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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.

Electrical tuning of photonic bandgap (PBG) of cholesteric liquid crystal (CLC) without deformation within the entire visible region at low voltages is not easy to achieve. This study demonstrates low-voltage-tunable PBG in full visible region with less deformation of the PBG based on smart materials of ferroelectric liquid crystal doped CLC (FLC-CLC) integrating with electrothermal film heaters. Experimental results show that the reflective color of the FLC-CLC can be low-voltage-tuned through entire visible region. The induced temperature change is induced by electrically heating the electrothermal film heaters at low voltages at near the smectic-CLC transition temperature. Coaxial electrospinning can be used to develop smart fibrous devices with FLC/CLC-core and polymer-shell which color is tunable in full visible region at low voltages.

We show that micron-scale two-dimensional (2D) honeycomb microwells can significantly improve the stability of blue phase liquid crystals (BPLCs). Polymeric microwells made by direct laser writing improve various features of the blue phase (BP) including a dramatic extension of stable temperature range and a large increase both in reflectivity and thermal stability of the reflective peak wavelength. These results are mainly attributed to the omni-directional anchoring of the isotropically oriented BP molecules at the polymer walls of the hexagonal microwells and at the top and bottom substrates. This leads to an omni-directional stabilization of the entire BPLC system. This study not only provides a novel insight into the mechanism for the BP formation in the 2D microwell but also points to an improved route to stabilize BP using 2D microwell arrays., Comment: 16 pages, 5 figures

The scientists in the field of liquid crystal (LC) have paid significant attention in the exploration of novel cholesteric LC (CLC) polymer template (simply called template) in recent years. The self-assembling nanostructural template with chirality can effectively overcome the limitation in the optical features of traditional CLCs, such as enhancement of reflectivity over 50%, multiple photonic bandgaps (PBGs), and changeable optical characteristics by flexibly replacing the refilling LC materials, and so on. This work fabricates two gradient-pitched CLC templates with two opposite handednesses, which are then merged as a spatially tunable and highly reflective CLC template sample. This sample can simultaneously reflect right- and left-circularly polarized lights and the tunable spectral range includes the entire visible region. By increasing the temperature of the template sample exceeding the clearing point of the refilling LC, the light scattering significantly decreases and the reflectance effectively increase to exceed 50% in the entire visible region. This device has a maximum reflectance over 85% and a wide-band spatial tunability in PBG between 400 nm and 800 nm which covers the entire visible region. Not only the sample can be employed as a wide-band spatially tunable filter, but also the system doping with two suitable laser dyes which emitted fluorescence can cover entire visible region can develop a low-threshold, mirror-less laser with a spatial tunability at spectral regions including blue to red region (from 484 nm to 634 nm) and simultaneous lasing emission of left- and right-circular polarizations.

This investigation is the first report of an optically stable and tunable laser using a quantum-dot (QD) embedded cholesteric liquid crystal (QD-CLC) microresonator with an added chiral-azobenzene moiety. The QD nanocrystal, CLC, and chiral-azobenzene play key roles in the highly stable fluorescence nano-emitter, microresonator, and photo-tuner in the photonic bandgap (PBG) of the CLC and associated lasing output, respectively. Experimental results show that both the PBG and lasing emission of the QD-CLC composite sample can be reversibly tuned under successive irradiation with UV and blue beams. The all-optical tunability of the laser is attributed to the successive elongation and shrinkage of the CLC pitch induced by UV- and blue beam-irradiation induced trans–cis and cis–trans back isomerizations of chiral-azobenzene, respectively. In addition, this composite laser has a high damage threshold (>85 μJ per pulse), and thus shows a uniquely high optical stability. This work has successfully opened up an opportunity for developing QD coherent light sources or lasers with good optical stability and tunability (e.g., optically stable and tunable single photon lasers).

Biosensors based on liquid crystal (LC) materials can be made by employing the sensitive interfacial effect between LC molecules and alignment layers on substrates. In the past, the optical texture observation method was used in the LC biosensor field. However, the method is limited by a complicated fabrication process and quantitative reproducibility of results that bv evidence that both the reliability and accuracy of LC biosensors need to be improved. In this report, we demonstrate that cholesteric LC (CLC) cells in which one substrate is coated with a vertically aligned layer can be used as a new sensing technology. The chirality of the single vertically anchored (SVA)/CLC biosensor was tested by detecting bovine serum albumin (BSA), a protein standard commonly used in the lab. The colors and corresponding spectrum of the SVA/CLC biosensor changed with the BSA concentrations. A detection limit of 1 ng/ml was observed for the SVA/CLC biosensor. The linear optical properties of the SVA/CLC biosensor produced cheap, inexpensive, and color-indicating detection of biomolecules, and may promote the technology of point-of-care devices for disease-related biomarker detection.

This work successfully develops a largely-gradient-pitched polymer-stabilized blue phase (PSBP) photonic bandgap (PBG) device with a wide-band spatial tunability in nearly entire visible region within a wide blue phase (BP) temperature range including room temperature. The device is fabricated based on the reverse diffusion of two injected BP-monomer mixtures with a low and a high chiral concentrations and afterwards through UV-curing. This gradient-pitched PSBP can show a rainbow-like reflection appearance in which the peak wavelength of the PBG can be spatially tuned from the blue to the red regions at room temperature. The total tuning spectral range for the cell is as broad as 165 nm and covers almost the entire visible region. Based on the gradient-pitched PSBP, a spatially tunable laser is also demonstrated in this work. The temperature sensitivity of the lasing wavelength for the laser is negatively linear and approximately −0.26 nm/°C. The two devices have a great potential for use in applications of photonic devices and displays because of their multiple advantages, such as wide-band tunability, wide operated temperature range, high stability and reliability, no issue of hysteresis, no need of external controlling sources and not slow tuning speed (mechanically).

The present study develops and investigates for the first time a unique spatial continuously tunable cone laser based on a dye-doped cholesteric liquid crystal (DDCLC) film with an LC-birefringence (Δn) gradient. A continuous Δn variation can be generated in a cell by diffusion and self-organization of CLC after four DDCLC mixtures with a discrete variation of Δn are successively injected into an empty cell. Not only the CLC photonic structure but also the lasing wavelength and the cone angle of the obtained conically symmetric emitted lasing ring can be tuned continuously by continuously changing the pumped position of the cell with an Δn gradient. The continuous tunabilities in the lasing wavelength and the corresponding emitted cone angle of the lasing ring are 605.8 → 568.1 nm and 29° → 50°, respectively, within a spatial interval of about 33 mm in the cell.

We present the simulation, fabrication, and optical characterization of low-index polymeric rod-connected diamond (RCD) structures. Such complex three-dimensional photonic crystal structures are created via direct laser writing by two-photon polymerization. To our knowledge, this is the first measurement at near-infrared wavelengths, showing partial photonic bandgaps for this structure. We characterize structures in transmission and reflection using angular resolved Fourier image spectroscopy to visualize the band structure. Comparison of the numerical simulations of such structures with the experimentally measured data show good agreement for both P- and S-polarizations.

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.

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.

A comparative study of the optical and electro-optic properties of polymer-stabilized blue phase liquid crystal (PS-BPLC) cells with photoalignment layers and rubbing alignment layers is performed. The optical microscope (OM) images of the photoaligned and rubbing-aligned cells depict that they have uniform PS-BPLC textures, while the post-processed OM images of the two cells reveal that the former has a more uniform PS-BPLC texture than the latter. The relatively uniform PS-BPLC texture in the photoaligned cell is verified by the reflective spectra of the two cells. The voltage-dependent transmittance curves and time-dependent transmittance curves of another two photoaligned and rubbing-aligned cells with in-plane-switching (IPS) electrodes indicate that the contrast ratio of the former is 41% larger than that of the latter, and the operation voltage (response time) of the former is 11 (10) % smaller than that of the latter. This result arises from the fact that the photoaligned cell with the IPS electrodes exhibits a relatively uniform PS-BPLC texture and relatively weak voltage-shielding effect. Therefore, the photoalignment can improve the electro-optic properties of PS-BPLC cells with alignment layers. The photoaligned and rubbing-aligned cells have no thermal hysteresis in low voltage regions due to the uniform surface alignment layers, and exhibit very low voltage-induced hysteresis in high voltage regions owing to no thermal hysteresis. Briefly, uniform surface alignment layers can eliminate the thermal hysteresis of PS-BPLC cells, suppressing their voltage-induced hysteresis. In other words, the hysteresis of PS-BPLC cells with alignment layers is independent of alignment methods as long as the alignment layers have strong surface anchoring energy.

This work reports, for the first time, a dielectrophoresis (DEP) effect-induced motion of liquid crystal (LC) droplets in an LC/monomer mixture sample with a poly-(N-vinyl carbazole) PVK-coated substrate without an external voltage. With the UV pre-irradiation of the PVK layer through a binary mask, a laterally non-uniform electric field can be induced between the pre-illuminated regions and the neighboring non-pre-illuminated PVK regions near the borders of the two regions. The phase separation occurs once the temperature is lower than 50 °C and the LC droplets can form in the sample. The pre-formed non-uniform field provides a DEP-like force to manipulate the small LC microdroplets in the pre-illuminated regions to effectively migrate to the adjacent non-pre-illuminated regions. The continuous supply of the LC from the pre-illuminated regions to the adjacent non-pre-illuminated regions significantly increases the diffraction efficiency of the grating sample. This study provides an insight into developing new external-voltage-free DEP-based devices that can be applied on various fields, such as photonics, displays, and biomedicines.

This work demonstrates the feasibility of a novel photosensitive and all-optically fast-controllable photonic bandgap (PBG) device based on a dye-doped blue phase (DDBP), embedded with a low-concentration azobenzene liquid crystal (azo-LC). PBG of the DDBP can be reversibly fast-tuned off and on with the successive illumination of a weak UV and green beams. UV irradiation can transform the trans azo-LCs into bend cis isomers, which can easily disturb LCs at the boundary between the double twisting cylinders (DTCs) and the disclinations, and, then, quickly destabilize BPI to become a BPIII-like texture with randomly-oriented DTCs. Doing so may quickly destroy the BP PBG structure. However, with the successive illumination of a green beam, the BPI PBG device can be fast-turned on, owing to the fast disappearance of the disturbance of the azo-LCs on the boundary LCs via the green-beam-induced cis → trans back isomerization. The response time and irradiated energy density for turning off (on) the BP PBG device under the UV (green) beam irradiation are only 120 ms (120 ms) and 0.764 mJ/cm(2) (2.12 mJ/cm(2)), respectively, which are a thousand-fold reduction in photoswitching a traditional cholesteric LC (CLC) PBG device based on similar experimental conditions (i.e., materials used, azo-LC concentration (1 wt%), spectral position of PBG peak, sample thickness, and temperature difference for a working temperature lower than the clearing one). The BP PBG device can significantly contribute to efforts to develop a photosensitive and all-optically fast-controlling LC laser.

A controllable multi-wavelength laser from a dye-doped cholesteric liquid crystal (DDCLC) cell is demonstrated by incorporating self-assembled polyhedral oligomeric silsesquioxane (POSS) nanoparticles (NPs). Multi-wavelength lasing emission is achieved by formation of multiple planar domains; this formation is dominantly influenced by the vertical alignment of NP clusters adsorbed on the substrate surface through a rapid thermal annealing process. The multi-wavelength lasing peaks are generated through the resultant effect of multiple longitudinal resonant modes of a Fabry–Perot etalon between the cell substrates and the amplification of fluorescence photons with the resonant wavelengths within the broadening long-wavelength edge of the reflection band of the multi-domain CLC. The amount of multi-wavelength lasing peaks can be controlled by changing the POSS NP concentration and the cooling rate of the cell. Furthermore, thermo-reversible control of the multi-wavelength lasing emission can be attained by controlling the thermally induced phase separation process of the POSS/DDCLC cell via a heating/cooling cyclic process.

This study systematically investigates the morphological appearance of azo-chiral dye-doped cholesteric liquid crystal (DDCLC)/polymer coaxial microfibers obtained through the coaxial electrospinning technique and examines, for the first time, their photocontrollable reflection characteristics. Experimental results show that the quasi-continuous electrospun microfibers can be successfully fabricated at a high polymer concentration of 17.5 wt% and an optimum ratio of 2 for the feeding rates of sheath to core materials at 25 °C and a high humidity of 50% ± 2% in the spinning chamber. Furthermore, the optical controllability of the reflective features for the electrospun fibers is studied in detail by changing the concentration of the azo-chiral dopant in the core material, the UV irradiation intensity, and the core diameter of the fibers. Relevant mechanisms are addressed to explain the optical-control behaviors of the DDCLC coaxial fibers. Considering the results, optically controllable DDCLC coaxial microfibers present potential applications in UV microsensors and wearable smart textiles or swabs.

This study elucidates electrically and all-optically controllable random lasers in dye-doped liquid crystals with adding a photoisomerizable dye. The lasing intensities and the energy thresholds of the random lasers can be electrically controlled below the Freedericksz transition threshold or all-optically controlled sequentially with a two-step exposure of UV and green beams. The below-threshold-electric- and all-optical controllabilities of the random lasers are attributable to the effective change of the spatial fluctuation of the orientational order and thus of the dielectric tensor of LCs by changing the electric-field-aligned order of LCs below the threshold and via the isothermal nematic-isotropic phase transition of LCs, respectively; thereby changing the diffusion constant and thus the scattering strength of the fluorescence photons in their recurrent multiple scattering. This can result in the change in the lasing intensity and thus the energy threshold of the random lasers.

A novel demonstration of an all-optically controllable dye-doped liquid crystal infiltrated photonic crystal fiber (DDLCIPCF) is presented. Overall spectral transmittance of the DDLCIPCF can decrease and then increase with a concomitant red-shift of the spectrum curve with increasing irradiation time of one UV beam. Continuing irradiation of one green beam following UV illumination on the DDLCIPCF can cause the transmission spectrum to recover completely. The reversible all-optical controllability of the photonic band structure of the fiber is attributable to the isothermal planar nematic (PN) → scattering (S) → isotropic (I) and I → S → PN state transitions of the LCs via the UV-beam-induced trans→cis and green-beam-induced cis→trans back isomerizations of the azo-dye, respectively, in the cladding of the DDLCIPCF. The photoinduced appearance of the S state and the variation of the index modulation between the core and the cladding of the fiber result in the variation of overall spectral transmittance and the shift of transmission spectrum, respectively.

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.

This work demonstrates, for the first time, an all-optically controllable distributed feedback (DFB) laser based on a dye-doped holographic polymer-dispersed liquid crystal (DDHPDLC) grating with a photoisomerizable dye. Intensity of the lasing emission can be reduced and increased by raising the irradiation intensity of one CW circularly-polarized green beam and the irradiation time of one CW circularly-polarized red beam, respectively. The all-optical controllability of the lasing emission is owing to the green-beam-induced isothermal nematic-->isotropic and red-beam-induced isothermal isotropic-->nematic phase transitions of the LCs via trans-->cis and cis-->trans back isomerizations of the azo-dye, respectively, in the LC-droplet-rich regions of the grating. The former (latter) mechanism can reduce (increase) the index modulation and thereby the coupling strength in the DFB grating, resulting in the decay (rise) of the lasing emission. Thermal effect is excluded from possible mechanisms causing such an optical controllability of the lasing emission.

Plasma immersion ion implantation (PIII) is a technique of material processing and surface modification, using controllable negative high voltage pulsed bias to attract the ion generated from the plasma. The method using PIII treatment quickly improves the performance of solar cell made of crystalline silicon, including monocrystalline, multicrystalline and polycrystalline silicon. Hydrogen ions are attracted and quickly implanted into solar cell under a predetermined negative pulse voltage, thus, the passivation of the crystal defects of the solar cell can be realized in a short period. Meanwhile, the properties of the antireflection layer can not be damaged as the proper operating conditions are used. Consequently, the series resistance can be significantly reduced and the filling factor increases as a result. Both the short-circuit and the open-circuit voltage can be increased. The efficiency can be enhanced.

This paper reports for the first time an electrically and thermally controllable nanoparticle (NP) random laser in a well-aligned dye-doped liquid crystal (DDLC) cell. Experimental results show that the random lasing emission is attributed to the amplification of the fluorescence via the multiple scattering of the randomly distributed NPs in the diffusion rout of the well-aligned DDLC cell. The random laser can be electrically and thermally controlled by varying the applied voltage and cell temperature, respectively. As the applied voltage is increased, the orientational change of the LCs from homogeneous to homeotropic texture decreases the dye absorption and thus the spontaneous fluorescence emission, resulting in the decrease of the random lasing emission. The random lasing intensity decreases with increasing temperature at the nematic phase and dramatically increases after the nematic→isotropic (N→I) phase transition. The result in the former stage is attributed to the decreases in the absorption and thus in the spontaneous fluorescence emission for the laser dyes because of the decrease in the order of the laser dyes with increasing temperature at the nematic phase. The result in the latter stage results from the significant decrease of the loss because of the disappearance for the strong leakage of the scattering fluorescence light through the boundaries of the LCs and the glass substrates after the N→I phase transition. Moreover, the anisotropy of the random lasing is crucially determined by two factors: the anisotropies in the spontaneous emission and the leakage of the scattering fluorescence light.

This work first reports wide-band tunable photonic bandgap (PBG) devices based on nematic-refilling cholesteric liquid crystal polymer template samples. By changing the type of refilling nematic liquid crystal (NLC) and sample cell gap, the PBG features of the template sample can be crucially influenced. A physical model related with the NLC infiltration into the template nanopores based on capillary action is used to qualitatively explain the tunable PBG features of the refilling template samples. In addition, a nearly full white (480 nm − 720 nm) spatially tunable PBG device based on a NLC-refilling template wedge cell is demonstrated.

In this Letter, we propose defect-mode lasing from a one-dimensional asymmetric photonic structure with dye-doped nematic liquid crystal as a central defect layer. The local field intensity of the distinguished single defect mode at the overlapped photonic band edges is drastically enhanced by the asymmetric structure consisting of two distinct multilayer photonic crystals. With high density of states of photons, effective output lasing emission and maximum input excitation are ensured. As a result, the single-mode lasing with a low excitation threshold of 0.2 μJ/pulse is achieved due to the combination of the defect layer and the photonic band edge effect.

This paper presents an optically wavelength-tunable and intensity-switchable dye-doped cholesteric liquid crystal (DDCLC) spherical microlaser with an azo-chiral dopant. Experimental results present that two functions of optical control - tunability of lasing wavelength and switchability of lasing intensity - can be obtained for this spherical microlaser at low and high intensity regimes of non-polarized UV irradiation, respectively. If the DDCLC microdroplet is subjected to weak UV irradiation, azo-chiral molecules may transform to the bent cis state at a low concentration rate. The effect can slightly decrease the local order of LCs and thus the helical twisting power of the CLC in the microdroplet. As a result, the CLC pitch may become slightly elongated, which will cause the gradual red-shift of both omnidirectional PBG and lasing emission of the DDCLC spherical microdroplet. In contrast, when the microdroplet is subjected to strong UV irradiation, numerous azo-chiral molecules may simultaneously change to bent cis-isomers to seriously disarrange the helical texture of the CLC, which will quickly deform the PBG and deactivate the lasing emission of the microdroplet. Prolonged irradiation of a blue beam after strong UV irradiation may cause the cis azo-chiral molecules quickly convert back rod-like trans-isomers, which may then regenerate the CLC Bragg onion and PBG structures and reactivate the lasing emission of the microdroplet. Optical control of the DDCLC spherical microlaser is realized on a scale of seconds and minutes when UV irradiation is strong and weak, respectively. The 3D DDCLC spherical microlaser is a highly promising controllable 3D micro-light source or microlaser (e.g., all-optical 3D single photon microlaser) for applications of 3D all-optical integrated photonics, laser displays, and biomedical imaging and therapy, and as a 3D UV microdosagemeter or microsensor.

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.

This study investigates, for the first time, an all-optically controllable random laser based on a dye-doped liquid crystal (DDLC) cell added with a photoisomerizable dye. Experimental results indicate that the lasing intensity of this random laser can be all-optically controlled to decrease and increase sequentially with a two-step exposure of one UV and then one green beam. All-optically reversible controllability of the random lasing emission is attributed to the isothermal nematic(N)--isotropic(I) and I--N phase transitions for LCs due to the UV-beam-induced trans--cis and green-beam-induced cis--trans back isomerizations of the photoisomerizable dye, respectively. The former and the latter can decrease and increase the spatial fluctuations of the order and thus of the dielectric tensor of LCs, respectively, subsequently increasing and decreasing the diffusion constant (or transport mean free path), respectively, and thus decaying and rising the scattering strength for the fluorescence photons in their recurrent multi-scattering process, respectively. The consequent decrease and increase of the lasing intensity for the random laser and thus the rise and descent of its energy threshold are generated, respectively.