Electro-optic Response Research Articles

Effect of ketyl radical on the structure and performance of holographic polymer/liquid-crystal composites

Holographic polymer/liquid-crystal composites, which are periodically ordered materials with alternative polymer-rich and liquid-crystal-rich phases, have drawn increasing interest due to their unique capabilities of reconstructing colored three-dimensional (3D) images and enabling the electro-optic response. They are formed via photopolymerization induced phase separation upon exposure to laser interference patterns, where a fast photopolymerization is required to facilitate the holographic patterning. Yet, the fast photopolymerization generally leads to depressed phase separation and it remains challenging to boost the holographic performance via kinetics control. Herein, we disclose that the ketyl radical inhibition is able to significantly boost the phase separation and holographic performance by preventing the proliferated diffusion of initiating radicals from the constructive to the destructive regions. Dramatically depressed phase separation is caused when converting the inhibiting ketyl radical to a new initiating radical, indicating the significance of ketyl radical inhibition when designing high performance holographic polymer composites.

Enhancement of effective polar anchoring strength and accelerated electro-optic switching in a two-dimensional hexagonal boron nitride/polyimide hybrid liquid crystal device.

The monolayer hexagonal boron nitride (h-BN) nanosheet is transferred onto an indium tin oxide (ITO)-coated glass substrate. This h-BN slide is placed together with a conventional planar-aligning polyimide (PI) slide to fabricate a liquid crystal (LC) cell, in which the LC achieves uniform planar alignment. The effective polar anchoring strength coefficient of this h-BN-based hybrid LC device is found to increase significantly compared to that of a standard PI/PI LC device. The presence of the monolayer h-BN nanosheet as an alignment agent increases the planar anchoring energy through the epitaxial interaction between the LC and the honeycomb structure of the two-dimensional h-BN lattice in this hybrid device. The amplified polar anchoring energy is found to accelerate the electro-optic response time of the LC in this h-BN-based hybrid device.

Dynamic Electric Field Alignment of Metal-Organic Framework Microrods.

Alignment of metal-organic framework (MOF) crystals has previously been performed via careful control of oriented MOF growth on substrates, as well as by dynamic magnetic alignment. We show here that bromobenzene-suspended microrod crystals of the MOF NU-1000 can also be dynamically aligned via electric fields, giving rise to rapid electrooptical responses. This method of dynamic MOF alignment opens up new avenues of MOF control which are important for integration of MOFs into switchable electronic devices as well as in other applications such as reconfigurable sensors or optical systems.

Pentaerythritol Derived Tetrapode Exhibiting a Nematic-Like Mesophase at Ambient Temperatures.

The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'NX ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the NX phase to exhibit an electrooptic response consistent with a nematic-like phase.

Simple fabrication of SWIR detectors based on wet deposition of carbon nanotubes and quantum dots

Over the last two decades, carbon based materials and especially carbon nanotubes (CNTs), were the subject of many studies, mainly due to their unique electrical, optical and mechanical properties (Ouyang et al., 2002; Dresselhaus et al., 2003; Dresselhaus et al., 1995). CNTs can combine electrical conductivity with wide absorption spectra, and can be produced in large scale (Danafar et al., 2009) [4]. These properties enable to realize CNTs in simple, low-cost detector. Here we present a proof-of-concept for such a detector operating at the short-wave infrared (SWIR) regime. We use a simple spray technique, which allows creating a large matrix of CNT bundles. Semiconducting quantum dots (QDs) were adsorbed on top of the CNTs, enhancing the sensitivity to the infrared regime. This regime is important for numerous applications in the civil, medical, defense and security fields. Controlled coupling between the QDs and the CNT matrix generates gate-like electro-optical response when light is absorbed. This proof-of-concept for a detector in the SWIR region is presented for large surfaces and substrates, while the responsivity and detectivity of the detector in a range of frequencies and wavelengths was evaluated.

Electro-Optical Switching of Dual-Frequency Nematic Liquid Crystals: Regimes of Thin and Thick Cells

Conventional display applications of liquid crystals utilize thin layers of mesogenic materials, typically less than 10 µm. However, emerging non-display applications will require thicker, i.e., greater than 100 µm, layers of liquid crystals. Although electro-optical performance of relatively thin liquid crystal cells is well-documented, little is known about the properties of thicker liquid crystal layers. In this paper, the electro-optical response of dual-frequency nematic liquid crystals is studied using a broad range (2–200 µm) of the cell thickness. Two regimes of electro-optical switching of dual-frequency nematics are observed and analyzed.

Influence of the type of phase sequence and polymer-stabilization on the physicochemical and electro-optical properties of novel high-tilt antiferroelectric liquid crystalline materials

Antiferroelectric liquid crystal (AFLC) mixtures with two types of phase sequence, isotropic - smectic A* - smectic C* - smectic CA* and isotropic - smectic CA*, respectively, have been formulated from components having biphenyl benzoate rigid cores, a non-chiral terminal chain with either alkoxy groups as well as perfluorinated parts on one side, and a chiral chain with different structures on the other side. The properties of the mixtures have been examined by means of polarized optical microscopy, differential scanning calorimetry, as well as UV–Vis-NIR spectrophotometry, and their electrooptic performance have been characterized in device cells, before and after polymer-stabilization of the structures. For all mixtures we observed unusually broad temperature ranges of the smectic CA* phase, extending from at least −30 °C to about +70 °C or more, molecular tilt angles of 42–44°, helical pitches of several micrometers, and a spontaneous polarization of about 200 nC/cm2. The polymer network has a profound impact on the relaxation to the anticlinic state, especially in the mixtures with a direct smectic SmCA*- isotropic phase transition. A symmetric electrooptic response, with both on and off response times of about 140 μs, was obtained in cell gaps of 1.5 μm, which is, to our knowledge, unique among contemporary available high tilt AFLC material systems.

Enhanced efficiency of electric field tunability in photonic liquid crystal fibers doped with gold nanoparticles.

In this paper, we present our recent research results on light propagation in photonic crystal fibers (PCFs) infiltrated with a 6CHBT nematic liquid crystal (LC) doped with 2-nm gold nanoparticles (NPs) with a concentration in the range of 0.01 - 0.5% wt. Electro-optical response times and thermal tuning of the investigated samples have been studied in detail. We have observed up to ~80% decrease of rise times for different concentrations of gold NPs in the LC. Moreover, a significant reduction of the Fréedericksz threshold voltage (up to 60%) has been observed for samples with higher concentrations of 2-nm gold NPs in 6CHBT.

Fast Electro-Optical Switching Mode in an LC Cell while Decreasing the Electrode Grating Period

An increase in the electro-optical switching speed with a decrease in the electrode grating period is observed in a cell with a homeotropically oriented nematic liquid-crystal (LC) layer and interdigitated electrodes. There are two modes in the electro-optical response: fast and slow. A numerical simulation has shown that the fast mode occurs because of the LC director deformation in the surface layer. On the contrary, the occurrence of slow mode is due to the fact that the deformation propagates much more slowly in the LC bulk. The observed effect has been demonstrated experimentally in the bidirectional switching mode using two crossed interdigitated electrodes, located at both sides of the LC layer.

Optically rewritable dynamic phase grating based on blue-phase-templated azobenzene liquid crystal.

We propose an optically rewritable dynamic phase grating based on polymer-templated azo liquid crystal in a blue-phase structure. The grating consists of alternating blue-phase and light-induced isotropic-phase regions, patterned by ultraviolet illumination. In the field-off state, the grating is hidden (showing no diffraction), due to index matching between the two phases. An index change is induced in the blue-phase regions when an external voltage is applied, while the refractive index of the isotropic-phase regions remains the same. The resulting periodic index modulation causes the grating to diffract light. The diffraction efficiency is independent of incident polarization, and the electro-optic response is in the sub-millisecond scale. Enabled by the reversible photoisomerism of the azobenzene, we demonstrate optical-patterning, -erasure, and re-patterning of a single liquid-crystal cell into different grating geometries.

Interference Phenomenon in Electro-Optic Response of Nematic in Domain Structure on Polycarbonate Surface

Interference Phenomenon in Electro-Optic Response of Nematic in Domain Structure on Polycarbonate Surface

Switchable Liquid Crystal Composite Windows Using Bacterial Cellulose (BC) Mat Substrates

Performance properties were investigated for transparency switching devices fabricated with nematic LC filled bacterial cellulose (BC) mats as electro-optical components. Biobased BC is a nanoporous bicontinuous network of cellulose fibers produced by bacteria and cultured to form mats of desired thickness. Devices made from BC mats with thicknesses of 6, 20, and 40 μm were compared. Furthermore, the electro-optical response of the devices provides insight into the ordering of the nematic LC within the bicontinuous BC matrix.

Broadband electro-optic frequency comb generation in a lithium niobate microring resonator.

Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communication, precision metrology, timing and spectroscopy1-9. At present, combs with wide spectra are usually generated by mode-locked lasers10 or dispersion-engineered resonators with third-order Kerr nonlinearity11. An alternative method of comb production uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability12-14. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. Here we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large EO response, ultralow optical loss and highly co-localized microwave and optical fields15, while enabling dispersion engineering. Our measured EO comb spans more frequencies than the entire telecommunications L-band (over 900 comb lines spaced about 10 gigahertz apart), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 hertz to 100 megahertz), and we use this feature to generate dual-frequency combs in a single resonator. Our results show that integrated EO comb generators are capable of generating wide and stable comb spectra. Their excellent reconfigurability is a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy16 to optical communications8.

Impedance and dielectric spectroscopy study of graphene-doped liquid crystal E7

The change in electrical conductivity and dielectric response of liquid crystal (LC) E7 by doping with graphene nanoparticles was experimentally studied by complex electrical impedance and dielectric spectroscopy over the frequency range from 0.1 Hz to 1 MHz of the applied electric field. In the measured graphene/E7 nanocomposite films (7 μm-thick, with planar orientation), graphene nanoflakes were dispersed in the nematic E7 at the concentration of 10−3 wt.%. Graphene nanodopants lead to a change of both electrical transport and dielectric permittivity of the LC E7 which should render improved electro-optic response of the considered room-temperature nanocomposite nematic films.

Electro-Optic Modulation in Hybrid Metal Halide Perovskites.

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro-optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on-chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in-plane birefringence and linear electro-optic response. Phenylmethylammonium lead chloride (PMA2 PbCl4 ) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA2 PbCl4 is reported and its EO coefficient is determined to be 1.40 pm V-1 . This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in-plane birefringence and electro-optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.

Reduced ionic effect and accelerated electro-optic response in a 2D hexagonal boron nitride planar-alignment agent based liquid crystal device

The presence of excess free-ion impurities in liquid crystals (LCs) gives rise to a number of problems in the electro-optical liquid crystal displays (LCDs), e.g., slow electro-optical responses and image sticking effects. Here we experimentally present that the two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheet can serve as a planar-alignment agent and as an ion-capturing agent at the same time in an electro-optic LC device. The 2D h-BN nanosheet is employed as a planar-alignment agent on one side of an LC cell, where the standard planar-aligning polyimide (PI) layer is used on the other side of the cell. The LC exhibits uniform planar-alignment in this h-BN/PI hybrid device. It is found that the free-ion impurities in the LC are significantly suppressed in this h-BN/PI hybrid cell compared to that in a standard PI/PI LC cell. The free-ion density is reduced in the hybrid cell due to the 2D h-BN nanosheet’s ion-capturing process. The reduction of ionic impurities results in an accelerated electro-optic response of the LC in the h-BN based hybrid cell—which may have potential application for faster electro-optic devices.

Design and synthesis of novel H-Shaped chromophore for enhanced nonlinear optical properties

A novel H-shaped nonlinear optical chromophore B based on julolidinyl donors were designed and synthesized through Williamson ether reaction. The chromophore B is composed of two traditional rod-shaped chromophores A linked to each other through an isolation group, forcing chromophore separation through steric interaction. Chromophore B showed good thermal stability and film-forming ability. Besides, the reduction of intermolecular electrostatic interactions allows for increased active chromophore number density in the formulation of a host-guest doping material, thus enhancing electro-optic response. Polymers doped with 35 wt% chromophore B have been poled to afford large electro-optic coefficient (r33) of 180 pm/V at 1310 nm, which is nearly two times higher than that obtained from the chromophore A (92 pm/V) at the doping concentration of 25 wt%. These results indicated that the H-shaped chromophore B has the advantages over rod-shaped chromophore A in transforming the microscopic hyperpolarizability into a macroscopic electro-optic coefficient.

Programmable plasmonic phase modulation of free-space wavefronts at gigahertz rates

Space-variant control of optical wavefronts is important for many applications in photonics, such as the generation of structured light beams, and is achieved with spatial light modulators. Commercial devices, at present, are based on liquid-crystal and digital micromirror technologies and are typically limited to kilohertz switching speeds. To realize significantly higher operating speeds, new technologies and approaches are necessary. Here we demonstrate two-dimensional control of free-space optical fields at a wavelength of 1,550 nm at a 1 GHz modulation speed using a programmable plasmonic phase modulator based on near-field interactions between surface plasmons and materials with an electrooptic response. High χ(2) and χ(3) dielectric thin films of either aluminium nitride or silicon-rich silicon nitride are used as an active modulation layer in a surface plasmon resonance configuration to realize programmable space-variant control of optical wavefronts in a 4 × 4 pixel array at high speed. A 4 × 4 pixel spatial light modulation scheme based on plasmonics offers high-speed spatial light modulation at the telecommunications wavelength of 1,550 nm.

Dynamics of the director reorientation and light modulation in helix-free ferroelectric liquid crystals

The dynamics of the director reorientation in new helix-free ferroelectric liquid crystals (FLC) is considered. These materials are specially designed helix-free FLCs with a rather low value of the spontaneous polarization (less than 50 nC/cm2) and high viscosity (from 0.3 to 1.0 Poise), which are characterized by a spatial periodic deformation of smectic layers in the absence of an electric field. FLC director reorientation is due to the motion of solitons – spatially localized waves of a stationary profile that arise in an alternating electric field upon transition to the Maxwellian mechanism of energy dissipation. A theoretical model is proposed for describing the spatial-periodic deformation of FLC and reorientation of its director. The frequency and field experimental dependences of FLC electro-optical response time are presented for the modulation of the light transmission with fastest response among all LC materials. The novel helix-free FLC are able to efficiently modulate the visible and near IR radiation at frequencies up to 7 kHz at the electric field strength of the order of 1-2 V/μm. The conditions for the continuous hysteresis-free electro-optical response were determined, and such a response was realized for the first time in the frequency range up to 6 kHz.

Optimizing low frequency electro-optic response of nematic liquid crystals

Optimized enhanced electro-optic (EO) response of nematic liquid crystals (NLC) driven by low modulation frequency AM electric signal is investigated. A perturbed non-uniform transmitted output signal was observed at modulating frequencies (fm) lower than 80 Hz due to the effect of free charges, and optimized uniform response is achieved by adding a small holding voltage to the AM driving signal. The minimum applied holding voltage required for attaining uniform response at frequencies <80 Hz decreases as the modulating signal frequency increase. As the frequency increases from 10 Hz to 70 Hz, the total response time decreases from 8.8 ms to 4.7 ms at field strength 1.2 V/μm and at holding voltage (Vh); 1.95 V and 0.7 V (peak voltage), respectively. By such way the response times is effectively stabilized and enhanced for NLC low frequency EO applications.