Electrode Gratings Research Articles

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.

High kt2 Exceeding 6.4% Through Metal Frames in Aluminum Nitride 2-D Mode Resonators.

This work describes a new method to enhance the electromechanical coupling coefficient ( kt2 ) of 2-D mode resonators (2DMRs). This approach exploits the multimodal excitation of different Lamb waves to achieve a quasi-uniform vertical displacement distribution in the electrode regions of 2DMRs. To do so, the 2DMRs reported in this work rely on the use of metal frame structures placed at the edges of each metal strip forming the electrode gratings. To demonstrate the effectiveness of this new technique in enhancing kt2 , we report the design and simulated performance through the finite-element methods of a 2.5-GHz 2DMR using metal frames. Our analysis proves that this device can simultaneously exhibit high kt2 in excess of 6.4% and wide lithographic frequency tunability in excess of 10%, thus approaching the reported performance of aluminum nitride (AlN) film bulk acoustic resonators (FBARs) operating within the same frequency range.

Electrical Evidence of the Tunable Electrical Bragg Bandgaps in Piezoelectric Plates.

A piezoelectric plate, poled along its thickness and supporting on its top and bottom surfaces a periodic grating of electrodes, is considered. An analytical model allowing band structure calculation is derived for the first symmetrical mode propagating along the length of the plate. Analytical results show that an electrical Bragg (EB) bandgap can be observed for this mode, depending on the electrical boundary conditions applied on the electrodes. This "EB bandgap" is associated with a discontinuity of the electric field between two successive unit cells. These results are validated with the numerical simulations based on the finite element method. Analytical and numerical results prove that the EB bandgap is highly tunable and can be optimized by changing the crystallographic orientation of the material. A simple tunable filter exploiting this bandgap is designed and fabricated. Experimental results of electrical impedance and electrical potential at the output together with a scanning laser vibrometer analysis are presented, which confirm the theoretical predictions.

Study of the vertically aligned in-plane switching liquid crystal mode in microscale periodic electric fields.

The ongoing interest in fast liquid crystal (LC) modes stimulated by display technology and new applications has motivated us to study in detail the in-plane switching (IPS) vertically aligned (VA) mode. We have studied how the decrease of the period of the interdigitated electrodes (down to sub-micrometer scale) influences the switching speed, especially the LC relaxation to the initial homeotropic state. We have found that there are two types of the relaxation: a fast relaxation caused by the surface LC sub-layer deformed in the vicinity of the electrodes and the slower relaxation of the bulk LC. The speed of the fast (surface) mode is defined by half of a period of the electrode grating, while the relaxation time of the bulk depends on the LC layer thickness and the length of the driving electric pulses. Thus, the use of the surface mode and the reduction of the electrode grating period can result in significant increase of switching speed compared to the traditional LC modes, where the bulk relaxation dominates in electrooptical response. We have studied thoroughly the conditions defining the surface mode applicability. The numerical simulations are in good agreement with experimental measurements.

A polarization-insensitive plasmonic photoconductive terahertz emitter.

We present a polarization-insensitive plasmonic photoconductive terahertz emitter that uses a two-dimensional array of nanoscale cross-shaped apertures as the plasmonic contact electrodes. The geometry of the cross-shaped apertures is set to maximize optical pump absorption in close proximity to the contact electrodes. The two-dimensional symmetry of the cross-shaped apertures offers a polarization-insensitive interaction between the plasmonic contact electrodes and optical pump beam. We experimentally demonstrate a polarization-insensitive terahertz radiation from the presented emitter in response to a femtosecond optical pump beam and similar terahertz radiation powers compared to previously demonstrated polarization-sensitive photoconductive emitters with plasmonic contact electrode gratings at the optimum optical pump polarization.

Stark effect and photoconductivity of fullerene С70 measured on samples with a submicron electrode grating and sandwich geometry

The electroabsorption (Stark effect) and photoconductivity of polycrystalline fullerene C70 in thinfilm samples with radically different geometries (sample 1 with interdigital Cr electrodes and sandwichlike sample 2 with SnO2 and Al electrodes) have been investigated. It is noteworthy that sample 1 is a submicron grating with 0.88-µm gaps between electrodes. When measuring electroabsorption and photoconductivity, the external voltage does not exceed 4 V. It was assumed that the small gap size may significantly affect the polycrystalline film morphology and surface effects and, as a consequence, the electroabsorption and charge carrier mobility. However, the results of the Stark effect measurements on samples with different geometries differ by only 25%, whereas the spectral photoconductivity values differ by factors of 1–2.5. The increase in photoconductivity may be related to the decrease in the carrier mobility in the sandwich sample or the increase in the number of carriers under the influence of atmospheric oxygen in the surface layer of C70 sample with interdigital electrodes.

Gains and Losses in PbS Quantum Dot Solar Cells with Submicron Periodic Grating Structures

Corrugated structures are integral to many types of photoelectronic devices, used essentially for the manipulation of optical energy inputs. Here, we have investigated the gains and losses incurred by this microscale geometrical change. We have employed nanostructured electrode gratings of 600 nm pitch in PbS colloidal quantum dot (PbS-CQD) solar cells and investigated their effect on photovoltaic properties. Solar cells employing grating structure achieved a 32% and 20% increase in short-circuit current density (Jsc) and power conversion efficiency, respectively, compared to nonstructured reference cells. The observed photocurrent increase of the structured devices mainly stems from the enhancement of photon absorption due to the trapping of optical energy by the grating structures. This optical absorption enhancement was particularly high in the near-infrared portion of the sun spectrum where PbS-based solar cells commonly present poor absorption. We have interestingly observed that the open-circuit vol...

Spectral characteristics of terahertz radiation from plasmonic photomixers.

We present a comprehensive analysis of spectral characteristics of terahertz radiation from plasmonic photomixers. We fabricate plasmonic photomixer prototypes with plasmonic contact electrode gratings on a low temperature grown GaAs substrate and characterize the spectral properties of the generated terahertz radiation by use of a heterodyne detection scheme. Our analysis shows that linewidth, stability, and frequency tuning range of the generated terahertz radiation are directly determined by linewidth, stability, and wavelength tuning range of optical pump beam and not affected by device geometry, substrate properties, optical pump power level and other operational settings. Our study indicates the crucial role of optical sources in realizing high performance terahertz spectroscopy and wireless communication systems based on plasmonic photomixers.

Plasmonic photoconductive detectors for enhanced terahertz detection sensitivity

A photoconductive terahertz detector based on plasmonic contact electrodes is presented. The use of plasmonic electrodes mitigates the inherent tradeoff between high quantum efficiency and ultrafast operation of the employed photoconductor, enabling significantly higher detection sensitivities compared to conventional photoconductive terahertz detectors. Prototypes of comparable photoconductive detectors with and without plasmonic contact electrode gratings were fabricated and characterized in a time-domain terahertz spectroscopy setup under the same operation conditions. The experimental results show that the plasmonic photoconductive detector offers more than 30 times higher terahertz detection sensitivities compared to the comparable conventional photoconductive detector without plasmonic contact electrodes over 0.1-1.5 THz frequency band.

Terahertz generation using plasmonic photoconductive gratings

A photoconductive terahertz emitter based on plasmonic contact electrode gratings is presented and experimentally demonstrated. The nanoscale grating enables ultrafast and high quantum efficiency operation simultaneously, by reducing the photo-generated carrier transport path to the photoconductor contact electrodes. The presented photoconductor eliminates the need for a short-carrier lifetime semiconductor, which limits the efficiency of conventional photoconductive terahertz emitters. Additionally, the photo-absorbing active area of the plasmonic photoconductive terahertz emitter can be increased without a significant increase in the capacitive loading to the terahertz radiating antenna, enabling high quantum-efficiency operation at high pump power levels by preventing the carrier screening effect and thermal breakdown. A plasmonic photoconductive terahertz emitter prototype based on the presented scheme is implemented and integrated with dipole antenna arrays on a semi-insulating In0.53Ga0.47As substrate. Emitted terahertz radiation is characterized in a terahertz time-domain spectroscopy setup, measuring a terahertz pulse width of 590 fs full-width at half maximum in response to 150 fs pump pulses at 925 nm.

A new capacitive displacement sensor with high accuracy and long-range

A new capacitive displacement sensor is designed and fabricated for measurement of a large displacement with very high accuracy. This sensor is a kind of linear encoder with an array of microelectrodes made by micromachining processes. The two patterned electrodes on the sensor substrates are assembled facing each other after being coated with thin dielectric film. Due to the resulting small gap size, it is highly sensitive to displacement but minimizes expected misalignments such as tilting error. The sensor fabricated as a sample has a grating of electrodes with a width of 20 μm which is coated with a diamond-like carbon (DLC) film 0.4 μm thick. The proposed sensor was tested to conclude that its resolution is 0.9 nm for the measuring range of 15 mm and that the linearity error is expected to be less than 0.0026% of the measurable range.

Prediction of the thermal sensitivity of surface acoustic waves excited under a periodic grating of electrodes

The prediction of the temperature sensitivity of surface acoustic wave (SAW) devices still requires improvement because the nature of the implemented surface modes and the devices' complexity strongly change from the early basic Rayleigh-wave-based devices. To address this problem, a theoretical analysis and a numerical tool have been developed to predict the thermal dispersion of general electro-acoustic devices. The proposed model accounts for the electrode contribution to the frequency-temperature law. The computed thermal sensitivities are compared to experimental results for different kinds of substrates and waves.

Minimum-loss short reflectors on 128/spl deg/ LiNbO/sub 3/

We consider the interaction of surface acoustic waves (SAWs) with short electrode gratings encompassing only few electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The qualifications of the reflectors are evaluated by comparing the part of incident SAW energy scattered by the structure into the bulk to the energy reflected back as a SAW.

Slowness curves and characteristics of surface acoustic waves propagating obliquely in periodic finite-thickness electrode gratings

The computation of the diffraction of surface acoustic waves (SAWs) on piezoelectric materials is complicated by the anisotropy of their propagation, and requires their slowness curves to be known precisely. In addition, in actual devices these slowness curves are strongly affected by the massive electrodes used for SAW generation. We investigate the characteristics of SAWs propagating obliquely in periodic finite-thickness electrode gratings, with the mass-loading effect taken into account. The velocity of the surface waves, as well as the attenuation, piezoelectric coupling and beam steering are obtained as a function of the propagation angle with respect to the grating axis. The slowness curves for SAWs propagating under periodic electrode gratings are compared with the slowness curves for the same SAWs propagating under a thick homogeneous metallic layer. Numerical examples are presented for the SAWs of 42.75°YX quartz and X112.2°Y lithium tantalate, and the leaky SAWs of 36°YX lithium tantalate.

Generally polarized acoustic waves trapped by high aspect ratio electrode gratings at the surface of a piezoelectric material

It has been shown theoretically and demonstrated experimentally that shear horizontal (SH) surface waves can exist when the surface of an isotropic substrate is perturbed by a strong corrugation, for instance consisting of deep grooves etched in the substrate, whereas these waves cannot exist without this perturbation. It is shown in this article that a periodic array of metallic electrodes (wires) exhibiting large aspect ratios deposited over a piezoelectric substrate give rise to surface acoustic waves with general polarization. The admittance of an interdigitated transducer, which is a basic tool for predicting the waves parameters, is calculated by a combination of finite element analysis and a boundary integral method. This approach has been extended to obtain the polarization of the acoustic waves. For different piezoelectric substrates, we predict various surface acoustic modes and their polarization. Along with mostly SH modes, we also find modes mostly polarized in the sagittal plane. We discuss the frequency behavior of the surface modes as a function of the electrode height compared to the period.

Analysis of nematic-liquid-crystal binary gratings with high spatial frequency

The behavior of nematic liquid crystals in spatially varying electrical fields is investigated. Director profiles of nematic-liquid-crystal phase gratings are simulated for different orientations of the liquid-crystal director field with respect to the electrode grating. With increasing spatial frequency of the grating the fringing electric field becomes important and the diffraction pattern changes. The optical properties are analyzed and measurements are performed.