Structural dynamics in atomic indium wires on silicon: From ultrafast probing to coherent vibrational control

Abstract

Light-control of structural dynamics at surfaces promises switching of chemical and physical functionality at rates limited only by the velocity of directed atomic motion. Following optical stimulus by femtosecond light pulses (1 fs = 10^-15 s), transient electronic and lattice excitations can drive phase transitions in solids. Coherent control schemes facilitate a selective transfer of optical energy to specific electronic or vibrational degrees of freedom, as exemplified by the steering of molecular reactions via optical pulse sequences in femtochemistry. However, a transfer of this concept from molecules to solids requires coupling of few decisive phonons to optical transitions in the electronic band structure, and a weak coupling to other lattice modes to maximize coherence times. In this respect, atomic indium wires on the (111) surface of silicon represent a highly attractive model system, with a Peierls-like phase transition between insulating (8×2) and metallic (4×1) structures, governed by shear and rotation phonons. This review provides a survey on our advances in the time-resolved probing and coherent vibrational control of the In/Si(111) surface. In particular, we discuss how coherent atomic motion can be harnessed to affect the efficiency and threshold of the phase transition. Starting from a description of the (8×2) and (4×1) equilibrium structures and key vibrational modes, we study the structural dynamics following single-pulse optical excitation of the (8×2) phase. Our results highlight the ballistic order-parameter motion in the nonequilibrium transition as well as the impact of microscopic heterogeneity on the excitation and subsequent relaxation of the metastable photo-induced (4×1) phase. Furthermore, we discuss our results on the combination of ultrafast low-energy electron diffraction (ULEED) with optical pulse sequences to investigate the coherent control over the transition, mode-selective excitation and the location of the transition state.