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arXiv2019

Optimization of Patterned Surfaces for Improved Superhydrophobicity Through Cost-Effective Large-Scale Computations

The growing need for creating surfaces with specific wetting properties, such as superhyrdophobic behavior, asks for novel methods for their efficient design. In this work, a fast computational method for the evaluation of patterned superhyrdophobic surfaces is introduced. The hydrophobicity of a surface is quantified in energy terms through an objective function. The increased computational cost led to the parallelization of the method with the Message Passing Interface (MPI) communication protocol that enables calculations on distributed memory systems allowing for parametric investigations at acceptable time frames. The method is demonstrated for a surface consisting of an array of pillars with inverted conical (frustum) geometry. The parallel speedup achieved allows for low cost parametric investigations on the effect of the fine features (curvature and slopes) of the pillars on the superhydophobicity of the surface and consequently for the optimization of superhyrdophobic surfaces.

physics.app-ph↗

arXiv2019

Diamond/c-BN HEMTs for power applications: A theoretical feasibility analysis

Diamond is a promising material for high-power electronic applications in both the dc and rf domains. However, the predicted advantages are yet to be realized for a number of technical challenges. In particular, n-type devices have not been feasible due to the large ionization energies and low thermodynamic solubility limits of n-dopants. Motivated by the recent advances in nonequilibrium processing, we propose and theoretically examine a diamond/c-BN HEMT that can circumvent the critical limitations. A first-principles calculation suggests the desired type-I alignment at the heterojunction of these two nearly lattice matched semiconductors. The investigation also illustrates that a large sheet carrier density in excess of $5\times10^{12}~\mathrm{cm^{-2}}$ can be induced in the undoped diamond channel by the gate bias. A subsequent analysis of a simple prototype design indicates that the proposed device can achieve large current drive (~10 A/cm), low $R_{on}$ ($\sim 0.05 ~ \mathrm{m}Ω\cdot \mathrm{cm}^2$), and high $f_T$ (~300 GHz) simultaneously.

physics.app-ph↗

arXiv2019

Wind Tunnel Tests of a Pitot-Static Tube Array to Estimate Wind Velocity

This paper examines the use of a pitot-static tube array (PSA) to estimate horizontal wind velocity in all directions. This method uses the readings from the PSA as inputs. No flight tests have been done, but this method could potentially be used to estimate wind velocity from small UAVs. To do this, UAV ground speed and attitude must be available as inputs. Wind tunnel data of the PSA was used to create an algorithm that estimates wind velocity even when the wind is not parallel with any of the pitot-static tubes. This paper discuses the wind tunnel data, the algorithm, and the application for small UAVs.

physics.flu-dyn↗

arXiv2019

Hydrodynamic Analysis and Responsivity improvement of a metal/semiconductor/metal plasmonic detector

Characteristics improvement of photon/plasmon detectors have been the subject of several investigations in the area of plasmonic integrated circuits. Among different suggestions, Silicon-based Metal-Semiconductor-Metal (MSM) waveguides are one of the most popular structures for implementation of high-quality photon/plasmon detectors in infrared wavelengths. In this paper, an integrated Silicon Germanium (SiGe) core MSM plasmon detector is proposed to detect lambda=1550 nm with internal photoemission mechanism. Performance characteristics of the new device are simulated with a simplified hydrodynamic model. In a specific bias point (V=3 V and the incident optical power of 0.31 mW), the output current is 404.3 uA (276 uA detection current and 128.3 uA dark current), responsivity is 0.89 A/W and the 3-dB electrical bandwidth is 120 GHz. Simulation results for the proposed Plasmon detector, in comparison with the empirical results of a reported Si-based MSM device, demonstrate considerable responsivity enhancement.

physics.app-ph↗

arXiv2019

A dynamic picture of energy conversion in photovoltaic devices

Studies of emerging photovoltaics, such as organic and perovskite solar cells, have recently shown that the separation of photo-generated charge carriers is correlated with non-thermal, coherent oscillations within the illuminated device. We consider this experimental evidence in light of results from the theory of open quantum systems that point to the need for a self-oscillating internal capacitor, acting as a microscopic piston, to explain how an illuminated solar cell operates as an autonomous heat engine. We propose a picture of work extraction by photovoltaic devices that supersedes the quasi-static descriptions prevalent in the literature. Finally, we argue that such a dialogue between condensed matter physics and quantum thermodynamics may offer a guide for the design of new energy transducers.

physics.app-ph↗

arXiv2019

The Core Design of a Small Modular Pressurised Water Reactor for Commercial Marine Propulsion

If international agreements regarding the need to significantly reduce greenhouse gas emissions are to be met then there is a high probability that the shipping industry will have to dramatically reduce its greenhouse gas emissions. For emission reductions from ships greater than around 40\% then alternatives to fossil fuels - such as nuclear energy - will very likely be required. A Small Modular Pressurised Water Reactor design has been developed specifically to meet the requirements of a large container ship with a power requirement of 110~MWe. Container ships have a number of requirements - including a small crew size and reduced outages associated with refuelling - that result in a greater focus on design simplifications, including the elimination of the chemical reactivity control system during power operation and a long core life. We have developed a novel, soluble-boron free, low power density core that does not require refuelling for 15 years. The neutronic and fuel performance behaviour of this system has been studied with conventional UO2 fuel. The size of the pressure vessel has been limited to 3.5 metres in diameter. Furthermore, to ensure the survivability of the cladding material, the coolant outlet temperature has been reduced to 285degC from 320degC as in conventional GWe-class PWRs, with a resulting reduction in thermal efficiency to 25%. The UO2 core design was able to satisfactorily meet the majority of requirements placed upon the system assuming that fuel rod burnups can be limited to 100 GWd/tHM. The core developed here represents the first workable design of a commercial marine reactor using conventional fuel, which makes realistic the idea of using nuclear reactors for shipping.

physics.app-ph↗

arXiv2019

Meeting the Scaling Challenge for Post-Silicon Nanoelectronics using CaF2 Insulators

Two-dimensional (2D) semiconductors have been suggested both for ultimately-scaled field-effect transistors (FETs) and More-than-Moore nanoelectronics. However, these targets can not be reached without accompanying gate insulators which are scalable into the nanometer regime. Despite the considerable progress in the search for channel materials with high mobilities and decent bandgaps, finding high-quality insulators compatible with 2D technologies has remained a challenge. Typically used oxides (e.g. SiO2, Al2O3 and HfO2) are amorphous when scaled, while two-dimensional hBN exhibits excessive gate leakages. To overcome this bottleneck, we extend the natural stacking properties of 2D heterostructures to epitaxial fluorite (CaF2), which forms a quasi van der Waals interface with 2D semiconductors. We report scalable single-layer MoS2 FETs with a crystalline CaF2 insulator of about 2 nm thickness, which corresponds to an equivalent oxide thickness of less than 1 nm. While meeting the stringent requirements of low leakage currents, our devices exhibit highly competitive performance and record-small hysteresis. As such, our results present a breakthrough for very large scale integration towards commercially competitive nano-electronic devices.

physics.app-ph↗

arXiv2019

Visible and infrared photocurrent enhancement in a graphene-silicon Schottky photodetector through surface-states and electric field engineering

The design of efficient graphene-silicon (GSi) Schottky junction photodetectors requires detailed understanding of the spatial origin of the photoresponse. Scanning-photocurrent-microscopy (SPM) studies have been carried out in the visible wavelengths regions only, in which the response due to silicon is dominant. Here we present comparative SPM studies in the visible ($λ$ = 633nm) and infrared ($λ$ = 1550nm) wavelength regions for a number of GSi Schottky junction photodetector architectures, revealing the photoresponse mechanisms for silicon and graphene dominated responses, respectively, and demonstrating the influence of electrostatics on the device performance. Local electric field enhancement at the graphene edges leads to a more than ten-fold increased photoresponse compared to the bulk of the graphene-silicon junction. Intentional design and patterning of such graphene edges is demonstrated as an efficient strategy to increase the overall photoresponse of the devices. Complementary simulations and modeling illuminate observed effects and highlight the importance of considering graphene's shape and pattern and device geometry in the device design.

cond-mat.mes-hall↗

arXiv2019

Rigorous Analytical Model for Metasurface Microscopic Design with Interlayer Coupling

We present a semianalytical method for designing meta-atoms in multilayered metasurfaces (MSs), relying on a rigorous model developed for multielement metagratings. Notably, this model properly accounts for near-field coupling effects, allowing reliable design even for extremely small interlayer spacings, verified via commercial solvers. This technique forms an appealing alternative to the common full-wave optimization employed for MS microscopic design to date.

physics.app-ph↗

arXiv2019

Propagation of Relativistic Shock Wave Induced by Laser Spark in Quiescent Air

Laser-induced breakdown has shown many potential applications in the various field of science and engineering. As the breakdown occurs in gas or aerosol, a rapid hydrodynamic expansion as shock (blast) wave initiated from the deposition location. The nature of the shock wave is one of the controlling factors in many physical processes; unfortunately, its nature has still not been clearly understood. In this study, an error was found during the calculation of shock wave properties with the classical non-relativistic approach. The error in calculation was due to the initial relativistic propagation of high-temperature plasma. Initially, the plasma and shock wave travel together with higher acceleration up to the point of inflation, and in later time shockwave dissociate itself from the plasma. However, this accelerating effect is neglected in the earlier studies. To address the spontaneous accelerating and deaccelerating nature of the shock wave, the theoretical details of the relativistic approach of shock wave propagation is presented.

physics.plasm-ph↗

arXiv2019

The lattice and electronic thermal conductivity of doped SnSe: a first-principles study

Recently, it has been found that crystalline tin selenide (SnSe) holds great potential as a thermoelectric material due to its ultralow thermal conductivity and moderate electronic transport performance. As thermoelectric application usually requires doped material, charge carriers can play a role in the thermal transport in doped SnSe, but such an effect has not been clearly elucidated in previous theoretical and experimental studies. Here we performed a fully first-principles study on the effects of electrons to the thermal transport in doped SnSe. The electron-phonon coupling (EPC) effects on both phonons and charge carriers were considered using the mode specific calculation in our work. It is found that for phonons, EPC are weak compared to the intrinsic phonon-phonon scattering even at high carrier concentrations and thus have negligible effects on the lattice thermal conductivity. The electronic thermal conductivity is not negligible when the carrier concentration is higher than $10^{19} cm^{-3}$ and the values can be as high as 1.55, 1.45 and 1.77 $Wm^{-1}K^{-1}$ on a, b and c axes, respectively, for $10^{20} cm^{-3}$ electron concentration at 300K. The Lorenz number of SnSe is also calculated and it is dependent on crystal orientations, carrier concentrations, and carrier types. The simple estimation of electronic thermal conductivity using Wiedemann-Franz law can cause large uncertainties for doped SnSe.

physics.app-ph↗

arXiv2019

Regular Self-Pulsations in External Cavity Mid-IR Quantum Cascade Lasers

We show theoretically and experimentally that a quantum cascade laser (QCL) in an external Fabry-Perot cavity (EC) can produce regular self-pulsations (SP) of the output intensity at frequencies ~75 GHz. We recognize that the propagation delay in EC provides QCL with a "memory" mechanism to preserve the regularity and coherence of the pulse train on the intervals significantly exceeding the sub-picosecond gain coherence and gain recovery times. These results may point to novel practical approaches for producing regular time-domain SPs and pulse trains in the Mid-IR QCLs. Otherwise, due to a multimode Risken-Nummedal-Graham-Haken (RNGH) instability, free-running FP QCLs exhibit SPs at Rabi flopping frequencies in the THz range, which in addition, reveal a quasi-periodic chaotic behavior if the cavity round trip time is a few picoseconds or more.

physics.optics↗

arXiv2019

Achieving sub-1 Ohm-mm Non-Recess S/D Contact Resistance in GaN HEMTs Utilizing Simple CMOS Compatible La/Ti/Al/Ti Metal Contacts

In this paper, we report the use of lanthanum (La) in S/D contacts of GaN HEMTs, achieving 0.97 Ohm-mm contact resistance without S/D recess. The HEMTs show well-behaved electrical characteristics and satisfactory reliability. Our studies show that La, a CMOS compatible metal, is promising to lower GaN HEMT S/D contact resistance. La's low work function (3.5 eV) is beneficial for reducing the barrier between the metals and GaN. The Ohmic contact formation mechanism involved was shown to be different from conventional Ti/Al films. Spherical-shaped high-La regions formed near the surface during annealing. La diffuses into the AlGaN layer, and the overlap of La and Al peaks is significantly increased compared with that before annealing.

physics.app-ph↗

arXiv2019

Light Enhanced Blue Energy Generation using MoS$_2$ Nanopores

Blue energy relies on the chemical potential difference generated between solutions of high and low ionic strength and would provide a sun-and-wind independent energy source at estuaries around the world. Converting this osmotic energy through reverse-electrodialysis relies on ion-selective membranes. A novel generation of these membranes is based on atomically thin MoS$_2$ membranes to decrease the resistance to current flow to increase power output. By modulating the surface charge by light we are able to raise the ion selectivity of the membrane by a factor of 5 while staying at a neutral pH. Furthermore, we find that the behavior of small nanopores is dominated by surface conductance. We introduce a formalism based on the Dukhin number to quantify these effects in the case of a concentration gradient system. As a consequence, the charges created by light illumination provoke two important changes. Increased surface charge at the pore rim enhances the ion selectivity and therefore larger osmotic voltage (dominating in small pores), while the increased surface charge of the overall membrane enhances the surface conductance and therefore the osmotic current (dominating in larger pores). The combination of these effects might be able to efficiently boost the energy generation with arrays of nanopores with varying pore sizes.

physics.app-ph↗

arXiv2019

High-sensitivity free space optical communications using low size, weight and power hardware

Free space optical communication systems with extremely high detector sensitivities are attractive for various applications with low size, weight and power requirements. For these practical systems, integrated hardware elements with small form factor are needed. Here, we demonstrate a communication link using a CMOS integrated micro-LED and array of single-photon avalanche diodes. These integrated systems provide a data rate of 100 Mb/s at a sensitivity of -55.2 dBm, corresponding to 7.5 detected photons per bit.

physics.app-ph↗

arXiv2019

Digitized Adjoint Method for Inverse Design of Digital Nanophotonic Devices

We present a digitized adjoint method for realizing efficient inverse design of "digital" subwavelength nanophotonic devices. We design a single-mode 3-dB power divider and a dual-mode demultiplexer to demonstrate the digitized adjoint method for single-object and dual-object optimizations, respectively. The optimization comprises three stages, a first stage of continuous variation for an "analog" pattern, a second stage of forced permittivity biasing for a "quasi-digital" pattern, and a third stage for a multi-level digital pattern. Compared with conventional brute-force method, the proposed digitized adjoint method can improve the design efficiency by about 5 times, and the performance optimization can reach approximately the same level using the ternary pattern. The digitized adjoint method takes the advantages of adjoint sensitivity analysis and digital subwavelength structure and creates a new way for efficient and high-performance design of compact digital subwavelength nanophotonic devices. This method could overcome the efficiency bottleneck of the brute-force method that is restricted by the number of pixels of a digital pattern and improve the device performance by extending a conventional binary pattern to a multi-level one, which may be attractive for inverse design of large-scale digital nanophotonic devices.

physics.optics↗

arXiv2019

Monolithic integration of broadband optical isolators for polarization-diverse silicon photonics

Integrated optical isolators have been a longstanding challenge for photonic integrated circuits (PIC). An ideal integrated optical isolator for PIC should be made by a monolithic process, have a small footprint, exhibit broadband and polarization-diverse operation, and be compatible with multiple materials platforms. Despite significant progress, the optical isolators reported so far do not meet all these requirements. In this article we present monolithically integrated broadband magneto-optical isolators on silicon and silicon nitride (SiN) platforms operating for both TE and TM modes with record high performances, fulfilling all the essential characteristics for PIC applications. In particular, we demonstrate fully-TE broadband isolators by depositing high quality magneto-optical garnet thin films on the sidewalls of Si and SiN waveguides, a critical result for applications in TE-polarized on-chip lasers and amplifiers. This work demonstrates monolithic integration of high performance optical isolators on chip for polarization-diverse silicon photonic systems, enabling new pathways to impart nonreciprocal photonic functionality to a variety of integrated photonic devices.

physics.app-ph↗

arXiv2019

Trap states impact photon upconversion in rubrene sensitized by lead halide perovskite thin films

The same optical and electronic properties that make perovskite thin films ideal absorber materials in photovoltaic applications are also beneficial in photon upconversion devices. In this contribution, we investigate the rubrene-triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA). To elucidate the role of trap states which affect the free carrier lifetimes, we fabricate MAFA perovskite thin films with three different thicknesses. By measuring the change in the photoluminescence properties under different excitation fluences, we find that the prevalent recombination mechanism shifts from monomolecular for thinner films to bimolecular recombination for thicker MAFA films, indicating a reduction in shallow trap-assisted recombination. The addition of rubrene shows a passivating effect on the MAFA surface, but adds an additional quenching pathway due to charge transfer to the triplet state of rubrene. We observe that the threshold for efficient triplet-triplet annihilation shifts to lower incident powers with increasing MAFA thickness, which suggests that the charge transfer to the triplet state competes with non-radiative trap filling. Hence, injection of free electrons and holes into the upconverting organic semiconductor can provide a new avenue for sensitization of rubrene, and may allow us to move away from the necessity of efficient excitonic singlet-to-triplet converters.

physics.app-ph↗