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In this work, a comparison of the microwave absorption properties of Opuntia ficus-indica cladodes, Agave atrovirens branches, and Cocos nucifera L. husk samples was performed. The study was carried out by inserting dry and powdered samples of these organic materials transversely and in the middle of a rectangular waveguide, for which scattering parameters S21 and S11 were measured to estimate the absorption coefficient. These measurements were compared to determine the material that behaves the best as a microwave absorber with a view to develop future low-cost and eco-friendly products by reusing agricultural waste. Specifically, Agave atrovirens sample showed the best performance, having an average value of absorption coefficient of 0.4218, while its maximum was 0.5792 at 9.706 GHz within the range from 8.005 to 13 GHz.

In this paper, we introduce a hexagonal array of vertical gold nanowires with enhanced antireflective properties in the UV–Vis and near-IR wavelengths. The array reduces the reflectance of the underlying silicon wafer, based on the plasmonic resonances that metals exhibit at the nanoscale. The design is optimized with the particle swarm optimization algorithm, an evolutionary algorithm able to drive the morphology of a nano-object towards an optimum. The nanowires’ optical properties are evaluated with numerical simulations. The reflectance of the optimized array decreases 100%, mainly in the visible wavelengths, unveiling its potential as an antireflective coating.

In this letter, the authors develop an optimized Seebeck nanoantenna design suitable for IR harvesting applications. The design is optimized via the so-called particle-swarm-optimization algorithm (PSO), an evolutionary algorithm able to drive the morphology of a nano-object towards an optimum. Along with the so-called nanoloading technique, efforts are subsequently addressed to understand the physical mechanisms behind the wave energy to voltage conversion, from both numerical and theoretical perspectives. In particular, the thermal and intrinsic impedance are considered to be the key issues beneath the device’s response.

This article presents the design, fabrication, and measurement of a square Koch fractal slot antenna for UHF band using both the FR4-G10 and Cuclad 250 substrates. Conveniently, this 56.56 cm full-length antenna possesses a geometry that allows it to be incorporated into the standardized 10 cm × 10 cm faces of the CubeSats. Furthermore, it is shown that both selected substrates exhibit an acceptable performance at the frequency of interest despite the economic cost difference and relative permittivity. Hence, the commercial FR4-G10 antenna substrate can be preferred because of its low-cost and admissible performance at 458 MHz, which is a frequency in the UHF band that is commonly used for telemetry, tracking, and command downlinks of CubeSats. Measurements show that the proposed antenna exhibits a reflection coefficient of −16.53 dB, a bandwidth of 22.62 MHz at −10 dB, a VSWR of 1.3508, a normalized impedance of 0.794 − j0.173 at 50 Ω, and a directivity of 2.24 dBi. The contribution of this work consists in the use of a fractal geometry to construct a low-cost slot antenna working at UHF frequencies over the limited area of the CubeSat faces and in order to optimize the area for an eventual coexistence with solar cells.

In this work the effects of filling the surface energy states in AlGaAs/GaAs heterostructures on the depletion layer are reported. The depletion layer width was varied from 49 to 10 nm as determined by Raman spectroscopy, allowing to discern their effect on the generation of Franz-Keldysh oscillations observed by photoreflectance spectroscopy. It is found that the photoreflectance modulation process of built-in electric fields at surface is negligible when the surface-levels are filled. This work demonstrates the relationship between the surface-states density, the surface states capability of capture carriers and the layer sequence of the heterostructure. These parameters need to be considered in order to get an adequate analysis of the photoreflectance spectrum of heterostructures. It is shown that if a nearly full-filled condition in the available surface energy levels is established, the Franz-Keldysh oscillations produced by the modulation of the built-in electric field intensity disappears as a result of the reduction in the photogenerated carrier density in the photoreflectance measurement.

Inspired by the extraordinary ability of metallic resonant nano-cavities to transmit light toward a substrate beneath them, the authors introduce quite a simple design of nano-cavities displaying enhanced antireflective properties in the UV–vis band. With the aid of an evolutionary algorithm, which is based on social rules, the authors drive the dimensions of a hexagonal periodic array of nano-cavities to induce an enhanced reflectance decrease in a silicon substrate as well as a strong increase in the transmittance through it. By using finite-element simulations in the real space, the physical mechanism behind the light propagation is subsequently unveiled.

In this article, the author leverages the concept of “input impedance” to determine in a proper manner the collective resonances of infrared devices based on square arrays of micro-dipoles, commonly obtained by the scattered field of devices under illumination. With the aid of finite-element simulations, the resistive and capacitive nature of the odd and even resonant modes of individual micro-dipoles is first unveiled. Subsequently, the micro-dipoles are incorporated into an array with lattice parameters (ax, ay), and the dependence of the emerging collective odd and even resonant modes, on the transverse and longitudinal dipolar interaction, is evaluated. The opposite wavelength shift of these modes is unveiled and the physical mechanisms behind their behavior are discussed. By analyzing the absorbance spectra of the micro-antenna arrays, the equivalence of optical resonances counterpart, in the short and open-circuit configurations, with the odd and even modes is presented. Finally, the effect on the array's performance that results from introducing highly resistive nano-bolometers is optimized by exploiting the natural high-resistance of the collective even modes.

The authors demonstrate both experimentally and numerically the high tunability that can be achieved in the optical response of sputtered multilayered metal-dielectric thin films. The wavelength dependence of both the transmission and reflectance is analyzed in terms of the geometrical parameters of transparent heterostructures consisting of alternating layers of Ag and SiO2. It is shown that these parameters can be adjusted to induce either a pass-band or a band-gap behavior in the visible range in a submicronic multilayered film.

The multi-resonant response of three-steps tapered dipole nano-antennas, coupled to a resistive and fast micro-bolometer, is investigated for the efficient sensing in the infrared band. The proposed devices are designed to operate at 10.6 μm, regime where the complex refractive index of metals becomes important, in contrast to the visible counterpart, and where a full parametric analysis is performed. By using a particle swarm algorithm (PSO) the geometry was adjusted to match the impedance between the nanoantenna and the micro-bolometer, reducing the return losses by a factor of 650%. This technique is compared to standards matching techniques based on transmission lines, showing better accuracy. Tapered dipoles therefore open the route towards an efficient energy transfer between load elements and resonant nanoantennas.

In this work we analyze dried mango slices by terahertz pulses, samples were prepared under laboratory control in accordance with the kinetic dry to preserve the organoleptic properties determining the initial moisture content with a thermobalance and dehydrating the samples in an oven with controlled humidity and temperature obtaining 10% final moisture in the thin perpendicular cut to fiber slice sample. The results show that it is possible to indicate the percent of moisture content in mango since the Tera-Hertz pulse attenuates its amplitude when it interacts with the sample, this attenuation is due to the polarization of the water molecules, resulting in a difference between the pulse emitted and the one received, besides that its thickness can be determined as a function of wave deface and optical properties.

A novel geometry of self-switching diode (SSD) is presented: the W-Shape geometry. A numerical model to evaluate the DC performance of the W-Shape SSD fabricated on AlGaAs/GaAs heterostructures that contains a two-dimensional electron gas (2DEG) is employed by the manipulation of the surface-states in the device. With this geometry the possibility of modifying the charge distribution in the nano-channel from one- and zero-dimensions by applying voltage between electrodes is explored. The DC current-voltage characteristics of the W-Shape showed a rectifier behavior. It is demonstrated that by changing the geometrical parameters of the W-SSD, the threshold voltage can be reduced and eventually be employed as THz detectors.

Metallic structures were evaluated to determine the one best matches impedance between lithographic antennas and microbolometers. The bestwas a 0.28 λ transmission line section between the antenna and the microbolometer, 12% longer than what RF theory sets, whose behaviour wascompared with other structures. The response was 360% higher.All Rights Reserved © 2015 Universidad Nacional Autonoma de Mexico, Centro de Ciencias Aplicadas y Desarrollo Tecnologico. This is anopen access item distributed under the Creative Commons CC License BY-NC-ND 4.0.

In this study, a noninvasive method based on infrared thermography is presented to obtain the resonant frequency of a dipole antenna in the microwave region. The study was performed by visualizing the temperature increase due to the Joule heating of the antenna fed by a voltage source. Results show a significant increase in dissipated heat at 830MHz where the resonance is expected. This method can be used to measure planar antennas noninvasive and in-situ without having to take them to a specialized antenna-measurement facility. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1610–1612, 2014

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Arrays of metallic thermocouples in the shape of spiral nanoantennas are proposed as infrared detectors, which use the thermoelectric properties of the metallic interfaces to generate electrical DC signals. The responsivity of these types of antennas is evaluated from both theoretical and numerical perspectives pointing out its potential as infrared sensors. Moreover, the same structures can be used to characterize the state of polarization of the optical near fields with a spatial resolution comparable to the wavelength.

In this article, the receiving properties of a fractal antenna coupled to a film of single-walled carbon nanotubes used as a bolometer are numerically and experimentally investigated in the megahertz frequency region. The numerical results show resonant frequencies at 420 and 1710 MHz, close to the 500 and 1600 MHz resonant frequencies obtained experimentally, the responsivity of the devices was calculated at 0.178 V/W at 610 MHz for an ibias of 0.3 μA. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1251–1253, 2012

The authors investigate the efficiency of a series of broadband rectennas designed to harvest the free-propagating electromagnetic energy at terahertz frequencies. We analyze by simulations the case of self-complementary square- and Archimedean-spiral antennas coupled to L-shaped self-switching diodes (L-SSDs). First, the geometry (i.e., the width and length of the channel) of the L-SSD was optimized to obtain a remarkable diode-like I–V response. Subsequently, the optimized L-SSD geometry was coupled to both types of spiral antennas and their characteristic impedance was studied. Finally, the energy conversion efficiency was evaluated for both rectenna architectures.

Thermo-electrical nanoantennas have been proposed as an alternative option for conversion solar energy harvesting applications. In this work, the response of a spiral broadband antenna has been obtained from numerical and theoretical simulations perspectives. The results show that this device exhibits a responsivity of 20mV/W under 117W/cm2, for a single-frequency radiation. We discuss strategies for enhanced efficiency.

We propose a mid-infrared device based on thermocouple optical antennas for light sensing and energy harvesting applications. We numerically demonstrate that antennas are able to generate low-power dc signals by beneficing of the thermoelectric properties of the metals that constitute them. We theoretically evaluate the optical-to-electrical conversion efficiency for harvesting applications and finally discuss strategies to increase its performance. Thermocouple optical antennas therefore open the route toward the design of photovoltaic devices., Comment: 16 pages, 5 figures

Dipole, bowtie, spiral and log-periodic optical nanoantennas were fabricated by e-beam lithography, measured by scattering scanning near-field optical microscopy (s-SNOM) and analyzed by numerical simulation in order to map their electric near-field distributions. These results provide important information about the near field of nanoantennas which is essential for the design of nanoantenna-phased arrays and integrated optics applications.

In this work, the influence of the surface depletion layer on the formation of a two-dimensional electron gas in AlGaAs/GaAs modulated doped heterostructures is studied. The authors explore a method for estimating the depletion region inside of the GaAs-based heterostructures by using the longitudinal optical and L- amplitude modes observed in Raman spectra, which are supported by the modeling results. The authors found that the position of the topmost doping layer changes the electron distribution in the heterostructure and decreases the influence of the depletion layer. Similar effects are perceived when an optimized solution of (NH4)2SX and isopropanol is employed. The authors present a method to evaluate the formation of a double two-dimensional electron gas in a heterostructure by the adequate use of modulation line in the photoreflectance spectroscopy.

In order to improve the rectification efficiency and current–voltage characteristics of self-switching diodes (SSD) the DC response is analyzed using technology computer aided design (TCAD). It is demonstrated that by varying geometrical parameters of L- and V-shaped SSDs or changing the dielectric permittivity of the trenches, a near zero threshold voltage can obtained, which is essential for energy harvesting applications. The carrier distribution inside the nanochannel is successfully simulated in two-dimensional mode for zero-, reverse-, and forward-bias conditions. This process allows for the evaluation of the effect of the lateral surface-charge on the formation and spatial distribution of the depletion region, in addition to, obtaining information on the physics of the SSD through the propose optimized geometries that were designed for tailoring and matching the desired frequencies of operation. The numerical results showed some insights for the improvement of the rectification efficiency and integration density using parallel SSD arrays.

In this letter, self-switching nanochannels have been proposed as an enabling technology for energy gathering in the terahertz (THz) regime. Such devices combine their diode-like behavior and high-speed of operation in order to generate DC electrical power from high-frequency signals. By using finite-element simulations, we have improved the sensitivity of L-shaped and V-shaped nanochannels based on InAlAs/InGaAs samples. Since those devices combine geometrical effects with their rectifying properties at zero-bias, we have improved their performance by optimizing their shape. Results show nominal sensitivities at zero-bias in the order of 40 V−1 and 20 V−1, attractive values for harvesting applications with square-law rectifiers.

Optical nanoantennas made of two metals are proposed to produce a Seebeck voltage proportional to the Stokes parameters of a light beam. The analysis is made using simulations in the electromagnetic and thermal domains. Each Stokes parameter is independently obtained from a dedicated nanoantenna configuration. S1 and S2 rely on the combination of two orthogonal dipoles. S3 is given by arranging two Archimedian spirals with opposite orientations. The analysis also includes an evaluation of the error associated with the Seebeck voltage, and the crosstalk between Stokes parameters. The results could lead to the conception of polarization sensors having a receiving area smaller than 10λ(2). We illustrate these findings with a design of a polarimetric pixel.

The near-field characteristics of single, double, and arrays of connected dipole nano-antennas coupled to bolometers were studied by infrared scattering scanning near-field optical microscopy (s-SNOM) and analyzed by numerical simulations. Results were consistent with classical antenna theory showing the expected π phase difference across the terminals of the dipoles. However, according to the observed differences between the measurements and simulations, the symmetry of the amplitude signal appeared to be sensitive with respect to the position of the bolometric element relative to the dipoles. The effect of the position of the bolometer on the associated near-field distribution suggests an influence on the coupling and efficiency of energy transfer into these detectors, which could be important for determining tolerances in the fabrication of such devices. These results show how near-field measurements in general can provide critical information to guide the design of nano-antennas, nano-antenna-phased arrays, and integrated photonic devices.

Optical antennas have been proposed as an alternative option for solar energy harvesting. In this work the power conversion efficiency of broadband antennas, log-periodic, square-spiral, and archimedian-spiral antennas, coupled to Metal-Insulator-Metal and Esaki rectifying diodes has been obtained from both theoretical and numerical simulation perspectives. The results show efficiencies in the order of 10(-6) to 10(-9) for these rectifying mechanisms, which is very low for practical solar energy harvesting applications. This is mainly caused by the poor performance of diodes at the given frequencies and also due to the antenna-diode impedance mismatch. If only losses due to antenna-diode impedance mismatch are considered an efficiency of about 10(-3) would be obtained. In order to make optical antennas useful for solar energy harvesting new rectification devices or a different harvesting mechanism should be used.