Electrotunable 180° achromatic linear polarization rotator based on a dual-frequency liquid crystal.

Linear polarization rotators have been widely used in optical systems. Commonly used polarization rotators are still beset by strong dispersion and thus restricted spectral bandwidth of operation. This leads to the development of achromatic or broadband alternatives, but most of them incorporate multiple waveplates for retardation compensation, which comes at the cost of increased complexity and reduced flexibility in operation and system design. Here, we demonstrate a single-element achromatic polarization rotator based on a thin film of dual-frequency chiral liquid crystal. The angle of polarization rotation is electrically tunable from 0° to 180° with low dispersion (±3°) in the entire visible spectrum, and a high degree of linear polarization (>95%) at the output.

Silicon-organic hybrid thermo-optic switch based on a slot waveguide directional coupler.

We propose and demonstrate a passively biased 2 × 2 thermo-optic switch with high power efficiency and fast response time. The device benefits from the highly concentrated optical field of a slot waveguide mode and the strong thermo-optic effect of a nematic liquid crystal (NLC) cladding. The NLC fills the nano-slot region and is aligned by the subwavelength grating inside. The measured power consumption and thermal time constant are 0.58 mW and 11.8 µs, respectively, corresponding to a figure-of-merit of 6.8 mW µs. The proposed silicon-organic hybrid device provides a new solution to design thermo-optic actuators having low power consumption and fast operation speed.

Reconfiguration of three-dimensional liquid-crystalline photonic crystals by electrostriction.

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.

Polarization-controlled chirped guided-mode resonance filter incorporating a hybrid splay-twist liquid crystal.

This work develops a tunable chirped guided-mode resonant (GMR) filter that has a hybrid splay-twist (HST) liquid crystal as a cladding layer. The GMR filter is a color reflector that strongly reflects light at the resonance wavelength, and its chirped grating structure supports tuning of the resonance peak over a wavelength range of over 50 nm. The HST-LC configuration serves as an achromatic polarization rotator that can rotate the axis of polarization of linearly polarized light by providing effective twist angles in the LC layer under an applied voltage. The HST-LC is used to change the direction of the polarization axis of the light that is reflected by the GMR filter; continuous angles of rotation of ∼90∘ are achieved and the linear polarization is retained under applied voltages. The proposed filter enables an ultrabroadband polarization rotation and still maintains a high degree of linear polarization, which allows more degrees of freedom in spectral and polarization controls.

Highly sensitive silicon photonic temperature sensor based on liquid crystal filled slot waveguide directional coupler.

A highly sensitive silicon photonic temperature sensor based on silicon-on-insulator (SOI) platform has been proposed and demonstrated. A two-mode nano-slot waveguide device structure cladded with a nematic liquid crystal (LC), E7, was adopted to facilitate strong light-matter interaction and achieve high sensitivity. The fabricated sensor was characterized by measuring the optical transmission spectra at different ambient temperatures. The extracted temperature sensitivities of the E7-filled device are 0.810 nm/°C around room temperature and 1.619 nm/°C near 50°C, which match well with simulation results based on a theoretical analysis. The results obtained represent the highest experimentally demonstrated temperature sensitivity for a silicon-waveguide temperature sensor on SOI platform. The slot waveguide directional coupler device configuration provides submicron one-dimensional spatial resolution and flexible selection in LC materials for designing temperature sensitivity and operational temperature range required by specific applications.

Voltage-controlled liquid crystal Pancharatnam-Berry phase lens with broadband operation and high photo-stability.

Pancharatnam-Berry phase optical elements (PBOEs) have received much attention due to their ability to generate complex structured light or to manipulate the shape of a light beam. This work demonstrates a tunable liquid crystal (LC) Pancharatnam-Berry (LCPB) lens using a simple and cost-effective PB phase hologram optical setup and thermal polymerization to form an irreversible photo-patterning alignment layer. The LCPB lens with high photo-stability supports ultra-broadband operation and provides a diffraction efficiency of ∼90% throughout the visible spectral range, achieved by applying the appropriate voltages. The LCPB lens functions as a convex or a concave lens, depending on the handedness of the circularly polarized incident light, so its image reduction and magnification functions are demonstrated, and its photo-stability is characterized. The fabrication of the proposed LC PBOEs is simpler and more cost-effective than previous methods, and the irreversible photo-patterning alignment layer that is formed by thermal polymerization allows larger operational bandwidths, supporting new applications.

Bistable switching of polarization-grating diffractions enabled by a front bistable twisted nematic film.

Bistable electrical switching of high-efficiency grating diffractions is realized by adding a front π-bistable twisted nematic (π-BTN) cell to a passive liquid crystal polarization grating (LCPG). The π-BTN cell can be switched between stable non- and 180°-twisted states, acting as a polarization converter that switches the polarization of a laser beam and thus changes the diffraction behavior of the LCPG. Both states of the π-BTN cell are stable and can be reversibly switched to each other by applying a voltage pulse of different frequencies. We experimentally demonstrate two bistable-switching operations: (i) the BTN-LCPG can either split or deflect the laser beam; and (ii) the BTN-LCPG can selectively diffract the laser beam to the +1st and -1st orders, while maintaining a high diffraction efficiency of ∼90%. With electrical switchability, a simple optical design, and low power consumption, the proposed BTN-LCPG concept is favorable in various applications.

Selective variable optical attenuator for visible and mid-Infrared wavelengths.

This work demonstrates a variable optical attenuator (VOA) using dynamic scattering mode (DSM) in ion-doped liquid crystals with negative dielectric anisotropy. The mechanism of attenuation comes from optical scattering, which is generated by the electrically induced instability of undulation of LC textures. Electric fields are applied to switch the initial transparent state of the designed VOA to scattering states, varying the transmittance. The electric field also changes the size of the scattering domain from the LC texture and causes the designed device to exhibit an ultra-broadband selective operation in a visible to mid-IR spectral range. Furthermore, the VOA can selectively block one visible or mid-IR wavelength of light while letting other light pass. Such a VOA has many superior optical switching properties, such as high on/off contrast, insensitivity to polarization, and spectral selectivity; therefore, it has the potential to be used in practical optical systems.

Broadband mid-infrared polarization rotator based on optically addressable LCs.

This work proposes a mid-infrared polarization rotator that incorporates a twisted nematic liquid crystal (TNLC) cell with a photo-controllable alignment layer. The TNLC device with a sufficient phase retardation can act as an achromic polarization rotation device over a wide wavelengths range and thus can rotate the polarization of a mid-IR laser beam. The photo-alignment technique enables TNLCs with arbitrary twisting angles to be generated by the use of visible polarized addressing light to control the directors of the photo-alignment layer. Therefore, arbitrary rotation angles of the polarization axis of a linearly polarized mid-IR laser beam can be realized. Moreover, the rewritable property and reliability of this polarization rotator are experimentally verified. The flexibility of polarization control for broadband mid-IR opens up a large range of potential mid-IR applications.

Full-color reflector using vertically stacked liquid crystal guided-mode resonators.

In this work, we proposed a full-color reflector using three stacked red (R), green (G), and blue (B) reflection gratings which are combined with the tunable 90° twisted nematic liquid crystals (TNLCs). The color reflector based on guided-mode resonance (GMR) gratings reflects strongly at the resonance wavelength. The optical reflectivity of GMR gratings can then be controlled by using 90° TNLCs to change the polarization of incident light. The optical characteristics and the chromaticity of the designed reflectors were evaluated by simulation. An individual RGB chip with/without LC was demonstrated experimentally. The fabricated GMR reflector for red exhibits a high TE/TM polarization ratio of >10:1 and 80% optical reflectivity at resonant wavelength, while the GMR reflector for blue only allows 60% optical reflectivity and a degraded polarization ratio of 3:1 mainly due to high optical absorption of silicon. Nevertheless, the silicon-based GMR reflector enables a wide reflection bandwidth, so a full-color reflector can be realized by vertically stacking RGB tunable reflectors. The proposed full-color reflector therefore exhibits a wide-gamut color space with low driving voltage of <3 V, showing its promise for use in energy-saving reflective information systems.

Full-color reflectance-tunable filter based on liquid crystal cladded guided-mode resonant grating.

This work proposes a tunable reflective guided-mode resonant (GMR) filter that incorporates a 90° twisted nematic liquid crystal (TNLC). The GMR grating acts as an optical resonator that reflects strongly at the resonance wavelength and as an alignment layer for LC. The 90° TNLC functions as an achromic polarization rotator that alters the polarization of incident light. The resonance wavelength and reflectance of such a filter can be controlled by setting the angle of incidence and driving the 90° TNLC, respectively. The designed filter exhibits a very large spectral shift in resonance wavelength from 710 to 430 nm, which covers the entire visible spectrum. The transmittance can be tuned to within 10 V at various resonance wavelengths. The hybrid GMR - LC filter is compact, has a simple design, and is easy to fabricated. It can therefore be used in practical applications.

Highly sensitive optical temperature sensor based on a SiN micro-ring resonator with liquid crystal cladding.

This work develops a sensitivity-enhanced optical temperature sensor that is based on a silicon nitride (SiN) micro-ring resonator that incorporates nematic liquid crystal (NLC) cladding. As the ambient temperature changes, the refractive index of the NLCs, which have a large thermal-optical coefficient, dramatically varies. The change in the refractive index of the NLC cladding that is caused by the temperature shift can alter the effective refractive index of the micro-ring resonator and make the resonance wavelength very sensitive to the ambient temperature. The temperature-sensitivity of the device with 5CB cladding for TM-polarized light was measured to be as high as 1nm/°C between 25 and 33 °C and over 2nm/°C at temperatures close to clearing temperature of the 5CB cladding. The temperature-sensitivity of the proposed device is at least 55 times that of the micro-ring resonator with air cladding, whose temperature-dependent wavelength shift for TM-polarized light is 18pm/ °C.

Electrically tunable high Q-factor micro-ring resonator based on blue phase liquid crystal cladding.

This work demonstrates an electrically tunable silicon nitride (SiN) micro-ring resonator with polymer-stabilized blue phase liquid crystals (PSBPLCs) cladding. An external vertical electric field is applied to modulate the refractive index of the PSBPLCs by exploiting its fast-response Kerr effect-induced birefringence. The consequent change in the refractive index of the cladding can vary the effective refractive index of the micro-ring resonator and shift the resonant wavelength. Crystalline structures of PSBPLCs with a scale of the order of hundreds of nanometers ensure that the resonator has a very low optical loss. The measured tuning range is 0.45 nm for TM polarized light under an applied voltage of 150V and the corresponding response time is in the sub-millisecond range with a Q-factor of greater than 20,000.

Bistable light-driven π phase switching using a twisted nematic liquid crystal film.

A light-activated optical phase switch was developed, exploiting the conversion between left-handed and right-handed twisted nematic liquid crystals. Theoretical and experimental analyses revealed that the handedness inversion of the twisted nematic film altered the optical phase of the output waves by π. Herein, the competition between the helical twisting powers of the two reverse-handed chiral dopants determines the handedness of the twisted nematic film. The photo-responsibility and the bistability are attributed to the azobenzene chromophores in one of the chiral additives.

Photo-switchable bistable twisted nematic liquid crystal optical switch.

This work demonstrates a photo-switchable bistable optical switch that is based on an azo-chiral doped liquid crystal (ACDLC). The photo-induced isomerization of the azo-chiral dopant can change the chirality of twisted nematic liquid crystal and the gap/pitch ratio of an ACDLC device, enabling switching between 0° and 180° twist states in a homogeneous aligned cell. The bistable 180° and 0° twist states of the azo-chiral doped liquid crystal between crossed polarizers correspond to the ON and OFF states of a light shutter, respectively, and they can be maintained stably for tens of hours. Rapid switching between 180° and 0° twist states can be carried out using 408 and 532 nm addressing light. Such a photo-controllable optical switch requires no specific asymmetric alignment layer or precise control of the cell gap/pitch ratio, so it is easily fabricated and has the potential for use in optical systems.

Optical bistability in a silicon nitride microring resonator with azo dye-doped liquid crystal as cladding material.

This investigation reports observations of optical bistability in a silicon nitride (SiN) micro-ring resonator with azo dye-doped liquid crystal cladding. The refractive index of the cladding can be changed by switching the liquid crystal between nematic (NLC) and photo-induced isotropic (PHI) states by. Both the NLC and the PHI states can be maintained for many hours, and can be rapidly switched from one state to the other by photo-induced isomerization using 532 nm and 408 nm addressing light, respectively. The proposed device exhibits optical bistable switching of the resonance wavelength without sustained use of a power source. It has a 1.9 nm maximum spectral shift with a Q-factor of over 10000. The hybrid SiN- LC micro-ring resonator possesses easy switching, long memory, and low power consumption. It therefore has the potential to be used in signal processing elements and switching elements in optically integrated circuits.

Random lasing in blue phase liquid crystals.

Random lasing actions have been observed in optically isotropic pure blue-phase and polymer-stabilized blue-phase liquid crystals containing laser dyes. Scattering, interferences and recurrent multiple scatterings arising from disordered platelet texture as well as index mismatch between polymer and mesogen in these materials provide the optical feedbacks for lasing action. In polymer stabilized blue-phase liquid crystals, coherent random lasing could occur in the ordered blue phase with an extended temperature interval as well as in the isotropic liquid state. The dependence of lasing wavelength range, mode characteristics, excitation threshold and other pertinent properties on temperature and detailed make-up of the crystals platelets were obtained. Specifically, lasing wavelengths and mode-stability were found to be determined by platelet size, which can be set by controlling the cooling rate; lasing thresholds and emission spectrum are highly dependent on, and therefore can be tuned by temperature.

Wavelength tunable infrared light source based on semiconductor-integrated liquid crystal filter.

This work proposes an electrically tunable infrared light source based on a new compact structure, i.e., an AlGaInAs semiconductor multiple quantum well (MQW) integrated with a liquid crystal Fabry-Pérot filter. The AlGaInAs MQW is used as a luminance layer that emits broadband light. By sandwiching the AlGaInAs and LC material with two conducting mirrors, the active light source with an optical filter can be tuned with a wide wavelength range. The filter filled with nematic liquid crystal enables continuous tuning of emission along the extraordinary mode and provides a 58 nm tuning range with a bias of 14 V. The simulation results of wavelength and tunability are consistent with the experimental results. Cholesteric liquid crystal with a planar texture is also used to examine the properties of the tunable light source. Under an electric field, all the helical liquid crystal molecules tend to be aligned parallel to the field. The variation of the refractive index is normal to the substrate surface, and the polarization-independent tuning range is 41 nm. The wide tuning range and the polarization properties observed when NLC and CLC are respectively incorporated into the AlGaInAs based Fabry-Pérot cavity suggest that this integration scheme has potential for applying to optical communication system.

Optically controllable bistable reflective liquid crystal display.

This Letter demonstrates a photo-addressable, bistable reflective liquid crystal display that is based on a dye-doped liquid crystal (DDLC). Bistable bright and dark states can be attained using the 45 deg twisted nematic (TN) and photo-induced isotropic states (PHI) of the DDLC, respectively. Both the 45 deg TN and PHI states can exist stably for tens of hours, and each can be rapidly switched to the other by the isomerization effect using UV and green light. A bistable reflective liquid crystal display is simply fabricated, easily operated, and rapidly switched. It therefore has the potential to be used in portable information systems.

Achiral Nanoparticle-Enhanced Chiral Twist and Thermal Stability of Blue Phase Liquid Crystals.

Blue phase liquid crystals (BPLCs) are chiral mesophases with 3D order, which makes them a promising template for doping nanoparticles (NPs), yielding tunable nanomaterials attractive for microlasers and numerous microsensor applications. However, doping NPs to BPLCs causes BP lattice extension, which translates to elongation of operating wavelengths of light reflection. Here, it is demonstrated that small (2.4 nm diameter) achiral gold (Au) NPs decorated with designed LC-like ligands can enhance the chiral twist of BPLCs (i.e., reduce cell size of the single BP unit up to ∼14% and ∼7% for BPI and BPII, respectively), translating to a blue-shift of Bragg reflection. Doping NPs also significantly increases the thermal stability of BPs from 5.5 °C (for undoped BPLC) up to 22.8 °C (for doped BPLC). In line with our expectations, both effects are saturated, and their magnitude depends on the concentration of investigated nanodopants as well the BP phase type. Our research highlights the critical role of functionalization of Au NPs on the phase sequence of BPLCs. We show that inappropriate selection of surface ligands can destabilize BPs. Our BPLC and Au NPs are photochemically stable and exhibit great miscibility, preventing NP aggregation in the BPLC matrix over the long term. We believe that our findings will improve the fabrication of advanced nanomaterials into 3D periodic soft photonic structures.