Ultrafast Pulse Excitation Research Articles

Single Broadband Phase-Shaped Pulse Stimulated Raman Spectroscopy for Standoff Trace Explosive Detection.

Recent success with trace explosives detection based on the single ultrafast pulse excitation for remote stimulated Raman scattering (SUPER-SRS) prompts us to provide new results and a Perspective that describes the theoretical foundation of the strategy used for achieving the desired sensitivity and selectivity. SUPER-SRS provides fast and selective imaging while being blind to optical properties of the substrate such as color, texture, or laser speckle. We describe the strategy of combining coherent vibrational excitation with a reference pulse in order to detect stimulated Raman gain or loss. A theoretical model is used to reproduce experimental spectra and to determine the ideal pulse parameters for best sensitivity, selectivity, and resolution when detecting one or more compounds simultaneously.

Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators.

Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.

Exploring the physics of efficient optical trapping of dielectric nanoparticles with ultrafast pulsed excitation.

Stable optical trapping of dielectric nanoparticles with low power high-repetition-rate ultrafast pulsed excitation has received considerable attention in recent years. However, the exact role of such excitation has been quite illusive so far since, for dielectric micron-sized particles, the trapping efficiency turns out to be similar to that of continuous-wave excitation and independent of pulse chirping. In order to provide a coherent explanation of this apparently puzzling phenomenon, we justify the superior role of high-repetition-rate pulsed excitation in dielectric nanoparticle trapping which is otherwise not possible with continuous-wave excitation at a similar average power level. We quantitatively estimate the optimal combination of pulse peak power and pulse repetition rate leading to a stable trap and discuss the role of inertial response on the dependence of trapping efficiency on pulse width. In addition, we report gradual trapping of individual quantum dots detected by a stepwise rise in a two-photon fluorescence signal from the trapped quantum dots which conclusively proves individual particle trapping.

Temporal focusing microscopy.

Axial localization of multiphoton excitation to a single plane is achieved by temporal focusing of an ultrafast pulsed excitation. We take advantage of geometrical dispersion in an extremely simple experimental setup, where an ultrashort pulse is temporally stretched and hence its peak intensity is lowered outside the focal plane of the microscope. Using this strategy, out-of-focus multiphoton excitation is dramatically reduced, and the achieved axial resolution is comparable to line-scanning multiphoton microscopy for wide-field excitation and to point-scanning multiphoton microscopy for line excitation. In this introduction, we provide a detailed description of the considerations in choosing the experimental parameters, as well as the alignment of a temporal focusing add-on to a multiphoton microscope. We also review current advances and applications for this technique.

Tip-Enhanced Raman Spectroscopy with Picosecond Pulses.

Tip-enhanced Raman spectroscopy (TERS) can probe chemistry occurring at surfaces with both nanometer spectroscopic and submolecular spatial resolution. Combining ultrafast spectroscopy with TERS allows for picosecond and, in principle, femtosecond temporal resolution. Here we couple an optical parametric oscillator (OPO) with a scanning tunneling microscopy (STM)-TERS microscope to excite the tip plasmon with a picosecond excitation source. The plasmonic tip was not damaged with OPO excitation, and TER spectra were observed for two resonant adsorbates. The TERS signal under ultrafast pulsed excitation decays on the time scale of 10 s of seconds; whereas with continuous-wave excitation no decay occurs. An analysis of possible decay mechanisms and their temporal characteristics is given.

Comment on “Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography”

In a recent letter (F. Carbone, P. Baum, P. Rudolf, et al., Physical Review Letters 100, 2008), graphite is reported to undergo a c-axis contraction on the time scale of few picoseconds (ps) after ultrafast pulsed laser excitation. The velocity of lattice contraction depended on the laser fluence. Furthermore, the lattice contraction is followed by large, non-thermal, lattice expansion of several picometers (pm) after few hundreds of ps. These results were interpreted based on the position of the (0014) diffraction spot in the glancing angle diffraction geometry off the graphite surface using ultrafast electron diffraction (UED). The lattice contraction and expansion corresponded to upward and downward movements of the diffraction spot respectively. Here, we show that ultrafast pulsed laser excitation of graphite produces large transient electric fields (TEFs). The TEFs deflect the electron beam in a manner similar to these reported and the neglect of TEFs likely led to erroneous conclusions.

Adding new dimensions to laser‐scanning fluorescence microscopy

We describe a novel method of optical imaging by exploiting simple ideas borrowed from pulsed optics. We show that the use of ultrafast pulsed one-photon excitation in laser-scanning fluorescence microscopy dramatically brings together several advantages offered by two widely used present day microscopic techniques, confocal and multi-photon fluorescence microscopy. The method appears as a novel tool in the context of laser-scanning fluorescence microscopy by having a 'built-in' 3D spatial resolution.

Non-Fourier heat transport in metal-dielectric core-shell nanoparticles under ultrafast laser pulse excitation

Relaxation dynamics of embedded metal nanoparticles after ultrafast laser pulse excitation is driven by thermal phenomena of different origins, the accurate description of which is crucial for interpreting experimental results: hot electron-gas generation, electron-phonon coupling, heat transfer to the particle environment, and heat propagation in the latter. Regarding this last mechanism, it is well known that heat transport in nanoscale structures and/or at ultrashort timescales may deviate from the predictions of the Fourier law. In these cases heat transport may rather be described by the Boltzmann transport equation. We present a numerical model allowing to determine the electron and lattice temperature dynamics in a spherical gold nanoparticle core under subpicosecond pulsed excitation as well as that in the surrounding shell dielectric medium. For this, we have used the electron-phonon coupling equation in the particle with a source term linked with the laser pulse absorption and the ballistic-diffusive equations for heat conduction in the host medium. Either thermalizing or adiabatic boundary conditions have been considered at the shell external surface. Our results show that the heat transfer rate from the particle to the matrix can be significantly smaller than the prediction of Fourier's law. Consequently, the particle-temperature rise is larger and its cooling dynamics might be slower than that obtained by using Fourier's law. This difference is attributed to the nonlocal and nonequilibrium heat conductions in the vicinity of the core nanoparticle. These results are expected to be of great importance for analyzing pump-probe experiments performed on single nanoparticles or nanocomposite media.

Enhanced terahertz radiation from high stacking fault density nonpolar GaN

Terahertz emission from high stacking fault density m-GaN has been observed using ultrafast pulse excitation. The terahertz signal exhibits a 360° periodicity with sample rotation and a polarity flip at 180°, characteristic of real carrier transport in an in-plane electric field parallel to the c axis induced by stacking fault (SF)-terminated internal polarization at wurtzite domain boundaries. The terahertz emission can be enhanced by several times relative to that from a SF-free m-GaN sample, for which the terahertz signal emanates from surface surge currents and diffusion-driven carrier transport normal to the surface and is independent of the c-axis orientation.

Gold nanoparticle assemblies: Thermal behaviour under optical excitation

The optical response of materials based on gold nanoparticle assemblies depends on many parameters connected to both material morphology and light excitation characteristics. The optical energy absorbed is then converted into heat through different nanoscale energy exchange mechanisms. This heating subsequently modifies itself the optical properties. We investigate the interplay between the optical and thermal responses of nanocomposite media under its theoretical aspect. In this first paper, the thermal response of gold nanoparticle assemblies under pulsed optical excitation is considered. Both conventional and original modelling approaches are presented. We first underline the role of electromagnetic interactions between particles in a dense assembly in its linear optical response. We then show how the interaction of light with matrix-embedded gold nanoparticles can result in the generation of thermal excitations through different energy exchange mechanisms. Finally, we demonstrate the possible significant influence of the heat carrier ballistic regime and phonon rarefaction in the cooling dynamics of an embedded gold nanoparticle subsequent to ultrafast pulsed laser excitation.

Ultrafast spin-dynamics in half-metallicCrO2thin films

Transient magneto-optic Kerr effect measurement utilizing ultrafast laser pulse excitation and detection method has been performed to study the spin dynamics in epitaxial $\mathrm{Cr}{\mathrm{O}}_{2}$ film. Different microscopic contributions leading to incoherent spin thermalization and relaxation on distinct time scales have been identified in an extended three-temperature model (3TM), by taking into account the nonthermal electrons. Interestingly, the spins and thermalized electrons seemed to have been decoupled in these measurements, which could be attributed to the half-metallic nature of the material.

Probing carrier dynamics in implanted and annealed polycrystalline silicon thin films using white light

Polycrystalline silicon thin film samples implanted and annealed at various temperatures have been studied using ultrafast laser pulse excitation. Nondegenerate pump-probe technique has been utilized to investigate carrier dynamics in the highly implanted samples at a relatively small fluence. A model based on two coupled differential equations has been used to fit the experimental data, giving a simple but adequate picture of the dynamics of this system. Basic sample parameters such as carrier trapping times, diffusion coefficient, and penetration depths have been extracted, providing a dependence on the annealing temperature for the samples under investigation.

Generation of transverse acoustic phonons in a GaAs/AlAs superlattice by ultrafast optical excitation

We have detected using superconducting bolometers the phonons generated by ultra-fast pulsed laser excitation of gallium arsenide/aluminium arsenide superlattice structures. In addition to the enhancement of the longitudinal acoustic (LA) phonon previously reported, we observe an enhancement of the signal due to ballistic transverse acoustic (TA) phonons when the condition for coherent phonon generation is satisfied, that is, the pump photon energy is equal to the exciton energy in the quantum wells. TA modes are not Raman active in (001) GaAs, therefore impulsively stimulated Raman scattering (ISRS), believed to be the mechanism for generation of LA modes, is unlikely to be responsible for the TA emission. We consider other processes which might be responsible for generation of the TA phonons.

Absorption and subsequent emission saturation of two-photon excited materials: theory and experiment

The saturation of two-photon absorption and the subsequent photoluminescence of matter under ultrafast laser pulse excitation are studied. For this purpose the experimental results of using two organic conjugated polymers as model substances are discussed theoretically. One model is based on the commonly used hyperbolic approach, whereas the new theory introduced correctly describes the saturation in two-photon spectroscopy and fits the experiment closely. The new model permits microscopic analysis of the absorption saturation process and makes possible an estimate of the two-photon absorption coefficient from the saturation intensity.

Resonant terahertz radiation from Tl2Ba2CaCu2O8+δ thin films by ultrafast optical pulse excitation

We have observed the free-space radiation of the resonant terahertz wave from c-axis oriented Tl2Ba2CaCu2O8+δ thin films by femtosecond optical pulse excitation under a radial magnetic field of about 100 Oe nearly parallel to the c axis of the thin film. The observed wave form showed clear oscillations up to 80 K. The frequency was shifted from ∼620 GHz at 24 K to ∼300 GHz at 80 K, corresponding to an increase in c-axis penetration depth, λc, from ∼26 μm to ∼53 μm, and disappeared above TC as expected for the Josephson plasma resonance.

Progress toward a better understanding of signal generation in laser-excited photothermal spectrometry of homogeneous samples

This article examines some recent experiments and theories that have advanced our understanding of the physical effects following thermal relaxation of the excited state. Experiments which show that the temperature-dependent refractive index is composed of density and pure-thermal terms are discussed. The importance of the purely thermal term is that it allows ultra-fast pulsed excitation sources to be used in photothermal refraction spectroscopy. This opens up new areas of applications in ultra-fast measurement. The effect of and progress towards the elucidation of specific volume changes are reviewed. Finally, recent progress in understanding nonlinear optical absorption and its effect on the photothermal signal are summarized.

The influence of high-frequency modes on two pulse spectroscopy

Theoretical analyses of molecular response to ultrafast pulse excitation often do not account for the influence of high frequency modes on the spectrum of the molecule. In a previous article [M. N. Kobrak, E. M. Hiller, and S. A. Rice, J. Chem. Phys. 105, 9403 (1996)], we have shown that the high frequency modes of the molecule can dramatically change the distribution of an observable in processes initiated by a single ultrashort pulse. In this article we extend our treatment to allow for two-pulse processes, and apply the results to two models which have a qualitative resemblence to bacteriorhodopsin.

Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metal-semiconductor interface

We have demonstrated a new method of generating pulses of freely propagating THz electromagnetic radiation. The resulting 380 fs pulses are the shortest directly measured THz pulses in free space to date and are more powerful than those generated by Hertzian dipoles or by resonant dipole antennas. Temporal features as short as 190 fs were observed on these THz radiation pulses and thereby, illustrate an ultrafast receiver response time.