Ultrafast Melting Research Articles

Ultrafast IR spectroscopy of ice–water phase transition

Superheating and bulk melting of neat H 2O ice are studied using 2-color IR spectroscopy with sub-picosecond pulses. The data are compared with similar experiments in 15 M HDO in D 2O. After calibration of our spectroscopic thermometer a maximum superheating of the solid phase up to 330 ± 10 K is observed after ultrafast temperature jumps. For energy depositions beyond the limit of superheating a strong evidence for partial melting in two steps is observed. The ultrafast thermal melting leads to formation of water clusters in the excited ice lattice and is followed by secondary melting at the generated phase boundaries. The latter process is found to consume energy amounts in agreement with the latent heat of melting and is accompanied by an accelerated temperature and pressure decrease of the residual ice component. The bottleneck of the initial melting process seems to be the thermalization time of the hydrogen-bonded network of 7.0 ± 1 ps.

Femtosecond Laser-Induced Nanowelding: Fundamentals and Applications

Fundamentals of femtosecond laser pulse and nanoparticles are analyzed by a two-temperature model. Ultrafast surface melting, surface nanoengineering and shock wave impact are evident in the surface of graphite by femtosecond irradiation. The interaction between femtosecond laser pulses and Au/Ag nanoparticles has been investigated. Two effects are identified at different intensities: photofragmentation at rather high intensity (~10 14 W/cm 2 ), nanojoining at low intensity (~10 10 W/cm 2 ). Photofragmentation forms a large number of tiny nanoparticles with an average size of tens of nanometers. Control over irradiation conditions at intensities near 10 10 W/cm 2 results in nanojoining of most of the nanoparticles. This nanojoining is obtained in both liquid solution and in solid state thin films assembled from nanoparticles. Nanojoining mechanism is further studied by joining Ag nanoparticles encapsulated by a polymer shell. Nonthermal melting is investigated. Nanojoined Au nanoparticles are expected to have numerous applications, such as probes for surface enhance Raman spectroscopy.

Time-resolved x-ray scattering from laser-molten indium antimonide

We demonstrate a concept to study transient liquids with picosecond time-resolved x-ray scattering in a high-repetition-rate configuration. Femtosecond laser excitation of crystalline indium antimonide (InSb) induces ultrafast melting, which leads to a loss of the long-range order. The remaining local correlations of the liquid result in broad x-ray diffraction rings, which are measured as a function of delay time. After 2 ns the liquid structure factor shows close agreement with that of equilibrated liquid InSb. The measured decay of the liquid scattering intensity corresponds to the resolidification rate of 1 m/s in InSb.

Molecular dynamics simulations of laser-induced damage of nanostructures and solids

A theoretical approach to treat laser induced femtosecond structural changes in covalently bonded nanostructures and solids is described. Our approach consists in molecular dynamic simulations performed on the basis of time-dependent, many-body potential energy surfaces derived from tight-binding Hamiltonians. The shape and spectral composition of the laser pulse is explicitly taking into account in a non-perturbative way. We show a few examples of the application of this approach to describe the laser damage and healing of defects in carbon nanotubes with different chiralities and the ultrafast nonequilibrium melting of bulk germanium, initiated by the laser-induced softening and destabilization of transversal acoustic phonon modes.

Dynamics of Photoinduced Charge-Density-Wave to Metal Phase Transition inK0.3MoO3

We present the first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast nonthermal CDW melting is achieved at the absorbed energy density that corresponds to the electronic energy difference between the metallic and CDW states. The results imply that on the subpicosecond time scale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.

Ultrafast photoinduced melting of spin-Peierls phase in potassium-tetracyanoquinodimethane

Ultrafast dynamics of photoinduced melting of spin-Peierls (SP) phase was investigated in K-tetracyanoquinodimethane by using fs pump-probe reflection spectroscopy. It was revealed that the photocarriers are localized with ∼70 fs and behave as nonmagnetic impurities, which lead to the melting of the SP phase within ∼ 400 fs. In the melting process of the SP phase, several kinds of coherent oscillations were observed, suggesting that the corresponding several phonon modes contribute to the stabilization of the SP phase.

Ultrafast Thermal Plasma Preparation of Solid Si Films with Potential Application in Photovoltaic Cells: A Parametric Study

An innovative method, namely ultrafast plasma surface melting, is developed to fabricate solid films of silicon with very high rates (150 cm2/min). The method is composed of preparing a suspension of solid particles in a volatile solvent and spreading it on a refractory substrate such as Mo. After solvent evaporation, the resulting porous layer is exposed to the flame tale of inductively coupled RF plasma to sinter and melt the surface particles and to prepare a solid film of silicon. It is shown that by controlling the flow dynamics and heat transfer around the substrate, and managing the kinetic parameters (i.e., exposure time, substrate transport speed, and reaction kinetics) in the reactor, we can produce solid crystalline Si films with the potential applications in photovoltaic cells industry. The results indicate that the optimum formation conditions with a film thickness of 250-700 μm is when the exposure time in the plasma is in the range of 5-12.5 s for a 100 × 50 mm large layer. By combining the Fourier’s law of conduction with the experimental measurements, we obtained an effective heat diffusivity and developed a model to obtain heat diffusion in the porous layer exposed to the plasma. The model further predicts the minimum and maximum exposure time for the substrate in the plasma flame as a function of material properties, the porous layer thickness and of the imposed heat flux.

Sub-10 nm Self-Enclosed Self-Limited Nanofluidic Channel Arrays

We report a new method to fabricate self-enclosed optically transparent nanofluidic channel arrays with sub-10 nm channel width over large areas. Our method involves patterning nanoscale Si trenches using nanoimprint lithography (NIL), sealing the trenches into enclosed channels by ultrafast laser pulse melting and shrinking the channel sizes by self-limiting thermal oxidation. We demonstrate that 100 nm wide Si trenches can be sealed and shrunk to 9 nm wide and that lambda-phage DNA molecules can be effectively stretched by the channels.

Anharmonic Noninertial Lattice Dynamics during Ultrafast Nonthermal Melting of InSb

We compute the potential energy surface of femtosecond-laser-excited InSb along the directions in which the crystal becomes soft. Using dynamical simulations the time dependence of the atomic coordinates is obtained. We find that at high excitation densities the anharmonicity of the potential energy surface becomes significant after approximately 100 fs. On the basis of our results we explain recent time-resolved x-ray diffraction experiments. We point out that an alternative model for ultrafast melting [A. M. Lindenberg, Science 308, 392 (2005)] is inconsistent with our calculations.

Ultrafast melting and resolidification of gold particle irradiated by pico- to femtosecond lasers

Ultrafast melting and resolidification of a submicron gold particle subject to pico- to femtosecond laser pulse are studied in this paper. The nonequilibrium heat transfer in the electrons and lattice is described using a two-temperature model, and the locations of the solid-liquid interface are determined using an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. The results showed that the maximum melting depth, peak interfacial temperature, and velocity increase with the decreasing particle size and pulse width or with the increasing laser fluence.

Repetitive ultrafast melting of InSb as an x-ray timing diagnostic

We have demonstrated the possibility of using repetitive ultrafast melting of InSb as a timing diagnostic in connection with visible-light pump∕x-ray probe measurements at high-repetition-rate x-ray facilities. Although the sample was molten and regrown approximately 1×106 times, a distinct reduction in time-resolved x-ray reflectivity could be observed using a streak camera with a time resolution of 2.5ps. The time-resolved x-ray reflectivity displayed this distinct decrease despite the fact that the average reflectivity of the sample had fallen to approximately 50% of its original value due to accumulated damage from the prolonged laser exposure. The topography of the laser-exposed sample was mapped using an optical microscope, a stylus profilometer, photoelectron microscopy, and a scanning tunneling microscope. Although the surface of the sample is not flat following prolonged exposure at laser fluences above 15mJ∕cm2, the atomic scale structure regrows, and thus, regenerates the sample on a nanosecond timescale. In the fluence range between 15 and 25mJ∕cm2, the laser power is sufficient to melt the sample, while regrowth occurs with a sufficiently good structure to allow the extraction of timing information via ultrafast time-resolved x-ray measurements. This can be applied for timing purposes at synchrotron radiation and x-ray free-electron laser facilities. It is also noteworthy that we were able to reproduce the fluence dependencies of melting depth and disordering time previously obtained in single-shot, nonthermal melting experiments with higher temporal resolution.

Ultrafast laser micromachining of 3C-SiC thin films for MEMS device fabrication

Femtosecond pulsed laser (800 nm, 120 fs) micromachining of thin films of 3C-SiC (β-SiC) semiconductor deposited on silicon substrate was investigated as a function of pulse energy (0.5 μJ to 750 μJ). The purpose is to establish suitable laser parametric regime for the fabrication of high accuracy, high spatial resolution and thin diaphragms for high-temperature MEMS pressure sensor applications. Etch rate, ablation threshold and quality of micromachined features were evaluated. The governing ablation mechanisms, such as thermal vaporization, phase explosion, Coulomb explosion and photomechanical fragmentation, were correlated with the effects of pulse energy. The results show that the etch rate is higher and the ablation threshold is lower than those obtained with nanosecond pulsed excimer laser ablation, suggesting femtosecond laser’s potential for rapid manufacturing. In addition, the etch rates were substantially higher than those achievable in various reactive ion and electrochemical etching methods. Excellent quality of machined features with little collateral thermal damage was obtained in the pulse energy range (1–10 μJ). The leading material removal mechanisms under these conditions were photomechanical fragmentation, ultrafast melting and vaporization. At very low pulse energies (<1 μJ), nanoscale material removal has occurred with the formation of nanoparticles that is attributed to Coulomb explosion mechanism. The effect of assist gas on the process performance at low and high energy fluences is also presented.

Ultrafast photoinduced melting of spin-Peierls phase in the organic charge-transfer compounds alkali-tetracyanoquinodimethane

Photoinduced phase transitions in spin-Peierls (SP) systems of alkali $(M=\mathrm{K},\mathrm{Na})$-tetracyanoquinodimethane (TCNQ) have been studied by a reflection-type femtosecond (fs) pump-probe spectroscopy. The SP phase is destabilized by the generation of photocarriers through the breaking of the spin-singlet states in dimers. It results in the decrease of the dimeric molecular displacements within a few hundred of femtoseconds over several tens of TCNQ molecules. It is accompanied by the displacive-type coherent oscillations, which consist mainly of three modes with the frequencies of 20, 49, and $90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ and of two modes with the frequencies of 49 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in Na-TCNQ. By taking into account the temperature dependence of the Raman scattering spectra, the mode with $20\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ and the modes with 49 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in Na-TCNQ are assigned to the phonon modes in the SP ground state, while the modes with 49 and $90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ are assigned to the local modes originating from the photoexcited states. Polarization dependence of the Raman scattering signals shows that the $20\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of K-TCNQ and the $49\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of Na-TCNQ are longitudinal optical (LO) modes, whereas the $99\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of Na-TCNQ is a transverse optical (TO) mode. Namely, the LO mode plays an important role on the stabilization of the SP phase in K-TCNQ, while both the LO and TO modes in Na-TCNQ. Such a difference is discussed by scrutinizing the difference of the crystal structures and the nature of the SP transitions in the two compounds.

Demonstration of the ultrafast nature of laser produced betatron radiation

This Letter aims to demonstrate the ultrafast nature of laser produced betatron radiation and its potential for application experiments. An upper estimate of the betatron x-ray pulse duration has been obtained by performing a time-resolved x-ray diffraction experiment: The ultrafast nonthermal melting of a semiconductor crystal (InSb) has been used to trigger the betatron x-ray beam diffracted from the surface. An x-ray pulse duration of less than 1ps at full width half-maximum (FWHM) has been measured with a best fit obtained for 100fs FWHM.

Phase inhomogeneities in the charge-orbital-ordered manganiteNd0.5Sr0.5MnO3revealed through polaron dynamics

Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperature-dependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.

Dynamics of Size-Selected Gold Nanoparticles Studied by Ultrafast Electron Nanocrystallography

We report the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with dynamical full-profile radial distribution functions determined with subpicosecond and picometer accuracies. In an ultrafast photoinduced melting, the nanoparticles are driven to a nonequilibrium transformation, characterized by the initial lattice deformations, nonequilibrium electron-phonon coupling, and, upon melting, the collective bonding and debonding, transforming nanocrystals into shelled nanoliquids. The displasive structural excitation at premelting and the coherent transformation with crystal/liquid coexistence during photomelting differ from the reciprocal behavior of recrystallization, where a hot lattice forms from liquid and then thermally contracts. The degree of structural change and the thermodynamics of melting are found to depend on the size of nanoparticle.

Theoretical Models and Qualitative Interpretations of Fs Laser Material Processing

In this paper a number of numerical models are presented which have been developed to describe the processes taking place at different time and length scales in different classes of materials under the irradiation by ultrashort laser pulses. A unified drift-diffusion approach for modeling chargecarrier transport in metals, semiconductors, and dielectrics allows to elucidate the dynamics of the electric field generated in the target due to photo-emission and to get insight into the origin of the Coulomb explosion process. The widely known two-temperature model is used to follow heating dynamics of irradiated matter and to analyze its phase transformations on the basis of thermodynamic concepts. Being modified for semiconductors, this model has allowed to establish the nature of high-energetic ion emission using laser pulse tailoring and to undertake a simplified modeling of consequences of ultrafast melting of silicon. A two-dimensional model of dielectric breakdown has made possible to uncover the mechanisms which enable the spatial modulation of the structures induced by temporally modulated laser pulses in wide-band-gap dielectric materials. A combined thermal/elasto-plastic model has provided a deep insight into the mechanisms and dynamics of the microbump and nanojet formation on nanosize gold films under femtosecond laser irradiation.

Photoinduced Melting of a Stripe-Type Charge-Order and Metallic Domain Formation in a Layered BEDT-TTF-Based Organic Salt

Photoinduced melting of charge-order (CO) in [bis(ethylenedithiolo)]-tetrathiafulvalene (BEDT-TTF) salts was investigated by femotosecond spectroscopy. Comparative studies on two polytypes exhibiting large [theta-(BEDT-TTF)2RbZn(SCN)_{4}] and small [alpha-(BEDT-TTF)2I3] molecular rearrangements through the CO transition were performed. Ultrafast melting of CO for both compounds demonstrates the major contribution of the electronic instability which is due to Coulomb interaction. The roles of the molecular rearrangements on the formation of the CO and the metallic domain are discussed on the basis of low-frequency lattice dynamics.

Influence of a nanosecond pulsed laser on aluminium alloys: Distribution of oxygen

Nanosecond laser pulses may produce both thermal melting (as femtosecond and picosecond pulses) or ultrafast nonthermal melting depending on the pulse fluence. This was demonstrated experimentally by Sokolowski-Tinten et al. [1], who found that the transformation of GaAs into its liquid state occurs within several tens of picoseconds at fluences close to the melt threshold due to thermal melting under highly superheated conditions or within several hundred femtoseconds via carrier excitation for very high fluences. The processes occurring under high energetic fs pulse irradiation could be described more precisely with the help of the theoretical work of Stampfli et al. In this work, a nanosecond pulsed laser (Nd:Yag) is used to irradiate an aluminum alloy sample. The oxygen distribution is studied as a function of distance in order to get an idea about the temperature distribution.

Ultrafast superheating and melting of bulk ice

The superheating of a solid to a temperature beyond its melting point, without the solid actually melting, is a well-known phenomenon. It occurs with many substances, particularly those that can readily be produced as high-quality crystals. In principle, ice should also be amenable to superheating. But the complex three-dimensional network of hydrogen bonds that holds water molecules together and gives rise to unusual solid and liquid properties strongly affects the melting behaviour of ice; in particular, ice usually contains many defects owing to the directionality of its hydrogen bonds. However, simulations are readily able to 'create' defect-free ice that can be superheated. Here we show that by exciting the OH stretching mode of water, it is possible to superheat ice. When using an ice sample at an initial temperature of 270 K, we observe an average temperature rise of 20 +/- 2 K that persists over the monitored time interval of 250 ps without melting.