Applications In Electro-optical Devices Research Articles

Physical attachment of NLO chromophores to polymers for great improvement of long-term stability

Dual D-Pi-A nunchaku-like structure chromophore was designed and synthesized to improve electro-optic (EO) materials׳ EO coefficients (r33) and long term stability by a simple doping system. The r33 values of poled APC/10% DIMER1 and APC/20% DIMER1 are 23pm/V and 45pm/V respectively. After annealed at 85°C for 500h, the poled film of APC/20% DIMER1 can retain more than 85% of its initial r33 value. All these characters showed us superior prospects of application in EO devices. Chromophore DIMER1 also showed us good thermal stability; its decomposition temperature (Td) was higher than 280°C, which is enough for its application in the EO devices. Also chromophore DIMER1 had good solubility in amorphous polycarbonate (APC); it could be dissolved in APC at high loading density (25%).

Photocurable, solventless, solution-processable, silica-nanoparticle-dispersed acryl hybrid materials.

Solventless silica-acryl hybrid materials composed of organically modified silica nanoparticles and acryl monomers were fabricated using a simple sol-gel process and solvent evaporation. The homogeneously dispersed silica-acryl hybrid materials can be formed into films through a simple solution-coating process and UV curing. The silica-acryl hybrid materials exhibited good photocurability with -90% conversion upon UV exposure and a high transmittance of above 95% in the visible wavelength region regardless of the silica nanoparticle content. In particular, the silica-acryl hybrid materials exhibited increased mechanical hardness and thermal stability directly proportional to the content of silica nanoparticles and exhibited a good surface roughness of -1 nm. Therefore, these silica-acryl hybrid materials are good candidates for coating applications in electronic, electro-optic, and energy devices.

Selective-Area Atomic Layer Deposition of Copper Nanostructures for Direct Electro-Optical Solar Energy Conversion

We investigate selective-area atomic layer deposition (ALD) of copper onto nanofabricated palladium structures for plasmonic applications in electro-optic conversion devices. We examine arrays of nanofabricated tip shaped electrodes for the formation of geometrically asymmetric tunnel junctions. We find ALD growth to be sensitive to sample pretreatment, and demonstrate that UV-ozone treatment is effective for initiating growth. We show that nucleation density and grain evolution are important properties of growth because ALD deposited topological features are on the same scale as the nanostructures. We predict electric-field intensities within the gap region between electrodes to scale exponentially with ALD growth. We establish the basis for ALD tuning of plasmonic nanostructures.

Fructose calcium chloride MOFs for NLO applications

Non-centrosymmetric Metal Organic Frameworks (MOF) based on coordination of carbohydrates on alkaline metal ion represent an ideal category of compounds to plan functional materials with interesting second-order non-linear optics properties. Two MOFs obtained from fructose and calcium chloride, [Ca(C6H12O6)(H2O)2]Cl2 (1) and [Ca(C6H12O6)2(H2O)2]Cl2·H2O (2), have been synthetized in ethanol and by solid-solid interactions and have been characterized with X-ray diffraction and IR and RAMAN spectroscopy. The two compounds have also been studied with DFT theoretical calculations to investigate their relative stability and foresee their optical properties. Finally, the second harmonic generation (SHG) properties have been evaluated by SHG microscopy. It has been demonstrated that the coordination of fructose on the calcium ion causes an improvement of the SHG intensity with respect to the fructose itself and that there is a strictly correlation between the calculated dipole moment and the first static hyperpolarizability values and intensity of the SHG signal. Furthermore, the remarkable difference in the SHG intensity of the two compounds studied, suggests that this physical property can be "controlled" not only by tuning the composition of the system, but also by the disposition of the MOF's building units in the crystal. The metal-carbohydrate based MOFs analyzed in this work have a favorable combination of thermal and chemical stability, transparency, and second-order optical non-linearity and are thus potential candidates for applications in electro-optic devices.

Theoretical study of ferroelectric nanoparticles using phase reconstructed electron microscopy

Ferroelectric nanostructures are important for a variety of applications in electronic and electro-optical devices, including nonvolatile memories and thin-film capacitors. These applications involve stability and switching of polarization using external stimuli, such as electric fields. We present a theoretical model describing how the shape of a nanoparticle affects its polarization in the absence of screening charges, and quantify the electron-optical phase shift for detecting ferroelectric signals with phase-sensitive techniques in a transmission electron microscope. We provide an example phase shift computation for a uniformly polarized prolate ellipsoid with varying aspect ratio in the absence of screening charges.

“Beating speckles” via electrically-induced vibrations of Au nanorods embedded in sol-gel

Generation of macroscopic phenomena through manipulating nano-scale properties of materials is among the most fundamental goals of nanotechnology research. We demonstrate cooperative “speckle beats” induced through electric-field modulation of gold (Au) nanorods embedded in a transparent sol-gel host. Specifically, we show that placing the Au nanorod/sol-gel matrix in an alternating current (AC) field gives rise to dramatic modulation of incident light scattered from the material. The speckle light patterns take form of “beats”, for which the amplitude and frequency are directly correlated with the voltage and frequency, respectively, of the applied AC field. The data indicate that the speckle beats arise from localized vibrations of the gel-embedded Au nanorods, induced through the interactions between the AC field and the electrostatically-charged nanorods. This phenomenon opens the way for new means of investigating nanoparticles in constrained environments. Applications in electro-optical devices, such as optical modulators, movable lenses, and others are also envisaged.

Electrochemical Characterization of Spiropyran Structures

Spiropyran (SP) is a type of photochromic molecules widely studied because of their interesting physical and chemical properties. Irradiation of spiropyrans with near UV-light induces heterolytic cleavage of the spiro bond, resulting in the possibility of their application in electro-optical molecular devices and electronic data storage. Although some reports conclude that the opening process also occurs upon electrochemical oxidation of the indolic nitrogen, a complete description of both oxidation and reduction processes, is yet unavailable. In this work, cyclic voltammetry was employed for characterizing nitrospiropyran structures. After oxidation of SP, a reduction signal at a potential value less negative than the original closed compound appeared when successive voltammetric cycles were performed. Based on the results obtained, possible mechanistic pathways are discussed.

Hybrid Plasmonic Structures: Design and Fabrication by Laser Means

Design and fabrication of hybrid plasmonic structures by laser means are investigated to extend flexible tuning of optical properties of metallic micro/nano-structures for high-sensitivity detection and enhancement of solar cell antireflection performance for high energy-conversion efficiency. It is shown that the identical micro/nano-structures in well-defined arrays fabricated by laser interference lithography can be used as a versatile platform to enhance fluorescence intensity of the molecules. Meanwhile, postprocessing of such structures via thermal annealing can result in ordered clusters of nanodots 50 nm in average size, which enhances Raman scattering intensity. A dramatic reduction in silicon surface reflectance is achieved via pulsed fiber laser texturing. The suppression of reflectance can be further improved by decorating the laser-textured Si surface with metallic nanoparticles by thermal annealing of metallic thin films deposited on the Si surfaces. This hybrid plasmonic structure scheme can achieve broadband (300-1000 nm) antireflection with a surface reflection as low as 5.5%. The improved broadband antireflection of the surfaces could have applications in solar energy, renewable energy, and electrooptical devices.

Optical absorption in asymmetric double quantum wells driven by two intense terahertz fields

Optical absorption is investigated for asymmetric double quantum wells driven by a resonant terahertz field and a varied terahertz field, both polarized along the growth direction. Rich nonlinear dynamics of the replica peak and the Autler-Townes splitting of various dressed states are systematically studied in undoped asymmetric double quantum wells by taking account of multiple factors, such as the frequency of the varied terahertz field and the strength of the resonant terahertz field. Each electron subband splits into two dressed states when the resonant terahertz field is applied in the absence of the varied terahertz field, the optical absorption spectrum shows the first-order Autler-Townes splitting of the electron subbands. When a varied terahertz field is added into the resonant system, the replica peak and the second-order Autler-Townes splitting of the dressed states near the band edge respectively emerge when the varied terahertz field is non-resonant and resonant with these dressed states. When the strength of the resonant terahertz field is increased, the first-order Autler-Townes double peaks and the replica peak in the optical absorption spectrum shift with the shifts of the dressed states. The presented results have potential applications in electro-optical devices.

Autler-Townes splitting and quantum confined Stark effect of sideband peak in asymmetric double quantum wells

Optical absorption is investigated by self-consistent density matrix approach in asymmetric double quantum wells driven by an intense terahertz field and a direct current electric field polarized along the growth direction. Rich nonlinear dynamic behaviors of sideband absorption peaks are systematically studied in undoped asymmetric double quantum wells. When only in presence of a resonant terahertz field, the Autler-Townes splitting of the sideband peaks becomes pronounced with increasing the strength of the terahertz field. Quantum confined Stark effect of sideband peaks is discussed when an invariant terahertz field and a direct current electric field are simultaneously applied to the quantum well. It is shown that the sideband peaks of the 1s main absorption peak undergo a red-shift and the sideband peaks of the 2s main absorption peak undergo a blue-shift with increasing intensity of the direct current electric field. The presented results have potential applications in electro-optical devices.

Transition metals in ZnO nanocrystals: Magnetic and structural properties

Currently, wide-gap ZnO nanoparticles bear important potential application in electro-optical devices, transparent ultraviolet protection films, and spintronic devices. We have studied the magnetic properties of nanocrystals of ZnO(Fe, Co, Mn) prepared by two methods of synthesis. We have used the microwave assisted hydrothermal synthesis and traditional wet chemistry method followed by calcination. The detailed structural characterization was performed by means of X-ray diffraction and micro-Raman spectroscopy measurements. The morphology of the samples was studied by means of SEM and TEM microscopy. The results of systematic measurements of AC magnetic susceptibility as a function of temperature and frequency as well as SQUID magnetization are presented. The SQUID magnetization measurements revealed a clear bifurcation of the FC and ZFC plots. Such behavior suggested superparamagnetic behavior above the blocking temperature. The dynamic magnetic measurements were performed at small AC magnetic field with amplitude not exceeding 5 Oe and different frequency values (from 7 Hz to 9970 Hz). For ZnO(Fe) and ZnO(Mn), the AC susceptibility maxima has been found for in-phase susceptibility Re(?) and for out of phase susceptibility Im(?). We analyzed the observed frequency dependence of the peak temperature in the AC susceptibility curve using the empirical parameter ? that is a quantitative measure of the frequency shift and is given by the relative shift of the peak temperature per decade shift in frequency, as well as Vogel- Fulcher law. We observed two different types of magnetic behavior, spin-glasslike behavior or superparamagnetic behavior, depending on the synthesis process. For ZnO(Co) nanocrystalline samples high temperature Curie-Weiss behavior in AC magnetic susceptibility was observed. We observed that the determined negative values of the Curie- Weiss temperature ? depend strongly on the nominal content of cobalt oxide. It was shown that for calcination method the values of ? increase with the increase of magnetic ion content indicating enhancement of predominance of antiferromagnetic interactions. For hydrothermal method the opposite effect was observed indicating the breakdown of predominance of aniferromagnetic coupling with the increase of nominal magnetic ion content. This paper gives an in-depth discussion of the structural and magnetic properties of ZnO nanocrystals in addition to the technological issues such as different methods of wet chemical synthesis.

Fabrication of size-tunable Cr nanodots and nanorings array by modified nanosphere lithography

This Letter is devoted to developing a novel and low-cost strategy for fabricating Cr triangular-shaped nanodot and nanoring array patterns by a distinct magnetron sputtering approach using nanosphere lithography-based technology. The size of polystyrene nanospheres had a direct influence on the size and period of Cr nanorings. The lateral sizes, inner sizes, wall thickness and wall height are controllable by varying the process conditions of reactive ion etching, magnetron sputtering and the lift-off process. The wettability can be manipulated by changing the size of the nanorings. This new approach will surely facilitate further exploration of metal nanorings for potential applications in electro-optic devices.

Photoinduced orientation in natural rubber

Azobenzene molecules and their derivatives have been widely investigated for their potential applications in optical and electrooptical devices. We have prepared a new guest–host system from natural rubber (NR) impregnated with azobenzene derivative Sudan Red B (SRB). The effects of stretching and immersion time on photoinduced orientation were investigated by birefringence signal measurements. We have found that the molecular orientation increase when the samples are stretched and decrease with the increase of immersion time. The first behavior was explained by using the random coil model and the latter was attributed to increase of the aggregation of SRB into NR matrix.

Solution-processed novel core–shell n–p heterojunction and its ultrafast UV photodetection properties

We demonstrate the feasibility of fabricating a solution-processed novel core–shell coaxial n–p nanorod (NR) heterojunction consisting of an n-type ZnO NR core and a p-type CuS shell through an ion-exchange route. The field emission scanning electron and high resolution transmission electron microscopy experiments show evidence of the formation of core–shell NR arrays manifesting a clear change in the side walls of the NRs, which become rougher due to CuS shell formation over ZnO NRs. The core–shell heterojunction shows diode behavior with an ideality factor of 5.8. The diode shows a very fast response time (<0.3 s) towards 365 nm ultraviolet (UV) light, which is an important criterion for the photodetection property. Furthermore, the main advantages of this solution-based method are its simplicity and low cost. This achievement demonstrates that such solution-processed novel core–shell n–p heterojunctions are promising candidates for applications in electro-optic devices.

Formation of nano-crystalline Si quantum dots in ZnO thin-films using a ZnO/Si multilayer structure

In this study, we propose to fabricate ZnO thin-films with nano-crystalline Si (nc-Si) quantum dots (QDs) embedded using a ZnO/Si multilayer structure by radio-frequency (RF) magnetron sputtering method. Our results show that a high Si sputtering power (PSi) can assist the formation of self-aggregated Si nano-clusters during deposition, which is helpful for the crystallization of nc-Si QDs and ZnO matrix during annealing. Great crystallinity and highly uniform size of nc-Si QDs are obtained for PSi of 110W. We experimentally demonstrate the formation of nc-Si QDs embedded in crystalline ZnO thin-films, which has a great potential for various electro-optical device applications.

Transparent Barium Strontium Titanate Ceramics Prepared by Spark Plasma Sintering

Transparent ferroelectric ceramics are valuable for many applications in electro-optical devices. In this paper, transparent barium strontium titanate (BST) ceramics were prepared by spark plasma sintering and thermal treatment. The microstructures and optical properties of the transparent BST samples were investigated. The densities of the BST samples were nearly theoretical and the samples were optically transparent. Both the transmittances at 633 nm of the Ba0.3Sr0.7TiO3 and Ba0.4Sr0.6TiO3 ceramics with a thickness of 0.3 mm are over 74%. The lead-free optical transparent ferroelectric BST ceramics are promising candidates for electro-optical applications.

Influence of Oxygen to Argon Ratio on the Properties of RF Magnetron Sputtered Ba0.7Sr0.3TiO3 Thin Films

The optical properties of thin films of barium strontium titanate (BST) are important due to its wide band gap, high refractive index and low absorption coefficient and hence find application in electro-optic and non-linear optical devices. Ba0.7Sr0.3TiO3 (BST) thin films were deposited by RF magnetron sputtering on fused silica and Pt/TiO2/SiO2/Si (PtSi) wafers at relatively low substrate temperature. The crystallisation of thin films strongly depends on O2/Ar ratio in the sputtering gas. The optical parameters like band gap (Eg), refractive index (n) were studied as a function of oxygen to argon ratio in the sputtering gas. The electrical properties of BST thin films deposited on PtSi substrates as a function of O2 to Ar ratio has also been studied. The films show 50% tunability with a minimum loss of 0.018 at 1 MHz when grown at the optimized O2 to Ar ratio.

Solution-chemistry approach to graphene nanostructures

Graphene has many novel optical and electrical properties desirable for applications in future electro-optical devices. Graphene nanostructures are especially attractive for their wide range of tunable properties. Here we describe the recent progress and challenges in the solution-chemistry approach to graphene nanostructures. This approach could not only lead to new materials with various well-defined properties, but also enable their production in large quantities.

Synthesis and Nonlinear Optical Properties of Novel Polyester with Enhanced Second Harmonic Generation Thermal Stability

). In the developments of NLO polymers for electro-optic device applicati ons, stabilization of electrically induced dipole alignment is one of important criteria; in this context, two approaches have been proposed to minimize the randomization of dipole alignment, namely the use cross-link-ing method

Second-order nonlinear optical response of zigzag BN single-walled nanotubes

A theory based on the two-band tight-binding approximation for $\ensuremath{\pi}$ electrons is developed to describe the second-order nonlinear optical (NLO) properties of arrays of uniformly sized and well-aligned boron-nitride single-walled nanotubes (BN-SWNTs) with a zigzag achiral structure. It is assumed that the coherent light beam at frequency $\ensuremath{\omega}$, incident upon the nanotube sample, is linearly polarized along the symmetry axis of the nanotubes. The long-axis NLO susceptibility ${\ensuremath{\chi}}^{(2)}(\ensuremath{\omega})$ of those nanotubes is calculated within the independent nanotube approximation and in neglecting local-field effects. Using the perturbation-theory formalism in the crystal-momentum representation, we derive an explicit analytic expression for the ${\ensuremath{\chi}}^{(2)}(\ensuremath{\omega})$ and apply it to study three distinct second-order NLO effects possible in the BN-SWNTs due to their noncentrosymmetric structure---namely, second-harmonic generation (SHG), linear electro-optical (LEO) effect, and nonlinear optical rectification (NOR). The theory is illustrated by numerical model calculations of the SHG, LEO, and NOR susceptibility spectra for several representative BN-SWNT ensembles consisting of large-diameter nanotubes. The calculated SHG spectra are found to be dominated by the highly peaked $2\ensuremath{\omega}$ resonance at half the band-gap energy of the BN-SWNTs, where the absorption of light is negligible. Distinct features are also found in the LEO and NOR susceptibility spectra, e.g., a sudden switching of the susceptibility from a positive peak value to a negative peak one in the near vicinity of the fundamental absorption edge. A fairly large magnitude of those susceptibilities, reaching the order of ${10}^{\ensuremath{-}7}\text{ }\text{esu}$ under off-resonant conditions and up to ${10}^{\ensuremath{-}6}\text{ }\text{esu}$ in the resonant case, suggests that BN-SWNTs are a promising material for various electro-optical device applications.