Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.

Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses

One of the great dream experiments in Science is to directly observe atomic motions as they occur. Femtosecond electron diffraction provided the first 'light' of sufficient intensity to achieve this goal by attaining atomic resolution to structural changes on the relevant timescales. This review covers the technical progress that made this new level of acuity possible and gives a survey of the new insights gained from an atomic level perspective of structural dynamics. Atomic level views of the simplest possible structural transition, melting, are discussed for a number of systems in which both thermal and purely electronically driven atomic displacements can be correlated with the degree of directional bonding. Optical manipulation of charge distributions and effects on interatomic forces/bonding can be directly observed through the ensuing atomic motions. New phenomena involving strongly correlated electron?lattice systems are also discussed in which optically induced changes in the potential energy landscape lead to ballistic structural changes. Concepts such as the structural order parameters are now directly observable at the atomic level of inspection to give a remarkable view of the extraordinary degree of cooperativity involved in strongly correlated electron?lattice systems. These recent examples, in combination with time-resolved real space imaging now possible with electron probes, are truly defining an emerging field that holds great promise to make a significant impact in how we understand structural dynamics.This article is dedicated to the memory of Professor David John Hugh Cockayne, a world leader in electron microscopy, who sadly passed away in December.

https://iopscience.iop.org/article/10.1088/0034-4885/74/9/096101/pdf

Femtosecond electron diffraction: heralding the era of atomically resolved dynamics

Using ultrafast, time-resolved, 1.54 angstrom x-ray diffraction, thermal and ultrafast nonthermal melting of germanium, involving passage through nonequilibrium extreme states of matter, was observed. Such ultrafast, optical-pump, x-ray diffraction probe measurements provide a way to study many other transient processes in physics, chemistry, and biology, including direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place.

http://nanofast.ucdavis.edu/Publications/Detection%20Nonthermal%20Melting%20Xray%20Diffrac_1999.pdf

Detection of nonthermal melting by ultrafast X-ray diffraction.

Measuring atomic-resolution images of materials with x-ray photons during chemical reactions or physical transformations resides at the technological forefront of x-ray science. New x-ray-based experimental capabilities have been closely linked with advances in x-ray sources, a trend that will continue with the impending arrival of x-ray-free electron lasers driven by electron accelerators. We discuss recent advances in ultrafast x-ray science and coherent imaging made possible by linear-accelerator-based light sources. These studies highlight the promise of ultrafast x-ray lasers, as well as the technical challenges and potential range of applications that will accompany these transformative x-ray light sources.

https://science.sciencemag.org/content/sci/316/5830/1444.full.pdf

Imaging Atomic Structure and Dynamics with Ultrafast X-ray Scattering

http://www.exp.physik.uni-due.de/vonderLinde/Publikationen/ICONO98.pdf

The physical mechanisms of short-pulse laser ablation

The effect of a dense electron-hole plasma on the stability of the diamond lattice of the crystalline group-IV elemental semiconductors C, Si, and Ge is examined with use of a tight-binding model. Such a plasma may result, for example, from a short, intense laser pulse. We find that the transverse-acoustic phonons of Si become soft if about 9% of the electrons are excited from the valence band into the conduction band. At higher densities of the electron-hole excitations the cubic symmetry of the diamond lattice is destroyed within less than 100 fs after the creation of the electron-hole plasma. This is much shorter than the time needed for the crystal to melt. The instability of the lattice then leads directly to a very rapid melting of the crystal structure. Our results are in agreement with recent experiments using pulsed lasers to induce disorder in crystalline Si surfaces. We obtain for C and Ge essentially the same theoretical results as for Si.

Theory for the instability of the diamond structure of Si, Ge, and C induced by a dense electron-hole plasma.

We extend our previous analysis of the ultrafast laser-induced instability of the diamond structure of semiconductors by including longitudinal optical-phonon distortions in addition to the instability of the transverse acoustic phonons. Generally, longitudinal optical distortions enhance the instability of the transverse acoustic phonons, increasing the kinetic energy of the atoms and the final lattice temperature. These phonons make a transition to a centrosymmetric structure of GaAs with metallic properties possible. We present results for the time dependence of the instability of Si for the case where 15% of the valence electrons have been excited into the conduction band. Thus, already 100 fsec after the excitation of the plasma the atoms are displaced about 1 \AA{} from their equilibrium position and their kinetic energy has increased to approximately 0.4 eV. Collisions between the atoms then lead to a rapid melting of the crystal. These results are in good agreement with recent experiments performed on Si and GaAs.

Time dependence of the laser-induced femtosecond lattice instability of Si and GaAs: Role of longitudinal optical distortions.

By extending our previous theory we analyze the observed time-resolved instability of the diamond lattice of silicon, which is induced by a dense electron-hole plasma and which may result from an intense laser pulse. The transverse acoustic phonons become instable if more than 8% of the valence electrons are excited into the conduction band. Furthermore, if 15--20 % of the valence electrons are excited, the average atomic displacement increases to more than 1 \AA{} within less than 200 fsec after the laser pulse. This very rapidly destroys the symmetries of the lattice and leads to a strong excitation of transverse phonons, resulting in a high temperature. Our results are in good agreement with experiments done on surfaces of silicon and GaAs.

P. Stampfli

Papers

Theory for the instability of the diamond structure of Si, Ge, and C induced by a dense electron-hole plasma.

Time dependence of the laser-induced femtosecond lattice instability of Si and GaAs: Role of longitudinal optical distortions.

Dynamical theory of the laser-induced lattice instability of silicon.