Electro-optic Response Research Articles

Mesomorphic, electro-optic and dielectric behavior of self-assembled nanocomposite materials: Nematic mixture doped with carbon coated cobalt nanoparticles

In the present study, different concentrations of carbon coated cobalt nanoparticles (0.25 wt% & 0.5 wt%) into eutectic nematic liquid crystal mixture (E7) have been dispersed and studied their mesomorphic, electro-optic and dielectric properties in planar as well as homeotropic aligned cells. Optical transmission method has been employed in planar aligned cell to study the electro-optic responses while dielectric spectroscopy in the frequency span 20 Hz-10 MHz has been performed in planar and homeotropic cells. Nearly 10°C increase in nematic-isotropic transition temperature is noticed at higher doping concentration. Dispersion of cobalt nanoparticles induces changes in dielectric permittivity, dielectric loss and relaxation frequency. Suppression of free ion concentration (in case of 0.5 wt% Co) suggests large dielectric anisotropy values. Entropy of the system is more or less affected with the nano inclusions but the free energy of the doped samples has been decreased. An improvement in threshold voltage, birefringence and order parameter by ∼ 10%, 15% & 5% respectively have been observed at low doping concentration. However, contrast ratio is reduced with the doping of cobalt nanoparticles. The present study shows that the doped systems have improved response than pure E7 and find potential applications in phase shifters as well as display devices.

Electro-optical response of polymer-dispersed liquid crystals doped with γ-Fe2O3 nanoparticles

ABSTRACT In this paper, the microstructures of the dilute magnetic PDLC films, doped with low concentrations of γ-Fe2O3 nanoparticles, have hardly changed, but their electric-optical responses have distinctly changed. The experimental results show that the threshold voltage and hysteresis decreased with the increasing concentration of γ-Fe2O3 nanoparticles. Furthermore, at the relatively low concentration of γ-Fe2O3 nanoparticles (samples (0) and (1) according to ϕm of 0 and 0.11%), the reciprocity of the rise time increased nonlinearly with the increasing square of the applied voltage. However, at the relatively high concentration of γ-Fe2O3 nanoparticles (samples (2) according to ϕm = 0.15%), the reciprocity of the rise time increased linearly with the square of the applied voltage. Moreover, the reciprocity of the decay time was independent of the applied voltage and decreased with increasing the concentration of γ-Fe2O3 nanoparticles. These characteristics are due to the adsorption of free ions by γ-Fe2O3 nanoparticles. As a result, the interfacial interaction between liquid crystal droplets and the polymer matrix decreased, and the effective electric field inside the liquid crystal droplets increased.

Polymer Stabilization of Uniform Lying Helix Texture in a Bimesogen-Doped Cholesteric Liquid Crystal for Frequency-Modulated Electro-Optic Responses.

A polymer network (PN) can sustain the uniform lying helix (ULH) texture in a binary cholesteric liquid crystal (LC) comprising a calamitic LC and a bimesogenic LC dimer. Upon copolymerization of a bifunctional monomer with a trifunctional monomer at a concentration of 5 wt% to create the desired polymer network structure, the PN-ULH was obtained with high stability and recoverability even when cycles of helical unwinding-to-rewinding processes were induced after the electrical or thermal treatment. Utilizing dielectric spectroscopy, the flexoelectric-polarization-dominated dielectric relaxation in the PN-ULH state was characterized to determine two frequency regions, f < fflexo and f > fdi, with pronounced and suppressed flexoelectric effect, respectively. It is demonstrated that the cell in the PN-ULH state can operate in the light-intensity modulation mode by the flexoelectric and dielectric effects at f < fflexo and phase-shift mode by the dielectric effect at f > fdi. Moreover, varying the voltage frequency from f < fflexo to f > fdi results in a frequency dispersion of transmittance analogous to that of flexoelectric-polarization-dominated dielectric relaxation. The unique combination of the bimesogen-doped cholesteric LC with a stable and recoverable PN-ULH texture is thus promising for developing a frequency-modulated electro-optic device.

Electro-optical and dielectric response of quantum dot-doped cholesteric liquid crystal composites

Electro-optical and dielectric response of quantum dot-doped cholesteric liquid crystal composites

Electro-Optic and Thermoelectric Response of Group Iii-Phosphides (Lip, Nap, and Kp) for Solar and Thermal Applications

Electro-Optic and Thermoelectric Response of Group Iii-Phosphides (Lip, Nap, and Kp) for Solar and Thermal Applications

Electro-optic characteristics of stabilized cholesteric liquid crystals with non-liquid crystalline polymer networks

Extensive prior research has explored the stabilization of the CLC phase with polymer networks. These prior efforts have demonstrated both tunable and switchable electro-optic reconfiguration of the selective reflection of the CLC phase. Recently, we and other groups have detailed that polymer stabilization of the CLC phase with liquid crystalline monomers retains "structural" chirality (e.g., the chiral phase templates the morphology of the achiral polymer network). Here, we demonstrate that structural chirality can be retained in aliphatic, non-liquid crystalline monomers. PSCLCs prepared by photoinitiated polymerization of aliphatic polymer networks exhibit reversible electro-optic responses. Facilitated by the retention of structural chirality in aliphatic stabilizing polymer networks, we explore the role of surface affinity and crosslink density in the transfer of structural chirality to the liquid crystal media.

Effect of Amorphous Crosslinker on Phase Behavior and Electro-Optic Response of Polymer-Stabilized Blue Phase Liquid Crystals.

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.

Hyperfine optical vector analysis of ultra- narrowband optical bandpass filters based on coherent two-tone sweeping and fixed low-frequency detection

A hyperfine optical vector analyzer (OVNA) is proposed and experimentally demonstrated to characterize ultra-narrowband optical bandpass filters (OBPFs). In this scheme, a coherent two-tone optical signal (TTOS) with a small frequency interval is generated to act as a probe light. Through finely sweeping the TTOS across the passband of the OBPF via electro-optic modulation, the magnitude and phase responses of the OBPF under test can be measured with a high signal-to-noise ratio based on fixed low-frequency detection. The frequency response measurement is immune to external disturbance. In the experiment, the magnitude and group delay responses of a fiber-based Fabry-Perot tunable filter (FFP-TF) and a fiber-based Fabry-Perot interferometer (FFP-I) with 3-dB bandwidths of 1.5 GHz and 60 MHz, respectively, are successfully measured. In addition, the measurement uncertainty is theoretically and experimentally analyzed. This method paves a way to characterize ultra-narrowband OBPFs with high out-of-band rejection ratios.

Design and Manufacture of Traveling-wave Electro-optic Modulator for X-band LFM Signal Generation

In this paper, a photonic-based microwave system technology is described, and a traveling-wave electro-optic modulator is designed and manufactured as a key component. The fabricated modulator is composed of a metal diffusion waveguide for optical transmission and a planar waveguide electrode on lithium niobate substrate for microwave transmission. The electro-optic response bandwidth of I and Q channels in a fabricated dual parallel Mach-Zehnder modulator were measured for 27.67 and 28.11 GHz, respectively. Photonic four times up-converted X-band frequency and linear frequency modulated signal were confirmed using the fabricated electro-optic modulator by S-band input signal. The confirmed broadband signal can be applied to a microwave system for surveillance and high-resolution ISAR imaging.

Exploring a Band-Edge Bragg Grating Modulator on a Hybrid Thin-Film Lithium Niobate Platform

In this work, we design, model, and characterize the Bragg grating modulator (BGM) on silicon-rich nitride and thin-film lithium niobate (SRN-TFLN) platform. In particular, the slow-light effect on the electro-optical modulation response is modeled and experimentally validated. The presented modulator shows a 3-dB electro-optical (E-O) bandwidth beyond 40 GHz, which agrees well with the simulation results. High-speed modulation up to 60 Gbps and 70 Gbps are experimentally obtained with extinction ratio (ER) of 4.7 dB and 2.7 dB, respectively. A measured modulation efficiency of 2.47 pm/V is also achieved. With optimized slow-light effect and proper design, the proposed BGM has great potential in high-speed data transmission.

Hyperbolic optics and superlensing in room-temperature KTN from self-induced k-space topological transitions

A hyperbolic medium will transfer super-resolved optical waveforms with no distortion, support negative refraction, superlensing, and harbor nontrivial topological photonic phases. Evidence of hyperbolic effects is found in periodic and resonant systems for weakly diffracting beams, in metasurfaces, and even naturally in layered systems. At present, an actual hyperbolic propagation requires the use of metamaterials, a solution that is accompanied by constraints on wavelength, geometry, and considerable losses. We show how nonlinearity can transform a bulk KTN perovskite into a broadband 3D hyperbolic substance for visible light, manifesting negative refraction and superlensing at room-temperature. The phenomenon is a consequence of giant electro-optic response to the electric field generated by the thermal diffusion of photogenerated charges. Results open new scenarios in the exploration of enhanced light-matter interaction and in the design of broadband photonic devices.

Dual-frequency electrically driven nematic microstructures confined to biaxial porous polymer membranes

We report the study of internal ordering and electro-optical response of dual-frequency nematic liquid crystals (DFNLCs) confined to microporous polyethylene terephthalate (PET) membranes. The DFNLCs are characterized by positive/negative signs of dielectric anisotropy in low/high-frequency electric fields E. Microscopic observations assisted by numerical simulations identified the escaped radial (ER) configurations of the nematic director field within the porous membrane for E = 0. Low/high-frequency electric fields were applied to the PET-DFNLC membrane to manipulate the internal nematic configuration. We found that the low/high-frequency electric fields transform the ER structure to the quasi axial/planar radial, resulting in the suppressed/increased propagation of near-infrared electromagnetic radiation through the composite material. The results are discussed for photonic applications.

High-performance and compact integrated photonics platform based on silicon rich nitride–lithium niobate on insulator

In this paper, a silicon rich nitride–lithium niobate on insulator hybrid platform with waveguides and several key components is proposed. The propagation loss of the silicon rich nitride–lithium niobate rib-loaded waveguide (300 × 300 nm2) is 0.86 dB/cm at 1550 nm. Passive devices, including adiabatic power splitters, multimode interferometer-based splitters, asymmetrical Mach–Zehnder interferometers, and Bragg grating filters, are fully designed and characterized. Moreover, we report a Mach–Zehnder modulator with high-speed modulation up to 120 GBaud without digital compensation. The electro-optical response with 1 dB roll-off at 40 GHz is obtained, and the 3-dB modulation bandwidth is predicted to be >100 GHz. Hence, the proposed platform enables high-performance passive and active devices with low loss and high integration density, making it a promising candidate for emerging photonics integrated circuits.

MXene-coated multi-response conductive film based on layer-by-layer assembly strategy for electromagnetic interference shielding

With the rapid development of wearable electronic products and smart devices, there is an urgent need to develop efficient electromagnetic interference (EMI)shielding materials to eliminate the resulting radiation pollution. Herein, hydrophobic and multi-responsive conductive composite film with core-shell structure and sandwich structure are constructed via simple in-situ polymerization and dip coating methods. The thin silicone coating can enhance the bonding fastness of the substrate and the conductive functional layer and endow the composite film excellent waterproof performance. The composite film with a thickness of 45 μm exhibits outstanding electrical conductivity of 62.15 S/cm and EMI shielding effectiveness (EMI SE) of 37.71 dB. Surprisingly, the composite film also exhibits good waterproof performance and excellent electro-optical thermal response, and has good stability in complex environments. This work proves the advantages of constructing multiple conductive interfaces inside the nanofiber film, and provides a new preparation strategy for efficient multifunctional shielding materials, which is expected to be widely used in the next generation of wearable electronic devices and wearable smart clothing.

Giant Enhancement of Excitonic Electro-optic Response in Trap-Reduced Organic Transistors

Organic field-effect transistors (FETs) exhibit excellent switching characteristics when they involve trap-eliminated semiconductor interfaces with highly hydrophobic gate dielectric layers. In this study, we investigate the excitonic electro-optic response by delocalized carrier accumulation at the trap-eliminated interfaces of pentacene single-crystal FETs. We find that gate-modulation (GM) imaging, which sensitively visualizes the variation in optical microscope images between the gate-on and gate-off states, exclusively reveals the unique enhancement of electro-optic response under the application of drain voltages (${V}_{d}$). The ${V}_{d}$-unbiased GM image exhibits a uniform spatial distribution, which is consistent with the accumulated carrier density in the channel. In contrast, the ${V}_{d}$-biased GM image presents a peculiar spatial distribution with fairly sharp increases around the edges of the source and drain electrodes. Furthermore, the following intriguing features are observed: (1) The sharp increase in the GM signal distribution around the electrode edge is similar to the lateral electric field distribution as measured by Kelvin probe force microscopy, and (2) the GM spectra, extracted from the respective GM images measured at different wavelengths, present a second-derivative-like shape that implies the broadening of exciton absorption. Based on these observations, we investigate the origin of this unique effect in terms of the enhanced violation of exciton coherence by delocalized carrier accumulations under drain bias. The gate-induced holes that are weakly bound to shallow traps should be detrapped by lateral electric fields, which eventually generate valence band holes and thus enhance the electro-optic response. These findings should elucidate the spatial coherence of the molecular excitons that are responsible for the various unique photoelectric characteristics of organic electronic devices.

Carbon Nanotubes Blended Nematic Liquid Crystal for Display and Electro-Optical Applications

In this paper, we investigate a commercial nematic liquid crystal (LC) mixture namely E7 dispersed with small concentrations of multi-walled carbon nanotubes (MWCNTs). The dielectric and electro-optical characterizations have been carried out in the homogeneously and vertically aligned LC cells. The electro-optical response of LC molecules has been enhanced by 60% after the addition of MWCNTs, which is attributed to the reduced rotational viscosity in the composites. MWCNTs act like barricades for ionic impurities by reducing them up to ∼34.3% within the dispersion limit of 0.05 wt%. The nematic–isotropic phase transition temperature (TNI) of the E7 LC has also been shifted towards the higher temperature, resulting in a more ordered nematic phase. The enhanced birefringence and orientational order parameter in the LC-MWCNTs are attributed to π-π electron stacking between the LC molecules and the MWCNTs. The outlined merits of the LC-MWCNTs composites evince their suitability for ultrafast nematic-based electro-optical devices.

Electro-Optic Activity in Excess of 1000 pm V-1 Achieved via Theory-Guided Organic Chromophore Design.

High performance organic electro-optic (OEO) materials enable ultrahigh bandwidth, small footprint, and extremely low drive voltage in silicon-organic hybrid and plasmonic-organic hybrid photonic devices. However, practical OEO materials under device-relevant conditions are generally limited to performance of ≈300 pm V-1 (10× the EO response of lithium niobate). By means of theory-guided design, a new series of OEO chromophores is demonstrated, based on strong bis(4-dialkylaminophenyl)phenylamino electron donating groups, capable of EO coefficients (r33 ) in excess of 1000 pm V-1 . Density functional theory modeling and hyper-Rayleigh scattering measurements are performed and confirm the large improvement in hyperpolarizability due to the stronger donor. The EO performance of the exemplar chromophore in the series, BAY1, is evaluated neat and at various concentrations in polymer host and shows a nearly linear increase in r33 and poling efficiency (r33 /Ep , Ep is poling field) with increasing chromophore concentration. 25 wt% BAY1/polymer composite shows a higher poling efficiency (3.9± 0.1 nm2 V-2 ) than state-of-the-art neat chromophores. Using a high-ε charge blocking layer with BAY1, a record-high r33 (1100± 100 pm V-1 ) and poling efficiency (17.8± 0.8nm2 V-2 ) at 1310nm are achieved. This is the first reported OEO material with electro-optic response larger than thin-film barium titanate.

Revealing intrinsic electro-optic effect in single domain Pb(Zr, Ti)O3 thin films

We deposited polar-axis-oriented tetragonal and rhombohedral single domain Pb(Zr, Ti)O3 (PZT) films on CaF2(100) substrates by inserting SrRuO3 (SRO)/LaNiO3 and SRO/SrTiO3/TiO2/CeO2 buffer layers. Both PZT films grew epitaxially and had a (001)- and (111)-domain with the remnant polarization and piezoelectric constant comparable to the theoretical values of PZT single crystals having the same compositions. The electro-optic (EO) response of the fabricated PZT films was constant with respect to the DC electric field and increased linearly with an increasing AC electric field, thus representing a typical linear EO response in single domain ferroelectrics. The measured EO coefficients were larger than the value for a single crystal of PbTiO3, i.e., one of the end members of PZT, but smaller than the values reported for polycrystalline and epitaxial PZT films with multiple domains. These findings show that the intrinsic EO effect is enhanced in PZT, which is similar to the enhancement seen in the dielectric and piezoelectric constants. Moreover, most of the reported EO response in PZT films is supported by additional extrinsic contributions.

A New Class of Chiral Nematic Phase with Helical Polar Order (Adv. Mater. 35/2021)

Liquid Crystals A new class of cholesteric phase with local polar order is realized by Fumito Araoka and Hiroya Nishikawa in article number 2101305. In general, the periodicity of the conventional cholesteric liquid crystal corresponds to the half-pitch of the helix, having head-and-tail symmetry in the local nematic director. On the other hand, the newly found polar cholesteric liquid crystal is characterized with a full-pitch periodicity of the helix due to the polar degeneracy in the local nematic director. This delivers some unprecedented properties, such as fast electro-optic response based on the photonic band modulated with polar helix deformation.

Dielectric and electro-optical responses of a dielectrically negative nematic liquid crystal doped with cationic surfactant

Cetyltrimethylammonium bromide (CTAB) was employed as an ionic dopant dispersed into a nematic liquid crystal characterized by its negative dielectric anisotropy. The electrohydrodynamic (EHD) effect in liquid crystal cells impregnated with various contents of CTAB was studied by dielectric spectroscopy and the corresponding electro-optical responses of the cells driven by an AC electric field were investigated. Compared with the undoped counterpart, the frequency range of the EHD regime after doping with 0.2-wt% CTAB into the liquid crystal was widened, and distinct optical textures, including dynamic scattering, stripes, and grids were unambiguously observed. The CTAB addition reduced the threshold voltage required for switching the planar state to other optical states generated by the EHD effect.