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arXiv2019

Giant Conductivity Modulation of Aluminum Oxide using Focused Ion Beam

Precise control of the conductivity of semiconductors through doping has enabled the creation of advanced electronic devices, similarly, the ability to control the conductivity in oxides can enable novel advanced electronic and optoelectronic functionalities. While this was successfully shown for moderately insulating oxides, such as In2O3, a reliable method for increasing the conductivity of highly insulating, wide bandgap dielectrics, such as aluminum oxide (Al2O3), has not been reported yet. Al2O3 is a material of significant technological interest, permeating diverse fields of application, thanks to its exceptional mechanical strength and dielectric properties. Here we present a versatile method for precisely changing the conductivity of Al2O3 . Our approach greatly exceeds the magnitude of the best previously reported change of conductivity in an oxide. Our method uses focused ion beam to produce conductive zones with nanoscale resolution within the insulating Al2O3 matrix. We investigated the source of conductivity modulation and identified trap-assisted conduction in the ion damage-induced defects as the main charge transport mechanism. Temperature-dependency of the conductivity and optical characterization of the patterned areas offer further insight into the nature of the conduction mechanism. We also show that the process is extremely reproducible and robust against moderate annealing temperatures and chemical environment. The record conductivity modulation, combined with the nanoscale patterning precision allows the creation of conductive zones within a highly insulating, mechanically hard, chemically inert, and bio-compatible matrix, which could find broad applications in electronics, optoelectronics, and medical implants

physics.app-ph↗
arXiv2019

Interplay of substrate inclination and wettability on droplet impact dynamics

Experimental investigations were carried out to elucidate the role of surface wettability and inclination on the post impact dynamics of droplets. Maximum spreading diameter and spreading time were found to decrease with increasing inclination angle and normal Weber number for superhydrophobic surfaces. The experiments on SH surfaces were found to be in excellent agreement with an existing analytical model, incorporated with the modifications for the oblique impact conditions. Energy ratios and elongation factor were also measured for different inclination angles. On inclined SH surfaces, different features like arrest of secondary droplet formation, reduced pinch off at the contact line and inclination dependent elongation mechanism were observed. Contrary to SH surfaces, hydrophilic surfaces show opposite trends of maximum spreading factor and spreading time with inclination angle and normal Weber number respectively. This was due to the dominance of tangential kinetic energy over adhesion energy and gravitational potential at higher inclination angles. Finally, colloidal solutions of nanoparticles were used to elucidate slip and disjoining pressure on SH and hydrophilic surfaces, respectively. Overall, the article provides a comprehensive picture of post impact dynamics of droplets on inclined surfaces encompassing a broad spectrum of governing parameters like Reynolds number, Weber number, degree of inclination and surface wettability.

physics.flu-dyn↗
arXiv2019

Magnetic interaction and anisotropy axes arrangement in nanoparticle aggregates can enhance or reduce the effective magnetic anisotropy

The magnetic response of nanostructures plays an important role on biomedical applications being strongly influenced by the magnetic anisotropy. In this work we investigate the role of temperature, particle concentration and nanoparticle arrangement forming aggregates in the effective magnetic anisotropy of Mn-Zn ferrite-based nanoparticles. Electron magnetic resonance and coercivity temperature dependence analyses, were critically compared for the estimation of the anisotropy. We found that the temperature dependence of the anisotropy follows the Callen-Callen model, while the symmetry depends on the particle concentration. At low concentration one observes only an uniaxial term, while increasing a cubic contribution has to be added. The effective anisotropy was found to increase the higher the particle concentration on magnetic colloids, as long as the easy axis was at the same direction of the nanoparticle chain. Increasing even further the concentration up to a highly packed condition (powder sample) one observes a decrease of the anisotropy, that was attributed to the random anisotropy axes configuration.

physics.app-ph↗
arXiv2018

Improving Performance of Organic Photodetectors based on Trap Assisted Photomultiplication through Adjusting Photogenerated Carrier Distributions

Organic photodetectors (OPDs) based on trap-assisted photomultiplication (PM) effect with external quantum efficiency (EQE) far exceeding 100% are quite appealing for achieving highly sensitive photodetection. A classic structure of PM-type OPDs is based on the active layer of P3HT:PC70BM with the weight ratio of 100:1, in which PC70BM forms islands and supply bulk electron traps for inducing strong interfacial band bending and thus hole tunneling from the Al electrode. In this paper, aiming for optimizing the PM effect, we study the photogenerated carrier distribution by tuning thickness of the active layer (P3HT:PC70BM). The combination effect of both the exciton generation and exciton dissociation processes affects the photogenerated carrier distribution, which ultimately determines the PM performances of OPDs. On the one hand, simulation reveals that the thinner the active layer, the stronger exciton generation near the Al electrode. On the other hand, the photoluminescence and surface morphology studies reflect that the active layer with thickness of 230 nm has the smooth surface and produces the highest exciton dissociation rate. The two effects result in the OPD with a 205 nm thick active layer has the champion EQE (105569%) and photoresponsivity (344 A/W), corresponding to an enhancement of 330% with respect to the OPD with a 325 nm thick active layer. Ascribed to the trade off of EQE and dark current, the detectivity performs differently at different wavelength ranges. At short wavelength range, the detectivity reaches the highest when the active layer is 205 nm thick, while the highest detectivity at long wavelength range is produced by the thickest active layer. Our work contributes to developing low cost organic photodetectors for detecting weak light signals.

physics.app-ph↗
arXiv2019

Low-threshold Stimulated Emission of Hybrid Perovskites at Room Temperature through Defect-Mediated Bound Excitons

Excitonic stimulated emission provides a promising mechanism and route to achieve low-threshold semiconductor lasers for micro-nano optoelectronic integrations. However, excitonic stimulated emission from quantum structure-free semiconductors has rarely been realised at room temperature due to the phase transition between excitonic and electron-hole plasma states. Herein, we show that through trap-state and band-edge engineering, bound exciton states can be stabilised within the hybrid lead bromide perovskite. Under modest pumping conditions, these states enable stimulated emission behaviour that exhibits a low threshold carrier density of only 1.6*1017 cm-3, as well as a high peak gain coefficient of ~1300 cm-1. This is the first time that bound exciton stimulated emission has been realised at room temperature from a quantum structure-free semiconductor. Not only does this open up new research horizons for perovskite materials, but also it has important implications for semiconductor excitonic physics and the development of next-generation optoelectronic applications.

physics.app-ph↗
arXiv2019

Amplitude-modulated pulses based phase-sensitive OTDR in distributed sensing

The detected frequency response range of phase-sensitive optical time domain reflectometry is limited by the sensing fiber length. In this paper, we demonstrate an amplitude modulated pulses based phase-sensitive OTDR to detect vibrations with high spatial resolution and wide frequency response range. The amplitude modulated pulses consisting of narrow pulses and background light are injected into the sensing fiber as probe light. The Rayleigh backscattering generated by the background light interferes at the detection end, presenting a phase sensitive response to external vibrations. The Rayleigh backscattering of narrow pulses are tracked to locate vibrations. The frequency information is obtained from the frequency domain analysis of the interference signal. The system can realize high frequency response with low repetition rate of probe pulses by one-end detection.

physics.app-ph↗
arXiv2019

Making the guided wave propagate unidirectional in periodic dielectric waveguide

Unidirectional waves are the waves propagating only in one direction and prohibited in opposite direction. This kind of waves can be surface waves realized by topological edge state or bulk waves in the media with time reversal and space inversion symmetry broken. Here, we introduce unidirectional waveguide modes in the periodic dielectric chain comprised of gyromagnetic rods. In this modes, the waves in the waveguide propagates unidirectional, but can be converted into bi-directional waves by changing the arrangement of rods. We show unidirectional guided waves result from the time-mirror symmetry broken of the chain. Experiments confirm the theoretical results of waveguide modes. A new vortex wave generator based on the unidirectional guided wave is demonstrated theoretically. Our work opens a new avenue for manipulating the electromagnetic wave in a simple and flexible way, and provides a new possibility for novel devices.

physics.app-ph↗
arXiv2019

Flexible Conductive Composites with Programmed Electrical Anisotropy Using Acoustophoresis

3D printing mechanically flexible composite materials with high electrical conductivity is currently hindered by the need to use high loading of conductive filler, which severely limits flexibility. Here, microstructural patterning of composite materials via acoustophoresis imparts these materials with high conductivity and flexibility simultaneously, filling a technology gap in the field. Acoustophoresis patterns filler particles into highly efficient percolated networks which utilize up to 97\% of the particles in the composite, whereas the inefficient stochastic networks of conventional dispersed-fiber composites utilize $<5$\%. These patterned materials have conductivity an order of magnitude higher than conventional composites made with the same ink, reaching 48\% the conductivity of bulk silver within the assembled silver-particle networks (at 2.6v\% loading). They also have low particle loading so that they're flexible, withstanding $>$500 bending cycles without losses in conductivity and changing conductivity only $5$\% within cycles on average (for 2.6v\% composites). In contrast, conventional unpatterned composites with the same conductivity require such high loading that they're prohibitively brittle. Finally, modulating the shape of the applied acoustic fields allows control over the anisotropy of the conductive networks and produces materials which are either 2-D conductive, 1-D conductive, or insulating, all using the same nozzle and ink.

physics.app-ph↗
arXiv2019

The Laboratory of Material Analysis with Ion Beams, LAMFI-USP

LAMFI is a laboratory dedicated to the development and application of ion beam techniques for the analysis of bulk materials and thin films. Its main facilities comprise an 1.7MV Pelletron tandem accelerator and two analytical setups, one mainly for Rutherford Backscattering Spectrometry (RBS), and channeling, and another, for Particle Induced X-ray Emission (PIXE) analysis. Since its beginning in March 1992, over 10 institutions involving about 68 researchers, 42 graduate and 9 undergraduate students have been direct or indirectly users at LAMFI.

physics.ins-det↗
arXiv2019

A highly sensitive magnetic sensor using a 2D van der Waals ferromagnetic material

Two-dimensional (2D) van der Waals ferromagnetic materials are emerging as promising candidates for applications in ultra-compact spintronic nanodevices, nanosensors, and information storage. Our recent discovery of the strong room temperature ferromagnetism in single layers of VSe2 grown on graphite or MoS2 substrate has opened new opportunities to explore these ultrathin magnets for such applications. In this paper, we present a new type of magnetic sensor that utilizes the single layer VSe2 film as a highly sensitive magnetic core. The sensor relies in changes in resonance frequency of the LC circuit composed of a soft ferromagnetic microwire coil that contains the ferromagnetic VSe2 film subject to applied DC magnetic fields. The sensitivity of the sensor reaches an extremely high value of 16x10^6 Hz/Oe, making it an excellent candidate for a wide range of magnetic sensing applications.

physics.app-ph↗
arXiv2019

Lens-Based Beamformer for Low-Complexity Millimeter-Wave Cellular Systems

Data rate requirements for cellular communications are expected to increase 1000-fold by 2020, compared to 2010. This is mainly because of the rapid increase in the number of wireless devices and data hungry applications per-device. This creates a formidable bandwidth crisis. Milimeter-wave (mmWave) systems with massive multiple-input multiple-output (MIMO) operation are two complementary concepts poised to meet this exploding demand, as verified by the wealth of investigations by both industry and academia. Nevertheless, existing MIMO processing techniques, requiring a dedicated radio-frequency (RF) up/down-conversion chain per antenna, results in prohibitively high complexity and cost of the mmWave prototypes. The primary objective of this paper is to present a complete feasibility study on alternative beamforming techniques targeted to reduce the system complexity without drastically compromising its performance. More precisely, at 28 GHz, we investigate the end-to-end performance of two different lens-based beamforming topologies. Our study is an amalgam of theoretical modelling, electromagnetic design, and prototype manufacturing, yielding a comprehensive mmWave, massive MIMO performance evaluation.

physics.app-ph↗
arXiv2019

Retrieval Approach for Determining Surface Susceptibilities and Surface Porosities of a Symmetric Metascreen from Reflection and Transmission Coefficients

Recently we derived generalized sheet transition conditions (GSTCs) for electromagnetic fields at the surface of a metascreen (a metasurface with a ``fishnet'' structure, i.~e., a periodic array of arbitrary spaced apertures in a relatively impenetrable surface). The parameters in these GSTCs are interpreted as effective surface susceptibilities and surface porosities, which themselves are related to the geometry of the apertures that constitute the metascreen. In this paper, we use these GSTCs to derive the plane-wave reflection ($R$) and transmission ($T$) coefficients of a symmetric metascreen, expressed in terms of these surface parameters. From these equations, we develop a retrieval approach for determining the uniquely defined effective surface susceptibilities and surface porosities that characterize the metascreen from measured or simulated data for the $R$ and $T$ coefficients. We present the retrieved surface parameters for metascreens composed of five different types of apertures (circular holes, square holes, crosses, slots, and a square aperture filled with a high-contrast dielectric). The last example exhibits interesting resonances at frequencies where no resonances exist when the aperture is not filled, which opens up the possibility of designing metasurfaces with unique filtering properties. The retrieved surface parameters are validated by comparing them to other approaches.

physics.app-ph↗
arXiv2019

Molybdenum oxide hole selective transport layer by hot wire oxidation-sublimation deposition for silicon heterojunction solar cells

In this article, a novel hot wire oxidation-sublimation deposition (HWOSD) technique was developed to prepare molybdenum oxide (MoOx) thin films with high quality. Silicon heterojunction (SHJ) solar cells with the HWOSD MoOx as a hole selective transport layer (HSL) were fabricated. Thickness of the MoOx layer and annealing process of the solar cells were studied and optimized. A power conversion efficiency up to 21.10% was achieved on a SHJ solar cell using a 14nm MoOx layer as the HSL. Dark current density-voltage-temperature (J-V-T) characteristics of the SHJ solar cell were measured at the temperatures from 200K to 380K. Transport processes including thermionic emission of electrons over the potential barrier and quantum assisted tunneling of holes through the gap states in the MoOx layer were proposed for the MoOx/n c-Si heterojunction. The investigation of the transport mechanisms provides us a better understanding of the characteristics of the novel SHJ solar cells and it is helpful for us to fully demonstrate the potential of such kind of solar cells in the future.

physics.app-ph↗
arXiv2019

Original Ti:Sa 10 kHz Front-end design delivering 17 fs, 170 mrad CEP stabilized pulses up to 5 W

We present a compact 10 kHz Ti:Sa front-end relying on an original double-crystal regenerative amplifier design. This new configuration optimizes the thermal heat load management, allowing producing a 110 nm large spectrum and maintaining a good beam profile quality. The front-end delivers up to 5 W after compression, 17 fs pulses with a 170 mrad shot-to-shot residual CEP noise.

physics.app-ph↗
arXiv2019

The Evidence of Cathodic Micro-discharges during Plasma Electrolytic Oxidation Process

Plasma electrolytic oxidation (PEO) processing of EV 31 magnesium alloy has been carried out in fluoride containing electrolyte under bipolar pulse current regime. Unusual PEO cathodic micro-discharges have been observed and investigated. It is shown that the cathodic micro-discharges exhibit a collective intermittent behavior which is discussed in terms of charge accumulations at the layer/electrolyte and layer/metal interfaces. Optical emission spectroscopy is used to determine the electron density (typ. 10 15 cm-3) and the electron temperature (typ. 7500 K) while the role of F-anions on the appearance of cathodic micro-discharges is pointed out. Plasma Electrolytic Oxidation (PEO) is a promising plasma-assisted surface treatment of light metallic alloys (e.g. Al, Mg, Ti). Although the PEO process makes it possible to grow oxide coatings with interesting corrosion and wear resistant properties, the physical mechanisms of coating growth are not yet completely understood. Typically, the process consists in applying a high voltage difference between a metallic piece and a counter-electrode which are both immersed in an electrolyte bath. Compare to anodizing, the main differences concern the electrolyte composition and the current and voltage ranges which are at least one order of magnitude higher in PEO 1. These significant differences in current and voltage imply the dielectric breakdown and consequently the appearance of micro-discharges on the surface of the sample under processing. Those micro-discharges are recognized as being the main contributors to the formation of a dielectric porous crystalline oxide coating. 2 Nevertheless, the breakdown mechanism that governs the appearance of those micro-discharges is still under investigation. Hussein et al. 3 proposed a mechanism with three different plasma formation processes based on differences in plasma chemical composition. The results of Jovovi{ć} et al. 4,5 concerning physical properties of the plasma seem to corroborate this mechanism, and also point out the importance of the substrate material in the plasma composition. 6 Compared with DC conducted PEO process, using a bipolar pulsed DC or AC current supply gives supplementary control latitude through the current waveform parameters. The effect of these parameter on the micro-discharges behavior has been investigated in several previous works. 2,3,7,8 One of the main results of these studies is the absence of micro-discharge during the cathodic current half-period. 9-11 Even if the cathodic half-period has an obvious effect on the efficiency of PEO as well as on the coating growth and composition, the micro-plasmas appear only in anodic half-period. Sah et al. 8 have observed the cathodic breakdown of an oxide layer but at very high current density (10 kA.dm-${}^2$), and after several steps of sample preparation. Several models of micro-discharges appearance in AC current have already been proposed. 1,2,8,12,13 Though cathodic micro-discharges have never been observed within usual process conditions, the present study aims at defining suitable conditions to promote cathodic micro-discharges and at studying the main characteristics of these micro-plasmas.

physics.app-ph↗
arXiv2019

High-performance dendritic metamaterial absorber for broadband and near-meter wave radar

Absorbing materials in ultra-high frequency (UHF) band has constantly been a major challenge. The size of the absorber in UHF band is large, whereas the resonant frequency band is narrow. According to Rozanov's theory, two kinds of composite metamaterial absorbers are designed to realize the requirements of low-frequency broadband metamaterial microwave absorber: the magnetic-metamaterial composite absorber1 (MA1) and the dielectric-metamaterial composite absorber 2 (MA2). In the range of approximately 300-1000MHz, both absorbers achieve absorption of over 90% and feature good adaptability to the incident angle of the incident wave. The absorbers also present good absorption rate of over 80% in the range of 0-45 degree. Processing samples of indium tin oxide (ITO) resistance film and polymethacrylimide (PMI) foam board feature simple preparation and low cost, and the most important thing is to consider the weight problem, which features certain advantages in terms of use.

physics.app-ph↗
arXiv2019

High-performance 2D 1xN T-junction Wavelength (De)Multiplexer Systems by Inverse Design

Previously proposed designs of integrated photonic devices have used the intuitive brute force approach or optimization methods that employ parameter search algorithms. However, a small parameter space and poor exploitation of the underlying physics have limited device performance, functionality, and footprint. In this paper, we propose efficient and compact 2D 1xN in-plane-incidence wavelength demultiplexers by using recently developed objective-first inverse design algorithm. Output ports in the presented 1xN photonic devices are located along the transverse to the input channel. Ultra-high device performance was achieved for the specific designs of 1x2, 1x4, and 1x6 wavelength (de)multiplexers with small footprints 2.80 um x 2.80 um, 2.80 um x 4.60 um, 2.80 um x 6.95 um, respectively. We used two approaches to binarization-level-set and binarization-cost-to obtain silicon wavelength demultiplexer considering fabrication constraints. For instance, the transmission efficiency of binarization-cost 1x2 demultiplexer was -0.30 dB for 1.31 um and -0.54 dB at 1.55 um while crosstalk at the operating wavelengths are negligibly small, i.e., -17.80 and -15.29 dB, respectively. Moreover, for the binarization-cost 1x4 demultiplexer, the transmission efficiency values were approximately -1.90 dB at 1.31, 1.39, 1.47, and 1.55 um as the crosstalk was approximately -13 dB. Furthermore, the objective-first algorithm was used to employ our demultiplexers as multiplexers which means the ports that were once used as inputs in demultiplexers are designed to be used as outputs. The inverse design approach that allows for the implementation of more than six output channels together with the proposed functionalities can help develop compact and manufacturable 2D 1xN couplers.

physics.app-ph↗