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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.

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.

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

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)

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.

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)

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)

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.

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)

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

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

Highly-localized focused laser spike (FLaSk) heating of polymer thin films is a resist- and developer-free alternative to 2D laser direct write for creating patterns on the single micron or, by exploiting overlap effects, submicron scale. The massive temporal and spatial thermal gradients and resulting thermal Marangoni stresses generated by FLaSk are an effective means for the directed dewetting and patterning of such films. Here, the general applicability of this technique to glassy amorphous polymer thin film systems is investigated through systematic investigation of film thickness, glass transition temperature, and polymer mobility. The results reveal that the important parameters are the film thickness (coupled to the optical heating effects through anti-reflection coating effects) and the high-temperature polymer melt mobility, allowing for generation of single features with linewidths of down to ~1 μm. Further, the introduction of spatial mobility variations by using polymer brushes, bilayers, and microphase separated block copolymers leads to additional profile manipulation effects (i.e. spontaneous 2D pattern generation and flattened top profiles).

A series of semicrystalline block copolymers (BCPs), poly(4-vinylpyridine)-block-poly(ε-caprolactone) (P4VP-PCL), with lamellar phases have been synthesized. P4VP-PCL BCP thin films with large-scale, oriented lamellar microdomains were obtained by rimming coating process followed by oscillated shearing using a homemade shear device. Owing to the vitrified P4VP microdomains and strongly segregated microphase separation, specific PCL crystalline chain orientation can be formed from the growth of anisotropic PCL crystallites under confinement so as to uniformly increase the birefringence of the BCP thin films. The enhanced birefringence corresponds well with the increase of PCL crystallinity. Consequently, the birefringence of the P4VP-PCL thin-films can be fine-tuned by PCL crystallization. The variation on the birefringence of the BCP thin films attributed to crystallization and melting is a reversible process with respect to temperature. The BCP thin films can thus be used as temperature-stimulated materials with controllable birefringence via crystallization kinetics.

The CO oxidation reaction is studied in this work using a controlled structure consisting of Au/CeO 2 multilayers, in the form of towers of 10 x 10 μm 2 base, with layer thicknesses in the nanometer range. This structure is prepared on a 3" Si wafer using reverse photolithography and vapor deposition in an e-beam chamber. The thickness of each layer is of nanometer dimensions and only the edges of the Au and CeO 2 layers are exposed to the reaction gas mixture. The CO oxidation reaction rate is found to scale with the total length of the Au/CeO 2 interfaces for nanotowers with the same total Au and CeO 2 surface areas. TEM and X-ray diffraction (XRD) analyses reveal highly stressed gold films in the nanotowers, the lattice strain being temperature and film thickness dependent. Deactivation with time-on-stream is commensurate with relaxation of the gold films, as measured by a drop in their lattice strain.

Interference lithography (IL) holds the promise of fabricating large-area, defect-free 3D structures on the submicrometer scale both rapidly and cheaply. A stationary spatial variation of intensity is created by the interference of two or more beams of light. The pattern that emerges out of the intensity distribution is transferred to a light sensitive medium, such as a photoresist, and after development yields a 3D bicontinuous photoresist/air structure. Importantly, by a proper choice of beam parameters one can control the geometrical elements and volume fraction of the structures. This article provides an overview of the fabrication of 3D structures via IL (e.g., the formation of interference patterns, their dependence on beam parameters and several requirements for the photoresist) and highlights some of our recent efforts in the applications of these 3D structures in photonic crystals, phononic crystals and as microframes, and for the synthesis of highly non spherical polymer particles. Our discussion concludes with perspectives on the future directions in which this technique could be pursued.

We investigated the acoustic properties in the sub-GHz frequency regime of a multilayer system comprising alternating 100-nm scale TiO2/poly(methyl methacrylate) (PMMA) layers through a laser photoacoustic method, impulsive stimulated thermal scattering (ISTS). The acoustic dispersion curves were determined, and the mechanical properties were extracted from the experimental results.

We demonstrate that in-plane 2D propagation and focusing of a laser-induced shock wave result in enhanced nano-crystallization of highly ordered pyrolytic graphite. Throughout the 2D shock focusing technique, which enables to clearly distinguish between the laser-induced and the shock-induced transformation/transition, our findings establish the role of the shock wave during the transformation/transition process. This configuration could open the way to an alternative path for laser shock fabrication of graphitic compounds and would give access to real time investigation of shock waves mediated phase transitions., United States. Army Research Office (W911NF-13-D-0001)

Single-wall carbon nanotubes (SWNTs) were fluorinated with F2 at 250 °C, and then some were heated under He to temperatures ranging from 100 to 400 °C to desorb the fluorine. The resulting samples were studied with X-ray photoelectron spectroscopy, Raman spectroscopy, IR spectroscopy, and electrical resistance measurements. The goals were to identify, as a function of fluorination and defluorination, the nature of the fluorine bonding, the damage to the tubes, the changes to the average tube diameter and proportions of metallic and semiconducting tubes, and the effects on the electrical conductivity and its mechanism. The stoichiometry of the as-fluorinated tubes was CF0.43. Most of the time, F atoms bonded covalently to one carbon atom. Smaller amounts of CF2 and CF3 were probably bonded primarily at various defect sites. The fluorinated single-wall nanotubes were highly insulating. Upon heating, the largest fluorine loss occurred between 200 and 300 °C, and defluorination was virtually complete at 400 °...

Two examples of selective electrochemical modification of individual nanoscale objects are presented, namely the electroless metallization of a plant virus, and the coupling of substituted phenyl residues to single-wall carbon nanotubes (SWCNTs). The electroless deposition of metal was achieved selectively on the inner or outer surface of tobacco mosaic virus (TMV) particles. Covalent modification of SWCNTs, deposited on a Si/SiO2 substrate, was performed successfully via reductive coupling of a 4-nitrobenzenediazonium salt in non-aqueous medium. An organic layer with a thickness of up to several nanometers could be deposited on isolated tubes and thin bundles. © 2002 Elsevier Science B.V. All rights reserved.

An overview of our latest findings on the photoionization of metallocarbohedrene clusters (Met-Cars) is presented. The ionization potentials (IPs) of titanium and zirconium Met-Cars of the stoichiometry Ti8−x Zr x C12 (x=0–4, 8) were determined by studying their photoionization efficiency spectra near threshold. It was found that the IPs are about 4 eV and decrease smoothly as the Ti/Zr ratio decreases. Comparison of the determined IPs with those predicted by theoretical calculations by other groups suggests that the geometry of the Met-Cars is that of the tetrahedral cage structure with T d symmetry, assuming a high degree of accuracy in the IP calculations. In another experiment, we investigated the mechanism of the delayed ionization of both single- and binary-metal Met-Cars. Using a reflectron time-of-flight mass spectrometer with a modified Wiley–McLaren double acceleration region, we measured ionization rates arising from the electron emission of Met-Cars composed of various transition metals under different photoionization conditions, which led to the conclusion that a thermionic emission mechanism dominantly contributes to the delayed ionization.

The 532 nm photodissociation of the three singly substituted, binary metal containing Metallocarbohedrenes, Ti7MC12+ (M=Y, Zr, or Nb), is investigated. The photofragments are identified through energy analysis of the fragment ions in a reflectron time-of-flight mass spectrometer, a new technique in photodissociation studies that results in a more accurate identification of the photofragment mass than what would normally be achievable through mass analysis using time-of-flight determinations. The findings reveal that the dominant mechanism proceeds through the loss of neutral Ti atoms for all clusters studied.

We apply time-resolved interferometric imaging to study laser-driven focused shock waves on the microscale. Shock waves are generated in a 10 μm-thick layer of water by sub-nanosecond laser pulses focused into a ring of 100 μm radius. Imaging is performed with a Mach-Zehnder interferometer by time-delayed femtosecond pulses. We obtain a series of images tracing the converging shock wave as it collapses to a focal point and then reemerges as a divergent shock wave eventually leaving behind a cavitation bubble at the focus. Quantitative analysis of interferograms yields density and shock velocity values that match the water Hugoniot data found in the literature. In a separate development, we captured the propagation of cracks in a glass substrate initiated by focused shock waves. The results open the prospect of spatially resolved studies of shock-compressed materials in a small-scale all-optical experiment.

However, up to now all of thesechemicalmodificationmethodswereperformedonlyonbulkSWCNTmaterial,notonindividualtubes.Herein we describe a versatile approach to the electro-chemicalmodificationofindividualcarbonnanotubes(smallSWCNT bundles), as demonstrated bythe attachment ofsubstituted phenyl groups. The electrochemical approach isparticularlysuitable for the chemical alteration of single-molecular objects, as their electrochemical potential, whichdeterminestheextentofreaction,canbedirectlyadjustedbyanappliedpotential.Ourresultsshowthatfunctionalgroupscan be controllablyattached to appropriatelycontactednanotubes,whichresultsinhomogeneousmolecularcoatingsofuptoseveralnanometersinthickness.Wepresenttwotypesof coupling reactions, working under oxidative (anodic) orreductive (cathodic) conditions. The corresponding electro-chemical reactions are illustrated schematicallyin Figure1.Inbothcasesaradicalspeciesisproducedonthesurfaceofthe nanotube, which attacks the carbon lattice to form acovalent bond. When the potential is applied for a suitablelength of time, polymerization of the radicals leads tomultilayer coating. This situation is similar to the multi-layer growth of aryl films observed in the electrochemicalmodification of highly oriented pyrolytic graphite(HOPG).

Lithographic alternatives to conventional layer-by-layer processes for the design of 3D structures such as photonic or phononic crystals often present a dichotomy: patterning control versus patterning area. We demonstrate a combined technique of large area interference lithography and local area direct write focused laser spike (FLaSk) annealing that can enable the microscale patterning of hierarchical structures defined in their morphology by the interference and defined in placement and shape by the direct write. This is accomplished by doping a commercial chemically amplified photoresist (SU-8) with an absorbing dye to provide thermal activation at a wavelength shifted from that causing UV crosslinking. In this way, the necessary post-exposure bake to complete the crosslinking of the resist is locally performed by the FLaSk laser, rather than globally on a hot plate. By utilizing the same experimental setup as used by a 3D direct write system, it is possible to integrate another level of patterning by enabling fully dense, arbitrarily written features on multiple length scales. Both experimental and simulated results of this novel processing method are shown.

Cylindrically or spherically focusing shock waves have been of keen interest for the past several decades. In addition to fundamental study of materials under extreme conditions, cavitation, and sonoluminescence, focusing shock waves enable myriad applications including hypervelocity launchers, synthesis of new materials, production of high-temperature and high-density plasma fields, and a variety of medical therapies. Applications in controlled thermonuclear fusion and in the study of the conditions reached in laser fusion are also of current interest. Here we report on a method for direct real-time visualization and measurement of laser-driven shock generation, propagation, and 2D focusing in a sample. The 2D focusing of the shock front is the consequence of spatial shaping of the laser shock generation pulse into a ring pattern. A substantial increase of the pressure at the convergence of the acoustic shock front is observed experimentally and simulated numerically. Single-shot acquisitions using a streak camera reveal that at the convergence of the shock wave in liquid water the supersonic speed reaches Mach 6, corresponding to the multiple gigapascal pressure range 30 GPa.

We present a new route for the fabrication of highly nonspherical complex multivalent submicron particles. This technique exploits the ability of holographic interference lithography to control geometrical elements such as symmetry and volume fraction in 3D lattices on the submicron scale. Colloidal particles with prescribed complex concave shapes are obtained by cleaving low volume fraction connected structures fabricated by interference lithography. Controlling which Wyckoff sites in the space group of the parent structure are connected assures specific “valencies” of the particles. Two types of particles, 2D “4-valent” and 3D “6-valent” particles are fabricated via this technique. In addition to being able to control multivalent particle shape, this technique has the potential to provide tight control over size, yield, and dispersity.

An electrochemical method is applied to attach functional groups to individual single‐wall carbon nanotubes. This approach allows both the reductive and oxidative coupling of substituted phenyl groups. Due to polymerization of the reactive species formed at the nanotube/electrolyte interface, coatings with a thickness of up to 20 nm are obtained. The thickness of the deposited layer can be effectively controlled by the magnitude and duration of the potential applied. By choosing appropriate additional substituents in the coupling reagent, the presented method offers the possibility to tune the surface properties of the modified nanotubes over a wide range.

We have developed a simple, efficient process for solubilization of single-wall carbon nanotubes (SWNTs) with amylose in aqueous DMSO. This process requires two important conditions, presonication of SWNTs and subsequent amylose treatment in an optimum mixture of DMSO/H2O. The former step separates SWNT bundles, and the latter step provides a maximum cooperative interaction of SWNTs with amylose, leading to the immediate and complete solubilization. The best solvent condition for this is around 10-20% DMSO, in which amylose assumes a random conformation or an interrupted helix. This indicates that the amylose helix is not the prerequisite for encapsulation of SWNTs. The SEM and AFM images of the encapsulated SWNTs manifest loosely twisted ribbons wrapping around SWNTs, which are locally intertwined as a multiple twist, but no clumps of the host amylose are seen on SWNT capsules.

For the next generation of phoXonic, plasmonic, opto-mechanical and microfluidic devices, the capability to create 3D microstructures is highly desirable. Fabrication of such structures by conventional top-down techniques generally requires multiple time-consuming steps and is limited in the ability to define features spanning multiple layers at prescribed angles. 3D direct write lithography (3DDW) has the capability to draw nearly arbitrary structures, but is an inherently slow serial writing process. Here we present a method, denoted focused proximity field nanopatterning (FPnP), that combines 3DDW with single or multiphoton interference lithography (IL). By exposing a thick photoresist layer having a phase mask pattern imprinted on its surface with a tightly focused laser beam, we produce locally unique complex structures. The morphology can be varied based on beam and mask parameters. Patterns may be written rapidly in a single shot mode with arbitrary positions defined by the direct write, thus exploiting the control of 3DDW with the enhanced speed of phase mask IL. Here we show the ability for this technique to rapidly produce arrays of "stand-up" far IR resonators.

The laser photoacoustic technique is used to generate and detect picosecond coherent acoustic vibrations in gold film media deposited on Si substrates. As a consequence of the gold crystallites’ canted orientation, the pump-probe picosecond transient reflectivity shows oscillations at the fundamental shear mode frequency. The shear character of the mode is suggested by its dispersion, by the dependence of the signal on the probe laser wavelength, and by x-ray texture analysis.

Covariance and correlation coefficient mapping techniques are utilized to investigate the formation mechanism of titanium carbon clusters in a laser vaporization source, and also to elucidate the details of fragmentation of these clusters under excitation with an intense femtosecond laser pulse (∼1015 W/cm2 at 120 fs). This study, and a previous investigation of the Coulomb explosion of ammonia clusters, demonstrates the potential which covariance mapping has in discerning the details of cluster formation and fragmentation processes. Particular attention is paid to the titanium metallocarbohedrene (Met-Car), Ti8C12.