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Nano-size distribution of birefringent liquid crystal droplets embedded in the polymer matrix gives rise to an optically isotropic liquid crystal (OILC) phase and the device utilizing OILC composite opens the possibility of alignment layer-free, flexible liquid crystal displays with fast response time. However, few critical issues such as high operating voltages and feeble light scattering are remained to be overcome before their widespread applications. To rectify the drawbacks, a small amount of conductive and anisotropic functionalized carbon nanotubes (f-CNTs) is doped into conventional OILCs. Consequently, switchable electro-optic properties of the proposed approach reveal that driving voltage is decreased by18.7%, response time becomes fast by 27%, and feeble light leakage is reduced to around 3.8 times, compared to conventional one, which helps the proposed approach to be competitive for display and photonic applications of film type of OILCs.

Photonic band gap materials have the ability to modulate light. When they can be dynamically controlled beyond static modulation, their versatility improves and they become very useful in scientific and industrial applications. The quality of photonic band gap materials depends on the tunable wavelength range, dynamic controllability, and wavelength selectivity in response to external cues. In this paper, we demonstrate an electrically tunable photonic band gap material that covers a wide range (241 nm) in the visible spectrum and is based on a monodomain blue-phase liquid crystal stabilized by nonmesogenic and chiral mesogenic monomers. With this approach, we can accurately tune a reflection wavelength that possesses a narrow bandwidth (27 nm) even under a high electric field. The switching is fully reversible owing to a relatively small hysteresis with a fast response time, and it also shows a wider viewing angle than that of cholesteric liquid crystals. We believe that the proposed material has the potential to tune color filters and bandpass filters. A material that reflects light of a specific and electrically controllable color has been created by researchers in South Korea and the USA. A structure made up of a repeating pattern can significantly influence the way a wave, such as sound or light, passes through it when the wavelength is similar to the pattern’s periodicity. This principle is the basis of photonic band gap materials, which can block transmission of light of a specific color. A team led by Min Su Kim from Johns Hopkins University, Baltimore, and Seung Hee Lee from Jeonbuk National University, Jeonju, produced a liquid-crystal-based photonic band gap material that reflects spectrally pure visible light. An applied electrical field could tune the specific color across a wide range of wavelengths. The material could be used for tunable color optical filters. An electrically tunable photonic band gap structure of monodomain blue phases shows the wide tunable band gap range of ~241 nm. A novel chiral monomer enables us to stabilize the blue phases and to induce electrostriction upon biased electric fields. This device also exhibits a color gamut 85% of NTSC with high color purity owing to narrow bandwidth of 31 nm.

We have demonstrated a paper-like diffractive film in which nano-structured liquid crystal droplets are embedded in elastomeric monomer incorporated polymer matrix by polymerization induced phase-separation. The film with voltage-tunable phase grating exhibits an optically isotropic phase with high transparency and an effective chromatic diffraction for an incident white light with sub-millisecond switching time. In addition, the proposed diffractive film is exhibiting excellent chemical stability against organic and inorganic solvents. In this paper, the diffraction properties of test films depending on incident polarization direction, wavelength, and spatial dispersion are characterized. Easy processing and optically isotropic nature of the film imparts potential applications to flexible electro-optic devices that can be widely implemented in wearable photonics.

Flexible devices require much higher reliability upon device driving. Because the voltage-driven liquid crystal devices show higher reliability than current-driven devices, flexible liquid crystal devices are relatively suit to open a new application fused with flexible electronics in displays and photonic devices. One plausible approach is to utilize polymer-dispersed liquid crystals with nanoscale droplets of liquid crystals dispersed in a polymer matrix. One of the most significant shortcomings with that approach is high operating voltage due to the strong molecular interaction between liquid crystals and polymer matrices, low fill factor of the system, and a voltage drop in the dielectric polymer matrix. Herein, we show a polymer composite that is blended with conductive PEDOT:PSS polymer to enhance the dielectric property of the system. The composite polymer matrix we show in this work provides a pathway of electric fields via the PEDOT:PSS when applying voltages and eventually reduces the voltage drop in the composite matrix. As a consequence of the enhanced electric field, the Kerr constant and transmittance of probe light passing through the sample mixing with 3.5 wt% of PEDOT:PSS increases up to 11.7 and 37.3% and the operating field decreases up to 17.2%. Furthermore, the rising times are enhanced up to 30% while the decaying times remain the same. We believe that this approach will greatly contribute to the realization of flexible liquid crystal devices.

We have demonstrated an electrically tunable less polarization sensitive and fast response nanostructured polymer dispersed liquid crystal (nano-PDLC) diffraction grating. Fabricated nano-PDLC is optically transparent in visible wavelength regime. The optical isotropic nature was increased by minimizing the liquid crystal droplet size below visible wavelength thereby eliminated scattering. Diffraction properties of in-plane switching (IPS) and fringe-field switching (FFS) cells were measured and compared with one another up to four orders. We have obtained a pore-type polymer network constructed by highly interlinked polymer beads at which the response time is improved by strong interaction of liquid crystal molecules with polymer beads at interface. The diffraction pattern obtained by transparent nano-PDLC film has several interesting properties such as less polarization dependence and fast response. This device can be used as transparent tunable diffractor along with other photonic application.

A liquid crystal device with optically isotropic liquid crystal (OILC) phase induced from polymerization-induced phase separation exhibit a fast response time and a proper dark state. Its electro-optic performances are highly influenced by a kind of materials and processing conditions. Here, the effect of materials and phase-separation on the electro-optic performance of OILC device has been investigated by utilizing the acrylate and the thiol-ene monomer mixtures. The optically isotropic phase was analyzed with scattering theory, and it was revealed that a novel polymer network structure of acrylate mixture is free of scattering and yields a higher on-state transmittance, enhanced by ~50%. By tuning the monomer ratio and UV intensity, an excellent transparent film was obtained and, in addition, the response time was improved by ~40%. The excellent black state and its flexibility can be applied to flexible liquid crystal photonic and display devices.

Feeble light leakage in a dark state of conventional optically isotropic liquid crystal (OILC) device has a strong impact on the contrast ratio of a liquid crystal (LC) device. In order to overcome such intrinsic problem, we proposed an OILC in which the LC directors inside droplets are twisted by introducing chirality. The light leakage is effectively suppressed by matching the refractive indices between LC and polymer matrix; consequently, we achieved a high contrast ratio, 1:1401. Interestingly, the on-state transmittance is enhanced by ~49% compared to conventional OILC. The response time was also improved and the hysteresis was suppressed to be negligible. The improved electro-optic performances of the proposed OILC device would give diverse applications in upcoming flexible display and various photonic devices.

We have demonstrated an ultra-fast switching and efficient polymer stabilized blue phase liquid crystal (PS-BPLC) diffraction grating utilizing a chiral monomer. We have obtained a 0.5 ms response time by a novel polymer stabilization method which is three times faster than conventional PS-BPLC. In addition, the diffraction efficiency was improved 2% with a much wider phase range and the driving voltage to switch the device is reduced. The polarization properties of the diffracted beam are unaffected by this novel polymer stabilization. This device can be useful for future photonic applications.