Modeling of Unwrapped Phase Defects in Modal Liquid Crystal Cylindrical Microlenses

Liquid crystal (LC) lenses have been the subject of research due to their advantage of focal length tunability that brings added value to applications typically based on conventional gradient index lenses. A novel approach for modeling the unwrapped phase defects in modal LC microlenses is presented. For solving the gradual voltage across the lenses, the proposed analytical method uses only circuit theory exploiting partial differential equations and conformal mapping techniques. The LC molecular ordering has been modeled on the basis of the continuum theory and optical response has been deduced through inspection of phase retardations. The model validity has been checked for predicting some defects of the modal microlenses in the phase of the lens design, comparing experimental characterization with simulation.This work was supported in part by the Ministerio de Ciencia e Innovación of Spain under Grant TEC2009-13991-C02-01 and in part by the Comunidad de Madrid under Grant FACTOTEM2 S2009/ESP-1781.Publicad

Optical vortices by a modal liquid crystal spiral phase plate

[Poster of]: 16th Topical Meeting on the Optics of Liquid Crystals. Sopot, 13th-18th of September, 2015, Poland.An ideal spiral phase plate based on liquid crystal and high resistivity layers is proposed and theoretically analyzed. The proposed structures generates a spiral-like voltage with simple voltage control...Authors acknowledge funding support from the Spanish Ministerio de Economía y Competitividad (grants nº TEC2013-47342-C2-2-R and nº TEC2013-50138-EXP) and the R&D Program SINFOTON S2013/MIT-2790 of the Comunidad de Madrid.No publicad

Design and experimental implementation of a multi-cloak paraxial optical system

Electromagnetic cloaking has being continuously pursued using a large variety of approaches. In recent years, this effect has been observed using either complex devices based on the so-called Transformation Optics or simple systems based on conventional optics with proper characteristics. In the latter case, a simple arrangement of lenses working in the paraxial regime can provide broadband visible cloaking in a wide area. In this work, we analyzed and generalized this method by proposing a five-lens system producing at least three potential invisible regions with a large cloaked area (>90% of the visual field). In particular, we developed the mathematical formalism and show, both numerically and experimentally, the successful operation of the cloaking system with the naked eye.AF-H wants also to express her grattitude to the Ministerio de Universidades for her predoctoral grant (FPU19/04133)

Characterization of high-power white leds for VLC applications

During the last years, visible light communications (VLC) have been proposed for providing connectivity while ensuring satisfactory illumination in both indoor environments and also specific outdoor scenarios without the need of deploying complex infrastructures for that purpose. Transmission for VLC is carried out through lightemitting diodes (LEDs), which correspond to semiconductors based on PN-junction materials with a direct gap. In this sense, the current flow plays a major role in the behavior and performance of these devices for VLC. Therefore, characterizing the electrical response of high-power white LED results mandatory for the successful implementation of VLC. At this point, it is worth noting that the electrical characterization is usually not available for high-power LEDs since, in fact, determining these characterization results challenging. In this sense, there exist some measurement instruments such as LCRs or impedance analyzers typically employed for characterizing materials and passive electrical components. However, these kinds of instruments are subject to a limited input impedance and a maximum value of forwarding current. In this work, the electrical characterization of the LED LXHL-BW02 of Luxeon is analyzed to show that typically commercial instrumentation for characterizing these devices is limited for high-power LEDs, which may provide polluted results when these limitations are not considered. After that, the characterization of the LXHL-BW02 based on a lock-in amplifier is proposed.This research was co-financed by Comunidad de Madrid and the FSE/FEDER Program under grant SINFOTON2-CM (S2018/NMT-4326), the Universidad Carlos III de Madrid under grant 2020/00038/001, and the Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (PID2019-109072RB-C31), and the European Regional Development Fund (ERDF) EXP 00119337/IDI-2019029

An enhanced method for dynamic characterization of high-power LEDs for visible light communication applications

Visible light communications (VLC) have been proposed for several applications beyond the traditional indoor scenarios, from vehicular to underwater communications. The common element in all these applications is the use of light-emitting diodes (LEDs) in which the forward current that flows through each LED plays a major role. Therefore, knowing the electrical equivalent of the LEDs is a useful tool for the proper design of the VLC systems. Currently, some measurement instruments exist, such as the LCR (inductance, capacitance, and resistance) meters or impedance analyzers to characterize the main parameters of the LEDs. However, these instruments and measurement procedures are subject to satisfying some requirements, such as a minimum value of the input impedance or the maximum forward current. In this work, the LED LXHL-BW02 is used to obtain its equivalent circuit, using different measurement methods and traditional methods of measurement with our proposed method. The equivalent model is implemented on the simulation tool LTSPICE. Our alternative method can be used for determining the electrical equivalent circuit of LEDs subject to high current variations at very high frequencies, in the MHz range, i.e., in an operating range for VLC applications.This research was co-financed by Comunidad de Madrid and the FSE/FEDER Program under grant SINFOTON2-CM (S2018/NMT-4326), the Madrid Government (Comunidad de Madrid) under the Multiannual Agreement with UC3M in the line of “Fostering Young Doctors Research” (GEOVEOLUZ-CM-UC3M), and in the context of the V PRICIT (Regional Programme of Research and Technological Innovation, and the Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (PID2019-109072RB-C31) and under the CDTI (Centre for the Development of Industrial Technology, Ministerio de Ciencia e Innovación) throughthe European Regional Development Fund (ERDF) EXP 00119337/IDI-2019029

Induced Magnetic Anisotropy in Liquid Crystals Doped with Resonant Semiconductor Nanoparticles

Currently, there are many efforts to improve the electrooptical properties of liquid crystals by means of doping them with different types of nanoparticles. In addition, liquid crystals may be used as active media to dynamically control other interesting phenomena, such as light scattering resonances. In this sense, mixtures of resonant nanoparticles hosted in a liquid crystal could be a potential metamaterial with interesting properties. In this work, the artificial magnetism induced in a mixture of semiconductor nanoparticles surrounded by a liquid crystal is analyzed. Effective magnetic permeability of mixtures has been obtained using the Maxwell-Garnett effective medium theory. Furthermore, permeability variations with nanoparticles size and their concentration in the liquid crystal, as well as the magnetic anisotropy, have been studied.This work has been supported by Ministerio de Economía y Competitividad of Spain (Grants nos. TEC2013-47342-C2-2-R, TEC2012-38901-C02-01, and TEC2013-50138-EXP) and the R&D Program SINFOTON S2013/MIT-2790 of the Comunidad de Madrid. Vicente Marzal acknowledges the Ministerio de Economía y Competitividad for FPI grant

Thickness-Dependent Coloration Properties of Glass Substrate Viologen-Based Electrochromic Devices

Proceedings of: E-MRS 2011 Fall Meeting. Symposium G. New trends in chromogenic materials and devices. Warsaw, Poland, September 19-23, 2009.Blue viologen-based electrochromic devices on glass substrates were constructed with different internal active layer thickness by means of a thermoplastic spacer (DuPontTM surlyn1702). Optical measurements, chronoamperometry, and cyclic voltammetry (CV) were the techniques used for characterization. Coloration properties such as switching times for coloring and bleaching processes, coloration efficiency, and effective charge density, previously studied for a single device, are now obtained for several devices with different thicknesses.This work was supported by the Comunidad Autónoma de Madrid under Grant S2009/ESP-1781, FACTOTEM-2.Publicad

Electrical Modeling of Tristate Antiferroelectric Liquid Crystal Devices

Proocedings of: 3rd International Workshop on Liquid Crystal for Photonics (LCP2010). Elche (Alicante), Spain. September 8-10, 2010Copyright 2011 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.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 voltagesCiencia e Innovación of Spain (Grant No. TEC2009–13991-C02–01) and Comunidad de Madrid (Grant No. S2009/ESP-1781).Publicad

Note: Series and parallel tunable resonators based on a nematic liquid crystal cell as variable capacitance

Copyright 2012 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Review of Scientific Instruments (2012), 83 (8). and may be found at http://dx.doi.org/10.1063/1.4738645In this work, tunable series and parallel resonators based on a nematic liquid crystal cell as variable capacitance are proposed and characterized. Tunable resonance 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 circuits. Tuning range in frequency obtained is around an octave.This work was supported in part by the Spanish Ministry of Science and Technology (TEC2009- 13991-C02-01) and the Comunidad de Madrid (FACTOTEM2-CM, REF.S2009/ESP- 1781).Publicad

Note: Phase-locked loop with a voltage controlled oscillator based on a liquid crystal cell as variable capacitance

Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Review of Scientific Instruments (2011), 82 (12). and may be found at http://dx.doi.org/10.1063/1.3666865A phase-locked loop is demonstrated using a twisted-nematic liquid crystal cell as a capacitance that can be varied as a function of applied voltage. The system is formed by a phase detector, a low-pass filter, as well as a voltage controlled oscillator including such variable capacitance. A theoretical study is proposed and experimentally validated. Capture and locked ranges of hundreds of kHz have been obtained for the configuration used in this circuit. An application as frequency demodulator using a practical implementation of this circuit has been demonstrated.This work was supported in part by the Spanish Ministry of Science and Technology (TEC2009-13991-C02-01) and the Comunidad de Madrid (FACTOTEM2-CM, REF.S2009/ESP-1781).Publicad