Optical and Physical Probing of Thermal Processes in Semiconductor and Plasmonic Nanocrystals

This article reviews thermal properties of semiconductor and emergent plasmonic nanomaterials, focusing on mechanisms through which hot carriers and phonons are produced and dissipated as well as the related impacts on optoelectronic properties. Elevated equilibrium temperatures, of particular relevance for implementation of nanomaterials in devices, affect absorptive and radiative transitions as well as emission efficiency that can present reversible and irreversible changes with temperature. In noble metal or doped semiconductor/insulator nanomaterials, hot carriers and lattice heating can substantially influence localized surface plasmon resonances and yield large ultrafast changes in transmission or strongly oscillatory coherences. Transient optical and diffraction characterizations enable nonequilibrium investigations of phonon dynamics and cooling such as lattice expansion and crystal phase stability. Timescales of nanoparticle thermalization with surroundings and transport of heat within films of such materials are also discussed.

Electronic and nuclear contributions to time-resolved optical and X-ray absorption spectra of hematite and insights into photoelectrochemical performance

Spectra show both transient photocarriers and lattice heating.

High-Speed Dicing of SiC Wafers by Femtosecond Pulsed Laser

A novel dicing technology that utilizes femtosecond pulsed lasers (FSPLs) are demonstrated as a high-speed and cost-effective dicing process for SiC wafers. The developed dicing process consists of cleavage groove formation on a SiC wafer surface by the FSPL, followed by chip separation by pressing a cleavage blade. The effective FSPL scan speed on the SiC surfaces was 33 mm/s. Kerf loss can be negligible in the developed FSPL dicing process. In addition, the residual lattice strain in the FSPL-diced SiC chips was comparably small to that of the conventional mechanical process using diamond saws, due to the absence of the lattice heating effect in femtosecond-laser processes.

Photoluminescence Study of the Driving Force for Stacking Fault Expansion in 4H-SiC

We investigated expansion velocities of Shockley stacking faults (SSFs) in 4H-silicon carbide under laser illumination using photoluminescence methods. The experiments showed that the velocity of SSF expansion or the glide velocity of SSF-bounding 30°-Si(g) partial dislocations (PD) is supralinearly dependent on the excitation intensity. We estimated sample temperature by analyzing the broadening of band-edge emission and concluded that the lattice heating by laser illumination is not the cause of the enhanced dislocation glide. The supralinear dependence can be accounted for by a photo-induced sign reversal of the effective formation energy of SSF acting as the driving force of SSF expansion under the illumination.

Observation of Ultrafast Lattice Heating in thin Metal Films using Time-resolved Electron Diffraction

We show that time-resolved electron diffraction is capable of revealing the ultrafast lattice heating in thin metal films following excitation by a femtosecond laser pulse. The build-up of the lattice temperature leads to a reduction of the diffraction intensity of the various diffraction orders due to the Debye-Waller-effect. We also observed a reduction of the transmitted (000)-signal which exhibits the same temporal evolution as the diffraction signals.