Electro-optic Substrate Research Articles

Design and Analysis of a PDLC-Based Reconfigurable Hilbert Fractal Antenna for Large and Fine THz Frequency Tuning.

The proposed reconfigurable radiating antenna design is based on the integration of a reconfigurable fractal antenna and electro-optic substrate material. This antenna can be adjusted to achieve either re-configurability or tunability in the desired frequency range for wireless systems. The electromagnetic characteristics of the fractal antenna are manipulated at both the level of fractal geometry, electrical length and dielectric substrate. The designed antenna features multiband responses, in which the geometry and length change create a large frequency shift and the dielectric change using polymer dispersed liquid crystal (PDLC) creates fine and/or continuous tuning. The far field and scattering properties of the antenna are analyzed using the Computer Simulation Technology (CST) Microwave Studio Suite. The proposed approach has successfully demonstrated reconfigurable switching for up to four frequency bands between 0.2 and 0.6 THz. The dielectric constant change in the PDLC substrate shows fine and continuous frequency tuning with an 8% maximum frequency shift when operating around 0.54 THz and a high directivity of 7.35 dBi at 0.54 THz and 8.43 dBi at 0.504 THz. The antenna can also realize a peak gain of 4.29 dBi at 0.504 THz in the extraordinary polarization state of PDLC. The designed antenna can be readily integrated in the current communication devices, such as satellites, smart phones, laptops, and other portable electronic devices, due to its compact geometry and IC compatible design. In satellite applications, the proposed antenna can play a significant role in terms of security. The antenna could be extremely useful for satellites that want to keep their information secret; by constantly switching their operating frequency, spy satellites can evade detection and data collection from enemy ears.

Broadband polarization-insensitive absorption by metasurface with metallic pieces for energy harvesting application

We present a broadband absorber in tri-layer plasmonic metamaterials with an electro-optic material as a substrate to fine tune the resonances. Broad resonant absorption peaks are achieved in a structure with metallic pieces. In the wavelength range from 300 nm to 2200 nm, the absorption spectral width is calculated to be 1078 nm, 566 nm, 282 nm, 119 nm, and 78 nm with absorption levels of 75%, 90%, 94.6%, 98.6%, and 99.4%, respectively. The total absorption efficiency, of the structure is evaluated to be 71.43%, which approaches to the absorption efficiency of a complex design with over 40-pair layers. The wide incident angle, polarization-insensitive, broadband absorption can be utilized for energy harvesting and solar thermals. Moreover, the use of electro-optic substrate has the advantage and potential to reduce the use of scaling structures and fabrication errors, and tune the resonant wavelengths to the desired range for imaging and infrared detection applications.

Broadband Wide-Angle Incident Light Absorption by Metallic Loop Metasurfaces Based on Electro-Optic Substrate

A metasurface absorber based on an electro-optic (EO) substrate is numerically investigated. Flat resonant peaks are achieved in both visible and infrared regions. The spectral widths are 1012 and 556 nm, respectively, at 70% and 90% of absorption level in the waveband of 500-2000 nm. The actual absorption efficiency is calculated as 71.56%, being about that of high-efficiency multilayer absorbers. The design is characterized for its thin-metallic substrate and a spectral width of 229 nm at 98% absorption level at a single resonant mode is observed. The EO substrate is used to tune the wavelengths to the desired region of absorption by varying the applied voltage in order to reduce the use of scaling and fabrication complexities. The broad resonances can be used for power harvesting and solar thermals. Moreover, the EO-based tunability can find applications in infrared detection and optical filtering.

Plasmonic Metasurface Absorber Based on Electro-Optic Substrate for Energy Harvesting.

A highly efficient and broad light absorber capable of wide-angle absorption in the visible and near infrared range is presented and numerically investigated for energy harvesting in a simple geometry. According to the calculated results, the proposed device has a peak absorption level of about 99.95%. The actual absorption efficiency is 76.35%, which is approaching that of complex multilayer absorbers with 88 layers working in the wavelength range of 300 nm to 2000 nm. The electro-optic material has the potential of shifting the absorption peak position, compensating fabrication errors and thus reducing the fabrication technique difficulties. Also, the high electro-optic tunability can be used for filters, infrared detection, and imaging applications. More directly, the proposed absorber can be potentially deployed in solar cells and solar thermals.

A Bragg grating embedded in a slab waveguide fabricated by the implantation of high-energy light ions in KLTN substrate

Electro-optically tunable transmission grating was imprinted in potassium lithium tantalate niobate by irreversible spatial patterning of the dielectric constant. While embedded into waveguided architecture, it provides a reliable and versatile building block for opto-electronic circuitry, capable of both active switching and multiplexing. Realization of such a block is critical for the fabrication of integrated photonic circuits in electro-optic substrates by means of Refractive Index Engineering by fast ion implantation.

Guided wave electrooptic and acoustooptic tunable filter apparatus and method

A two-port guided wave tunable filter in a birefringent electrooptic and/or acoustooptic substrate material includes two 3-port, symmetric Y-branch beam splitters connected by two waveguide sections in which phase-matched polarization coupling occurs, with an input port and an output port. The optical path difference between the beam splitters is half an optical wavelength, and the polarization coupling regions between the beam splitters are relatively displaced by an odd integral multiple of half the spatial period of the perturbation responsible for the coupling. In one embodiment, an electrooptic tunable filter, the polarization coupling in the waveguides is caused by a spatially periodic strain-inducing film and tuning results from an applied electric field. In another embodiment, an acoustooptic tunable filter, polarization coupling results from a surface acoustic wave and tuning is accomplished by changing the acoustic frequency. Alternatively, four port electrooptic and acoustooptic tunable filters are formed by replacing the 3-port beam splitters with 4-port directional couplers, where in each of the directional couplers the splitting ratio for TE input polarization plus the splitting ratio for TM input polarization is substantially equal to one.

Microwave modelling of optical diffraction grating devices

Design methods for obtaining shaped radiation patterns and beam control from optical diffraction gratings are presented, based on equivalent radiating currents and measurements of radiation leakage on the grating itself or microwave models, the latter offering many advantages as an initial design process prior to optical manufacture. Under certain assumptions the design method represents a unified process that is applicable to both linear and annular gratings. Microwave model measurements on copper strip linear and annular gratings substantiate the design methods and illustrate the polarisation properties enforced by the grating and substrate structure. Novel ideas concerning possible electronic beam scanning using linear gratings on electrooptic substrates and also the creation of eye-safe broad beams using random phase hologram gratings on optical D-fibres, are examined using the design methods.

Single-mode/multimode waveguide electro-optic grating coupler modulator

A waveguide electro-optic grating coupler modulator based on a single straight channel waveguide and an interdigital modulation electrode structure on an electro-optic substrate, has been reported. Both single-mode and multimode operations are possible. Preliminary experiments have demonstrated the modulation on a proton exchanged LiNbO3 waveguide device. Modulation performed on other electro-optic waveguides, such as GaAs, LiTaO3, and EO polymers, are also possible.

Ｙ‐Ｂａ‐Ｃｕ‐Ｏ系超伝導体と電気光学結晶との反応性

We investigated the reactivity of YBa2Cu3Oy (YBCO) with electrooptic crystals, such as LiNbO3 (LN), LiTaO3 (LT), Sr0.75Ba0.25Nb2O6 (SBN), KTa0.65Nb0.35O3 (KTN), Ba2NaNb5O15 (BNN). The YBCO powder mixed with 5wt% electrooptic crystal was heated at 700°C for 8hr, 16hr or at 900°C for 8hr in air. The reactivity of LT, LN, KTN with YBCO was considerably high, and that of BNN was relativly low.The YBCO thin films were deposited on LN and BNN substrates through an RF magnetron sputtering method. The YBCO thin films on BNN were c-axes oriented with Tc(zero)=72.5K and c=11.70A. The thickness of the reaction layer for BNN was one order of magnitude thinner than that for LN. From these results, the BNN single crystal was concluded to be suitable for an electrooptic substrate for YBCO thin film fabrication.

Quasi-static analysis of electrooptic modulators by the method of lines

A quasi-TEM analysis using the method of lines is implemented to calculate the electric field anywhere within a layered guided wave structure at appropriately low frequencies. The analysis provides the electric fields and transmission-line parameters for a shielded structure having any number of coplanar electrodes within an arbitrary number of anisotropic dielectric layers, the principal axes of which are parallel to the axes of the structure. The practical application considered is the calculation of the phase shift of a guided optical wave within an electrooptic modulator that has a dielectric buffer layer between the electrodes and electrooptic substrate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quasi-optics of a thin-film polarization converter

A planar thin isotropic film sandwiched between an isotropic cover and an electro-optic substrate crystal can support transverse electric and transverse magnetic guided modes. When an external electric field is applied to the substrate and is harmonically modulated suitably in the propagation direction, the two modes have a phase-matched interaction and exchange energy in the propagation direction. The governing equations of this interaction are deduced, using a quasi-optical technique for two orientations of the substrate crystal, and are analyzed to yield the requirement for the complete polarization conversion. Representative numerical results are obtained to show that for practically realizable film thicknesses, complete polarization conversion can be achieved in acceptable values of the interaction length and with acceptable frequency selectivity.

Voltage-induced optical waveguide modulator in lithium niobate

A type of optical modulator first proposed by D.J. Channin (1971) in which an optical waveguide is induced in an electrooptic substrate by applying voltage between two parallel coplanar electrodes is reinvestigated. It is found that such devices will work for voltages much smaller than those used by Channin. The devices are fabricated with much smaller gaps between the electrodes and have an optical buffer layer to isolate the optical field from the electrodes. The theory of operation is developed and the device is modeled mathematically. The theory predicts that the optical field distributions of the modes of these devices become more highly confined with increasing voltage. >

Optical Guided-Wave Devices

Optical fiber is rapidly becoming the transmission medium of choice for new telecommunication systems. For a true lightwave network to evolve, however, optical control devices such as optical switches and multiplexer-demultiplexers will be essential. Optical guided-wave devices built with photolithographic fabrication techniques and electro-optic substrates are compact, low drive power devices that provide these functions. Research is particularly advanced on integrated-optic devices based on waveguides formed by titanium diffused into lithium niobate. Switch arrays, tunable filters, and high-speed modulators have been demonstrated and used in research systems experiments.

Reactive ion beam etching of PLZT electrooptic substrates with repeated self-aligned masking

Deep channels in electrooptic substrates are often desired for optical fiber coupling and the fabrication of embedded electrodes in waveguides and spatial light modulators. While ion beam etching can be used to form channels with vertical wall profiles, the achievable etch depth is often rate-limited by the substrate/mask etch ratio. In this paper we compare etch rates of the electrooptic material (Pb,La)(Zr,Ti)O3 under argon-ion beam etching and CF4 and C2F6 reactive ion beam etching (RIBE). Then, as an alternative to the use of thick or multilayered masks, of limited resolution, we present a method using backside exposure to apply repetitively a self-aligned photoresist mask for multiple RIBE runs. This procedure has been used to fabricate channels of 7-μm depth in the electrooptic material.

Electrooptic sampling in GaAs integrated circuits

Electrooptic sampling has been shown to be a very powerful technique for making time-domain measurements of fast electronic devices and circuits. Previous embodiments relied on a hybrid connection between the device under test and a transmission line deposited on an electrooptic substrate such as LiTaO 3 . The hybrid nature of this approach leads to device packaging difficulties and can result in measurement inaccuracies and performance degradation at very high frequencies. Since GaAs is electrooptic and an attractive material for high speed devices, we have devised an approach of direct electrooptic sampling of voltage waveforms in the host semiconductor. In this paper, we review the principles and limitations of electrooptic sampling and discuss this new noninvasive technique for electronic probing with applications to characterizing high speed GaAs circuits and devices.

Subpicosecond electro-optic sampling using coplanar strip transmission lines

Electrical sampling with a temporal resolution of less than 500 fs has been achieved using coplanar transmission lines on electro-optic substrates. The improved performance over the previously demonstrated balanced stripline is due to the reduced dimensions, i.e., distance between the electrodes and width of the electrodes, achievable with photolithographic technique. By further reducing the line dimensions to 10–20-μm range, this technique makes the temporal resolution of electrical waveforms in the range of 100 fs possible.

Guided wave electrooptic devices for logic and computation

The use of guided wave optical components for performing logic and computation is proposed. The basic logic gates are switches and intensity modulators fabricated on an electrooptic substrate. The states of the switches and modulators are determined by electrical signal voltages. The output signal from each of the gates is intensity-modulated light, while inputs can be electrical signals or a combination of electrical and optical signals. Individual gates can be interconnected on a substrate by dielectric waveguides, so that sequential operations are possible. This is illustrated by a configuration for a serial-parallel binary adder. The propagation delay, as determined by the optical pathlength, is calculated to be of the order of 20-40 psec/gate, assuming that the substrate material is lithium niobate, and the logic voltage level is 5 V.

Electrooptic Bragg-deflection modulators: theoretical and experimental studies

We have studied the mechanism of Bragg diffraction in electrooptic thin-film lightguides and examined in detail the effectiveness of interaction between the electric field and optical field. Expressions for the full deflection voltage and the power per bandwidth are obtained and applied to various modulator structures using out-diffused, metal-diffused, epitaxial, and sputtered-film (electrooptic substrate) waveguides. An expression for minimum interaction length to ensure better than 30-dB extinction ratio is derived. The theoretical results are checked with experimental observations from an electrooptic substrate modulator we fabricated, and good agreement is obtained.

The efficiency of thin-film optical-waveguide modulators using electrooptic films or substrates

This paper describes a new method for characterizing thin-film modulators which utilize TE <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\rightleftarrows</tex> TM wave conversion. The conversion is described in terms of a coupled wave model. The coupling constant <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</tex> describes the strength of the coupling between the TE and TM waves. It is expressed in the form <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k = k_{pw}C</tex> where k <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pw</inf> is the coupling constant of plane waves traveling through the relevant bulk medium. The coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</tex> contains the parameters which characterize the guide. The model allows the comparison of structures utilizing electrooptic films and electrooptic substrates. Our results show that in general for single mode films (those approximately a half-wavelength thick), modulators of either type are about equally effective. It is tacitly assumed that k <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pw</inf> is identical for each structure. For films which can support several modes (films a few wavelengths thick), the coefficient for devices utilizing electrooptic substrates approach zero. In contrast <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</tex> approaches unity for similar thick-film devices which use electrooptic films.

Chalcogenide glasses as passive thin film structures for integrated optics

One approach to obtaining electro-optic or magneto-optic modulation in an integrated optic structure is to use a transparent film on an “active” (electro- or magneto-optic) optical substrate. Since the most useful active optical materials have relatively high refractive indices which preclude the use of oxide glasses as the transparent thin film, we have investigated the possibility of using higher index chalcogenide glasses in this configuration. Preliminary waveguiding studies were performed on a model system consisting of an As2S3 film on a LiNbO3 substrate which demonstrated the feasibility of using chalcogenide glasses in this regard. The optical and mechanical properties of a suitable glass system, As-Ge-S, were then characterized. The characterization consisted of developing empirical formulas based on the type of bonding present in the glass which can be used to predict the relevant properties of a glass as a function of its composition. These formulas allow selection of optimum glass compositions to match a variety of electro-optic and magneto-optic substrates.