Dye-doped Cholesteric Liquid Crystal Cell Research Articles

Optimization of dye mixing for achromatic transmittance control with a dye-doped cholesteric liquid crystal cell

Transmittance-control liquid crystal (LC) devices require a high transmittance difference between their transparent and opaque states. Dye-doped cholesteric LC (CLC) cells have been widely used for transmittance control because they provide the highest transmittance difference among LC modes. However, the color of a CLC cell is different from that of a homogeneously-aligned LC cell. In this paper, we present a systematic approach to find the optimal dye concentrations for the desired black color in a dye-doped CLC cell. To achieve the desired black color in a dye-doped CLC cell, we consider its transmission spectrum in our numerical calculations. The optimal concentration for each individual dye is found through an iterative process. We demonstrate that a dye-doped CLC cell designed by considering its transmission spectrum can provide a true black color in the CIE 1931 color space.

Lasing properties of a cholesteric liquid crystal containing aggregation-induced-emission material.

We demonstrate band edge lasing action from a cholesteric liquid crystal (CLC) containing an aggregation-induced-emission (AIE) dye as gain material. AIE materials do not suffer aggregation-caused quenching, have strong resistance to photobleaching, and can show large Stokes shift. The amplified spontaneous emission (ASE) and lasing emission of the dye-doped CLC cell have been characterized, the lasing threshold has been estimated, and its resistance to photobleaching has been measured. AIE materials with their unique properties are especially suitable for acting as gain materials in liquid crystal lasers where defect structures lower the threshold for nanoscale aggregation effects. Our studies have shown that such AIE-dye-doped CLC is capable of lasing action with unusually large Stokes shift at moderate threshold with strong resistance to photobleaching.

Simultaneous control of haze and transmittance using a dye-doped cholesteric liquid crystal cell

We propose a light shutter device using dye-doped cholesteric liquid crystals for a high-visibility see-through display. In the focal-conic state, the proposed device can perfectly block the background image through simultaneous use of light scattering and absorption effects in a single-layered structure. By switching the proposed device placed at the backside of a see-through display, we can choose transparent or high-visibility display modes in a see-through display.

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.

Performance evolution of color cone lasing emissions in dye-doped cholesteric liquid crystals at different fabrication conditions.

This work investigates the performance evolution of color cone lasing emissions (CCLEs) based on dye-doped cholesteric liquid crystal (DDCLC) cells at different fabrication conditions. Experimental results show that the energy threshold (E(th)) and relative slope efficiency (η(s)) of the lasing signal emitted at each cone angle (0°-35°) in the CCLE decreases and increases, respectively, when the waiting time in a homogenously rubbed aligned DDCLC cell is increased from 0 hr to 216 hr (9 days). This result occurs because defect lines gradually shrink with the anchoring of the surface alignment when the waiting time is increased. Hence, the scattering loss decreases, and the dwelling time of the fluorescence photons in the resonator increases, which in turn enhances the CCLE performance. With the aligned cell given the pretreatment of a rapid annealing processing (RAP), the waiting time for obtaining an optimum CCLE can markedly be reduced sixfold. The surface alignment of the DDCLC cell also plays a necessary role in generating the CCLE. This work provides an insight into the temporal evolution of the performance for the CCLE laser and offers a method (RAP) of significantly speeding up the formation of a CCLE laser with optimum performance.

Optically controllable transflective spatial filter with high- and low-pass or notch- and band-pass functions based on a dye-doped cholesteric liquid crystal film

This study developed an optically controllable transflective spatial filter in a dye-doped cholesteric liquid crystal (DDCLC) film. The mechanism to induce the spatial filter is attributable to the photoisomerization-induced controllability of the redshift of the reflection band in the DDCLC cell. At various pumped intensities, different spatial distributions of the diffraction pattern of the object can be selected to be filtered, such that high- and low-pass or notch- and band-pass transmitted and reflected images can be simultaneously obtained, respectively.

Chiroptical Properties Induced in Chiral Photonic‐Bandgap Liquid Crystals Leading to a Highly Efficient Laser‐Feedback Effect

Highly efficient lasing action is obtained by exploiting the chiroptical characteristics of the ground and excited states of an achiral fluorescent dye doped in a cholesteric liquid crystal (CLC) medium (see figure). The highly efficient lasing action from the oblique dye-doped CLC cell is produced by tuning the ellipticity angle of the excitation beam while taking into consideration the molecular helical sense of the cholesteric host.

Spatially tunable laser emission in dye-doped photonic liquid crystals

A spatially tunable laser emission of the dye-doped cholesteric liquid crystal (CLC) cell using a one-dimensional temperature gradient is demonstrated. The photoexcitation of dye-doped CLC device using a frequency-doubled pulsed Nd: yttrium–aluminium–garnet laser gives rise to laser emission in the yellow-red spectral range. The lasing wavelength is widely tunable from 577 to 670 nm by shifting the position of the dye-doped CLC cell with respect to the pumping beam. The lowest excitation energy and maximum lasing efficiency occur at λ∼605nm which corresponds to the peak fluorescence emission of the dye.

Doubling the optical efficiency of a chiral liquid crystal laser using a reflector

A simple method for doubling the laser emission efficiency of Nd:yttrium-aluminum-garnet pumped cholesteric liquid crystal (CLC) photonic band edge lasers is demonstrated. By adding a mirror or a CLC reflector behind the dye-doped CLC cell, the lasing intensity is nearly doubled. For the mirror reflector, due to the mixing of right-handed and left-handed circular polarizations, the output laser is basically unpolarized. However, for the CLC reflector, a partially coherent laser can be built in which either right-handed or left-handed circularly polarized laser could dominate the output, depending on the handedness of the cholesteric structure.

Lowering the Lasing Threshold by Introducing Cholesteric Liquid Crystal Films to Dye-Doped Cholesteric Liquid Crystal Cell Surfaces

The effect of the introduction of polymer cholesteric liquid crystal (PCLC) films on the threshold of dye-doped cholesteric liquid crystal (CLC) distributed feedback (DFB) cavity lasing has been investigated. A PCLC film used to reflect a pump beam brings about the efficient use of incident energy, whereas a PCLC film used to reflect the emission contributes to amplifying the stimulated emission. As a result, the cell, in which both PCLC films are introduced, gains about a 60% reduction in the lasing threshold. It is also found that a lasing threshold exists not only for the excitation energy but also for the emission intensity. Namely, the lasing starts to occur at a certain emission level irrespective of the cell structures.

Phototunable photonic bandgap in a chiral liquid crystal laser device

This letter describes the phototunable photonic bandgap of cholesteric liquid crystal (CLC) doped with a fluorescent dye to generate the distributed feedback effect. Photoirradiation of the dye-doped CLC cell under UV light at 254 nm resulted in continuous changes in the chiral photonic bandgap from 550 to 720 nm due to photolysis reaction of the cholesteryl iodide embedded in the CLC host. We showed that the laser oscillation wavelength could be controlled in a wide wavelength range from 610 to 700 nm by photoassisted adjustment of CLC photonic bandgaps.