Near-field Response Research Articles

Alloying: A Platform for Metallic Materials with On-Demand Optical Response.

Metallic materials with engineered optical properties have the potential to enhance the performance of energy harvesting and storage devices operating at the macro- and nanoscale, such as solar cells, photocatalysts, water splitting, and hydrogen storage systems. For both thin films and subwavelength nanostructures, upon illumination, the coherent oscillation of charge carriers at the interface with a dielectric material gives rise to resonances named surface plasmon polariton (SPP) and localized surface plasmon resonance (LSPR), respectively. These resonances result in unique light absorption, scattering, and transmission responses over the electromagnetic spectrum, which, in turn, can be exploited to tailor the behavior of active metallic components in optoelectronic devices containing Ag, Au, Cu, Al, Mg, among other metals. The wavelength in which the resonances occur primarily depends on the metal itself (i.e., the dielectric function or permittivity), the dielectric medium surrounding the metals, and the size, geometry, and periodicity of the metallic nanostructures. Nevertheless, the aforementioned parameters allow a limited modulation of both SPP and LSPR over a narrow window of frequencies. To overcome this constraint, we have proposed and realized the alloying of metals via physical deposition methods as a paradigm to almost arbitrarily tuning their optical behavior in the UV-NIR, which leads to permittivity values currently not available. Our approach offers an additional knob, chemical composition, to engineer light-matter interactions in metallic materials. This Account highlights recent progress in using alloying as a pathway to control the optical behavior of metallic thin films and nanostructures for energy harvesting and storage applications, including (photo)catalysis, photovoltaics, superabsorbers, hydrogen storage, among other systems. We choose to primarily focus on the optical properties of the metallic mixtures and in their near- to far-field responses in the UV-NIR range of the spectrum as they represent key parameters for materials' selection for the devices above. By alloying, it is possible to obtain metallic materials with LSPR not available for pure metals, which can enable the further control of the electromagnetic spectrum. First, we discuss how the permittivity of binary mixtures of coinage metals (Au, Ag, and Cu) can be tailored based on the chemical composition of their pure counterparts. Second, we present how novel metallic materials can be designed through band structure engineering through density functional theory (DFT), a paradigm that could benefit from artificial intelligence methods. Concerning alloyed thin films, we discuss the promise of earth-abundant metals and provide an example of the superior performance of AlCu in superabsorbers. In the realm of nanostructures, we focus the discussion on physical deposition methods, where we provide a detailed analysis of how chemical composition can affect the far- and near-field responses of metallic building blocks. Finally, we provide a brief outlook of promising next steps in the field.

Effects of finite water depth and lateral confinement on ships wakes and resistance

Wash waves produced by ships disintegrate river banks and coastal lines. This phenomenon of bank erosion is mainly due to the height of the waves. Various factors govern the formation of these waves and their amplitudes: the geometry of the water channel, the shape and the speed of the boat, etc.. These factors play an important role on the wave generation, in addition on the resistance of the ship and so on its fuel consumption. Whether to study the impact of wash waves on the ship's environment or its resistance, the analysis of the generated wake is essential. Hence a fine characterization of the wave field is necessary. This study proposes a comparison of wakes generated by two generic ships based on a Wigley hull with block coefficients 0.67 and 0.89 respectively representative of maritime and fluvial ships. The wakes generated in deep water and confined water configurations have been measured for different Froude numbers by a non-intrusive optical stereo-correlation method, giving access to a detailed and complete definition of the generated wave fields. The resistance of the ship hulls has been measured in deep and confined water configurations with a hydrodynamic balance. The results permit one to study the influence of both hull and water channel geometries on the ship wake, on the amplitude of the far-field generated waves and on the near-field hydrodynamic response. Moreover, resistance curves are obtained for both configurations and highlight the effect of both hull and water channel geometries on the resistance coefficient of the ship. A comparison of the resistance curves with or without the ship trim is conducted and shows the influence of the trim on the resistance coefficient in the different ship speed regimes.

Multi-functional terahertz metamaterial using symmetrical and asymmetrical electric split-ring resonator

Abstract We present the electromagnetic coupling effects in four types of electric split-ring resonators (eSRRs). These four designs of eSRRs are based on SRR tailored with single-, dual-, triple-, and quad-split, respectively. The near-field electromagnetic coupling responses indicate the resonance caused from the inductive-capacitive (LC) resonance between eSRRs. According to the patterns of eSRR are symmetrical or asymmetrical configurations, these eSRRs exhibit polarization-dependence, polarization-independence, resonant frequency shifting, resonant intensity modulation, and resonant switching characteristics, respectively. To increase the flexibility and applicability of eSRRs, two patterned eSRR layers are designed on elastic substrate for the enhancement of the resonant tunability. The designs of eSRRs open an avenue to a new direction for future optical applications, which offer significant promise in flexible electronics applications. This work provides future tunable THz designs based on flexible actuation mechanism to deform the geometrical dimensions between eSRRs microstructures.

(Invited) Nano-FTIR Correlation Nanoscopy for Organic and Inorganic Material Analysis

Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) is a scanning probe approach to optical microscopy and spectroscopy, bypassing the ubiquitous diffraction limit of light to achieve a spatial resolution below 20 nanometers. s-SNOM employs the strong confinement of light at the apex of a sharp metallic atomic force microscopy (AFM) tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip enables the extraction of the optical properties (dielectric function) of the sample directly below the tip and yields nanoscale resolved images simultaneous to topography [1]. In addition, the technology has been advanced to enable Fourier-Transform Infrared Spectroscopy on the nanoscale (nano-FTIR) [2] using broadband radiation from the visible spectral range to THz frequencies. Recently, the combined analysis of complex nanoscale material systems by correlating near-field optical data with information obtained by other scanning probe microscopy (SPM)-based measurement methodologies has gained significant interest. For example, the material-characteristic nano-FTIR spectra of a phase-separated polystyrene/low-density polyethylene (PS/LDPE) polymer blend verifies sharp material interfaces by measuring a line profile across a ca. 1 µm sized LDPE island (Fig1). Near-field reflection/absorption imaging at 1500cm-1 of the ca. 50nm thin film allows to selectively highlight the distribution of PS in the blend and simultaneously map the mechanical properties like adhesion of the different materials [3,4]. Further, results will be presented that correlate the near-field optical response of semiconducting samples like graphene (2D) or functional SRAM devices (3D) in different frequency ranges (mid-IR & THz) to Kelvin Probe Force Microscopy (KPFM) measurements. Thus, s-SNOM systems represent an ideal platform to gain novel insights into complex material systems by different near-field and AFM-based method. [1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004). [2] F. Huth, et al., Nano Lett. 12, 3973 (2012). [3] B. Pollard, et al., Beilstein J. of Nanotechn. 7, 605 (2016). [4] I. Amenabar, et al., Nature Commun. 8, 14402 (2017). Figure caption: Fig 1. Near-field correlation nanoscopy of a thin PS/LDPE polymer film, highlighting the phase separation of the materials by nano-FTIR measurements as well as the different mechanical properties of the polymers. Figure 1

Effects of stimulus cosine onset properties on bottlenose dolphin (Tursiops truncatus) auditory brainstem responses.

Although the auditory brainstem response (ABR) is known to be an onset response, the specific relationship between stimulus onset properties and the resulting ABR is not well understood. In this study, the effects of stimulus onset on dolphin ABR were examined by measuring ABRs in six bottlenose dolphins while systematically manipulating rise time and plateau sound pressure of cosine-enveloped noise bursts. Noise bursts were spectrally "pink" with frequency content from 10 to 160 kHz, rise times from 32 μs to 4 ms, and plateau sound pressure levels from 102 to 138 dB re 1 μPa. Envelope rise time and plateau sound pressure alone were found to be poor predictors for ABR peak amplitudes and latencies. Peak amplitudes were well described by the envelope sound pressure at the end of a 260-μs window; however, best-fits to the data across ABR peaks were obtained when the window start time was allowed to vary. Peak latencies were best described by the maximum value of the second derivative of the pressure envelope. These results are consistent with single-unit and nearfield response data for terrestrial mammals and indicate that stimuli with rise times greater than 260 μs are non-optimal with respect to maximizing ABR amplitudes.

Underwater explosion of slender explosives: Directional effects of shock waves and structure responses

The shape of a high-explosive is an important factor to consider in the near-field and contact explosion, but this has been rarely studied previously. In this paper, we first conduct experiments to study the directional effects of the underwater explosion shock wave of slender explosives. A numerical model based on the multi-material arbitrary Lagrangian Eulerian (MM–ALE) technique is then developed to study the influence of the explosive slender ratio on the generated shock waves. The numerical model is validated by experiment data. Then, tests are conducted with constant-mass cylindrical explosives with slender ratios ranging from 2.0 to 9.2. By comparing with the center-detonated spherical explosive, the peak pressure, pulse duration, impulse and energy flux of the underwater explosion of slender explosives are analyzed in detail. Furthermore, the effectiveness of the fluid-structure interaction algorithm is verified through the experimental data of a plate subjected to underwater contact explosion. Finally, the response of plates subjected to the underwater explosion of a slender explosive at different azimuths is studied. The interaction between the plate and the directional loads is analyzed. We conclude that the loads and structure responses of the near-field underwater explosion of slender explosives are different from those of spherical explosives. The analyses and results provide a reference for the near-field underwater explosion loads and structural response studies.

Near-Field Spectral Response of Optically Excited Scanning Tunneling Microscope Junctions Probed by Single-Molecule Action Spectroscopy.

The near-field spectral response of metallic nanocavities is a key characteristic in plasmon-assisted photophysical and photochemical processes. Here, we show that the near-field spectral response of an optically excited plasmonic scanning tunneling microscope (STM) junction can be probed by single-molecule reactions that serve as a nanoscale sensor detecting the local field intensity. Near-field action spectroscopy for the cis ↔ cis tautomerization of porphycene on a Cu(110) surface reveals that the field enhancement in the STM junction largely depends on microscopic structures not only on the tip apex, but also on its shaft. Using nanofabrication of Au tips with focused ion beam, we show that the spectral response is strongly modulated through the interference between the localized surface plasmon in the junction and propagating surface plasmon polariton generated on the shaft. Furthermore, it is demonstrated that the near-field spectral response can be manipulated by precisely shaping the tip shaft.

Broadband coherent hyperspectral near-field imaging of plasmonic nanostructures.

We develop a coherent hyperspectral near-field microscope using a combined nano-Fourier Transform Infra-Red (FTIR) spectroscope and a scattering Scanning Near-field Optical Microscope (s-SNOM) illuminated by an ultra-broadband few-cycle femtosecond source, spanning a spectrum from 660 to 1050 nm. Using this spatio-spectral approach, we resolve hyperspectral near-field response of a single plasmonic nano-antennas over 450 nm bandwidth with a spatial resolution of 40 nm and a spectral resolution of 50 cm-1. In particular, we identify the electric near-field spatial distribution of the dipole resonant mode of various nano-antennas and observe, in accordance with previous theoretical reports, that those are spectrally red-shifted from their far-field response. Moreover, we are able to spectrally and spatially differentiate the near-field distribution of the dipole and quadrupole modes at the single nanoparticle level. Being coherent and short-pulsed, our technique opens the path for optical ultrafast characterization and control of light-matter interaction at the nanoscale.

Thermo-acoustic radiation of free-standing nano-thin film in viscous fluids

In this paper, the theory of thermo-acoustic wave emission and propagation is further generalized to consider media viscosity effects. The fully coupled thermo-acoustic field theory and solution for a free-standing nano-thin film in viscous fluid are further developed where fluid viscosity was usually ignored in many early works on thermo-acoustic interaction. The effects of heat loss, heat capacity and heat exchange are also considered in this investigation. In addition to examining the double frequency effect when a sinusoidal alternating current acts on the nano thin film, we have successfully decoupled the thermal wave propagation from the acoustic effect by reformulating the governing differential equations. With appropriate simplification, an analytical prediction for near-field and far-field acoustic pressure response with viscosity effect is derived. We illustrate in some practical examples that the analytical predictions are in excellent agreement with experiments. Further referring to some past works, an analytical solution of higher-order accuracy with respect to the wavenumber omitted in the previous studies is obtained. Although the viscous effect may have limited influence in the vicinity of the thermo-acoustic source, it is vital for the subsequent propagation of acoustic waves. This work may have shown great potentials for the design of thermo-acoustic projectors for new underwater sonars as compared with the electro-acoustic traditional designs that are induced by membrane vibrations.

Fundamental Limits to Near-Field Optical Response over Any Bandwidth

We develop an analytical framework to derive upper bounds to light--matter interactions in the optical near field, where applications ranging from spontaneous-emission amplification to greater-than-blackbody heat transfer show transformative potential. Our framework connects the classic complex-analytic properties of causal fields with newly developed energy-conservation principles, resulting in a new class of power--bandwidth limits. These limits demonstrate the possibility of orders-of-magnitude enhancement in near-field optical response with the right combination of material and geometry. At specific frequency and bandwidth combinations, the bounds can be closely approached by canonical plasmonic geometries, with the opportunity for new designs to emerge away from those frequency ranges. Embedded in the bounds is a material "figure of merit," which determines the maximum response of any material (metal/dielectric, bulk/2D, etc.), for any frequency and bandwidth. Our bounds on local density of states (LDOS) represent maximal spontaneous-emission enhancements, our bounds on cross density of states (CDOS) limit electromagnetic-field correlations, and our bounds on radiative heat transfer (RHT) represent the first such analytical rule, revealing fundamental limits relative to the classical Stefan--Boltzmann law.

Cooperative field localization and excitation eigenmodes in disordered metamaterials

We investigate numerically and experimentally the near-field response of disordered arrays comprising asymmetrically split ring resonators that exhibit strong cooperative response. Our simulations treat the unit cell split ring resonators as discrete pointlike oscillators with associated electric and magnetic point dipole radiation, while the strong cooperative radiative coupling between the different split rings is fully included at all orders. The methods allow to calculate local field and Purcell factor enhancement arising from the collective electric and magnetic excitations. We find substantially increased standard deviation of the Purcell-enhancement with disorder, making it increasingly likely to find collective excitation eigenmodes with very high Purcell factors that are also stronger for magnetic than electric excitations. We show that disorder can dramatically modify the cooperative response of the metamaterial even in the presence of strong dissipation losses as is the case for plasmonic systems. Our analysis in terms of collective eigenmodes paves a way for controlled engineering of electromagnetic device functionalities based on strongly interacting metamaterial arrays.

Hybrid nanostructure of SiO2@Si with Au-nanoparticles for surface enhanced Raman spectroscopy.

In this study, a structure of large-area orderly-arranged SiO2@Si core-shell nanoparticles decorated with Au nanoparticles was fabricated for surface-enhanced Raman spectroscopy (SERS). This hybrid structure features light confinement in the Si shells and a uniform distribution of localized electric hot spots. FDTD simulations were carried out to examine the near-field enhancement response of this structure. Results indicate that the strongly enhanced local electric field is attributed to the WGM-LSPR coupling, that is, the coupling of the whispering gallery mode (WGM) of Si nanoshells with the localized surface plasmon resonance (LSPR) of Au nanoparticles. The excitation of WGM comes primarily from the magnetic response of the Si shell with a minor modification by its electric response. The WGM-LSPR coupling of the structure is tunable through the change of geometric parameters of SiO2@Si particles. Raman scattering measurements were conducted on the samples fabricated, which agree well with the simulated results. The measured data gave a SERS G factor of ∼2 × 108 and showed highly sensitive and reproducible SERS signals of R6G with a high spatial uniformity on a 2 × 2 cm2 substrate consisting of an array of SiO2@Si (D = ∼220 nm/290 nm) particles whose outer surfaces were scattered with d = ∼20 nm Au particles.

Near-Field and Far-Field Thermal Emission of an Individual Patch Nanoantenna.

The far-field spectral and near-field spatial responses of an individual metal-insulator-metal nanoantenna are reported, using thermal fluctuations as an internal source of the electromagnetic field. The far-field spectra, obtained by combining Fourier transform infrared spectroscopy with spatial modulation based on a light falloff effect in a confocal geometry, have revealed two distinct emission peaks attributed to the excitation of the fundamental mode of the nanoantenna at two distinct wavelengths. Superresolved near-field images of the thermally excited mode have been obtained by thermal radiation scanning tunneling microscopy. Experimental results are supported by numerical simulations showing that it is possible to excite the same mode at different wavelengths near a resonance of the insulating dielectric material forming the antenna.

Direct Characterization of Near-Field Coupling in Gap Plasmon-Based Metasurfaces.

Metasurfaces based on gap surface-plasmon resonators allow one to arbitrarily control the phase, amplitude, and polarization of reflected light with high efficiency. However, the performance of densely packed metasurfaces is reduced, often quite significantly, in comparison with simple analytical predictions. We argue that this reduction is mainly because of the near-field coupling between metasurface elements, which results in response from each element being different from the one anticipated by design simulations, which are commonly conducted for each individual element being placed in an artificial periodic arrangement. In order to study the influence of near-field coupling, we fabricate meta-elements of varying sizes arranged in quasi-periodic arrays so that the immediate environment of same size elements is different for those located in the middle and at the border of the arrays. We study the near-field using a phase-resolved scattering-type scanning near-field optical microscopy (s-SNOM) and conducting numerical simulations. By comparing the near-field maps from elements of the same size but different placements we evaluate the near-field coupling strength, which is found to be significant for large and densely packed elements. This technique is quite generic and can be used practically for any metasurface type in order to precisely measure the near-field response from each individual element and identify malfunctioning ones, providing feedback to their design and fabrication, thereby allowing one to improve the efficiency of the whole metasurface.

Tip-Enhanced Second Harmonic Generation: an Approach for Hyper-Raman Spectroscopy

The near-field optical response generated at the apex of a scanning probe in tip-enhanced Raman spectroscopy (TERS) enhances and confines the interaction of light and matter to a few nanometers, thus breaking the diffraction limit of light while preserving the wealth of chemical and structural information. With the way paved to deploy Raman spectroscopy to surface nanostructures, we now investigate the most favorable experimental conditions and possible limitations for tip-enhanced hyper-Raman spectroscopy (TEHRS). In this two-photon-one phonon process, detected around the second harmonic of the excitation wavelength, selection rules different from those of the conventional Raman effect apply, thus holding promise for an almost comprehensive vibrational signature of the sample. TEHRS signal intensity critically depends on the second harmonic generation (SHG) efficiency. The aim of this work is to outline the experimental conditions that are best suited to obtain TEHRS signals in the challenging context of an extremely low scattering cross section. Using a silver tip and non-centrosymmetric PbTiO3 film on platinum substrate as a sample system in our simulations, we show a strong dependence of the intensity of SHG on the polarization of the incident light, the tip radius, the presence of a non-centrosymmetric sample, and the excitation power. We further discuss the effect of tip-sample distance for early-stage (proof of concept) TEHRS investigations and found 40% of total power dissipation for 1 nm distance.

Dependence of bottlenose dolphin (Tursiops truncatus) auditory brainstem responses on noise burst rise time, amplitude, and envelope shape

A series of experiments examined the dependence of dolphin auditory brainstem response (ABR) peak amplitudes and latencies by stimulus level, rise times, and envelope shape. Stimuli were spectrally “pink” noise bursts with frequency content from 10 to 160 kHz, rise times that varied from 32 μs to 4 ms, and plateau sound pressure levels from 102 to 138 dB re 1 μPa. Cosine and cosine-cubed rise (and fall) envelopes were tested, and results were compared to those obtained for dolphins using noise bursts with linear rise envelopes [Finneran et al., J. Acoust. Soc. Am. 143, 2914–2921 (2018)]. Results for the cosine and cosine-cubed envelopes showed that ABR peak amplitudes were a function of envelope sound pressure at the end of a fixed integration window of approximately 260 μs. This is congruent with the previous findings for linear envelopes. The ABR peak latencies for the cosine and cosine-cubed envelopes were dependent on the second derivative of the envelope pressure function, as opposed to the first derivative of the pressure function for linear envelopes. These data are consistent with single-unit and nearfield response data for terrestrial mammals. [Work supported by Navy Living Marine Resources and SSC Pacific Naval Innovative Science and Engineering Programs.]

How to calculate the pole expansion of the optical scattering matrix from the resonant states

We present a formulation for the pole expansion of the scattering matrix of open optical resonators, in which the pole contributions are expressed solely in terms of the resonant states, their wavenumbers, and their electromagnetic fields. Particularly, our approach provides an accurate description of the optical scattering matrix without the requirement of a fit for the pole contributions or the restriction to geometries or systems with low Ohmic losses. Hence, it is possible to derive the analytic dependence of the scattering matrix on the wavenumber with low computational effort, which allows for avoiding the artificial frequency discretization of conventional frequency-domain solvers of Maxwell's equations and for finding the optical far- and near-field response based on the physically meaningfull resonant states. This is demonstrated for three test systems, including a chiral arrangement of nanoantennas, for which we calculate the absorption and the circular dichroism.

Polarity of the Fano Resonance in the Near‐Field Magnetic‐Dipole Response of a Dielectric Particle Near a Conductive Surface

AbstractFano resonances are observed in the near‐field electric‐field spectra of dielectric particles sustaining magnetic dipole resonances and located in the vicinity of a conductive surface (reflector). The polarity of these Fano resonances is defined by the relative positions of the resonant particle, the observation points, and the reflector. This paper studies two cases corresponding to common near‐field measurement techniques, where the electric field is detected by aperture‐based probes and scattering probes. In the first case, as the particle moves away from the reflector, the polarity of the observed Fano resonance reverses every quarter‐of‐a‐wavelength of the particle's magnetic resonance. In the second case, in addition to the periodic change of polarity, a phase shift of π is detected between the fields detected at the opposite sides of the resonant particle. This work proves, theoretically, that the observed Fano resonance polarities arise due to the interference between the external electric‐field standing wave and the resonant fields generated by the particle's magnetic dipole moment. The findings are supported by a recent terahertz experiment and the gigahertz experiment presented in this paper.

Spatial and Frequency Selective Plasmonic Metasurface for Long Wavelength Infrared Spectral Region

AbstractThe development of novel approaches that control absorption and emission operating in the long wavelength infrared (LWIR) spectral region is of fundamental importance for many applications, such as remote temperature sensing, thermal imaging, radiation cooling, environmental monitoring, and night vision. A high performance plasmonic metasurface–based absorber for the LWIR spectral region is presented. In the design, a pyroelectric thin film, poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) copolymer, is introduced as spacer, that offers the device not only multiple selective high absorption bands but also promising potential for application in optoelectronics. The angle‐resolved optical responses show that the absorption effect is sensitive to the incident angles and can be controlled by the periodicity, indicating that the design can function as optical devices with directional and frequency‐selective absorption/emission characteristics. By employing near‐field optical microscopy, both the near‐field amplitude and phase optical responses of the absorber are investigated at resonant wavelength, thereby providing direct experimental evidence to verify the nature of the absorption effect. To further demonstrate the versatility of the design, a particular metasurface patterned by the building blocks of the absorber is fabricated. 2D hyperspectral images show that such a patterned structure exhibits both frequency and spatially selective absorption.

Underwater explosion of cylindrical charge near plates: Analysis of pressure characteristics and cavitation effects

In this paper, a coupled scheme utilizing advantages of the Runge–Kutta discontinuous Galerkin (RKDG) method and finite elements is applied to investigate cavitation induced by rarefaction waves during a near-field underwater explosion of cylindrical charge. A high-order RKDG method has advantages of an accurate shock capturing. So, it was used to solve a governing Eulerian equation for a compressible fluid. A finite-element method (FEM) was suitable to deal with problems of a shock response of structures and, therefore, applied for structural analysis. The suggested method was used to study pressure characteristics and cavitation effects of underwater explosions of cylindrical charges near single/double plates. First, a cavitation model was introduced in the RKDG method, and a numerical model of a high-pressure bubble in a cylinder was developed. The obtained numerical results were compared with the known solution in order to verify the validity of the suggested method. Second, a RKDG-FEM model of underwater explosion of a spherical charge near a plate was developed; its results for maximum deflection at the centre of the plate were compared with experimental data to prove the effectiveness of the coupled algorithm. Then, this algorithm was employed to simulate the process of underwater explosions of cylindrical charges near a single plate. Here, effects of different parameters - thickness of the plate and a distance between the charge and the plate - on pressure and cavitation characteristics were studied. Finally, a numerical model of double plates subjected to a near-field underwater explosion was developed. Cavitation evolution and its effect on shock-wave loading were analysed. Additionally, the effect of the distance between two plates was studied. The suggested analysis and its results provide a reference for load characteristics of near-field underwater explosions and shock response of structures.