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arXiv2019

Tunable MEMS VCSEL on Silicon substrate

We present design, fabrication and characterization of a MEMS VCSEL which utilizes a silicon-on-insulator wafer for the microelectromechanical system and encapsulates the MEMS by direct InP wafer bonding, which improves the protection and control of the tuning element. This procedure enables a more robust fabrication, a larger free spectral range and facilitates bidirectional tuning of the MEMS element. The MEMS VCSEL device uses a high contrast grating mirror on a MEMS stage as the bottom mirror, a wafer bonded InP with quantum wells for amplification and a deposited dielectric DBR as the top mirror. A 40 nm tuning range and a mechanical resonance frequency in excess of 2 MHz are demonstrated.

physics.app-ph↗

arXiv2019

Electromagnetic design of a superconducting electric machine with bulk HTS material

The use of high-temperature superconductors in electric machines offers potentially large gains in performance compared to conventional conductors, but also comes with unique challenges. Here, the electromagnetic properties of superconducting electric machines with bulk HTS trapped-field magnets in the rotor and conventional copper coils in the stator are investigated. To this end, an analytical model of the electromagnetic field in radial air-gap synchronous electric machines is developed and validated, taking into account the specific difficulties that occur in the treatment of machines with bulk HTS. Using this model, the influence of pole pair number, stator winding thickness, rotor surface coverage, and air gap width on the machine's Esson coefficient is calculated. In contrast to numerical simulations, the method presented here can provide results within minutes, making it particularly useful for work in early development and systems engineering, where large parameter spaces must be investigated quickly.

physics.app-ph↗

arXiv2019

Inverting Singlet and Triplet Excited States using Strong Light-Matter Coupling

In organic microcavities, hybrid light-matter states can form with energies that differ from the bare molecular excitation energies by nearly 1 eV. A timely question, given recent advances in the development of thermally activated delayed fluorescence materials, is whether strong light-matter coupling can be used to invert the ordering of singlet and triplet states and, in addition, enhance reverse intersystem crossing (RISC) rates. Here, we demonstrate a complete inversion of the singlet lower polariton and triplet excited states. We also unambiguously measure the RISC rate in strongly-coupled organic microcavities and find that, regardless of the large energy level shifts, it is unchanged compared to films of the bare molecules. This observation is a consequence of slow RISC to the lower polariton due to the delocalized nature of the state across many molecules and an inability to compete with RISC to the dark exciton reservoir, which occurs at a rate comparable to that in bare molecules.

physics.app-ph↗

arXiv2019

Rapid and unambiguous determination of the open time for waterborne paint films with Laser Speckle Imaging

In drying coatings, the time span between the deposition of the wet film and the formation of an uniform solid harbors a procession of complex phenomena each of which carves the final properties of the dry paint film. Attaining a better control of the drying process requires the availability of a tool that allows monitoring the attending dynamics. The experimental access to the dynamics underlying the drying process is challenging as paint films are often opaque and the phenomena accompanying the drying process usually populate a multitude of time and length scales. To circumvent these obstacles we deploy Laser Speckle Imaging (LSI) as a technique that allow us to quantitatively probe the internal dynamics of drying waterborne paints and unambiguously determine a crucial handling parameter such as the open time. We develop a set of scaling relations which accurately predict the behavior of the experimentally determined open time as a function of a parameter set that governs the drying process. Additionally, we harness the big temporal dynamic range of LSI to monitor phenomena that inhabit the later stages of the drying process. Remarkably, we not only capture the film formation time, the time point in which phase inversion takes place, but also microscopically detect the deformation of polymeric particles that precedes it.

physics.app-ph↗

arXiv2019

Achromatic Non-Interferometric Single Grating Neutron Phase Imaging

We demonstrate a simple single grating beam modulation technique, which enables the use of a highly intense neutron beam for phase imaging and thus spatially resolved structural correlation measurements in full analogy to quantum interference based methods. In contrast to interferometric approaches our method is intrinsically achromatic and provides unprecedented flexibility in the choice of experimental parameters. Utilizing merely a macroscopic absorption mask unparalleled length scales become accessible. The results presented, including application to a variety of materials, establish a paradigm shift in phase imaging.

physics.app-ph↗

arXiv2019

Relationship Between Macrostep Height and Surface Velocity for a Reaction-Limited Crystal Growth Process

This work examined the effect of macrostep height on the growth velocity of a vicinal surface during reaction- (interface-) limited crystal growth under non-equilibrium steady state conditions. The Monte Carlo method was employed, based on a restricted solid-on-solid (RSOS) model with point-contact-type step-step attraction (termed the p-RSOS model). Although this is a simple lattice model, the model surface shows a variety of distinctive configurations depending on the temperature and the driving force for crystal growth. The results demonstrate that the surface velocity decreases as the height of the faceted macrostep increases. In addition, the significant variation in surface velocity recently reported by Onuma {\it et al.} in a study based on 4H-SiC was reproduced. This work also shows that the terrace slope, elementary step velocity and elementary step kinetic coefficient are all affected by the faceted macrostep height.

physics.chem-ph↗

arXiv2018

Device Platform for Electrically Reconfigurable Dielectric Metasurfaces

Achieving an electrically tunable phased array optical antenna surface has been a principal challenge in the field of metasurfaces. In this letter, we demonstrate a device platform for achieving reconfigurable control over the resonant wavelength of a subwavelength optical antenna through free-carrier injection. We engineer and grow, using molecular beam epitaxy, a heterostructure of In1-xAlxAs/InAs/AlyGa1-ySb layers designed to achieve large amplitude and phase modulation of light by maximizing the refractive index change in regions of resonant field enhancement The p-i-n layers are grown on a heavily doped n-InAs layer which forms a reflecting substrate to confine the Mie resonances within the nanowires of the index tunable layers. We outline the fabrication process developed to form such tunable metasurface elements using a four-step projection lithography process and a self-aligned vertical dry etch. We experimentally demonstrate the operation of an electrically reconfigurable optical antenna element where the resonant wavelength blue shifts by 200nm only during carrier-injection. We extrapolate the experimentally measured InAs refractive index shifts to show we can achieve nearly π phase shift in a metasurface array. This solid-state device platform enables us to contact each resonant element independently to form a truly reconfigurable Fourier optical element with the promise of arbitrary control of the electromagnetic wavefront at the subwavelength scale.

physics.app-ph↗

arXiv2018

Miura-ori tube metamaterial with tunable dynamic property

Structures and/or materials with engineered functionality, capable of achieving targeted mechanical responses reacting to changes in external excitation, have various potential engineering applications, e.g. aerospace, oceanographic engineering, soft robot, and several others. Yet tunable mechanical performance is normally realized through carefully designing the architecture of structures, which is usually porous, leading to the complexity of the fabrication of the structures even using the recently emerged 3D printing technique. In this study we show that origami technique can provide an alternative solution to achieving the aim by carefully stacking the classical Miura sheets into the Miura-ori tube metamaterial and tuning the geometric parameters of the origami metamaterial. By combining numerical and experimental methods, we have demonstrated that an extremely broad range of natural frequency and dynamic response of the metamaterial can be achieved. The proposed structure can be easily fabricated from a single thin sheet made of one material and simultaneously owns better mechanical properties than the Miura sheet.

physics.app-ph↗

arXiv2019

Experimental and Theoretical Realization of Zenneck Wave-based Non-Radiative, Non-Coupled Wireless Power Transmission

A decade ago, non-radiative wireless power transmission re-emerged as a promising alternative to deliver electrical power to devices where a physical wiring proved to be unfeasible. However, existing approaches are neither scalable nor efficient when multiple devices are involved, as they are restricted by factors like coupling and external environments. Zenneck waves are excited at interfaces, like surface plasmons and have the potential to deliver electrical power to devices placed on a conducting surface. Here, we demonstrate, efficient and long range delivery of electrical power by exciting nonradiative waves over metal surfaces to multiple loads. Our modeling and simulation using Maxwells equation with proper boundary conditions shows Zenneck type behavior for the excited waves and are in excellent agreement with experimental results. In conclusion, we physically realize a radically different power transfer system, based on a wave, whose existence has been fiercely debated for over a century.

physics.app-ph↗

arXiv2019

Transport and Phototransport in ITO Nanocrystals with Short to Long-Wave Infrared Absorption

Nanocrystals are often described as an interesting strategy for the design of low-cost optoelectronic devices especially in the infrared range. However the driving materials reaching infrared absorption are generally heavy metalcontaining (Pb and Hg) with a high toxicity. An alternative strategy to achieve infrared transition is the use of doped semiconductors presenting intraband or plasmonic transition in the short, mid and long-wave infrared. This strategy may offer more flexibility regarding the range of possible candidate materials. In particular, significant progresses have been achieved for the synthesis of doped oxides and for the control of their doping magnitude. Among them, tin doped indium oxide (ITO) is the one providing the broadest spectral tunability. Here we test the potential of such ITO nanoparticles for photoconduction in the infrared. We demonstrate that In2O3 nanoparticles presents an intraband absorption in the mid infrared range which is transformed into a plasmonic feature as doping is introduced. We have determined the cross section associated with the plasmonic transition to be in the 1-3x10-13 cm2 range. We have observed that the nanocrystals can be made conductive and photoconductive due to a ligand exchange using a short carboxylic acid, leading to a dark conduction with n-type character. We bring further evidence that the observed photoresponse in the infrared is the result of a bolometric effect.

physics.app-ph↗

arXiv2018

Great enhancement of strong-field ionization in femtosecond-laser subwavelength-structured fused silica

A wavelength-degenerate pump-probe spectroscopy is used to study the ultrafast dynamics of strong-field ionization in femtosecond-laser subwavelength-structured fused silica. The comparative spectra demonstrate that femtosecond-laser subwavelength structuring always give rise to great enhancement for strong-field ionization as well as third-order nonlinear optical effects, which is the direct evidence of great local field enhancement in subwavelength apertures of fs-laser highly-excited surface. In short, the study shows the prominent subwavelength spatial effect of strong-field ionization in femtosecond-laser ablation of dielectrics, which greatly contributes to the well-known "incubation effect".

physics.app-ph↗

arXiv2018

Measuring micro-displacements of specular surfaces using speckle interferometry

The displacement field of an object surface can be measured by using speckle interferometry. This technique is based on the phenomenon of laser speckle and consists in correlating speckle interferograms taken after and before the deformation of the surface. The main requirement is that the surface under study must be optically rough to generate the speckle patterns to be correlated. In this paper, we present a very simple and intuitive method based on speckle interferometry to measure out-of-plane displacements on specular reflecting surfaces that generate no speckle patterns. The method consists in a modified digital speckle pattern interferometer which requires no special equipment other than that used in conventional speckle interferometers. The proposed method could find useful application in the measurement of thermal deformation of mirrors.

physics.app-ph↗

arXiv2018

Scaling Law for Criticality Conditions in Heterogeneous Energetic Materials under Shock Loading

Initiation in heterogeneous energetic material (HEM) subjected to shock loading occurs due to the formation of hot spots. The criticality of the hot spots governs the initiation and sensitivity of HEMs. In porous energetic materials, collapse of pores under impact leads to the formation of hot spots. Depending on the size and strength of the hot spots chemical reaction can initiate. The criticality of the hot spots is dependent on the imposed shock load, void morphology and the type of energetic material. This work evaluates the relative importance of material constitutive and reactive properties on the criticality condition of spots. Using a scaling-based approach, the criticality criterion for cylindrical voids as a function of shock pressure, Ps and void diameter, Dvoid is obtained for two different energetic material HMX and TATB. It is shown that the criticality of different energetic materials is significantly dependent on their reactive properties.

physics.app-ph↗

arXiv2019

Design and Fabrication of an Ultra-low Noise Ag-AgCl Electrode

We have developed a new method for fabrication of ultra-low noise silver-silver chloride (Ag-AgCl) reference electrode with dramatically high stability, useful in biomedical precise measurements. Low noise level and high stability are the most important features when evaluating reference electrode for example when used as a half-cell at electrochemical cells. The noise was measured for bare Ag-AgCl electrode pairs in a saline electrolyte without any junction. The noise spectral density for such measurements increases at low frequencies due to flicker (1/f) noise. Thermal noise from the real part of the electrode impedance is always lower than 1/f noise and is shown as offset voltage in noise spectrum. Electrode noise is highly correlated to its offset voltage. Moreover, the variance of the potential difference in our measurements is correlated to the noise level of electrodes. In this paper, offset voltage spectrum and the variance of data while doing measurements are proposed for identification of noise in our electrodes. The results presented suggest a new process for fabrication of electrode pairs with differential noise potential of 16 nanovolt root mean square at frequency of 1 Hz.

physics.chem-ph↗

arXiv2019

Ferroelectric multiple-valued logic units

Employing many-valued logic (MVL) data processing allows to dramatically increase the performance of computing circuits. Here we propose to employ ferroelectrics for the material implementation of MVL units basing on their ability to pin the polarization as a sequence of multi-stable states. Two conceptual ideas are considered. As the first system, we suggest using the strained ferroelectric films that can host the polarization states, allowing the effective field-induced multilevel switching between them. As the second one, we propose to employ the ferroelectric nano-samples that confine the topologically protected polarization textures, which may be used as MVL structural elements. We demonstrate that these systems are suitable for engineering of the 3-, 4- and even 5-level logic units and consider the circuit design for such elements.

physics.app-ph↗

arXiv2019

Enhancement of Photoemission on P-type GaAs using Surface Acoustic Waves

We demonstrate that photoemission properties of GaAs photocathodes (PCs) can be altered by surface acoustic waves (SAWs) generated on the PC surface due to dynamical piezoelectric fields of SAWs. Simulations with COMSOL indicate that electron effective lifetime in p-doped GaAs may increase by a factor of 10x to 20x. It implies a significant, by a factor of 2x to 3x, increase of quantum efficiency (QE) for GaAs PCs. Essential steps in device fabrication are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of I-V characteristic of the SAW device, and ability to survive high-temperature annealing.

physics.app-ph↗

arXiv2019

Effect of the Thickness on the Fracturing Behavior of Discontinuous Fiber Composite Structures

In this study, we investigate experimentally and numerically the mode I intra-laminar fracture and size effect of Discontinuous Fiber Composites (DFCs) as a function of the structure thicknesses. By testing geometrically-scaled Single Edge Notch Tension (SENT) specimens a notable structure size effect on the nominal strength of DFCs is identified. As the specimen size increases, the nominal strength decreases. For small specimens, we find a limited size effect with enhanced pseudo-ductility and a strong divergence from Linear Elastic Fracture Mechanics (LEFM). For sufficiently large specimen sizes, the scaling of the nominal strength follows closely LEFM with a strong brittle failure. As the thickness increases, the size effect decreases. We identify the fracture energy and the effective size of the fracture process zone as a function of the thickness of the structure. To do so, we integrate equivalent fracture mechanics and stochastic finite element modeling. Experimentally, we collect the nominal strength of geometrically-scaled Single Edge Notch Tension (SENT) specimens. The numerical stochastic model captures the complex, inhomogeneous mesostructure of DFCs by explicitly generating the platelets. From the integrated analysis, it is found that the fracture energy depends significantly on the structure thickness. It is shown to increase gradually up to 2 mm and saturates after 3 mm to a value of 57.77 N/mm, which is 4.81 times larger than a typical aluminum alloy.

physics.app-ph↗

arXiv2019

Microstructure, hardness and mechanical properties of two different unalloyed tantalum wires deposited via Wire + Arc Additive Manufacture

An innovative way of producing large-scale unalloyed tantalum parts, based on the Wire + Arc Additive Manufacturing process, has been developed in this study. Two different unalloyed tantalum wires have been used to deposit 200-mm-long structures in tantalum. The effect of the wire chemistry on microstructure, hardness, porosity, mechanical properties and strain localisation has been investigated. The deposits showed high integrity and excellent mechanical properties, with yield strength, ultimate tensile strength and elongation as high as 234 MPa, 261 MPa, and 36 %, respectively. Indeed, yield strength was higher than commercially available tantalum, even though, in this study, the grains were large and had a high aspect ratio. Wire + Arc Additive Manufacture has clearly shown the potential to produce tantalum components with relatively low cost and reduced lead time, thus offering a new robust and viable manufacturing route.

physics.app-ph↗