Real-Time Observation of a Coherent Lattice Transformation into a High-Symmetry Phase

Abstract

†These authors contributed equally to this work Excursions far from their equilibrium structures can bring crystalline solids through collective transformations including transitions into new phases that may be transient or long-lived. Direct spectroscopic observation of far-from-equilibrium rearrangements provides fundamental mechanistic insight into chemical and structural transformations, and a potential route to practical applications, including ultrafast optical control over material structure and properties. However, in many cases photoinduced transitions are irreversible or only slowly reversible, or the light fluence required exceeds material damage thresholds. This precludes conventional ultrafast spectroscopy in which optical excitation and probe pulses irradiate the sample many times, each measurement providing information about the sample response at just one probe delay time following excitation, with each measurement at a high repetition rate and with the sample fully recovering its initial state in between measurements. Using a single-shot, real-time measurement method, we were able to observe the photoinduced phase transition from the semimetallic, lowsymmetry phase of crystalline bismuth into a high-symmetry phase whose existence at high electronic excitation densities was predicted based on earlier measurements at moderate excitation densities below the damage threshold. Our observations indicate that coherent lattice vibrational motion launched upon photoexcitation with an incident fluence above 10 mJ/cm 2 in bulk bismuth brings the lattice structure directly into the high-symmetry configuration for tens of picoseconds, after which carrier relaxation and diffusion restore the equilibrium lattice configuration.

Nanosecond Laser Switching of Phase‐Change Random Metasurfaces with Tunable ON‐State

“…[46,47] These works triggered a vivid discussion about ultrafast transient solid-solid phase transitions, which were further investigated by numerous groups, reporting evidence for nonthermal melting, [48] transient formation of a high-symmetry solid phase in ref. [49]. However, these works also showed that such transient states are short-lived (tens of picosecond time scale), require femtosecond excitation to be triggered, and are accompanied by electron-phonon relaxation and thermal diffusion in longer time scales.…”

Snapshots of a light-induced metastable hidden phase driven by the collapse of charge order

“…A 4f imaging systems was used to relay the signal and reference grids onto a CCD (Hamamatsu Orca-ER). These grids were then combined, binned and unfolded to generate a pump-probe transient reflectivity trace measured in each laser shot [18,30].…”

“…Singleshot time-resolved spectroscopy techniques, on the other hand, can capture, with a single pump laser shot, the real-time dynamical evolution of slowly reversible and irreversible processes. [12,18,19] Single-shot measurements thereby can offer unique mechanistic insights into the formation of metastable hidden phases.…”

Revealing thermally-activated nucleation pathways of diffusionless solid-to-solid transition

“…iffusionless solid-to-solid transitions (martensitic transition) have been widely observed in both hard materials, such as metals, alloys, and ceramics [1][2][3][4][5][6][7][8][9][10] , and soft-matter systems, such as protein solutions and colloidal suspensions [11][12][13][14] . Despite the variety of these systems with various softness and symmetry [4][5][6][7][8][9][10][11][12][13][14] , one common intriguing feature is the formation of rich microstructures via complex nucleation and growth pathways. Nuclei formation has so far been believed to be athermal because immense strain energy is required to overcome a high barrier, which is the case for the steel's martensitic transition.…”