The iterative algorithm of Feldman for heat flow in layered structures is solved in cylindrical coordinates for surface heating and temperature measurement by Gaussian-shaped laser beams. This solution for the frequency-domain temperature response is then used to model the lock-in amplifier signals acquired in time-domain thermoreflectance measurements of thermal properties.

Analysis of heat flow in layered structures for time-domain thermoreflectance

The femtosecond optical response of noble metal nanoparticles and its connection to the ultrafast electron dynamics are discussed in light of the results of high-sensitivity femtosecond pump−probe experiments. The physical origins of the nonlinear responses in the vicinity of the surface plasmon resonance and interband transition threshold are analyzed using extension of the theoretical models used in the bulk materials. These responses contain information on the electron interaction processes (electron−electron and electron−phonon scattering) that can thus be directly investigated in the time domain. Their size and environment dependences are discussed, and the results are compared to the ones in the bulk materials. Time-resolved techniques also permit direct study of the vibrational modes of metal nanoparticles and, in particular, the determination of their damping, which is a sensitive probe of the nature of the surrounding matrix and of the interface quality.

Ultrafast Electron Dynamics and Optical Nonlinearities in Metal Nanoparticles

We review recent advances in experimental methods for high spatial-resolution and high time-resolution thermometry, and the application of these and related methods for measurements of thermal transport in low-dimensional structures. Scanning thermal microscopy (SThM) achieves lateral resolutions of 50 nm and a measurement bandwidth of 100 kHz: SThM has been used to characterize differences in energy dissipation in single-wall and multi-wall carbon nanotubes. Picosecond thermoreflectance enables ultrahigh time-resolution in thermal diffusion experiments and characterization of heat flow across interfaces between materials; the thermal conductance G of interfaces between dissimilar materials spans a relatively small range, 20<G<200 MW m -2 K -1 near room temperature. Scanning thermoreflectance microscopy provides nanosecond time resolution and submicron lateral resolution needed for studies of heat transfer in microelectronic, optoelectronic and micromechanical systems. A fully-micromachined solid immersion lens has been demonstrated and achieves thermal-radiation imaging with lateral resolution at far below the diffraction limit, <2 μm. Microfabricated metal bridges using electrical resistance thermometry and joule heating give precise data for thermal conductivity of single crystal films, multilayer thin films, epitaxial superlattices, polycrystalline films, and interlayer dielectrics. The room temperature thermal conductivity of single crystal films of Si is strongly reduced for layer thickness below 100 nm. The through-thickness thermal conductivity of Si-Ge and GaAs-AlAs superlattices has recently been shown to be smaller than the conductivity of the corresponding alloy. The 3ω method has been recently extended to measurements of anisotropic conduction in polyimide and superlattices. Data for carbon nanotubes measured using micromachined and suspended heaters and thermometers indicate a conductivity near room temperature greater than diamond.

https://nanoheat.stanford.edu/sites/default/files/publications/A57.pdf

Thermometry and Thermal Transport in Micro/Nanoscale Solid-State Devices and Structures

Tailoring the thermal conductivity of nanostructured materials is a fundamental challenge for nano- and microelectronics heat management It is now demonstrated how to modify the thermal conductivity of SiGe by engineering nanodot inclusions in regions as short as 15 nm A similar approach could used on other materials, extending the range of thermal conductivities available

/pdf/precise-control-of-thermal-conductivity-at-the-nanoscale-4szesujuif.pdf

Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers

Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.

Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses.

We report results obtained using an optical interferometric detection in picosecond ultrasonics. A comparison between signals detected by reflectometric and interferometric measurements was done in various systems: thin films, multilayers and particles in colloids.

Interferometric detection in picosecond ultrasonics

Ultrasonics signals at frequencies 5.7±0.1 and 6.8±0.1GHz are measured in two organelles of a single vegetal cell in vitro with a picosecond ultrasonic technique. Using standard values for cell optical index, ultrasound velocities of 1.6±0.1 and 2.0±0.1μm∕ns are measured from several signals recorded in the vacuole and in the nucleus of a single Allium cepa cell, respectively. A 1μm lateral and 0.25μm depth resolution is attained.

/pdf/in-vitro-picosecond-ultrasonics-in-a-single-cell-58dc3tm1dp.pdf

In Vitro picosecond ultrasonics in a single cell

There are several classes of coherent phonons whose generation mechanisms are completely different: impulsive stimulated Raman scattering, Brillouin oscillations, and coherent longitudinal-acoustic-phonon Bragg reflection. All of these are investigated in Si/ SiGe superlattices using a picosecond ultrasonics technique at room temperature. Bragg reflection for a single vibrational mode is related to gaps in the phonon-dispersion relation at the center and the boundary of the folded Brillouin zone. The two lowest minigaps are observed at 283 and 527 GHz. Theoretical calculations of phonon-dispersion relation and phonon reflection rate are given to explain experimental results.

/pdf/coherent-phonons-in-si-sige-superlattices-4xf08hja6y.pdf

Coherent phonons in Si/ SiGe superlattices

The picosecond acoustic technique is a pump and probe method associated with a lock-in amplification scheme. It involves the excitation of the sample by an ultrashort optical pulse and the monitoring of the subsequent relaxation processes by a weaker pulse delayed with respect to the former. In particular, information about the temperature evolution a short duration after the pump pulse is absorbed can be obtained. We examine here the different time scales involved in such an experiment. We show that the photothermal signal is superimposed on a continuous background originating from the modulation of the pump beam for synchronous detection. We also derive expressions for the cooling rate of both bulk and layered specimens after an instantaneous heating and we show experimentally that this macroscopic approach is valid after a delay of several picoseconds.

Clément Rossignol

Papers

Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses.

Interferometric detection in picosecond ultrasonics

In Vitro picosecond ultrasonics in a single cell

Coherent phonons in Si/ SiGe superlattices

Photothermal properties of bulk and layered materials by the picosecond acoustics technique