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. Regardingthis 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 us to determine the electron and lattice temperature dynamics in a spherical gold nanoparticle core under subpicosecond pulsed excitation, as well as that of 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 conduction 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

Off-resonance field enhancement by spherical nanoshells

We study light scattering by spherical nanoshells consistent of metal/dielectric composites. We consider two geometries of metallic nanoshell with dielectric core, and dielectric coated metallic nanoparticle. We demonstrate that for both geometries the local field enhancement takes place out of resonance regions ("dark states"), which, nevertheless, can be understood in terms of the Fano resonance. At optimal conditions the light is stronger enhanced inside the dielectric material. By using nonlinear dielectric materials it will lead to a variety nonlinear phenomena applicable for photonics applications

Anisotropic Decay Dynamics of Photoexcited Aligned Carbon Nanotube Bundles

We have performed polarization-dependent ultrafast pump-probe spectroscopy of a film of aligned single-walled carbon nanotube bundles. By taking into account imperfect nanotube alignment as well as anisotropic absorption cross sections, we quantitatively determined distinctly different photo-bleaching dynamics for polarizations parallel and perpendicular to the tube axis. For perpendicular polarization, we observe a slow (1.0-1.5 ps) relaxation process, previously unobserved in randomly-oriented nanotube bundles. We attribute this slower dynamics to the excitation and relaxation of surface plasmons in the radial direction of the nanotube bundles.Comment: 4 pages, 3 figure

On-command enhancement of single molecule fluorescence using a gold nanoparticle as an optical nano-antenna

We investigate the coupling of a single molecule to a single spherical gold nanoparticle acting as a nano-antenna. Using scanning probe technology, we position the particle in front of the molecule with nanometer accuracy and measure a strong enhancement of more than 20 times in the fluorescence intensity simultaneous to a 20-fold shortening of the excited state lifetime. Direct comparison with three-dimensional calculations allow us to decipher the contributions of the excitation enhancement, spontaneous emission modification, and quenching. Furthermore, we provide direct evidence for the role of the particle plasmon resonance in the modification of the molecular emission.Comment: 5 pages, 4 figures. submitted to Phys.Rev.Lett. 12/04/200

Gardens of life: Multifunctional and ecosystem services of urban cemeteries in Central Europe and beyond—Historical, structural, planning, nature and heritage conservation aspects

Cemeteries are often seen as monofunctional spaces for burial and mourning and, within the dynamically changing urban fabric, as a planning conundrum. Long periods of stability have also turned these untouched and hidden places into refugia for nature and wildlife. In booming and dense cities with high land use pressures and housing shortages, in particular, as the amount of burial ground needed per citizen decreases and burial cultures change, the cemetery has become a contested nature, as a simultaneous space of emotion, commerce and community. We revisited the diversity and ontogenesis of cemeteries, and the interactions with neighboring uses of the urban matrix. Our review demonstrates a wide range of different ecosystem services of urban cemeteries, beyond potential as hotspots of culture and biodiversity. We highlight their multifunctional character and the need for a holistic and trans-disciplinary evaluation using multistakeholder approaches to further develop cemeteries as a crucial element of sustainable urban landscapes.Peer Reviewe

Dynamics of femtosecond laser-induced melting and amorphization of indium phosphide

7 pages, 5 figures.-- PACS: 64.70.Dv; 81.30.Fb; 61.80.Ba; 78.66.Fd; 61.82.FkLaser-induced melting and resolidification of single-crystalline indium phosphide (InP) upon irradiation with 150 fs laser pulses at 800 nm has been investigated by means of real-time-reflectivity measurements with subnanosecond time resolution. Melting of the surface is observed to occur very rapidly on a time scale shorter than our experimental resolution while the lifetime of the liquid phase is several tens of nanoseconds. As a result of the subsequent rapid solidification process, a thin layer of amorphous material with a thickness of several tens of nanometers is formed on the surface. The formation of this amorphous layer has been observed for every fluence above the melting and below the ablation threshold. The evolution of the reflectivity has been modeled for several different solidification scenarios and compared to the experimental results. This comparison shows that solidification proceeds interfacially from the solid interface towards the surface. A lower limit for the critical solid-liquid interface velocity for amorphization in this compound semiconductor has been estimated to be in the range of 1–4 m/s.This work has been partially supported by the EU in the frame of the TMR Project XPOSE (Grant No. HPRN-CT- 2000-00160). S.M.W. acknowledges the funding in the frame of the same project. J.B. acknowledges the funding of the CSIC through a contract in the frame of the I3P programme (Ref. I3P-PC2002), co-funded by the European Social Fund.Peer reviewe

Calculating Nonlocal Optical Properties of Structures with Arbitrary Shape

In a recent Letter [Phys. Rev. Lett. 103, 097403 (2009)], we outlined a computational method to calculate the optical properties of structures with a spatially nonlocal dielectric function. In this Article, we detail the full method, and verify it against analytical results for cylindrical nanowires. Then, as examples of our method, we calculate the optical properties of Au nanostructures in one, two, and three dimensions. We first calculate the transmission, reflection, and absorption spectra of thin films. Because of their simplicity, these systems demonstrate clearly the longitudinal (or volume) plasmons characteristic of nonlocal effects, which result in anomalous absorption and plasmon blueshifting. We then study the optical properties of spherical nanoparticles, which also exhibit such nonlocal effects. Finally, we compare the maximum and average electric field enhancements around nanowires of various shapes to local theory predictions. We demonstrate that when nonlocal effects are included, significant decreases in such properties can occur.Comment: 30 pages, 12 figures, 1 tabl

Light propagation in nanorod arrays

We study propagation of TM- and TE-polarized light in two-dimensional arrays of silver nanorods of various diameters in a gelatin background. We calculate the transmittance, reflectance and absorption of arranged and disordered nanorod arrays and compare the exact numerical results with the predictions of the Maxwell-Garnett effective-medium theory. We show that interactions between nanorods, multipole contributions and formations of photonic gaps affect strongly the transmittance spectra that cannot be accounted for in terms of the conventional effective-medium theory. We also demonstrate and explain the degradation of the transmittance in arrays with randomly located rods as well as weak influence of their fluctuating diameter. For TM modes we outline the importance of skin-effect, which causes the full reflection of the incoming light. We then illustrate the possibility of using periodic arrays of nanorods as high-quality polarizers.Comment: 6 pages, 7 figure

Photoemission Electron Microscopy as a tool for the investigation of optical near fields

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO$_{x}$). Photoemission was induced by illumination with a Hg UV-lamp (photon energy cutoff $\hbar\omega_{UV}=5.0$ eV, wavelength $\lambda_{UV}=250$ nm) and with a Ti:Sapphire femtosecond laser ($\hbar\omega_{l}=3.1$ eV, $\lambda_{l}=400$ nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.Comment: 10 pages, 4 figures; submitted to Physical Review Letter

Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles

We present a microscopic model for surface-enhanced Raman scattering (SERS) from molecules adsorbed on small noble-metal nanoparticles. In the absence of direct overlap of molecular orbitals and electronic states in the metal, the main enhancement source is the strong electric field of the surface plasmon resonance in a nanoparticle acting on a molecule near the surface. In small particles, the electromagnetic enhancement is strongly modified by quantum-size effects. We show that, in nanometer-sized particles, SERS magnitude is determined by a competition between several quantum-size effects such as the Landau damping of surface plasmon resonance and reduced screening near the nanoparticle surface. Using time-dependent local density approximation, we calculate spatial distribution of local fields near the surface and enhancement factor for different nanoparticles sizes.Comment: 8 pages, 6 figures. Considerably extended final versio