Understanding and utilizing the transport of thermal energy at the nanoscale via various excitations such as hot electrons, lattice vibrations and spin waves is a fundamental scientific challenge that is relevant to all research fields of the group: It touches plasmonics as the origin of the broadening of optical resonances and is highly relevant to control chemical reactivity. In ultrafast lattice dynamics, the heat transport limits the available spatio-temporal shape of the photoinduced stresses driving the strain waves. Anharmonic interactions of phonons determine phononic heat transport via Umklapp-processes and govern the nonlinear strain propagation at large amplitudes. In ultrafast magnetism, nanoscale heat enables „heat-assisted magnetic recording (HAMR)“ and it is important to calibrate the contributions to the specific heat by spins, electrons and phonons. After all, heat controls the phase transitions in magnetically ordered materials and in ferroelectrics. We believe that ultrafast control of heat and strain at the nanoscale can be exploited for the realization of ultrafast ferroelectric switching processes.
Pudell J.E., Mattern M., Hehn M., Malinowski G., Herzog M., and Bargheer M.
Heat Transport without Heating?—An Ultrafast X‐Ray Perspective into a Metal Heterostructure
Rewarded as Inside Back Cover article:
Advanced Functional Materials 30, 2070304 (2020).
When the spatial dimensions of metallic heterostructures shrink below the mean free path of its conduction electrons, the transport of electrons and hence the transport of thermal energy by electrons continuously changes from diffusive to ballistic. Electron–phonon coupling sets the mean free path to the nanoscale and the time for equilibration of electron and lattice temperatures to the picosecond range. A particularly intriguing situation occurs in trilayer heterostructures combining metals with very different electron–phonon coupling strength: Heat energy deposited in few atomic layers of Pt is transported into a nanometric Ni film, which is heated more than the Cu film through which the heat is released. Femtosecond pump‐probe experiments with hard X‐ray pulses provide a layer‐specific probe of the heat energy. A purely diffusive two‐temperature model with increased thermal conductivity of hot electrons excellently reproduces the observed signals from all three layers. At the time when the Ni lattice is maximally heated, no significant heat has entered the Cu lattice. This phenomenon would be enhanced in thinner layers where ballistic transport dominates. In this context it is shown that purely diffusive transport can lead to a linear time‐to‐length dependence that must not be misinterpreted as ballistic transport.
Sarhan R. M., Koopman W., Pudell J.-E., Stete F., Rössle M., Herzog M., Schmitt C. N. Z., Liebig F., Koetz J., and Bargheer M.
Scaling up Nanoplasmon Catalysis: The Role of Heat DIssipation
Nanoscale heating by optical excitation of plasmonic nanoparticles offers a new perspective of controlling chemical reactions, where heat is not spatially uniform as in conventional macroscopic heating but strong temperature gradients exist around microscopic hot spots. In nanoplasmonics, metal particles act as a nanosource of light, heat, and energetic electrons driven by resonant excitation of their localized surface plasmon resonance. As an example of the coupling reaction of 4-nitrothiophenol into 4,4′-dimercaptoazobenzene, we show that besides the nanoscopic heat distribution at hot spots, the microscopic distribution of heat dictated by the spot size of the light focus also plays a crucial role in the design of plasmonic nanoreactors. Small sizes of laser spots enable high intensities to drive plasmon-assisted catalysis. This facilitates the observation of such reactions by surface-enhanced Raman scattering, but it challenges attempts to scale nanoplasmonic chemistry up to large areas, where the excess heat must be dissipated by one-dimensional heat transport.
Pudell J.-E., Maznev A. A., Herzog M., Kronseder M., Back C. H., Malinowski G., Reppert A. v., and Bargheer M.
Layer specific observation of slow thermal equilibration in ultrathin metallic nanostructures by femtosecond X-ray diffraction
Ultrafast heat transport in nanoscale metal multilayers is of great interest in the context of optically induced demagnetization, remagnetization and switching. If the penetration depth of light exceeds the bilayer thickness, layer-specific information is unavailable from optical probes. Femtosecond diffraction experiments provide unique experimental access to heat transport over single digit nanometer distances. Here, we investigate the structural response and the energy flow in the ultrathin double-layer system: gold on ferromagnetic nickel. Even though the excitation pulse is incident from the Au side, we observe a very rapid heating of the Ni lattice, whereas the Au lattice initially remains cold. The subsequent heat transfer from Ni to the Au lattice is found to be two orders of magnitude slower than predicted by the conventional heat equation and much slower than electron–phonon coupling times in Au. We present a simplified model calculation highlighting the relevant thermophysical quantities.