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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.

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 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.

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.

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.

In this letter, we evaluate the conversion efficiency of thermally isolated Seebeck nanoantennas by numerical simulations and discuss their uses and scope for energy harvesting applications. This analysis includes the simple case of titanium-nickel dipoles suspended in air above the substrate by a 200 nm silicon dioxide membrane to isolate the heat dissipation. Results show that substantially thermal gradients are induced along the devices leading to a harvesting efficiency around 10-4 %, 400 % higher than the previously reported Seebeck nanoantennas. In the light of these results, different optimizing strategies should be considered in order to make the Seebeck nanoantennas useful for harvesting 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.