Light-matter interaction is crucial in many application domains of nanophotonics, including biosensing, trapping at the nanoscale, nonlinear optics, and lasing. Many approaches, mainly based on photonic and plasmonic resonant structures, have been investigated to enhance and tailor the interaction, but those based on all-dielectric metasurfaces have several unique advantages: low loss, easy excitation and readout, possibility of engineering the optical field distribution with many degrees of freedom, and electric tuning. Here we show that properly designed all-dielectric metasurfaces can support silicon-slot quasi-bound states in the continuum modes resonating in the near-infrared, strongly confining light in air and, consequently, enhancing light-matter interaction. Some samples of the designed metasurface have been fabricated in a silicon-on-sapphire wafer by e-beam lithography and reactive ion etching. The optical characterization of the chip has confirmed the excitation of the quasi-bound state in the continuum resonant modes, with measured Q-factor values exceeding 700.
All-dielectric metasurfaces have attracted increasing attention due to their negligible losses and sharper resonances compared to their metallic counterparts. In this work, we numerically studied the optical performance of a novel alldielectric metasurface based on complementary split-ring resonators (CSRRs), in which ultrathin slots were periodically etched in a thin silicon layer. The proposed CSRR metasurface exhibits two multipolar resonances in the near-infrared (NIR) window. Moreover, a quasi-bound state in the continuum (quasi-BIC) with an ultra-high quality factor can be excited by breaking the symmetry of the structure. Taking advantage of the high-quality factor quasi-BIC mode and its sensitivity to the superstrate medium refractive index (S = Δλres/Δn), we design an ultra-high figure of merit (FoM = S/FWHM) refractive index sensor for biomedical applications. By three-dimensional finite element method (3D-FEM), we evaluate the sensitivity of the sensing device to the variation of the superstrate refractive index in the range 1.31-1.33, which is typical for aqueous solutions. Our simulations reveal that a sensitivity of S ~ 155 nm RIU-1 and an extraordinary FoM ~ 387500 RIU-1 can be achieved using the ultra-narrow quasi-BIC resonance in the CSRR metasurface structure. The proposed approach opens new paths to develop flat biochemical sensors with high accuracy and real-time performance.
In this work, a novel technique to create adaptive liquid crystal lenses and other optical components is proposed and demonstrated. This proposal avoid all of the previous techniques disadvantages, a simple fabrication process and low voltage control is required, and thin lenses can be obtained. The novelty of the proposal, resides in a micro-structured indium tin oxide, designed to transmit the voltage homogeneously across the entire surface of the active area. This design is composed of two main elements, a transmission line that generates a voltage gradient, and a series of combs that distribute the voltage across the entire active area. Two different apertures are designed. One of this designs is fabricated and measured to demonstrate the viability of the idea. This novel structure open new venues of research in phase-only LC optical devices.
In this work, a novel method to obtain all-dielectric toroidal response metasurfaces in the W-band and THz range is demonstrated. Two designs are proposed, a symmetric and asymmetric disk metasurface. The first design is intended to corroborate the theoretical analysis, demonstrating the excitation of a strong toroidal mode resonance at 93.2 GHz. Then, the second design is used to demonstrate that symmetry-breaking variations in the disk dimensions, could lead to birefringent metasurfaces, affecting the polarization of the impinging light. Two structures are designed, a polarization beam splitter and a polarization converter. Such devices are difficult to obtain at the target frequency range with low absorption, so they could be of particular interest for the next generation of 5G communications and THz devices.
A localized surface plasmon resonance based fiber optic sensor for temperature sensing has been analyzed theoretically. The effects of the size of the spherical metal nanoparticle on the performance of the sensor have been studied in detail. The high sensitivity of localized surface plasmon resonances to refraction index changes, in collaboration with the high thermo-optic coefficients of Liquid Crystal materials, has result in a fiber optical sensor with high temperature sensitivity. This sensitivity has been demonstrated to be dependent on nanoparticle size. Maximum sensitivities of 4nm/°C can be obtained for some specific temperature ranges. The proposed sensor will be low cost, and will have all the typical advantages of fiber optic sensors.
A free-space laser communication system has been designed and partially developed as an alternative to standard RF
links from UAV to ground stations. This project belongs to the SINTONIA program (acronym in Spanish for low
environmental-impact unmanned systems), led by BR&TE (Boeing Research and Technology Europe) with the purpose
of boosting Spanish UAV technology.
A MEMS-based modulating retroreflector has been proposed as a communication terminal onboard the UAV, allowing
both the laser transmitter and the acquisition, tracking and pointing subsystems to be eliminated. This results in an
important reduction of power, size and weight, moving the burden to the ground station. In the ground station, the ATP
subsystem is based on a GPS-aided two-axis gimbal for tracking and coarse pointing, and a fast steering mirror for fine
pointing. A beacon-based system has been designed, taking advantage of the retroreflector optical principle, in order to
determine the position of the UAV in real-time. The system manages the laser power in an optimal way, based on a
distance-dependent beam-divergence control and by creating two different optical paths within the same physical path
using different states of polarization.
A new electrical equivalent circuit (EEC) has been proposed to model antiferroelectric liquid crystal devices. This circuit includes a constant phase element to take into account the ferroelectric part of the dielectric response in these devices. Electrical characterization of samples has been carried out using a specific experimental protocol based on impedance spectroscopy. The parameters of waveforms used in impedance measurements have been optimized. The procedure to obtain the components of the EEC has also been explained. Finally, the EEC has been validated by comparing experimental and simulated impedance results. A reasonable agreement between both of them has been obtained in a wide frequency range for all selection voltages.
New liquid crystals applications are being developed apart from optics, imaging and display systems. A liquid
crystal cell can be modeled as a nonlinear capacitor, being its capacitance varied as a function of the applied
electric field. A feedback resonator composed of a capacitance and an inductance presents an oscillation
frequency depending on the value of these components. A tunable feedback resonator based on a nematic
liquid crystal cell as variable capacitance is presented. The circuit has been electrically modeled and validated
with experimental measurements. Oscillation frequencies in the range of kHz have been obtained for the
combination of the inductance and the liquid crystal cell (capacitance) used in the proposed circuit. New
applications for this circuit are currently under study.
Liquid crystals are a growing technology bringing solutions for a number of applications in high performance displays
featuring video-rate, color and high resolution images, and in prototypes of photonic devices. Electrooptic response of
antiferroelectric liquid crystals (AFLC) might be superior to nematic liquid crystals that are been customarily employed
nowadays. AFLC show reduced time response being promising candidates for portable multimedia devices, optical
routing applications, among others.
In this work, temperature and frequency dependence of impedance measurements, in passive devices of commercial
antiferroelectric liquid crystals, has been studied.
Measurements of the temperature dependence of optical transmission have been obtained. 1Hz triangular waveforms
with different amplitude have been applied to the devices to carry out such characterization.
Simultaneous measurements of optical transmission and electrical impedance have been performed. Specific addressing
schemes have been tested in order to obtain the optimum electrooptical performance. Display blanking takes place when
a saturation pulse is applied. Results achieved show that increasing temperature shifts the dynamic range of the analogue
grayscale towards lower voltages. Impedance analysis of these devices upon switching has been performed as well.
Temperature and frequency dependence of the impedance measurements have been characterized. Negative phase
responses show there is a combined capacitive and resistive behavior. As the frequency increases the capacitive effect
grows. Magnitude shows a linear decrease on a log-log frequency scale. As temperature increases, phase profile becomes
slight more complex. New capacitive effects are suggested in a model of the electric response of AFLC cells at low
frequencies.
In this paper, different optical configurations, simple and compound, based on ring resonators and using liquid crystals as
the tuning or controlling block, are reported. Specific attention is given to their application as tunable filters and
wavelength switches, in this last case as part of complex matrix fabrics for all optical switching. Simulations of
integrated optics ring resonators are developed using FullWave by R-Soft. In the compound configurations that use serial
micro ring resonators it is implemented a 3 port reconfigurable demultiplexer in a compact cross-grid configuration.
Theoretical 10 nm tuning is reported on SOI substrate with nematic liquid crystals. Optimization of the critical coupling
condition by changing the evanescent coupling length is also reported. Theoretical analysis is presented to identify and
emphasize the design parameters of each configuration as regards its application of interest. The reported structures can
be developed in integrated optic technologies; all designs corresponds to state of the art integrated optics technology. A
revision of photonics circuits with equivalent components already developed is reported.
Liquid crystals (LCs) have focused a great attention from industrial and scientific community in the last decades. The
property of ferroelectricity in liquid crystals was first claimed in 1975. Five years later so-called surface-stabilized
ferroelectric liquid crystals were described, which caused a surge in industrial interest because of their promising electro-optical
applications. Additionally, antiferroelectricity in liquid crystals (AFLCs) was also identified in 1989. This kind of
devices show interesting electrooptical properties such as tri-state switching, fast response, intrinsic analogue gray scale,
wide viewing angle, among others, which are appropriate for high-end video display applications.
The performance of the AFLC displays is determined by their electrical and optical behavior. In order to measure some
electrical characteristics such as the switching currents and electrical loops (polarization-voltage), an A/D
instrumentation system has been specially designed and implemented. A first approach of this system was reported
elsewhere. However, new components were introduced and functional blocks of such version were modified in order to
improve the S/N ratio. It is well known that to perform measurements of electric current ranged in the pA-nA, a specific
and usually expensive equipment should be used. This work presents an enhanced A/D instrumentation system which is
able to measure with reasonably precision small amplitude values of switching currents in AFLC displays. Moreover, the
system can also carry out the temporal integration of the switching current allowing to obtain the electrical hysteresis of
these devices.
We present a microcontrolled optoelectronic system to measure online the average velocity of a projectile impacting on aircraft and spacecraft structures. The projectile velocity can vary in the range from subsonic to supersonic. The implemented optical system is based on three optical barriers that are crossed by the projectiles before impacting on the structural elements. A simple optoelectronic prototype is constructed and tested. The flight times among the three optical barriers are obtained by the microcontroller system. The measured velocity accuracy in test conditions is better than 1%. A high-brightness liquid crystal display is used to show messages of the system configuration and also the measured projectile velocity. To ensure safety in the experimental impact tests, serial RS-232 communication can be used to monitor this information using a remote computer. The system is highly robust since it is able to measure projectile velocity even when an optical barrier is missed by failure in either the optical transmitter or the receptor.
New all-plastic electrochromic devices have been manufactured using commercial PEDOT foils and classical polymer electrolyte electrodeposition techniques. Several devices with different areas have been electrically characterized by using an impedance spectroscopy technique. After a brief description of the manufacturing process, we discuss electrical properties with the size of the device. Real and imaginary parts of the impedance have been plotted as a function of frequency. Some interesting conclusions have been derived from those plots, and some improvements in manufacturing process of this kind of devices are also proposed.
A Variable Optical Attenuator (VOA) based on a Polymer-Dispersed Liquid Crystal (PDLC) Cell is presented. The VOA's developed prototype has been successfully tested at 660nm, 850nm and 1300nm. This ability makes it strongly recommended for networks using Perfluorinated Gradual Index Polymer Optical Fiber (PF GI-POF) technology. The prototype has revealed a better than 12dBs dynamic range and losses of <1.2dB. In addition, the VOA presents a very low power consumption and non-dependendence with polarization. In order to avoid the dependence with temperature, an electrooptical feedback is incorporated to the device, by means of a microcontroller system. Electrically controllable intermediate transmission levels can be selected this way. The response time is in the milisecond range. The present feedback prototype, developed with POF technology, includes lenses, PDLC, 1x2 POF couplers and fotodiodes in the optical part, and a microcontroller system where the feedback processing is carried out. No polarizers are required so that optical losses are minimal. Polymer-Dispersed Liquid Crystals are formed by microdroplets of liquid crystal embedded in a flexible matrix, and sandwiched between transparent electrodes. This structure scatters strongly the light. When an AC electrical field is applied to the film the material becomes transparent. A largest dynamic range could be achieved designing conveniently the radius of the microdroplets. No-dependence with polarization, high transmittance when activated, and large dynamic range within a wide range of optical wavelengths make PDLC the most appropriate liquid crystal technology for VOAs fabrication.
Flat panel display technology constitutes the fastest growing segment of the semiconductor industry. Presently, the majority of display related research is focused on the application of electrooptic effects due to the ease and efficiency of molecular reorientation with an applied voltage. One such area of study is the surface stabilized ferroelectric liquid crystal (SSFLC) display, which has many advantages over conventional cathode ray tubes as well as other types of liquid crystal displays. Antiferroelectric liquid crystal (AFLC) displays have unique electrooptical properties such as tristate switching behaviour, fast response, intrinsic analogue grey scale and a wide viewing angle that lead these materials in very attractive candidates for their potential use in high-resolution flat panel displays and microdisplays for computers and TV. To become a competitive display technology, they should work at video frequency and give full colour and a significant number of the grey levels. These features depend on the AFLC material used in the display device as well as the addressing schemes employed.
We present a new programmable driver for addressing passive matrix AFLC displays based on a microcontroller system. This prototype was built with commercial electronic subsystems and it is able to range voltage levels for the row selection of ±40V and has a minimum time resolution of 5 microseconds to shape the frame complete (selection pulse, bias, well and reset slots). The driver can address a 16x160 pixels AFLC display. A grey scale will showed in a preliminary 4x4 pixels AFLC display by using this prototype.
Liquid crystals are customarily used in several kinds of flat panel displays. Besides usual nematic liquid crystals, smectic tristate antiferroelectric liquid crystals have shown analogue grayscale and full color video rate at high-end devices with passive multiplexing. These devices ultimately are intended to be applied to small size devices on microdisplay applications. When a symmetric driving signal is applied, the electrooptic response of the devices usually consists of two symmetric hysteresis lobes. An asymmetric hysteresis cycle can be developed by using dissimilar aligning layers onto the two glass plates of the cell. This kind of devices can lead to analogue optical multistability, i.e., devices whose optical transmission may be arbitrarily set and maintained reducing or eliminating the bias voltage. In this work, a study of the asymmetric behavior of cells filled with commercial antiferroelectric liquid crystal is presented. Optical hysteresis cycles have been obtained applying a low frequency triangular waveform to the devices. Analogue grayscales have been generated only at one lobe of the hysteresis cycle. Electrical characterization has been carried out measuring the switching current of the cells test. Multiplexed driving waveforms have been applied with and without bias voltage in order to evaluate the stability of the optical transmission for video rate working. Results demonstrating analogue optical multistability on asymmetric antiferroelectric cells have been obtained. Narrow dynamic ranges, compatible with standard electronics for dynamic grayscale in data columns have been found. Preliminary measurements of the frequency dependence of impedance have been obtained on the capacitive device.
Electrochromic (EC) materials are used mainly for domotic applications, such as transparency controlled windows or rear-view mirrors in cars. The device construction is a sandwich of electrochemical compounds, which change their optical properties when applying voltage. Although the changes that are used in the applications take place in the visible, there are also changes in the near infrared region. In the last years, some works have proposed their use in fiber optic applications, mainly as optical modulators or VOAs (Variable Optical Attenuator). EC devices have usually slow responses (several seconds) and low transmittance range, specially the organic ones. The slow response is the major drawback for their use as modulators. But in NIR transmittance ranges, there are promising results in materials like ruthenium or PEDOT (poly(3,4-ethylenedioxythiophene)). In this work, we will study the possible use in VOAs of new EC devices developed with the minimum number of layers, by their response in telecommunications wavelengths. These devices are manufactured in such a way that the integration in fiber optic devices is an easy task. The minimum number of layers and the easy construction are improvements over the existing possibilities. PEDOT is the EC material on these devices, and different manufacturing ways are compared in order to detect the best possible candidate to use.
In this paper, different optical configurations based on ring resonators and a reflective section in the feedback path, are proposed in order to show the feasibility of their applications in different fields. Additionally, the use of optical amplification in some of the configurations offers the possibility of improving the device insensibility to fabrication tolerances and gives flexibility to the designs. In the compounds configurations that use reflective elements in the ring resonators, such as Sagnac configurations and Bragg Gratings, tuning is achieved by changing the coupling ratio of a coupler apart from conventional tuning by changing the equivalent loop length, using temperature or injection current. Applications as tunable filters in DWDM networks and lasers will be discussed. The reported structures can be integrated in InP or silicon technology, because photonics circuits with equivalent components have already been developed. Some of them are a monolithically integrated Sagnac interferometer for an all-optical controlled-NOT gate. Integrated optic devices have higher free spectral ranges, thus complying with applied standards on DWDM networking. Theoretical analysis is presented to identify and emphasize the design parameters of each configuration as regards its application of interest. Measurements on fibre optic prototypes for showing the principle of operation and validating the theoretical models are reported in [24].
A 2x2 optical switch for plastic optical fibre (POF) has been developed, able to work for both 660 and 850nm simultaneous and independently of the input light's polarization, improving previous developments. The device has four bidirectional optical ports, and is able to switch from each port to any other. In this way, there are three operation modes: straight (each input connected to the corresponding output), crossed (inputs and outputs crosses) and closed (inputs connected on the one part, and output connected on the other part). As the device is bidirectional, inputs and outputs are interchangeable. The switching process is carried out by a set of Polarized Beam Splitters, Liquid Crystal cells, λ/4 plates, lens and mirrors. An electronic circuitry has been developed to control the state of the optical switch, which is shown in a Liquid Crystal Display. The system has been tested for both 660nm and 850nm, and the optical switch exhibits miliseconds switching times, an optical interchannel crosstalk better than -25 dB, and low power consumption. Applications of the switch include systems where a redundant path is needed to guarantee communication, such as safety systems in automobiles, LANs, telemedicine, heavy machinery in the industry along with coarse WDM GI (graded index) POF networks. Device size reduction is under development.
The control of transmittance and its use in glazing have powered the research on electrochromic devices in the last decade. The search for new materials to be used in different applications is a goal today. We characterize new electrochromic devices following an all-plastic construction. Poly(ethylendioxy-tiophene) (PEDOT) is used between plastic substrates and sandwiched symmetrically with a poly(ethylene oxide)-based polymer electrolyte. This multilayer acts as an electrochromic device without the necessity for an ITO layer. Several devices are tested to determine their electro-optical characteristics using cyclic voltammetry and complex impedance spectroscopy techniques, among others. Additionally, a protocol of electrical and optical characterization is proposed. The devices switch between 0 and 3 V with no dependence on the electric field direction, due to their layer construction symmetry. Impedance and voltammetry measurements show a hysteretic behavior from which charge injection can be derived. Finally, transmittance changes above 14% are measured at a 600-nm wavelength, so that further research must be done in materials and sticking process to improve the contrast. Bleaching and coloring times of 20 to 30 s make these devices potential candidates for applications in domotics, environment control or vision, and color filters.
Novel demultiplexer configurations based on concatenated Mach-Zehnders and a ring resonator are presented. These new devices exhibit high channel isolation, flexibility, no loss, and megahertz-adjustable full width at half maximum (FWHM) suitable for narrow channel spacing. General design equations are described. Examples of a 1×4-2.5-GHz demultiplexer and a 2.5-GHz/5-GHz interleaver are reported. These designs have channel isolation >40 dB, 128 MHz FWHM, 10 dB gain. These devices can be conveniently implemented using the silicon or InP integrated optic technologies. A fabrication tolerance analysis is carried out. The worst figures for the previous designs are 220 MHz FWHM, 23-dB channel isolation, and 5.7-dB gain and can be readjusted with a gain control.
A system for measuring liquid level in multiple tanks using optical fiber technology has been developed. Oil field service industry or any sector requiring liquid level measurements in inflammable atmospheres can be benefited from this intrinsically safe technology. Three different models considering various effects have been derived and tested on two prototypes. The first model use punctual emitters and divergence, the second model use finite emitters with paraxial approach, and the third model use a constant power rays distribution in the emitters using with each one of the rays the Snell's law, to take in count the optical aberrations. A Monte-Carlo method is used to fit the experimental data and obtain the models parameters. The simplest model is demonstrated to be accurate enough for a proper correlation between the experimental data and the fitted curve in a range of 2 m.
Influence of first order mode excitation in input/output waveguides of multimode interference couplers is analyzed. Emphasis in common mode rejection ratio penalty in 3-dB couplers as part of balance coherent detection receivers is encountered. Design analysis is carried out in InGaAsP/InP buried waveguides. Coupler behavior is modified basically when first order mode is excited at input waveguides. A drastically reduction of 25% couplers width tolerance is derived from first order mode partial presence (10% of full input optical power) in accessing waveguides. Some guidelines about width tolerance improvement in restricted interference couplers against general interference couplers is also reported. Couplers dependence on fabrication tolerance was described as a function of power distribution between different modes in multimode section.
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