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This study investigates and develops for the first time an electrically tunable liquid-crystal–polymer composite laser with a symmetric sandwich structure. This structure includes two identical pol... more...

This study applies a low-cost solvothermal method to synthesize all-inorganic (lead-free cesium tin halide) perovskite quantum dots (AIPQDs) and to fabricate AIPQD-doped lasers with cholesteric liquid crystal (CLC) lasing cavities. The lasers present highly qualified lasing features of low threshold (150 nJ/pulse) and narrow line width (0.20 nm) that are attributed to the conjunction of the suppression of photoluminescence (PL) loss caused by the quantum confinement of AIPQDs and the amplification of PL caused by the band-edge effect of the CLC-distributed feedback resonator. In addition, the lasers possess highly flexible lasing-wavelength tuning features and a long-term stability under storage at room temperature and under high humidity given the protective role of CLC. These advantages are difficult to confer to typical light-emitting perovskite devices. Given these merits, the AIPQD-doped CLC laser device has considerable potential applications in optoelectronic and photonic devices, including lighting, displays, and lasers. more...

This work presents an electro- and photo-controllable spatial filter that is based on a liquid crystal (LC) film with a photoconductive layer. The controllable spatial filter can be formed because of the controllability of the photoelectro-induced screen effect of the space charge in the LC cell. An applied dc voltage or incident pumped intensity can be controlled to enable different spatial distributions of the diffraction pattern of the target object to be selected for filtering by the LC cell, such that various reconstructed images can be obtained. A simulation using Fourier analysis is developed, and its results agree closely with experimental results. Additionally, the LC spatial filter has the extra advantage of controllable low or high filtering functions: they are controlled by switching the configuration between normally black and normally white modes. more...

This study demonstrates for the first time a continuously tunable photonic bandgap (PBG) of wide spectral range based on a blue phase (BP) wedge cell. A continuously shifting PBG of the BP wedge cell occurs due to the thickness gradient of the wedge cell at a fixed temperature. The wedge cell provides a gradient of boundary force on the LCs and thus forms a distribution of BP crystal structure with a gradient lattice. Additionally, a spatially tunable lasing emission based on a dye-doped BP (DDBP) wedge cell is also demonstrated. The tunable band of the PBG and lasing emission is about 130 nm and 70 nm, respectively, which tuning spectral ranges are significantly wider than those of CLC and DDCLC wedge cells, respectively. Such a BP device has a significant potential in applications of tunable photonic devices and displays. more...

This paper reports on the color-tunable emission from organic semiconductor films composed of Alq3 doped with DCM. The composite system was excited using an ultraviolet laser to enable the emission of light from green to red regions of the visible spectra based on the nonradiative Forster transfer mechanism. A cholesteric liquid crystal film was used as a blue emitter, while simultaneously serving as a temperature-controlled optical filter based on Bragg reflection to tune the emission color of the organic semiconductor films. The corresponding color points on the CIE chromaticity diagram located near the black-body locus exhibited a wide color-tuning range within a narrow temperature range. more...

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. more...

In this study, we demonstrate the fabrication of an electrically switchable Fresnel lens based on surface-stabilized ferroelectric liquid crystals (SSFLCs) with polymer networks. The Fresnel lens was fabricated by injecting a monomer-doped ferroelectric liquid crystal into an extremely thin cell and exposing the cell to ultraviolet light through a Fresnel-zone-plate mask. The fabricated Fresnel lens consisted of the SSFLC and polymer-stabilized SSFLC structures. The focusing effect can be switched on by applying an appropriate voltage, and the characteristics of FLCs enable a rapid response on the order of milliseconds. more...

This study shows the results of a photonic band-edge laser using dye-doped cholesteric liquid crystals (CLCs) combined with silver (Ag) nanoparticles. When the Ag nanoparticle surface plasmon resonance wavelength matched the excitation source wavelength, the large optical fields provided by surface plasmons increased the fluorescence of dye molecules by enhancing the molecular excitation rate, achieving a low lasing threshold and high pumping efficiency. more...

This study demonstrates a tunable Fresnel lens in an azo-dye-doped liquid crystal (ADDLC) film using an interference technique. One Fresnel-patterned green beam using a Sagnac interferometer irradiated the UV-illuminated ADDLC cell, yielding a concentric zone plate distribution with homeotropic and isotropic structures in bright and dark regions of the green interference pattern. The proposed Fresnel lens is polarization independent, focus tunable, and the focusing efficiency of the device can be optically controlled. more...

This study investigated optically controllable biphotonic gratings (BGs) in azo-dye-doped cholesteric liquid crystals. The BGs were formed under the illumination of one green beam with the simultaneous irradiation of an interference field generated by two coherent red beams. This study ascribes the formation of the BGs to the green-beam-induced dye reorientation and elongation of the helical pitch through trans-cis isomerization and red-beam-induced suppression of dye reorientation and elongation of the helical pitch by cis-trans back isomerization. The diffraction characteristics strongly depended on the helical pitch of the cholesteric structure, the polarization state of the probe beam, and the relative intensity of the green and red beams. Application of the finite-difference time-domain method demonstrated that the model of photoinduced distortion of the cholesteric liquid crystal structure satisfactorily explains this dependence. more...

This paper reports the electrical tuning of a lasing in a liquid crystal (LC) sandwich structure. A dye-doped nematic LC (NLC) layer is sandwiched between two CLC layers to act as a phase retarder with the CLC layers acting as cavity mirrors, for the selective reflection of light in the photonic band with the same sense of helix handedness as that of the CLC layers. The transmittance spectrum of the sandwich cell provides a large range of modulation due to the wavelength dependent nature of phase retardation between the optical eigenmodes in the NLC layer. Lasing occurs at wavelengths corresponding to the maximum transmittance within the reflection band of the CLC layers. The application of voltage to the NLC layer makes it possible to shift the wavelengths of maximum transmittance, thereby tuning the wavelength of lasing. In these experiments, an applied voltage of 1.25 V was sufficient to shift the lasing peak wavelength by approximately 47 nm. more...

This study presents a theoretical analysis and experimental demonstration of an electrically controllable Fresnel lens in a 90° twisted nematic liquid crystal cell. The cell gap was chosen to satisfy the Gooch-Tarry conditions, and therefore, the polarization rotation effect was valid regardless of the incident polarization direction. The polarization sensitivity of the diffraction efficiency of the 90° twisted nematic Fresnel lens was dependent on the applied voltage regime. Theoretical calculations effectively explain the experimental results. more...

This study demonstrated a color-filter Fresnel lens in polymer-stabilized cholesteric liquid crystals (PSCLCs). The liquid crystals in the odd zones were in a planar cholesteric texture, the Bragg reflection band of which was located in the infrared region. The even zones had a focal conic texture. The linearly polarized white light passing through the planar texture underwent optical activity. The light with the polarization perpendicular to the transmission axis of an analyzer behind the PSCLC Fresnel lens was eliminated. Changing the orientation of the transmission axis of the analyzer can select the color of the focusing image. more...

A tunable transflective Fresnel lens is demonstrated by using an interference technique based on an azobenzene-doped cholesteric liquid crystal (AzoCLC) film. The AzoCLC film was irradiated simultaneously by ultraviolet light and one Fresnel-patterned green beam derived from a Sagnac interferometer. The linearly polarized incident beam, the wavelength of which was in the Bragg reflection band of the AzoCLC, was simultaneously transmitted and reflected from the planar textures in the binary zone plate, and constructively added and focused in the transmissive and reflective directions, respectively. Focusing efficiency was controlled by adjusting the intensity of the Fresnel-patterned green beam. more...

This investigation elucidates for the first time electrically controllable random lasers below the threshold voltage in dye-doped liquid crystal (DDLC) cells with and without adding an azo-dye. Experimental results show that the lasing intensities and the energy thresholds of the random lasers can be decreased and increased, respectively, by increasing the applied voltage below the Fréedericksz transition threshold. The below-threshold-electric-controllability of the random lasers is attributable to the effective decrease of the spatial fluctuation of the orientational order and thus of the dielectric tensor of LCs by increasing the electric-field-aligned order of LCs below the threshold, thereby increasing the diffusion constant and decreasing the scattering strength of the fluorescence photons in their recurrent multiple scattering. This can result in the decrease in the lasing intensity of the random lasers and the increase in their energy thresholds. Furthermore, the addition of an azo-dye in DDLC cell can induce the range of the working voltage below the threshold for the control of the random laser to reduce. more...

This work investigates a novel color cone lasing emission (CCLE) based on a one-dimensional photonic crystal-like dye-doped cholesteric liquid crystal (DDCLC) film with a single pitch. The lasing wavelength in the CCLE is distributed continuously at 676.7-595.6 nm, as measured at a continuously increasing oblique angle relative to the helical axis of 0-50 degrees . This work demonstrates that lasing wavelength coincides exactly with the wavelength at the long wavelength edge of the CLC reflection band at oblique angles of 0-50 degrees . Simulation results of dispersion relations at different oblique angles using Berreman's 4X4 matrix method agrees closely with experimental results. Some unique and important features of the CCLE are identified and discussed. more...

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. more...

This investigation elucidates one- and biphotonic reorientational nonlinearities in dye-doped nematic liquid crystals (DDNLCs) and dye-doped cholesteric liquid crystals (DDCLCs) using the Z-scan technique. Three cases—DDNLCs stimulated by one green beam polarized linearly parallel and perpendicular to the LC director and DDCLCs stimulated by one circularly polarized green beam—are considered without and with simultaneous stimulation by one linearly polarized red beam. Green-beam-induced dye reorientation through trans-cis isomerization and red-beam-induced suppression of dye reorientation by cis-trans back isomerization are responsible for the one- and biphotonic nonlinearities, respectively. The nonlinear refractive indices obtained herein are two orders of magnitude larger than those obtained elsewhere [Fuh et al., Opt. Exp. 13, 10634 (2005)]. The induced biphotonic nonlinearities can be modulated by controlling the intensity of the red beam in different ways due to the difference among the capabilities... more...