arXiv subjects

Simon Wall

Publications and source records attributed to Simon Wall.

21 records · Page 2Linked to original sources

arXiv2015

Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials

The extreme electro-optical contrast between crystalline and amorphous states in phase change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise due to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a unique combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 femtoseconds due to a rapid depletion of electrons from resonantly-bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2 ps time constant. The optical changes are an order-of-magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

cond-mat.mtrl-sci↗

arXiv2014

Witnessing the formation and relaxation of massive quasi-particles in a strongly correlated electron system

The non-equilibrium semiconductors physics is based on the paradigm that different degrees of freedom interact on different timescales. In this context the photo-excitation is often treated as an impulsive injection of electronic energy that is transferred to other degrees of freedom only at later times. Here, by studying the ultrafast particles dynamics in a archetypal strongly correlated charge-transfer insulator (La2CuO4), we show that the interaction between electrons and bosons manifest itself directly in the photo-excitation processes of a correlated material. With the aid of a general theoretical framework (Hubbard Holstein Hamiltonian), we reveal that sub-gap excitation pilots the formation of itinerant quasi-particles which are suddently dressed (<100 fs) by an ultrafast reaction of the bosonic field.

cond-mat.str-el↗

arXiv2012

Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, in order to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. We show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

cond-mat.str-el↗