Explore the vast range of titles available.

Chiral nematic liquid crystals are self-organized helical superstructures in which the helices can stand or lie, and lie in either a uniform or a random way; here, the helices are reversibly driven from a standing arrangement to a uniform lying arrangement and then rotated in-plane—solely by light. Manipulation of cholesteric liquid crystals This paper reports the manipulation of the helix axis accompanied by handedness inversion of an optically tunable, self-organized helical superstructure (a cholesteric liquid crystal) in three dimensions. Practical applications of chiral nematic liquid crystals (also known as cholesteric liquid crystals), rely on variation of the pitch length of these helices, or switching the helical axis between parallel or perpendicular to a substrate. This is typically achieved application electric or magnetic fields. Quan Li and colleagues now report such a manipulation using only light as a stimulus. Using this method, they achieve two-dimensional beam steering without the need for complex, multicomponent integrated systems, and they also construct a photoswitchable diffraction grating using a bilayer cell design. Chiral nematic liquid crystals—otherwise referred to as cholesteric liquid crystals (CLCs)—are self-organized helical superstructures that find practical application in, for example, thermography 1 , reflective displays 2 , tuneable colour filters 3 , 4 and mirrorless lasing 5 , 6 . Dynamic, remote and three-dimensional control over the helical axis of CLCs is desirable, but challenging 7 , 8 . For example, the orientation of the helical axis relative to the substrate can be changed from perpendicular to parallel by applying an alternating-current electric field 9 , by changing the anchoring conditions of the substrate, or by altering the topography of the substrate’s surface 10 , 11 , 12 , 13 , 14 , 15 , 16 ; separately, in-plane rotation of the helical axis parallel to the substrate can be driven by a direct-current field 17 , 18 , 19 . Here we report three-dimensional manipulation of the helical axis of a CLC, together with inversion of its handedness, achieved solely with a light stimulus. We use this technique to carry out light-activated, wide-area, reversible two-dimensional beam steering—previously accomplished using complex integrated systems 20 and optical phased arrays 21 . During the three-dimensional manipulation by light, the helical axis undergoes, in sequence, a reversible transition from perpendicular to parallel, followed by in-plane rotation on the substrate surface. Such reversible manipulation depends on experimental parameters such as cell thickness, surface anchoring condition, and pitch length. Because there is no thermal relaxation, the system can be driven either forwards or backwards from any light-activated intermediate state. We also describe reversible photocontrol between a two-dimensional diffraction state, a one-dimensional diffraction state and a diffraction ‘off’ state in a bilayer cell.

Natural self-assembled three-dimensional photonic crystals such as blue-phase liquid crystals typically assume cubic lattice structures. Nonetheless, blue-phase liquid crystals with distinct crystal symmetries and thus band structures will be advantageous for optical applications. Here we use repetitive electrical pulses to reconfigure blue-phase liquid crystals into stable orthorhombic and tetragonal lattices. This approach, termed repetitively applied field, allows the system to relax between each pulse, gradually transforming the initial cubic lattice into various intermediate metastable states until a stable non-cubic crystal is achieved. We show that this technique is suitable for engineering non-cubic lattices with tailored photonic bandgaps, associated dispersion and band structure across the entire visible spectrum in blue-phase liquid crystals with distinct composition and initial crystal orientation. These field-free blue-phase liquid crystals exhibit large electro-optic responses and can be polymer-stabilized to have a wide operating temperature range and submillisecond response speed, which are promising properties for information display, electro-optics, nonlinear optics, microlasers and biosensing applications. Repetitive electrical pulse stimulation of blue-phase liquid crystals promotes their reconfiguration into stable non-cubic structures with promising electro-optical responses for display technologies.

Soft-matter-based photonic crystals like blue-phase liquid crystals (BPLC) have potential applications in wide-ranging photonic and bio-chemical systems. To date, however, there are limitations in the fabrication of large monocrystalline BPLCs. Traditional crystal-growth process involves the transition from a high-temperature disordered phase to an ordered (blue) phase and is generally slow (takes hours) with limited achievable lattice structures, and efforts to improve molecular alignment through post-crystallization field application typically prove ineffective. Here we report a systematic study on the molecular self-assembly dynamics of BPLC starting from a highly ordered phase in which all molecules are unidirectionally aligned by a strong electric field. We have discovered that, near the high-temperature end of the blue phase, if the applied field strength is then switched to an intermediate level or simply turned off, large-area monocrystalline BPLCs of various symmetries (tetragonal, orthorhombic, cubic) can be formed in minutes. Subsequent temperature tuning of the single crystal at a fixed applied field allows access to different lattice parameters and the formation of never-before-seen monoclinic structures. The formed crystals remain stable upon field removal. The diversity of stable monocrystalline BPLCs with widely tunable crystalline symmetries, band structures, and optical dispersions will significantly improve and expand their application potentials. Soft-matter-based photonic crystals like blue-phase liquid crystals (BPLC) have potential applications but face fabrication limitations for large monocrystalline structures. The authors report a method to rapidly form large-area monocrystalline BPLCs with various symmetries and tunable properties through an application of electric fields and temperature control.

Although there have been intense efforts to fabricate large three-dimensional photonic crystals in order to realize their full potential, the technologies developed so far are still beset with various material processing and cost issues. Conventional top-down fabrications are costly and time-consuming, whereas natural self-assembly and bottom-up fabrications often result in high defect density and limited dimensions. Here we report the fabrication of extraordinarily large monocrystalline photonic crystals by controlling the self-assembly processes which occur in unique phases of liquid crystals that exhibit three-dimensional photonic-crystalline properties called liquid-crystal blue phases. In particular, we have developed a gradient-temperature technique that enables three-dimensional photonic crystals to grow to lateral dimensions of ~1 cm (~30,000 of unit cells) and thickness of ~100 μm (~ 300 unit cells). These giant single crystals exhibit extraordinarily sharp photonic bandgaps with high reflectivity, long-range periodicity in all dimensions and well-defined lattice orientation. Conventional fabrication approaches for large-size three-dimensional photonic crystals are problematic. By properly controlling the self-assembly processes, the authors report the fabrication of monocrystalline blue phase liquid crystals that exhibit three-dimensional photonic-crystalline properties.

Thermoplasmonics deals with the generation and manipulation of nanoscale heating associated with noble metallic nanoparticles. To this end, gold nanoparticles (AuNPs) are unique nanomaterials with the intrinsic capability to generate a nanoscale confined light‐triggered thermal effect. This phenomenon is produced under the excitation of a suitable light of a wavelength that matches the localized surface plasmonic resonance frequency of AuNPs. Liquid crystals (LCs) and hydrogels are temperature‐sensitive materials that can detect the host AuNPs and their photo‐induced temperature variations. In this perspective, new insight into thermoplasmonics, by describing a series of methodologies for monitoring, detecting, and exploiting the photothermal properties of AuNPs, is offered. From conventional infrared thermography to highly sophisticated temperature‐sensitive materials such as LCs and hydrogels, a new scenario in thermoplasmonic‐based, next generation, photonic components is presented and discussed. Moreover, a new road in thermoplasmonic‐driven biomedical applications, by describing compelling and innovative health technologies such as on‐demand drug‐release and smart face masks with smart nano‐assisted destruction of pathogens, is proposed. The latter represents a new weapon in the fight against COVID‐19. Gold nanoparticles are unique nanomaterials with the intrinsic capability to produce a considerable amount of heating upon illumination with a suitable resonant laser beam. The photo‐induced heat generated by gold nanoparticles—thermoplasmonic effect—offers several opportunities in different research fields ranging from advanced optical components to high‐precision medical applications. These cross‐disciplinary possibilities bring novel insight into light‐controlled applications.

This article provides a brief overview of the research on localized optical states called Tamm plasmons (TPs) and their potential applications, which have been extensively studied in recent decades. These states arise under the influence of incident light at the interface between a metal film and a medium with the properties of a Bragg mirror, or between two media with the properties of a Bragg mirror. The localization of the states in the interfacial region is a consequence of the negative dielectric constant of the metal and the presence of a photonic band gap of the Bragg reflector. Optically, TPs appear as resonant reflection dips or peaks in the transmission and absorption spectra in the region corresponding to the photonic band gap. The relative simplicity of creating a Tamm structure and the significant sensitivity of TPs to its parameters make them attractive for applications. The formation of broadband and tunable TP modes in hybrid structures containing, in particular, rugate filters and porous distributed Bragg reflectors are considered. Considerable attention is paid to TP designs that include liquid crystals, which allow for the remote tuning of the TP spectrum without the mechanical restructuring of the system. The application of TPs in sensors, thermal emitters, absorbers, laser generation, and the experimental capabilities of TP-liquid crystal devices are also discussed.

Absorption, reflection, and transmission coefficients of the hybrid structure formed by a metal film and a holographic polymer–liquid crystal grating (HPLCG) are theoretically studied in the spectral region of the HPLCG band gap. HPLCG cells consist of four alternating layers, two layers of polymer and two layers of the same liquid crystal (LC), but with different orientations of the LC director. The appearance of reflection, transmission, and absorption peaks in the HPLCG band gap due to the excitation of optical Tamm states (OTSs) at the metal film–HPLCG interface is investigated. The dependence of the spectral manifestation of OTSs on the parameters of the hybrid structure is also studied. A comparison is made with the corresponding results for the case when HPLCG cells of a hybrid structure consist of one polymer layer and one LC layer (two-layer HPLCG).

Blue phase liquid crystals (BPLCs) composed of double-twisted cholesteric helices are promising materials for use in next-generation displays, optical components, and photonics applications. However, BPLCs are only observed in a narrow temperature range of 0.5–3 °C and must be stabilized with a polymer network. Here, we report on controlling the phase behavior of BPLCs by varying the concentration of an amorphous crosslinker (pentaerythritol triacrylate (PETA)). LC mixtures without amorphous crosslinker display narrow phase transition temperatures from isotropic to the blue phase-II (BP-II), blue phase-I (BP-I), and cholesteric phases, but the addition of PETA stabilizes the BP-I phase. A PETA content above 3 wt% prevents the formation of the simple cubic BP-II phase and induces a direct transition from the isotropic to the BP-I phase. PETA widens the temperature window of BP-I from ~6.8 °C for BPLC without PETA to ~15 °C for BPLC with 4 wt% PETA. The BPLCs with 3 and 4 wt% PETA are stabilized using polymer networks via in situ photopolymerization. Polymer-stabilized BPLC with 3 wt% PETA showed switching between reflective to transparent states with response times of 400–500 μs when an AC field was applied, whereas the application of a DC field induced a large color change from green to red.

The dynamic electro‐optic (EO) response of polymer‐stabilized cholesteric liquid crystals prepared using unpolarized UV light (U–PSCLC), such as reflection bandwidth broadening and either red or blue tuning of the reflection peak, has been previously reported. Herein, recent efforts to use a polarized single argon‐ion laser beam to create PSCLCs (L–PSCLCs) with higher‐order reflections are described. The L–PSCLCs exhibit a primary reflection peak in the near‐infrared (NIR) regime and a second‐order reflection band with a narrow bandwidth in the visible regime that results from a deformed in‐plane CLC helical structure. The initial positions of the reflection bands are adjusted by the chiral dopant concentrations of the CLC mixture, and red, green, and blue reflection colors from the second‐order Bragg reflection are demonstrated. The primary and the second‐order reflection bands can be shifted to longer wavelengths by application of a DC electric field. The reflection efficiency of the higher‐order reflection notch increases with polymer concentration, which affects the degree of in‐plane deformation and fixation of the CLC helix. Modeling is used to further explain the formation of the higher‐order reflection bands of PSCLCs observed experimentally. An electrically tunable second‐order reflection color is demonstrated in the positive Δε polymer‐stabilized cholesteric liquid crystals (PSCLCs). The second‐order reflection peak shifts from 570 to 740 nm as the DC voltage increases to 150 V, and a color change from green to red is observed.

A new class of liquid crystals is reported that undergoes light-induced ordering and order-increasing phase transitions; possible applications include ophthalmic devices, such as variable transmission sunglasses. Leading lights As a rule, photoresponsive liquid crystals such as azobenzenes become structurally disordered on exposure to light. In a few instances, however, the reverse has been reported, so that in certain conditions the crystals can become more ordered. Here, Kosa et al . demonstrate a new class of naphthopyran-based liquid crystals that takes the latter behaviour much further, displaying a variety of ordering transitions: from isotropic to nematic, from isotropic to cholesteric and from nematic to smectic. With appropriate functionalization of the naphthopyran dyes, their isotropic-to-nematic transition results in a clear to strongly absorbing dichroic state. These properties could be useful in a variety of applications, not least in ophthalmic devices such as polarized variable-transmission sunglasses. Liquid crystals are traditionally classified as thermotropic, lyotropic or polymeric, based on the stimulus that governs the organization and order of the molecular system 1 . The most widely known and applied class of liquid crystals are a subset of thermotropic liquid crystals known as calamitic, in which adding heat can result in phase transitions from or into the nematic, cholesteric and smectic mesophases. Photoresponsive liquid-crystal materials and mixtures can undergo isothermal phase transitions if light affects the order parameter of the system within a mesophase sufficiently. In nearly all previous examinations, light exposure of photoresponsive liquid-crystal materials and mixtures resulted in order-decreasing photo-induced isothermal phase transitions 2 . Under specialized conditions, an increase in order with light exposure has been reported, despite the tendency of the photoresponsive liquid-crystal system to reduce order in the exposed state 3 , 4 , 5 , 6 , 7 . A direct, photo-induced transition from the isotropic to the nematic phase has been observed in a mixture of spiropyran molecules and a nematic liquid crystal 8 . Here we report a class of naphthopyran-based materials that exhibit photo-induced conformational changes in molecular structure capable of yielding order-increasing phase transitions. Appropriate functionalization of the naphthopyran molecules leads to an exceedingly large order parameter in the open form, which results in a clear to strongly absorbing dichroic state. The increase in order with light exposure has profound implications in optics, photonics, lasing and displays and will merit further consideration for applications in solar energy harvesting. The large, photo-induced dichroism exhibited by the material system has been long sought in ophthalmic applications such as photochromic and polarized variable transmission sunglasses.