Femtosecond-tunable measurement of electron thermalization in gold

ULTRAFAST DYNAMICS OF GOLD NANORODS: TUNING BETWEEN PHOTO-BLEACHING AND PHOTO-INDUCED ABSORPTION

We report ultrafast electron dynamics in gold nanorods investigated using 80 fs laser pulses centered at 1.57 eV. Five types of nanorod colloidal suspensions in water having their longitudinal surface plasmon peak (E LSP ) on either side of the laser photon energy (EL) have been studied. For E LSP > EL, photo-induced absorption with single decay time constant is observed. On the other hand, for E LSP < EL, photo-bleaching is observed having bi-exponential decay dynamics; the faster one between 1–3 ps and slower one between 7 ps to 22 ps both of them increasing almost linearly with the difference |EL – E LSP |. These time constants increase linearly with the pump intensity. Simulations have been carried out to understand the interplay between photo-bleaching and photo-induced absorption.

Engineering metallic nanostructures for plasmonics and nanophotonics

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

Plasmon dynamics in colloidal Cu₂-xSe nanocrystals

The optical response of metallic nanostructures after intense excitation with femtosecond-laser pulses has recently attracted increasing attention: such response is dominated by ultrafast electron-phonon coupling and offers the possibility to achieve optical modulation with unprecedented terahertz bandwidth. In addition to noble metal nanoparticles, efforts have been made in recent years to synthesize heavily doped semiconductor nanocrystals so as to achieve a plasmonic behavior with spectrally tunable features. In this work, we studied the dynamics of the localized plasmon resonance exhibited by colloidal Cu(2-x)Se nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pump-probe experiments in the near-infrared, with ∼200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong nonlinearities in the plasmonic absorption due to the much lower carrier density of Cu(2-x)Se compared to noble metals, which led to ultrafast control of the probe signal with modulation depth exceeding 40% in transmission.

All-optical ultrafast control of beaming through a single sub-wavelength aperture in a metal film

We propose an ultrafast all-optical technique to control and beam the light emerging from a sub-wavelength slit in a planar gold film by exciting a transient grating in the area around the slit. A FDTD model is used to show how excitation of surface plasmon polaritons by the grating governs the beaming process. Both the grating and the beaming effect are shown to decay on a picosecond time-scale. An on-off contrast of 5 dB is obtained for the beaming, with a divergence angle of only 2.4 degrees.

Ultrafast all-optical coupling of light to surface plasmon polaritons on plain metal surfaces

We propose and demonstrate an ultrafast all-optical method to couple light to surface plasmon polaritons on planar gold films. By interfering two 150 fs, 810 nm pulses we excite a transient grating in the temperature of the free electrons of the metal, resulting in a grating in the dielectric function, and leading to a 1 ps launch window for plasmonic excitation. We use pump-probe experiments to identify these ultrashort plasmonic excitations between 520 and 570 nm.

Ultrafast spatiotemporal relaxation dynamics of excited electrons in a metal nanostructure detected by femtosecond-SNOM

Ultrahigh spatiotemporal resolved pump-probe signal near a gold nano-slit is detected by femtosecond-SNOM. By employing two-color pump-probe configuration and probing at the interband transition wavelength of the gold, signal contributed by surface plasmon polariton is avoided and spatiotemporal evolvement of excited electrons is successfully observed. From the contrast decaying of the periodical distribution of the pump-probe signal, ultrafast diffusion of excited electrons with a time scale of a few hundred femtoseconds is clearly identified. For comparison, such phenomenon cannot be observed by the one-color pump-probe configuration.

Aberration effects on femtosecond pulses generated by nonideal achromatic doublets

There are three main effects that affect the femtosecond pulse focusing process near the focal plane of a refractive lens: the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper we study in detail these effects generated by nonideal achromatic doublets based on a Fourier-optical analysis and Seidel aberration theory considering lens material, wavelength range, lens surface design, and temporally and spatially uniform and Gaussian intensity distributions. We show that the residual chromatic aberration in achromatic lenses, which has been neglected so far, has a considerable effect on the focusing of pulses shorter than 20 fs in the spectral range between the UV and IR, 300 to 1100 nm, and is particularly important in the blue and UV spectral range. We present a general fitted function for an estimation of the pulse stretching parameter, which depends only on the numerical aperture and focal length of the doublet as well as the wavelength of the carrier of the pulse.

Femtosecond surface plasmon interferometry

We demonstrate femtosecond plasmonic interferometry with a novel geometry. The plasmonic microinterferometer consists of a tilted slit-groove pair. This arrangement allows for (i) interferometric measurements at a single wavelength with a single microinterferometer and (ii) unambiguous discrimination between changes in real and imaginary parts of the metal dielectric function. The performance is demonstrated by monitoring the sub-picosecond dynamics of hot electrons in gold.

Ultrafast control of grating-assisted light coupling to surface plasmons

We demonstrate subpicosecond control over the coupling of free-space radiation to surface-plasmon polaritons using 830 and 500 nm period gold gratings. Thermal changes to the electron distribution following irradiation by 100 fs, 810 nm pulses produce a shift of the 570 nm plasmon resonance by approximately 0.75 nm with reflectivity change up to 6% and decay time of approximately 1 ps.

Wavelength dispersion of nonlinear dielectric function of Cu nanoparticle materials

The wavelength dispersions of third-order nonlinear optical response for Cu nanoparticle materials have been experimentally evaluated from transient spectra measured with the pump-probe method. The evaluated dispersions were analyzed on hot electron contribution using the Maxwell-Garnett approximation with the Drude model for intraband transition and first principles calculation for interband transition. The wavelength dispersion didn't directly reflect the dispersion of a local electric field factor. The interband transition term in hot electron contribution strongly dominated the dispersion around the surface Plasmon resonance by Fermi smearing. Intrinsic interband contribution to the nonlinearity was suggested from the analysis. Particle-size and host-medium dependence of the nonlinearity were also simulated.

Transient optical response of ultrafast nonequilibrium excited metals: effects of electron-electron contribution to collisional absorption

Approaching energy coupling in laser-irradiated metals, we point out the role of electron-electron collision as an efficient control factor for ultrafast optical absorption. The high degree of laser-induced electron-ion nonequilibrium drives a complex absorption pattern with consequences on the transient optical properties. Consequently, high electronic temperatures determine largely the collision frequency and establish a transition between absorptive regimes in solid and plasma phases. In particular, taking into account umklapp electron-electron collisions, we performed hydrodynamic simulations of the laser-matter interaction to calculate laser energy deposition during the electron-ion nonequilibrium stage and subsequent matter transformation phases. We observe strong correlations between optical and thermodynamic properties according to the experimental situations. A suitable connection between solid and plasma regimes is chosen in accordance with models that describe the behavior in extreme, asymptotic regimes. The proposed approach describes as well situations encountered in pump-probe types of experiments, where the state of matter is probed after initial excitation. Comparison with experimental measurements shows simulation results which are sufficiently accurate to interpret the observed material behavior. A numerical probe is proposed to analyze the transient optical properties of matter exposed to ultrashort pulsed laser irradiation at moderate and high intensities. Various thermodynamic states are assigned to the observed optical variation. Qualitative indications of the amount of energy coupled in the irradiated targets are obtained.

Gold nanoshell photomodification under a single-nanosecond laser pulse accompanied by color-shifting and bubble formation phenomena

Laser-nanoparticle interaction is crucial for biomedical applications of lasers and nanotechnology to the treatment of cancer or pathogenic microorganisms. We report on the first observation of laser-induced coloring of gold nanoshell solution after a one nanosecond pulse and an unprecedentedly low bubble formation (as the main mechanism of cancer cell killing) threshold at a laser fluence of about 4 mJ cm(-2), which is safe for normal tissue. Specifically, silica/gold nanoshell (140/15 nm) suspensions were irradiated with a single 4 ns (1064 nm) or 8 ns (900 nm) laser pulse at fluences ranging from 0.1 mJ cm(-2) to 50 J cm(-2). Solution red coloring was observed by the naked eye confirmed by blue-shifting of the absorption spectrum maximum from the initial 900 nm for nanoshells to 530 nm for conventional colloidal gold nanospheres. TEM images revealed significant photomodification of nanoparticles including complete fragmentation of gold shells, changes in silica core structure, formation of small 20-30 nm isolated spherical gold nanoparticles, gold nanoshells with central holes, and large and small spherical gold particles attached to a silica core. The time-resolved monitoring of bubble formation phenomena with the photothermal (PT) thermolens technique demonstrated that after application of a single 8 ns pulse at fluences 5-10 mJ cm(-2) and higher the next pulse did not produce any PT response, indicating a dramatic decrease in absorption because of gold shell modification. We also observed a dependence of the bubble expansion time on the laser energy with unusually very fast PT signal rising (∼3.5 ns scale at 0.2 J cm(-2)). Application of the observed phenomena to medical applications is discussed, including a simple visual color test for laser-nanoparticle interaction.

Ultrafast studies of gold, nickel, and palladium nanorods

Steady state and ultrafast transient absorption studies have been carried out for gold, nickel, and palladium high aspect ratio nanorods. For each metal, nanorods were fabricated by electrochemical deposition into approximately 6 microm thick polycarbonate templates. Two nominal pore diameters(10 and 30 nm, resulting in nanorod diameters of about 40 and 60 nm, respectively) were used, yielding nanorods with high aspect ratios (>25). Static spectra of nanorods of all three metals reveal both a longitudinal surface plasmon resonance (SPR(L)) band in the mid-infrared as well as a transverse band in the visible for the gold and larger diameter nickel and palladium nanorods. The appearance of SPR(L) bands in the infrared for high aspect ratio metal nanorods and the trends in their maxima for the different aspect ratios and metals are consistent with calculations based on the Gans theory. For the gold and nickel samples, time resolved studies were performed with a subpicosecond resolution using 400 nm excitation and a wide range of probe wavelengths from the visible to the mid-IR as well as for infrared excitation (near 2000 cm(-1)) probed at 800 nm. The dynamics observed for nanorods of both metals and both diameters include transients due to electron-phonon coupling and impulsively excited coherent acoustic breathing mode oscillations, which are similar to those previously reported for spherical and smaller rod-shaped gold nanoparticles. The dynamics we observe are the same within the experimental uncertainty for 400 nm and infrared (5 microm) excitation probed at 800 nm. The transient absorption using 400 nm excitation and 800 nm probe pulses of the palladium nanorods also reveal coherent acoustic oscillations. The results demonstrate that the dynamics for high aspect ratio metal nanorods are similar to those for smaller nanoparticles.

Near-field two-photon-induced photoluminescence from single gold nanorods and imaging of plasmon modes

We investigated the two-photon-induced photoluminescence properties of single gold nanorods by scanning near-field spectroscopy. The process was found to be initiated by a sequential one-photon absorption for creating a pair of an electron and a hole in the sp and d bands. Photoluminescence is then radiated when the electron near the Fermi surface recombines with the hole near the X and L symmetry points. The polarization characteristics of emitted photons from the X and L regions were found to be different. These characteristics can be understood by the crystalline structure and the band structure of the gold nanorod. We found characteristic spatial oscillatory features along the long axis of the nanorods in photoluminescence excitation images. The images were well reproduced by density-of-states maps of the nanorods calculated with Green's dyadic method and were attributed to the spatial characteristics of the wave functions of the plasmon modes inside the nanorods.

Influence of interband transitions on electron-phonon coupling measurements in Ni films

The reduction in size and the increase in speed of opto- and magnetoelectronic devices is making the probability of nonequilibrium electron-phonon phenomena greater, leading to increased thermal resistance in these devices. The measurement of electron-phonon coupling in materials in these devices is becoming increasingly important for accurate thermal management. Here femtosecond thermoreflectance is used to measure the electron-phonon coupling factor in thin Ni films of varying thickness grown on Si and glass substrates. The thermoreflectance response is measured at 1.3 and 1.55 eV, yielding drastically different responses due to the Fermi-level transition at 1.3 eV in Ni. The influence of this transition on the thermoreflectance response results in a measurement of the electron-phonon coupling factor that is twice as high as that recorded in previous measurements that were unaffected by the Fermi-level transition.

Highly efficient avalanche multiphoton luminescence from coupled Au nanowires in the visible region

We demonstrate highly efficient avalanche multiphoton luminescence (MPL) from ordered-arrayed gold nanowires (NWs) with low time-average excitation intensity, Iexc (5.0-9.1 kW/cm2). The intensity of avalanche MPL, IMPL, is about 10(4) times larger than that of three-photon luminescence, the slope partial differential log IMPL/ partial differential log Iexc of avalanche MPL reaches as high as 18.3, and the corresponding polarization dependence of IMPL has a form of cos50 phip. The emission dynamics of avalanche MPL and three-photon luminescence are also studied comparatively. These observations indicate that the highly efficient avalanche MPL is attributed to the giant enhancement and coupling of longitudinal surface plasmon resonance of ordered-arrayed gold NWs.

COHERENT EXCITATION OF VIBRATIONAL MODES IN METALLIC NANOPARTICLES

Excitation of metal nanoparticles with subpicosecond laser pulses causes a rapid increase in the lattice temperature, which can impulsively excite the phonon modes of the particle that correlate with the expansion coordinates. The vibrational periods depend on the size, shape, and elastic constants of the particles. Thus, time-resolved spectroscopy can be used to examine the material properties of nanometer-sized objects. This review provides a brief overview of the steady-state and time-resolved electronic spectroscopy of metal particles, which is important for understanding why vibrational motion appears in transient absorption traces. I also describe how the vibrational modes observed in the experiments are assigned, and what information can be obtained from the measurements. Our work has been mainly concerned with noble metal particles (gold and silver) in aqueous solution. The different shapes that have been examined to date include spheres, rods, and triangles, all with different sizes.

Hot Adsorbate-Induced Retardation of the Internal Thermalization of Nonequilibrium Electrons in Adsorbate-Covered Metal Nanoparticles

Femtosecond transient absorption spectroscopy has been used to investigate the electron-electron scattering dynamics in sulfate-covered gold nanoparticles of 2.5 and 9.2 nm in diameter. We observe an unexpected retardation of the absolute internal thermalization time compared to bulk gold, which is attributed to a negative feedback by the vibrationally excited sulfate molecules. These hot adsorbates, acting as a transient energy reservoir, result from the back and forth inelastic scattering of metal nonequilibrium electrons into the pi orbital of the sulfate. The vibrationally excited adsorbates temporarily govern the dynamical behavior of nonequilibrium electrons in the metal by re-emitting hot electrons. In other terms, metal electrons reabsorb the energy deposited in the hot sulfates by a mechanism involving the charge resonance between the sulfate molecules and the gold NPs. The higher surface-to-volume ratio of sulfate-covered gold nanoparticles of 2.5 nm leads to a stronger inhibition of the internal thermalization. Interestingly, we also note an analogy between the mechanism described here for the slow-down of electron-electron scattering in metal nanoparticles by the hot adsorbates and the hot phonon-induced retardation of hot charge carriers cooling in semiconductors.

Ultrafast Cooling of Photoexcited Electrons in Gold Nanoparticle−Thiolated DNA Conjugates Involves the Dissociation of the Gold−Thiol Bond

Using UV-visible extinction spectroscopy and femtosecond pump-probe transient absorption spectroscopy, we have studied the effect of femtosecond laser heating on gold nanoparticles attached to DNA ligands via thiol groups. It is found that femtosecond pulse excitation of the DNA-modified nanoparticles at a wavelength of 400 nm leads to desorption of the thiolated DNA strands from the nanoparticle surface by the dissociation of the gold-sulfur bond. The laser-initiated gold-sulfur bond-breaking process is a new pathway for nonradiative relaxation of the optically excited electrons within the DNA-modified gold nanoparticles, as manifested by a faster decay rate of the excited electronic distribution at progressively higher laser pulse energies. The experimental results favor a bond dissociation mechanism involving the coupling between the photoexcited electrons of the nanoparticles and the gold-sulfur bond vibrations over one involving the conventional phonon-phonon thermal heating processes. The latter processes have been observed previously by our group to be effective in the selective photothermal destruction of cancer cells bound to anti-epidermal growth factor receptor-conjugated gold nanoparticles.

Size-Dependent Ultrafast Electronic Energy Relaxation and Enhanced Fluorescence of Copper Nanoparticles

The energy relaxation of the electrons in the conduction band of 12 and 30 nm diameter copper nanoparticles in colloidal solution was investigated using femtosecond time-resolved transient spectroscopy. Experimental results show that the hot electron energy relaxation is faster in 12 nm copper nanoparticles (0.37 ps) than that in 30 nm copper nanoparticles (0.51 ps), which is explained by the size-dependent electron-surface phonon coupling. Additional mechanisms involving trapping or energy transfer processes to the denser surface states (imperfection) in the smaller nanoparticles are needed to explain the relaxation rate in the 12 nm nanoparticles. The observed fluorescence quantum yield from these nanoparticles is found to be enhanced by roughly 5 orders of magnitude for the 30 nm nanoparticles and 4 orders of magnitude for the 12 nm nanoparticles (relative to bulk copper metal). The increase in the fluorescence quantum yield is attributed to the electromagnetic enhancement of the radiative recombination of the electrons in the s-p conduction band below the Fermi level with the holes in the d bands due to the strong surface plasmon oscillation in these nanoparticles.

Nonlinear absorption of ultrashort laser pulses in thin metal films

Self-consistent simulations of the ultrafast electron dynamics in thin metal films were performed. A regime of nonlinear oscillations was observed that corresponds to ballistic electrons bouncing back and forth against the films' surfaces. When an oscillatory laser field is applied to the film, the field energy is partially absorbed by the electron gas. Maximum absorption occurs when the period of the external field matches the period of the nonlinear oscillations, which, for sodium films, lies in the infrared range. Possible experimental implementations are discussed.

Ultrafast generation of ferromagnetic order via a laser-induced phase transformation in FeRh thin films

It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations.

Structures and Dynamics of Self-Assembled Surface Monolayers Observed by Ultrafast Electron Crystallography

In this communication, we report our first study of self-assembled adsorbates on metal surfaces. Specifically, we studied single-crystal clean surfaces of Au(111) with and without a monolayer of reaction involving the assembly of 2-mercaptoacetic acid from 2,2'-dithiodiacetic acid. We also studied monolayers of iron hemes. With ultrafast electron crystallography, we are able to observe and isolate structural dynamics of the substrate (gold) and adsorbate(s) following an ultrafast temperature jump.

Ultrafast chemical interface scattering as an additional decay channel for nascent nonthermal electrons in small metal nanoparticles

The use of 4.2 nm gold nanoparticles wrapped in an adsorbates shell and embedded in a TiO2 metal oxide matrix gives the opportunity to investigate ultrafast electron-electron scattering dynamics in combination with electronic surface phenomena via the surface plasmon lifetimes. These gold nanoparticles (NPs) exhibit a large nonclassical broadening of the surface plasmon band, which is attributed to a chemical interface damping. The acceleration of the loss of surface plasmon phase coherence indicates that the energy and the momentum of the collective electrons can be dissipated into electronic affinity levels of adsorbates. As a result of the preparation process, gold NPs are wrapped in a shell of sulfate compounds that gives rise to a large density of interfacial molecules confined between Au and TiO2, as revealed by Fourier-transform-infrared spectroscopy. A detailed analysis of the transient absorption spectra obtained by broadband femtosecond transient absorption spectroscopy allows separating electron-electron and electron-phonon interaction. Internal thermalization times (electron-electron scattering) are determined by probing the decay of nascent nonthermal electrons (NNEs) and the build-up of the Fermi-Dirac electron distribution, giving time constants of 540 to 760 fs at 0.42 and 0.34 eV from the Fermi level, respectively. Comparison with literature data reveals that lifetimes of NNEs measured for these small gold NPs are more than four times longer than for silver NPs with similar sizes. The surprisingly long internal thermalization time is attributed to an additional decay mechanism (besides the classical e-e scattering) for the energy loss of NNEs, identified as the ultrafast chemical interface scattering process. NNEs experience an inelastic resonant scattering process into unoccupied electronic states of adsorbates, that directly act as an efficient heat bath, via the excitation of molecular vibrational modes. The two-temperature model is no longer valid for this system because of (i) the temporal overlap between the internal and external thermalization process is very important; (ii) a part of the photonic energy is directly transferred toward the adsorbates (not among "cold" conduction band electrons). These findings have important consequence for femtochemistry on metal surfaces since they show that reactions can be initiated by nascent nonthermal electrons (as photoexcited, out of a Fermi-Dirac distribution) besides of the hot electron gas.

Electron-phonon scattering in metal clusters

Electron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect.

Optical properties and ultrafast dynamics of metallic nanocrystals

Noble metal particles have long fascinated scientists because of their intense color, which led to their application in stained glass windows as early as the Middle Ages. The recent resurrection of colloidal and cluster chemistry has brought about the strive for new materials that allow a bottoms-up approach of building improved and new devices with nanoparticles or artificial atoms. In this review, we discuss some of the properties of individual and some assembled metallic nanoparticles with a focus on their interaction with cw and pulsed laser light of different energies. The potential application of the plasmon resonance as sensors is discussed.

Coherent magnetization rotation and phase control by ultrashort optical pulses in CrO(2) thin films

We have applied photoexcitation by ultrashort laser pulses to single crystal thin CrO(2) films to trigger coherent transient magnetization rotation on a subnanosecond time scale, in macroscale single domains. Moreover, by applying the photoexcitation by pairs of temporally separated pump pulses, the transient precession of the magnetization can be phase controlled, depending on the time separation between the pulses. The mechanism behind the photoexcitation originates from the modulation of the magnetocrystalline anisotropy by nonthermal hot electron spins.