisomerization, the reflection bands of the CLCs are modulated using purple and green laser beams. The CLC reflectors are integrated into an optical gate that can be used to modulate output spectra. We also apply the integrated system in optical switching and logic.

Dual-frequency bistable reflective cholesteric liquid crystal displays based on chiral-flexoelectric polarisation dispersion

A new approach to switching between states in reflective cholesteric liquid crystal displays is demonstrated that relies on the dispersion in the cholesteric material's dielectric properties due to flexoelectricity. Flexoelectric polarisation allows the device to be switched into a weakly scattering focal-conic state at low frequencies, while at higher frequencies the device is driven into the reflective Grandjean state. The non-conventional dual-frequency effect allows driving between states in both directions. The cross-over frequency can be as low as 300 Hz, orders of magnitude smaller than other dual-frequency effects. Devices of various reflective colours are demonstrated and have favourable contrast ratios, viewing angles, and switching behaviours at room temperature. The technique potentially affords a greater flexibility in surface alignment conditions, driving schemes, material parameters, and use of polymer networks in cholesteric devices than other switching methods.