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Flatbands have become a cornerstone of contemporary condensed-matter physics and photonics. In electronics, flatbands entail comparable energy bandwidth and Coulomb interaction, leading to correlated phenomena such as the fractional quantum Hall effect and recently those in magic-angle systems. In photonics, they enable properties including slow light

Capturing and analyzing cometary coma dust lead to elucidate the origin of water and organics within the Solar System. For future sample return missions of fragile organic microparticles from a cometary nucleus, rendezvous operations will become more favorable than flyby missions because the comet rendezvous can reduce impacting velocity of cometary dust particles slow enough to capture them intact, rather than hypervelocity flyby sampling like the Stardust mission to the Comet Wild 2. At JAXA/ISAS, we are developing a core technology for sample return of microparticles ejected at as a lower velocity as an order of 0.1 m/s to 100 m/s after rendezvous with a cometary nucleus. We have devised “Vertically Aligned Carbon Nanotube (VA-CNT)” carpets as an effective capture medium for such a purpose. The VA-CNT carpets can amplify van der Waals force with impacting particles due to the large contact area and can capture intact the microparticles of sub-mm in size or smaller in the vacuum space environment while preserving its adhesive strength. In this study, we modelled capturing mechanism of microparticles on the VA-CNT carpets by the impact analysis software “LS-DYNA” to further improve its particle capture performance. The stress-strain constitutive laws for the VA-CNT carpets were derived via indentation and inputted to our simulations that were consistent with impact experiment results. The simulations reveal that the mechanical property of the VA-CNT carpets is the key for the improvement of its particle capture performance suitable for sampling the cometary dust.

International audience; Herein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering.

Background Impact experiments, routinely performed at the macroscale, have long been used to study mechanical properties of materials. Microscale high-velocity impact, relevant to applications such as ballistic drug delivery has remained largely unexplored at the level of a single impact event. Objective In this work, we study the mechanical behavior of polymer gels subjected to high-velocity microparticle impact, with strain rates up to 107 s−1, through direct visualization of the impact dynamics. Methods In an all-optical laser-induced particle impact test, 10–24 μm diameter steel microparticles are accelerated through a laser ablation process to velocities ranging from 50 to 1000 m/s. Impact events are monitored using a high-speed multi-frame camera with nanosecond time resolution. Results We measure microparticle trajectories and extract both maximum and final penetration depths for a range of particle sizes, velocities, and gel concentrations. We propose a modified Clift-Gauvin model and demonstrate that it adequately describes both individual trajectories and penetration depths. The model parameters, namely, the apparent viscosity and impact resistance, are extracted for a range of polymer concentrations. Conclusions Laser-induced microparticle impact test makes it possible to perform reproducible measurements of the single particle impact dynamics on gels and provides a quantitative basis for understanding these dynamics. We show that the modified Clift-Gauvin model, which accounts for the velocity dependence of the drag coefficient, offers a better agreement with the experimental data than the more commonly-used Poncelet model. Microscale ballistic impact imaging performed with high temporal and spatial resolution can serve as direct input for simulations of high-velocity impact responses and high strain rate deformation in gels and other soft materials.

Shock waves generated by laser pulses have been gaining attention for biological and medical applications in which shock-induced cell membrane deformation influences cell permeation. However, the mechanisms through which the deformation of cell membranes affects permeability remain mostly unknown because of the difficulty of observing in real time the transient and dynamic behaviors of the shock waves and the cells. Here we present an all-optical measurement method that can quantitatively capture the pressure distribution of the propagating shock wave and simultaneously monitor the dynamic behavior of cell membranes. Using this method, we find that the profile of the shock wave dictates the cell membrane permeation. The results suggest a possible mechanism of membrane permeation where sharp pressure gradients create pores on the membrane. Our measurement will foster further understanding of the interaction of shock waves with cells, while the proposed mechanism advances biological and medical applications of shock waves. While shock waves are widely used in clinical and biological research due to their ability to deform the cell’s membrane and its permeability, the mechanisms of such interaction are still unclear. Here, the authors propose a method that allows to monitor the dynamic deformation of a cell’s membrane in response to shock waves and its effect on permeability.

Bombardment of materials by high-energy particles often leads to light emission in a process known as scintillation. Scintillation has widespread applications in medical imaging, x-ray nondestructive inspection, electron microscopy, and high-energy particle detectors. Most research focuses on finding materials with brighter, faster, and more controlled scintillation. We developed a unified theory of nanophotonic scintillators that accounts for the key aspects of scintillation: energy loss by high-energy particles, and light emission by non-equilibrium electrons in nanostructured optical systems. We then devised an approach based on integrating nanophotonic structures into scintillators to enhance their emission, obtaining nearly an order-of-magnitude enhancement in both electron-induced and x-ray–induced scintillation. Our framework should enable the development of a new class of brighter, faster, and higher-resolution scintillators with tailored and optimized performance.

We show in both theory and experiment that flatband photonic resonances can control and boost free-electron radiation, as validated by enhancement, band, and polarization-shaping measurements.

We demonstrate how Implosion Fabrication, a new three-dimensional nanofabrication technique, enables the realization of three-dimensional photonic devices at optical wavelengths. We realize two- and three-dimensional optical crystals of hydrogel-embedded silver meta-atoms.

We develop a general framework to enhance and control X-ray scintillation by embedding nanophotonic structures into scintillators. We demonstrate 10-fold scintillation enhancement in a conventional scintillator, showing the potential of our technique for X-ray imaging.

When impinging on optical structures or passing in their vicinity, free electrons can spontaneously emit electromagnetic radiation, a phenomenon generally known as cathodoluminescence. Free-electron radiation comes in many guises: Cherenkov, transition, and Smith–Purcell radiation, but also electron scintillation, commonly referred to as incoherent cathodoluminescence. While those effects have been at the heart of many fundamental discoveries and technological developments in high-energy physics in the past century, their recent demonstration in photonic and nanophotonic systems has attracted a great deal of attention. Those developments arose from predictions that exploit nanophotonics for novel radiation regimes, now becoming accessible thanks to advances in nanofabrication. In general, the proper design of nanophotonic structures can enable shaping, control, and enhancement of free-electron radiation, for any of the above-mentioned effects. Free-electron radiation in nanophotonics opens the way to promising applications, such as widely tunable integrated light sources from x-ray to THz frequencies, miniaturized particle accelerators, and highly sensitive high-energy particle detectors. Here, we review the emerging field of free-electron radiation in nanophotonics. We first present a general, unified framework to describe free-electron light–matter interaction in arbitrary nanophotonic systems. We then show how this framework sheds light on the physical underpinnings of many methods in the field used to control and enhance free-electron radiation. Namely, the framework points to the central role played by the photonic eigenmodes in controlling the output properties of free-electron radiation (e.g., frequency, directionality, and polarization). We then review experimental techniques to characterize free-electron radiation in scanning and transmission electron microscopes, which have emerged as the central platforms for experimental realization of the phenomena described in this review. We further discuss various experimental methods to control and extract spectral, angular, and polarization-resolved information on free-electron radiation. We conclude this review by outlining novel directions for this field, including ultrafast and quantum effects in free-electron radiation, tunable short-wavelength emitters in the ultraviolet and soft x-ray regimes, and free-electron radiation from topological states in photonic crystals.

A monofilament fiber spun from individual carbon nanotubes is an arbitrarily long ensemble of weakly interacting, aligned, discrete nanoparticles. Despite the structural resemblance of carbon nanotube monofilament fibers to crystalline polymeric fibers, very little is known about their dynamic collective mechanics, which arise from van der Waals interactions among the individual carbon nanotubes. Using ultrafast stroboscopic microscopy, we study the collective dynamics of carbon nanotube fibers and compare them directly with nylon, Kevlar, and aluminum monofilament fibers under the same supersonic impact conditions. The in situ dynamics and kinetic parameters of the fibers show that the kinetic energy absorption characteristics of the carbon nanotube fibers surpass all other fibers. This study provides insight into the strain-rate-dependent strengthening mechanics of an ensemble of nanomaterials for the development of high-performance fibers used in body armor and other protective nanomaterials possessing exceptional stability in various harsh environments.

This study elucidates the influence of molecular attributes on the observed dynamic stiffening in select two-component polyurethanes upon high strain-rate impact. Unlike typical segmented elastomers, polyurethanes consisting of poly(tetramethylene oxide), PTMO, and a diisocyanate, but without a chain extender, are investigated. The hexamethylenediisocyanate (HDI)-based polyurethane, HDI−PU, exhibits crystallinity and a much higher ambient storage modulus, as determined by dynamic mechanical analysis at 1 Hz, than that of 4,4′-methylenediphenyldiisocyanate (MDI)-based polyurethane, MDI−PU. In contrast, MDI−PU exhibits a higher glass transition temperature than that of HDI−PU, and a greater dynamic stiffening against silica micro-particle impacts at strain rates between 107 and 108 s−1. The variation in dynamic stiffening corroborates well the observed dynamics at the molecular level, as determined via solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The presence of a slower-dynamics component in MDI−PU, as evidenced in the 13C ssNMR dipolar dephasing time, is used to explain the observed enhanced dynamic stiffening response.

Optical metasurfaces have been heralded as the platform to integrate multiple functionalities in a compact form-factor, potentially replacing bulky components. A central stepping stone towards realizing this promise is the demonstration of multifunctionality under several constraints (e.g. at multiple incident wavelengths and/or angles) in a single device -- an achievement being hampered by design limitations inherent to single-layer planar geometries. Here, we propose a general framework for the inverse design of volumetric 3D metaoptics via topology optimization, showing that even few-wavelength thick devices can achieve high-efficiency multifunctionality. We embody our framework in multiple closely-spaced patterned layers of a low-index polymer. We experimentally demonstrate our approach with an inverse-designed 3d-printed light concentrator working at five different non-paraxial angles of incidence. Our framework paves the way towards realizing multifunctional ultra-compact 3D nanophotonic devices., 16 pages, 4 figures

We develop a general framework to describe non-equilibrium radiation by materials in nanophotonic structures (such as photoluminescence/cathodoluminescence/scintillation). We demonstrate the concept experimentally, enhancing and shaping cathodoluminescence from a silica photonic crystal.

We investigate Implosion Fabrication, a technique which prints arbitrary 3D nanostructures, as a new platform for nanophotonics. We show that optical properties of printed materials are tunable by characterizing the reflectivity of printed silver.

We demonstrate new axisymmetric inverse-design techniques that can solve problems radically different from traditional lenses, including \emph{reconfigurable} lenses (that shift a multi-frequency focal spot in response to refractive-index changes) and {\emph{widely separated}} multi-wavelength lenses ($\lambda = 1\,\mu$m and $10\,\mu$m). We also present experimental validation for an axisymmetric inverse-designed monochrome lens in the near-infrared fabricated via two-photon polymerization. Axisymmetry allows fullwave Maxwell solvers to be scaled up to structures hundreds or even thousands of wavelengths in diameter before requiring domain-decomposition approximations, while multilayer topology optimization with $\sim 10^5$ degrees of freedom can tackle challenging design problems even when restricted to axisymmetric structures., Comment: 13 pages, 6 figures

High-velocity microparticle impacts are relevant to many fields from space exploration to additive manufacturing and can be used to help understand the physical and chemical behaviors of materials under extreme dynamic conditions. Recent advances in experimental techniques for single microparticle impacts have allowed fundamental investigations of dynamical responses of wide-ranging samples including soft materials, nano-composites, and metals, under strain rates up to 108 s-1. Here we review experimental methods for high-velocity impacts spanning 15 orders of magnitude in projectile mass and compare method performances. This review aims to present a comprehensive overview of high-velocity microparticle impact techniques to provide a reference for researchers in different materials testing fields and facilitate experimental design in dynamic testing for a wide range of impactor sizes, geometries, and velocities. Next, we review recent studies using the laser-induced particle impact test platform comprising target, projectile, and synergistic target-particle impact response, hence demonstrating the versatility of the method with applications in impact protection and additive manufacturing. We conclude by presenting the future perspectives in the field of high-velocity impact.

The nature of polymer deformation depends on the ability of the chain segments to respond to the applied load at the imposed loading rate. When the polymer response time is significantly longer than the loading duration, the polymer responds in a brittle manner. Polystyrene, for example, is a brittle, glassy solid at room temperature and absorbs very little energy during deformation. Here we show unexpected, thickness and strain-rate-dependent deformation processes in thin polystyrene films at extreme axisymmetric tensile deformation rates. The impact of a supersonic micro-projectile initiates crazing, yielding, and adiabatic heating leading to extensive plastic flow of a load-bearing viscoelastic melt prior to perforation and film rupture. The less entangled, more mobile near-surface regions of these freestanding films favorably modify these processes, increasing the specific energy absorption as thickness decreases at the highest impact velocity. This results in unprecedented energy absorption at extreme strain rates in what is normally considered a brittle material.

Recent advances in the fabrication of nanostructures and nanoscale features in metasurfaces offer new prospects for generating visible light emission from low-energy electrons. Here we present the experimental observation of visible light emission from low-energy free electrons interacting with nanoscale periodic surfaces through the Smith–Purcell (SP) effect. We demonstrate SP light emission from nanoscale gratings with periodicity as small as 50 nm, enabling the observation of tunable visible radiation from low-energy electrons (1.5 to 6 keV), an order of magnitude lower in energy than previously reported. We study the emission wavelength and intensity dependence on the grating pitch and electron energy, showing agreement between experiment and theory. Our results open the way to the production of SP-based nanophotonics integrated devices. Built inside electron microscopes, SP sources could enable the development of novel electron–optical correlated spectroscopic techniques and facilitate the observatio...

Recent observations of the high-velocity impact response in poly (urethane urea), PUU, elastomers has inspired a new inquiry on whether enabling molecular mechanisms could benefit dynamic impedance optimization at the interface of a glass/polymer bilayer, particularly at the moment of impulse interaction. In this work, we investigate the molecular influence on dynamic impedance using microballistic measurements on two bulk elastomers, a PUU and a polyurea, PU. Upon impact at strain rates ∼108/s, PUU exhibits a moderate improvement in resistance against penetration than PU, that is more pronounced at higher speeds. The variation in dynamic stiffening corroborates well with the corresponding segmental dynamics data determined via broadband dielectric relaxation. Meanwhile, we calculate the shock impedance from the shock velocity data derived from the respective shock Hugoniot to discern the efficacy of dynamic impedance optimization between PUU and glass. New insight on molecular attributes will guide glass/polymer interface designs.

The dynamic deformation response of select model poly(urethane urea) elastomers (PUU) at high strain rates is investigated via an all-optical laser-induced projectile impact test (LIPIT). LIPIT measurements allow the direct visualization of the impact of micro-projectiles (silica spheres) on substrates and in-situ characterization, including depth of penetration and the extent of rebound of the micro-projectiles. PUUs are proven to be robust and the silica spheres are observed to rebound from them upon impact. In addition, for PUUs a strong correlation was noted between the coefficient of restitution and the maximum depth of penetration. Also, the coefficient of restitution data is comparable to that of glassy polycarbonate (PC), which is in great contrast to the comparison of the corresponding ambient storage modulus data obtained via dynamic mechanical analysis at 1 Hz. We hypothesize that high-rate deformation-induced glass transition is a plausible molecular relaxation mechanism towards macroscopic, dynamic stiffening/strengthening in PUUs. Keywords: Laser-induced particle impact test (LIPIT); Micro-ballistics; Poly(urethane urea) elastomers; Coefficient of restitution; Depth of penetration; Segmental dynamics; High-rate deformation-induced glass transition, United States. Army Research Office (Grant r W911NF-13-D-0001)

Metallic microparticles can acquire remarkable nanoscale morphologies after experiencing high velocity collisions, but materials science regarding the extreme events has been limited due to a lack of controlled experiments. In this work, collision dynamics and nonlinear material characteristics of aluminum microparticles are investigated through precise single particle collisions with two distinctive substrates, sapphire and aluminum, across a broad range of collision velocities, from 50 to 1,100 m/s. An empirical constitutive model is calibrated based on the experimental results, and is used to investigate the mechanics of particle deformation history. Real-time and post-impact characterizations, as well as model based simulations, show that significant material flow occurs during the impact, especially with the sapphire substrate. A material instability stemming from plasticity-induced heating is identified. The presented methodology, based on the use of controlled single particle impact data and constitutive models, provides an innovative approach for the prediction of extreme material behavior.

Active metasurfaces with novel visible and infrared (vis/IR) functionalities represent an exciting, growing area of research. Rectification of vis/IR frequencies would produce needed direct current (DC) with no inherent frequency limitation (e.g. no semiconducting bandgap). However, controlling the materials and functionality of (nano)rectennas for rectifying 100 s of THz to the visible regime is a daunting challenge, because of the small features and simultaneously the need to scale up to large sizes in a scalable platform. An active metasurface of a planar array of nanoscale antennas on top of rectifying vertical diodes is a 'nanorectenna array' or 'microrectenna array' that rectifies very high frequencies in the infrared, or even higher frequencies up to the visible regime. We employ a novel strategy for forming optical nanorectenna arrays using scalable patterning of Au nanowires, demonstrate strong evidence for spectral-selective high-frequency rectification, characteristic of optical antennas. We discover a previously unreported out-of-equilibrium electron energy distribution, i.e. hot electrons arising from plasmonic resonance absorption in an optical antenna characterized by an effective temperature, and how this effect can significantly impact the observed rectification.

We describe a high-speed single-shot multi-frame interferometric imaging technique enabling multiple interferometric images with femtosecond exposure time over a 50 ns event window to be recorded, following a single laser-induced excitation event. The stroboscopic illumination of a framing camera is made possible through the use of a doubling cavity that produces a femtosecond pulse train that is synchronized to the gated exposure windows of the individual frames of the camera. The imaging system utilizes a Michelson interferometer to extract phase and ultimately displacement information. We demonstrate the method by monitoring laser-induced deformation and the propagation of high-amplitude acoustic waves in a silicon nitride membrane. The method is applicable to a wide range of fast irreversible phenomena such as crack branching, shock-induced material damage, cavitation, and dielectric breakdown., U.S. Army Research Office (Contract W911NF-18-2-0048)

Extracting light from silicon is a longstanding challenge in modern engineering and physics. While silicon has underpinned the past 70 years of electronics advancement, a facile tunable and efficient silicon-based light source remains elusive. Here, we experimentally demonstrate the generation of tunable radiation from a one-dimensional, all-silicon nanograting. Light is generated by the spontaneous emission from the interaction of these nanogratings with low-energy free electrons (2–20 keV) and is recorded in the wavelength range of 800–1600 nm, which includes the silicon transparency window. Tunable free-electron-based light generation from nanoscale silicon gratings with efficiencies approaching those from metallic gratings is demonstrated. We theoretically investigate the feasibility of a scalable, compact, all-silicon tunable light source comprised of a silicon Field Emitter Array integrated with a silicon nanograting that emits at telecommunication wavelengths. Our results reveal the prospects of a CMOS-compatible electrically-pumped silicon light source for possible applications in the mid-infrared and telecommunication wavelengths., Extracting light from silicon is a longstanding challenge. Here, the authors report an experimental demonstration of free-electron-driven light emission from silicon nanogratings and investigates the feasibility of a compact, all-silicon tunable light source integrated with a silicon field emitter array.

This paper presents a novel approach to launch single microparticles at high velocities under low vacuum conditions. In an all-optical table-top method, microparticles with sizes ranging from a few microns to tens of microns are accelerated to supersonic velocities depending on the particle mass. The acceleration is performed through a laser ablation process and the particles are monitored in free space using an ultra-high-speed multi-frame camera with nanosecond time resolution. Under low vacuum, we evaluate the current platform performance by measuring particle velocities for a range of particle types and sizes, and demonstrate blast wave suppression and drag reduction under vacuum. Showing an impact on polyethylene, we demonstrate the capability of the experimental setup to study materials behavior under high-velocity impact. The present method is relevant to space applications, particularly to rendezvous missions where velocities range from tens of m/s to a few km/s, as well as to a wide range of terrestrial applications including impact bonding and impact-induced erosion.

Free electron radiation such as Cerenkov, Smith--Purcell, and transition radiation can be greatly affected by structured optical environments, as has been demonstrated in a variety of polaritonic, photonic-crystal, and metamaterial systems. However, the amount of radiation that can ultimately be extracted from free electrons near an arbitrary material structure has remained elusive. Here we derive a fundamental upper limit to the spontaneous photon emission and energy loss of free electrons, regardless of geometry, which illuminates the effects of material properties and electron velocities. We obtain experimental evidence for our theory with quantitative measurements of Smith--Purcell radiation. Our framework allows us to make two predictions. One is a new regime of radiation operation---at subwavelength separations, slower (nonrelativistic) electrons can achieve stronger radiation than fast (relativistic) electrons. The second is a divergence of the emission probability in the limit of lossless materials. We further reveal that such divergences can be approached by coupling free electrons to photonic bound states in the continuum (BICs). Our findings suggest that compact and efficient free-electron radiation sources from microwaves to the soft X-ray regime may be achievable without requiring ultrahigh accelerating voltages., Comment: 7 pages, 4 figures

Segmented polyurethanes exhibit versatile mechanical properties, where their outstanding resilience and toughness enable material designs with high energy-absorption capability. However, there is a lack of fundamental understanding regarding the underlying molecular pathways toward rapid dissipation of high-pressure fields. Here, we design a set of 4,4′-methylenediphenyldiisocyanate (MDI)−butanediol (BDO)−poly(tetramethylene oxide) (PTMO)-based polyurethanes and elucidate the influence of composition on thermal transition characteristics, crystallinity, segmental dynamics of PTMO, as well as high-strain-rate impact response on the microscale. Furthermore, simulations of shock compression, performed using an isotropic, constant-stress Hugoniostat method, comparing a MDI-BDO-PTMO-based polyurethane with polyethylene models of varying crystallinity suggest that the high-rate mechanical response is dominated by a soft domain response, which in turn can be sensitive to specific interactions present in the PTMO component that are not present in PE.

Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression., Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001)

This study investigates the incorporation of zirconia particles in polyurea matrix to form hybrid composites. Pristine zirconia particles in the monoclinic phase are first premixed with an oligomeric diamine−a reactant for polyurea−as an alternative method of surface functionalization that does not require the use of organosilanes. Interfacial interactions between the zirconia particles and polyurea are revealed via attenuated total reflectance-Fourier transform infrared spectroscopy, where a shift and broadening of the urea-carbonyl stretching and an increase in relative intensity of the corresponding disordered vs. ordered N–H stretching are shown in hybrid composites vs. bulk polyurea. The composition dependence of the viscoelastic relaxation characteristics and dynamic storage modulus in hybrid composites as determined by dynamic mechanical analysis are also elaborated upon. Additionally, we evaluate the potential of exploiting zirconia-polyurea hybrids to enabling dynamic impedance optimization at the interface of a select bi-layer construct, particularly when considering the material response under dynamic loading environments.

Deformation-induced glass transition in segmented elastomers has been proposed to allow highly desirable enhanced energy dissipation. In this study, we investigate the temperature-dependent microscale impact response of polyurea at a fixed impact velocity. We observe a local elevated impact energy absorption around 115 °C, which is attributed to the glass-to-rubber transition temperature under the present high-rate dynamic loading. Dielectric spectroscopy was performed, and the soft-segmental α2-relaxation was extracted and fit with a Havriliak-Negami function. The α2-relaxation frequency at 115 °C correlates well with an order-of-magnitude estimate of the equivalent frequency of deformation. This work further supports the importance of the dynamical Tg as an important consideration in the design of impact resistant materials., Army Research Office and Army Research Laboratory (Contract W911NF-18-2-0048)

We demonstrate single-shot multi-frame imaging of quasi-2D cylindrically converging shock waves as they propagate through a multi-layer target sample assembly. We visualize the shock with sequences of up to 16 images, using a Fabry-Perot cavity to generate a pulse train that can be used in various imaging configurations. We employ multi-frame shadowgraph and dark-field imaging to measure the amplitude and phase of the light transmitted through the shocked target. Single-shot multi-frame imaging tracks geometric distortion and additional features in our images that were not previously resolvable in this experimental geometry. Analysis of our images, in combination with simulations, shows that the additional image features are formed by a coupled wave structure resulting from interface effects in our targets. This technique presents a new capability for tabletop imaging of shock waves that can be extended to experiments at large-scale facilities.

Dynamic fracture of borosilicate glass through focusing of high-amplitude nanosecond surface acoustic waves (SAWs) at the micron scale is investigated in an all-optical experiment. SAWs are generated by a picosecond laser excitation pulse focused into a ring-shaped spot on the sample surface. Interferometric images capture the SAW as it converges towards the center, focuses, and subsequently diverges. Above a laser energy threshold, damage at the acoustic focal point is observed. Numerical calculations help us determine the time evolution of the stress distribution. We find that the glass withstands a local tensile stress of at least 6 GPa without fracture. Keyword: Dynamic fracture; Surface acoustic waves; Interferometry; Glass, United States. Army Research Office (Agreement W911NF-13-D-001 and W911NF-18-2-0048), United States. Office of Naval Research (Grant N000141512694), United States. Department of Energy (Award DE-SC0018091)

Materials with magneto-optic (MO) properties have enabled critical fiber-optic applications and highly sensitive magnetic field sensors. While traditional MO materials are inorganic in nature, new generations of MO materials based on organic semi-conducting polymers could allow increased versatility for device architectures, manufacturing options, and flexible mechanics. However, the origin of MO activity in semiconducting polymers is far from understood. In this paper, we report high MO activity observed in a chiral helical poly-3-(alkylsulfone)thiophene (P3AST), which confirms a new design for the creation of giant Faraday effect with Verdet constants up to (7.63±0.78)×104 deg T−1 m−1 at 532 nm. We have determined that the sign of the Verdet constant and its magnitude are related to the helicity of the polymer at the measured wavelength. The Faraday rotation and the helical conformation of P3AST are modulated by thermal annealing, which is further supported by DFT and MD simulations. Our results demonstrate that helical polymers exhibit enhanced Verdet constants, and expand the previous design space for polythiophene MO materials that was thought to be limited to highly regular lamellar structures. The structure property studies herein provide insights for the design of next generation MO materials based upon semiconducting organic polymers.

Scientific Reports 6: Article number: 25577; published online: 09 May 2016; updated: 16 February 2018 This Article contains a typographical error in the legend of Figure 2: “The vertical scale bars are 20 μm.” should read: “The vertical scale bars are 24 μm.” The calculations and results were unaffected by this error.

Swift electrons moving closely to a periodic structure can generate far-field radiation. The radiated light is usually polarized in the direction of electron propagation. We have demonstrated manipulation of this polarization with properly designed metasurfaces.

We derive and experimentally validate limits of electron radiation and energy loss. We show slow electrons generate stronger radiation than relativistic ones at subwavelength separations and bound states in the continuum enable order-of-magnitude radiation enhancement.

We experimentally demonstrate the generation of tunable radiation from silicon-only periodic structures in the near-infrared (up to 1550nm). Spontaneous emission from low-energy electrons (down to 2 keV) is recorded in a modified scanning electron microscope, matching our theoretical predictions.

In this study a single laser pulse spatially shaped into a ring is focused into a thin water layer, creating an annular cavitation bubble and cylindrical shock waves: an outer shock that diverges away from the excitation laser ring and an inner shock that focuses towards the center. A few nanoseconds after the converging shock reaches the focus and diverges away from the center, a single bubble nucleates at the center. The inner diverging shock then reaches the surface of the annular laser-induced bubble and reflects at the boundary, initiating nucleation of a tertiary bubble cloud. In the present experiments, we have performed time-resolved imaging of shock propagation and bubble wall motion. Our experimental observations of single-bubble cavitation and collapse and appearance of ring-shaped bubble clouds are consistent with our numerical simulations that solve a one-dimensional Euler equation in cylindrical coordinates. The numerical results agree qualitatively with the experimental observations of the appearance and growth of large bubble clouds at the smallest laser excitation rings. Our technique of shock-driven bubble cavitation opens interesting perspectives for the investigation of shock-induced single-bubble or multibubble cavitation phenomena in thin liquids.

Focusing of high-amplitude surface acoustic waves leading to material damage is visualized in an all-optical experiment. The optical setup includes a lens and an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern at the surface of a gold-coated glass substrate. Optical excitation induces a surface acoustic wave (SAW) that propagates in the plane of the sample and converges toward the center. The evolution of the SAW profile is monitored using interferometry with a femtosecond probe pulse at variable time delays. The quantitative analysis of the full-field images provides direct information about the surface displacement profiles, which are compared to calculations. The high stress at the focal point leads to the removal of the gold coating and, at higher excitation energies, to damage of the glass substrate. The results open the prospect for testing material strength on the microscale using laser-generated SAWs., United States. Army Research Office. Institute for Soldier Nanotechnologies (contract number W911NF-13-D-0001), Centre national de la recherche scientifique (France) (grant Projet International de Coopération Scientifique)

The emission of light from electrons passing near a grating includes resonant plasmonic features and Smith-Purcell collective excitations. We observe both and distinguish between them, finding surprising robustness to disorder in the Smith-Purcell radiation.

Recent advances in the fabrication of nanostructures and nanoscale features in metasurfaces offer a new prospect for generating visible, light emission from low energy electrons. In this paper, we present the experimental observation of visible light emission from low-energy free electrons interacting with nanoscale periodic surfaces through the Smith-Purcell (SP) effect. SP radiation is emitted when electrons pass in close proximity over a periodic structure, inducing collective charge motion or dipole excitations near the surface, thereby giving rise to electromagnetic radiation. We demonstrate a controlled emission of SP light from nanoscale gold gratings with periodicity as small as 50 nm, enabling the observation of visible SP radiation by low energy electrons (1.5 to 6 keV), an order of magnitude lower than previously reported. We study the emission wavelength and intensity dependence on the grating pitch and electron energy, showing agreement between experiment and theory. Further reduction of structure periodicity should enable the production of SP-based devices that operate with even slower electrons that allow an even smaller footprint and facilitate the investigation of quantum effects for light generation in nanoscale devices. A tunable light source integrated in an electron microscope would enable the development of novel electron-optical correlated spectroscopic techniques, with additional applications ranging from biological imaging to solid-state lighting., 16 pages, 4 figures

We present experimental results demonstrating multiple order Smith-Purcell radiation in high aspect ratio Silicon Nanowires structures using low-energy electrons (2.5–10keV). These produce emission spanning the visible, paving the way to a fully tunable ultraviolet source.

Electrons interacting with plasmonic structures can give rise to resonant excitations in localized plasmonic cavities and to collective excitations in periodic structures. We investigate the presence of resonant features and disorder in the conventional Smith-Purcell effect (electrons interacting with periodic structures) and observe the simultaneous excitation of both the plasmonic resonances and the collective excitations. For this purpose, we introduce a new scanning-electron-microscope-based setup that allows us to probe and directly image new features of electron-photon interactions in nanophotonic structures like plasmonic crystals with strong disorder. Our work creates new possibilities for probing nanostructures with free electrons, with potential applications that include tunable sources of short-wavelength radiation and plasmonic-based particle accelerators., United States. Army Research Office (Institute for Soldier Nanotechnologies. Contract W911NF-13-D-0001), Israel Science Foundation (Grant 1310/13), German-Israeli Project Cooperation, National Science Foundation (U.S.). Graduate Research Fellowship Program (Award 1122374), National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-1419807), Seventh Framework Programme (European Commission) (Grant 328853- MC-BSiCS)

Block copolymer (BCP) thin films have shown high potential as a pattern transfer medium for ultra-fine ( tens of millisecond time scale by creating a very large driving thermal gradient (estimated as 100–750 K/μm or, temporally, 3000–75,000 K/s), enhanced by incorporation of solvent vapor (here toluene) swelling of the BCP film. The extent of the thermal effects suggests that the role of solvent may extend beyond increasing the mobility of the BCP film to enhancing both the thermal gradient and also potentially the surface energy gradients, providing a thermocapillary shear mechanism. Further, enhanced domain alignment is greatest at higher scan speed, indicating as well the importance of the temporal thermal gradient.

This study investigates the molecular dependencies of dynamic stiffening and strengthening through comparison of high strain rate impact responses of various polyurethanes and polyureas. We use an in-house designed tabletop microimpact experimental platform—the laser-induced particle impact test—to perform high strain rate impacts and measure the corresponding material response. Dynamic mechanical analysis and differential scanning calorimetry are used to show that glass transition temperature is a useful predictor of the impact response at ambient temperatures. Meanwhile, solid-state nuclear magnetic resonance spectroscopy identifies segmental dynamics as an important determinant of the variation in both dynamic stiffening and strengthening. The impact responses of polyurethanes and polyureas both show clear dependencies on the molecular weight of the soft segment. This comparison suggests the state of intermolecular hydrogen bonding plays a key role in dynamic stiffening and strengthening. This study aims to identify the molecular dependencies of the impact response and establish a foundation for further design and testing of optimal high strain rate characteristics in synthetic elastomers. Keywords: materials analysis; impact testing; lasers; microscopy; velocity measurement; nuclear magnetic resonance; spectroscopy; polymers; mechanical testing

Prof. L. C. Kimerling Department of Materials Science and EngineeringMaterials Processing Center Massachusetts Institute of Technology 77 Massachusetts Ave, Cambridge, MA, 02139, USA Dr. J. Michel Materials Processing Center Massachusetts Institute of Technology 77 Massachusetts Ave, Cambridge, MA, 02139, USA Prof. E. L. Thomas Department of Mechanical Engineering and Materials Science Rice University 6100 Main Street, Houston, TX, 77005, USA E-mail: elt@rice.edu

Understanding high–velocity microparticle impact is essential for many fields, from space exploration to medicine and biology. Investigations of microscale impact have hitherto been limited to post–mortem analysis of impacted specimens, which does not provide direct information on the impact dynamics. Here we report real–time multi–frame imaging studies of the impact of 7 μm diameter glass spheres traveling at 700–900 m/s on elastomer polymers. With a poly(urethane urea) (PUU) sample, we observe a hyperelastic impact phenomenon not seen on the macroscale: a microsphere undergoes a full conformal penetration into the specimen followed by a rebound which leaves the specimen unscathed. The results challenge the established interpretation of the behaviour of elastomers under high–velocity impact., United States. Office of Naval Research (ONR DURIP Grant No. N00014-13-1-0676), United States. Army Research Office (Grant W911NF-13-D-0001)