Electro-optical Characteristics Research Articles

Electro-optical synaptic characteristics of ferroelectric liquid crystals for artificial intelligence.

We propose electro-optical synaptic devices using surface-stabilized ferroelectric liquid crystals. Typical synaptic characteristics were observed for varying pulse time intervals, numbers of pulses, and signal voltages. Plasticity only occurred when pulses were applied at intervals shorter than the response time of the ferroelectric liquid crystal. Moreover, the plasticity increased with a higher pulse voltage and number of pulses. This demonstrates the importance of repeated learning. The synaptic weights required to make connections through learning in an artificial neural network can be determined by tuning the pulse signal. We discuss the high-speed computational potential of optical neuromorphic devices using liquid crystals.

Research on Electro-Optical Characteristics of Infrared Detectors with HgCdTe Operating at Room Temperature.

This paper presents a thorough analysis of the current-voltage characteristics of uncooled HgCdTe detectors optimized for different spectral ranges. HgCdTe heterostructures were grown by means of metal-organic chemical vapor deposition (MOCVD) on GaAs substrates. The obtained detector structures were measured using a Keysight B1500A semiconductor device analyser controlled via LabVIEW for automation. The experimental characteristics were compared with numerical calculations performed using the commercial platform SimuAPSYS (Crosslight). SimuAPSYS supports detector design and allows one to understand different mechanisms occurring in the analysed structures. The dark current density experimental data were compared with theoretical results at a temperature of 300 K for short, medium, and long wavelength infrared ranges. The dark current density of detectors optimized for different wavelengths was determined using various generation-recombination mechanisms. Proper matching between experimental and theoretical data was obtained by shifting the Shockley-Read-Hall carrier lifetime and the Auger-1 and Auger-7 recombination rates. Exemplary spectral responses were also discussed, giving a better insight into detector performance. The matching level was proven with a theoretical evaluation of the zero-bias dynamic resistance-area product (R0A) and the current responsivity of the designed detectors.

Electrical and optical properties of electro chemical luminescence device with quasi solid electrolyte and ZnO/TiO2 electrode

In this work, a quasi-solid electrolyte process that can be simply manufactured for an electrochemical luminescence (ECL) device with a ZnO/TiO2 electrode was proposed, and the electro-optical characteristics of the fabricated ECL cells were investigated. The electrolyte was prepared by adding poly (vinylidene fluoride-co-hexa-fluoropropylen) with the Ru(bpy)3(PF6)2 as a luminescence material in 1:4 weigh ratio in 1 g of Acetonitrile (ACN). The ZnO/TiO2 films as an electrode were prepared by RF sputtering system. The ZnO and TiO2 targets were set up in co-sputtered system and FTO glass size 2 × 2 cm2 was used as a substrate. Argon gas flow rating as the working gas was fixed at 30 sccm. First, the ZnO layer is deposited on the FTO substrate at 100 W of RF power and at 10 mTorr of Argon gas pressure. TiO2 layer is deposited on the ZnO layer while increasing the sputtering time by 10 min. The morphological and optical properties of the thin films as electrode were confirmed using scanning electron microscope (SEM, Hitachi Su70), X-ray diffraction (XRD, Rigaku D/max 2100H) and UV–vis spectrometer (DH-2000-BAL Ocean Optics). The performance of proposed ECL devices were evaluated by varying operation voltage of 0.5 V from 2 V to 5 V. As the result, we can achieve high performance of device having a wide operating voltage margin and improved luminance intensity.

Exploration of the structural, vibrational, electronic, mechanical and thermal properties of Ru4Al3B2 and Ru9Al3B8: a DFT study.

The structural, dynamical, electro-optical, mechanical, and thermal characteristics of the newly synthesized intermetallic compounds Ru4Al3B2 and Ru9Al3B8 have been studied under ambient and elevated pressure through density functional theory (DFT). The obtained lattice parameters of the compounds are consistent with the experimental values. The metallic character of these compounds is established by the band structure and density of states (DOS). The electronic charge density distribution and bond analysis imply that Ru4Al3B2 and Ru9Al3B8 have mainly both ionic and covalent bonding. The non-negative phonon dispersion frequency of the compounds reaffirms their dynamical stability. Both compounds are tough as well as have high melting points, and hence, can be applied in harsh conditions. Mechanical properties are significantly improved under pressure. Thermal barrier coating (TBC) is a possible field of application for both compounds. The different thermal properties such as the Debye temperature (ΘD), Grüneisen parameter (γ), melting temperature (Tm), minimum thermal conductivity (Kmin) and lattice thermal conductivity (κph) of these compounds have been studied to figure out the suitable application areas in thermally demanding situations. The pressure and temperature dependent bulk modulus (B) and other thermodynamic properties have also been analyzed, which suggested that the present compounds are strong candidates for device applications at high temperature and pressure. Owing to their high optical absorptivity and reflectivity in the UV region, they are also candidates for UV-based applications. Furthermore, they also have applicability in the fields of electronics, aviation, energy storage, and supercapacitor devices for their superior electronic, thermal and mechanical properties.

Self-aligned bilayer inkjet printing process for reducing shadow area by auxiliary electrodes in OLED lighting

Auxiliary electrodes are a key component for uniform luminance of large area organic light emitting diode (OLED) lighting. A shadow area is created by the auxiliary electrodes covering the light emitting area of ​​the OLED, reducing the light emitting area. To minimize the shadow area of ​​ printed auxiliary electrodes, we developed a self-aligned bilayer inkjet printing (SABP) process. A silver-insulator bilayer was self-aligned by a hydrophobic bank where the ink did not spread. Through the SABP process, the insulator was covered with a width that was about 400 nm wider than the width of the Ag electrode. In addition, electro-optical characteristics such as luminance, current density, current efficiency, and external quantum efficiency of 40 × 40 mm 2 OLED lighting were measured to confirm the effect of the fabricated auxiliary electrodes. The luminance non-uniformity of the OLED lighting with the printed auxiliary electrode was 17%. For OLED lighting, inkjet-printed auxiliary electrodes provide potentials comparable to those of vacuum-deposited auxiliary electrodes. • Self-aligned bilayer printing patterns the silver-insulator of an auxiliary electrode for OLED lighting. • The auxiliary electrode is printed with a height of 1.09 μm and a width of 21.1 μm. • The insulator covers the width of the Ag electrode with a margin of 400 nm wide. • Self-aligned bilayer printing is effective in reducing a shadow area by auxiliary electrodes. • The luminance non-uniformity of OLED lighting with printed auxiliary electrodes is 17%.

Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment

Zinc oxide (ZnO) films reportedly exhibit a rubbing effect for liquid crystal (LC) alignment. In this study, we investigated the LC alignment characteristics of solution-derived ZnO films according to the number of rubbing iterations. Uniform and homogeneous LC alignments were achieved on the rubbed ZnO films, regardless of the number of rubbing iterations. As the number of rubbing iterations increased, the surface energies of the rubbed ZnO films were similar to 42.20 mJ/m2, but the polar energy increased from 4.288 mJ/m2 to 6.470 mJ/m2. Additionally, the electro-optical characteristics of the twisted-nematic (TN) LC cells fabricated by rubbed ZnO films showed the best performance when the number of rubbing iterations was at five. By confirming that the ZnO film with improved physical, chemical, and electro-optical characteristics compared to the polyimide film achieved a perfect LC alignment through the conventional rubbing process, it indicates that the ZnO film can be an excellent substitute for the LC alignment film. In addition, it is expected that it can be applied to an LC-based virtual reality (VR)/augmented reality (AR) display system that requires a very fast response time through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process.

Si-target power dependence on the microstructure, mechanical behavior, and electro-optical characteristics of magnetron sputtered Six(TiZrHf)1-xN coatings

Si-doped (TiZrHf)N was cosputtered from TiZrHf and Si targets on Si substrates in N2/Ar atmosphere. The effect of Si-target power on crystal, microstructural, mechanical, and electro-optical characteristics was examined. The Si content raised to 8.74 at.% significantly with increased Si target power to 70 W. The Si-free coatings had a V-shaped columnar structure with voided boundaries and a (111)-preferred direction. With increased Si-target power, the grain size and residual compressive stress initially increased and then decreased. The microstructure of the coatings obviously became compact and eventually transformed into a nanofibrous structure with (200) preferred orientation. When the Si-target power rised up to 50 W, the Si content reached 2.60 at.% and the coating demonstrated a maximum hardness of 34.8 GPa. The electro-optical properties of coatings greatly deteriorated with Si content regardless of structural evolution. The electrical resistivity of the coatings increased from 52 μΩ cm to 6250 μΩ cm, and their light reflectivity at 0.62 eV dropped from 83.5% to 33.6%.

Preparation and Applications of Rare-Earth-Doped Ferroelectric Oxides

Ferroelectric oxides possess abundant fascinating physical functionalities, such as electro-optic, acousto-optic, and nonlinear optical characteristics, etc. However, most pristine ferroelectric oxides exhibit no efficient luminescent properties due to the indirect and wide bandgap. Rare-earth-doped phosphors demonstrate advantages such as sharp emission bandwidths, large Stokes shift, high photonstability, and low toxicity. The combination of rare-earth ions and ferroelectric oxides has shown great potential in optical sensing, lighting, solar cells, and other applications. Rare-earth-doped ferroelectric oxides exhibit efficient upconversion or downconversion luminescence in the range of ultraviolet (UV) to near-infrared (NIR) regions. In this article, the preparation process of rare-earth-doped ferroelectric oxides and the preparation methods of thin films are introduced. Their recent applications in optical sensing, lighting, and solar cells are highlighted. The review concludes with a brief summary of all related branches and discusses the potential direction of this field.

Anisotropic Surface Formation Based on Brush-Coated Nickel-Doped Yttrium Oxide Film for Enhanced Electro-Optical Characteristics in Liquid Crystal Systems

This paper introduces anisotropic nickel yttrium oxide (NYO) film formed by the brush coating technique. X-ray photoelectron spectroscopy confirmed well-formed NYO film after the curing process, and the morphology of the surface was investigated using atomic force microscopy. The shear stress driven from brush hair movements caused the nano/micro-grooved anisotropic surface structure of NYO. This anisotropic surface induced uniform liquid crystal (LC) alignment on the surface, which was confirmed by pre-tilt angle analysis and polarized optical microscopy. The contact angle measurements revealed an increase in hydrophilicity at higher temperature curing, which contributed to homogenous LC alignment. The NYO film achieved good optical transmittance and thermal stability as an LC alignment layer. In addition, the film demonstrated good electro-optical properties, stable switching, and significantly enhanced operating voltage performance in a twisted-nematic LC system. Therefore, we expect that this brush coating method can be applied to various inorganic materials to achieve an advanced LC alignment layer.

Impact of terminal group of organic dopant on liquid crystal-based electro-optic device

In this study, the influences of terminal group of organic dopant in nematic liquid crystals (LCs) are investigated. A novel organic molecule, N-(4-Fluoro-2-methylphenyl)benzenemethanamine (BFM) with fluorine terminal group and a similar one, N-benzyl-2-methyl-4-nitroaniline (BNA) with nitro terminal group are doped into the nematic LCs, separately. The electro-optical characteristics of the BFM-doped and BNA-doped LC cells are demonstrated. The LC cells with organic dopings have the lower threshold and operation voltages than the pristine LC cell, due to the enhanced electrical torque of LCs. The fluorine terminal atoms in the BFM-doped LC cell prevents the drastic absorbance at short visible wavelengths and suppresses the free-ions in the BNA-doped LC cell significantly. BFM-doped and BNA-doped LC cells have the fall times that are 5.79X and 5.05X faster than pristine LC cell, respectively, owing to decreased viscosity and threshold voltage and increased local electric field. Furthermore, density functional theory calculation confirms that organic dopant enhances the dipole moment, polarizability, polarizability anisotropy and hence the dielectric anisotropy of LC mixture. The competition between the dipole moment and free-ions of organic molecule-LC composite has a crucial influence on the electro-optical properties of LC cell. BFM doping not only prevents the color shift but also improves the operation voltage and response time of LC cell significantly.

Electro-Optical Characteristics of Polymer Dispersed Liquid Crystal Doped with MgO Nanoparticles.

In this paper, inorganic oxide MgO nanoparticles-doped polymer dispersed liquid crystal (PDLC) films were made from a mixture of the prepolymer, SLC1717 liquid crystal, and MgO nanoparticles by the polymerization induced phase separation (PIPS) process. To observe the effect of MgO concentration, PDLC was dispersed with 0.2, 0.4, 0.6, and 0.8 wt.% MgO. Electro-optical properties of the films have been investigated using LCD parameter meter and Scanning Electron Microscope (SEM) at room temperature. It is established that MgO nanoparticles affect the microstructure of PDLC films significantly because of the formed agglomerates of MgO nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the driving voltage for doped systems with MgO nanoparticles. When the doping amount of MgO is 0.8 wt.%, the threshold voltage (Vth) is reduced to about 7.5 V. Therefore, MgO-doped PDLC is expected to become an excellent choice in the field of energy-saving.

Influence of the thickness of frontal platinum metallic layer on the electro-optical characteristics of GaN-based Schottky ultraviolet photodetectors

In this work, we evaluated the effect of the thickness of frontal metallic layer on the electro-optical characteristics of an n-type gallium nitride (n-GaN)-based Schottky barrier ultraviolet (UV) detector using device modeling and numerical simulations. Comparison of the current density-voltage characteristics J(V) calculated for different metals demonstrated that platinum (Pt) is the most suitable metal to form Schottky contacts. The obtained results show that the thickness of the frontal platinum Schottky contact highly affects the spectral responsivity of the detector in the considered UV range of 0.2…0.4 µm. In particular, the detector responsivity at room temperature can reach the peak value of 0.208 A·W–1 at the wavelength of 0.364 µm and the semi-transparent Pt layer as thin as 1 nm. Afterward, it gradually decreases with the increase of the metal layer thickness down to 0.147 A·W–1 for the thickness of the Pt layer of 100 nm.

Investigation of the electro-optical characteristics of GaAs/AlGaAs multiple quantum well grown by metal-organic vapor phase epitaxy

In this study, the photoluminescence measurements of GaAs/Al0.25Ga0.75As multi-quantum-wells heterojunction structure grown on n+-GaAs substrates by Metal Organic Vapor Phase Epitaxy (MOVPE) method are investigated. By dropping 5145 Å wavelength laser light on the sample at room temperature and low temperatures, the transitions between the bands in the structure and the changes in these transitions under the different electric fields and temperatures are observed. In addition, by making theoretically developed self-consistent potential calculations, the subband energy levels and their corresponding wave functions of the structure under the electric field and without the electric field are calculated. The obtained numerical results were found to be in full agreement with the experimental measurements and theoretical calculations.

Adaptive PAM-4/PAM-8 graphene-based electro-optical modulator integrated into a polymer waveguide platform for data-center communication

This paper presents a new concept of a graphene-based adaptive modulator for multilevel amplitude modulation on polymer technology platform. This component consists of an electro-optical modulator with a distributed five segments capacitive structure, based on two single-layers of graphene (SLG) and a thin layer of silicon nitride (Si3N4) in the form (SLG-Si3N4-SLG), which shows a dynamic configuration of the modulation scheme, PAM-4/PAM-8, by the applied logic voltage sequence (3Vpp) for an operating wavelength at 1550 nm. Considering an extinction ratio of 5.6 dB for PAM-4, and 6.0 dB for PAM-8, a compact device with 179.6 µm total length, and the maximum simulated cut-off frequency of 48.12 GHz with 122.9 fF of capacitance and total device resistance of 26.9 Ω, were achieved. This cut-off frequency results in a transmission rate of 96.24 Gbps at PAM-4 and 144.4 Gb/s at PAM-8. The simulated insertion loss is 12.0 dB (PAM-4) and 11.5 dB for PAM-8, and the power consumption is 0.54 pJ/bit for PAM-4 and 0.81 pJ/bit for PAM-8. Combining an innovative design with the unique electro-optical characteristics widely demonstrated by the materials used in the construction of the proposed adaptive device, allows it to be seen as a candidate to contribute to the nanotechnology integration and high-performance communication requirements.

Application of TiO2 nanoparticle and polyimide blend alignment layer in liquid crystal lens

Suspension of colloidal nanoparticles (NPs) in liquid crystals (LCs) is a common technology to modify the electro-optical characteristics of a LC device. Unfortunately, NP aggregation spontaneously proceeds over time through the dipolar attractive interaction between NPs and results in the light scattering and distortion of the nematic order, which degrades the performance of the LC device. Here, rutile titanium dioxide (TiO2) NPs were suspended in the alignment layer rather than LCs to prevent the proceeding NP aggregation. During the deposition of the alignment layer, the NPs were hardened in the alignment layer with a hard bake process. The blend alignment layer was utilized to develop a hole-patterned liquid crystal (HLC) lens. This layer enhances the polar anchoring of LCs because of the intermolecular interaction between NP and LC and reduces the ion density and rotational viscosity of the LC mixture through harvested effect of free-ions. It also assists in spreading the fringing electric field toward lens center by TiO2 NP contribution. These results speed up the dynamic response of the HLC lens. The introduction of blend alignment layer decreases the focus and defocus times of the HLC lens by 15% and 34%, respectively. Moreover, the operation voltage of the HLC lens for achieving the shortest focal length is reduced by ∼40%. The decreased switching time is verified by calculating the transient phase conversions of HLC lenses with various viscosities. The calculated results correspond to the experimental results. The blend alignment layer did not degrade the lens and focusing qualities of the HLC lens. Long-term observation revealed that the NP aggregation state remained similar for the LC sample with NPs suspended in the alignment layer. By contrast, NP aggregation spontaneously proceeded and connected networks appeared with time for the LC sample with NPs suspended in LCs. The blend alignment layer can improve the operation voltage and dynamic response of a LC device and prevent device deterioration over time.

Detailed electro-optical modeling of thermally-activated delayed fluorescent OLEDs with different host-guest concentrations

OLED device optimization often relies on time-consuming trial-and-error experiments. While the photoluminescence quantum yield can serve as a first indicator to find the best performing host-guest ratio, this quantity does not consider the impact of the latter on the charge transport in the full device. Herein, we analyse four thermally activated delayed fluorescence OLEDs with varied host-guest ratio in the emissive layer. These devices were characterized and modelled in steady-state, under transient conditions and in the frequency domain. In this set of devices charge injection into and transport inside the emissive layer plays a crucial role in the performance. Evidenced by a particular transient electroluminescence turn-off overshoot, we show that for the 5% guest concentration device electron and hole transport occurs mainly on the host molecules, with guest molecules acting as trap states. For the other devices with higher guest concentration, we find that transport occurs mainly on the guest molecules. As a second step, a fit of the luminance efficiency is performed, with which we can extract the triplet-triplet annihilation and triplet-polaron annihilation rates. By comparing the extracted parameters, we found that they increase with increasing concentration of guest molecules. Moreover, we were able to identify triplet-polaron quenching from holes to be limiting the luminance efficiency at low current, while triplet-polaron quenching from electrons and triplet-triplet annihilation are the dominant non-radiative decay processes in the high current regime. Overall, we demonstrate that model-based analysis of steady-state, transient and frequency domain data obtained for a thermally activated delayed fluorescence OLED allows to get a deeper understanding of the efficiency limiting factors for various host-guest concentrations and driving currents. • Electro-optical characteristics of various OLEDs is well described by the simulation. • Charge transport occurs on TADF guest molecules only at large guest concentrations. • The polarity of TPBi can be captured with impedance and CELIV techniques. • Simulations of the efficiency roll-off allow to quantify quenching processes. • Triplet quenching processes scale empirically with the log of guest concentration.

Electro-optical characteristics of polymer-dispersed liquid crystal containing copper (II) phthalocyanine as a function of UV irradiation time

We describe a method of dispersing copper (II) phthalocyanine that can expand the application range of polymer-dispersed liquid crystals (PDLCs) among smart-window technologies. Property changes were monitored during the polymerization-induced phase separation method as the ultraviolet (UV) irradiation time was increased from 70 to 175 s. With increasing UV irradiation time, the number of droplets in the polymer matrix within the PDLC increased, while the droplet size decreased, as observed by polarized optical and low vacuum-scanning electron microscopies. Voltage–transmittance and response time were measured to observe the effects on electro-optical properties; the threshold and saturation voltages increased with increasing UV-irradiation time. The falling time of the response decreased with increasing irradiation time; a fast switching time of < 20 ms was observed in all cases. This study provides the performance of the produced PDLC varies according to the UV irradiation time, and conditions and processes necessary for producing blue PDLC are presented.

Magnetron-sputtered gallium–magnesium–zinc oxide transparent semiconductor thin films: Structural, optical and electrical investigation

Thin transparent semiconductor films of gallium–magnesium–zinc oxide (GaMgZnO) have been prepared with magnetron-sputtering technique on quartz glass substrates. We have investigated the effects of sputtering pressure on the crystallization, micro-structure, electrical and optical features of the deposited films. The results indicated that all the deposited films were polycrystalline in nature having a hexagonal wurtzite-type crystal structure with a preferred grain orientation in the (002) direction. The sputtering pressure has a significant impact on the structure and electro-optical characteristics of the deposited films. The sample prepared under sputtering pressure of 3.6 Pa possesses the best crystalline quality and electro-optical performance, with the minimum dislocation density (3.69×1014linesm−2), tensile stress (8.26×107 Pa), resistivity (1.53×10−3Ωcm), the maximum mean visible transmittance (87.71%) and the highest figure of merit (4.73×103Ω−1cm−1). Additionally, the external extrapolation method is employed to evaluate the optical band gap. The gap value is between 3.41 and 3.44 eV, which is larger than the optical band gap of pure ZnO.

Evaluation of the Characteristics of Cholesteric Liquid Crystal Diffuser Element Applied in Multi-Focal Display Architectures

Solid-state multi-focal and volumetric technologies highlight the future of 3D-display development. One of the most convenient implementations of multi-focal 3D displays are stacks of transparent liquid crystal displays. In this work, the core element is dissected—a switching optical diffuser element based on cholesteric liquid crystals, playing the role of a transparent display. In the present study, high-speed synchronized optical spectroscopy is used. We analyzed the kinetic and electro-optical characteristics of the diffuser element, the operation of which is based on the switching between diffuse and transparent states of this element. The underlying aim of this study was to investigate ways to improve some of these characteristics. It has been found that the transient peak in the optical transmission during field-off state, which is reducing the intensity of the light scattered by the diffuser element, is likely not associated to the assumed formation of the transient planar state. As the origin of this peak, we suggest a transient state possessing uniform lying helix structure, formed due the material flow taking place in the cell during relaxation of the liquid crystal. The role of the contacting surface’s pre-tilt angle in the switching process of the liquid crystal diffuser was established.

Interdependency among electro-optic characteristics and absorption coefficient of homeotropically aligned liquid crystal considering Beer’s law theory

Interdependency among electro-optic characteristics and absorption coefficient of homeotropically aligned liquid crystal considering Beer’s law theory