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

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. The 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 requirement 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 are able to observe the photoinduced phase transition from the semimetallic, low-symmetry phase of crystalline bismuth into a high-symmetry phase whose existence at high electronic excitation densities is 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/cm2 in bulk bismuth brings the lattice structure directly into the high-symmetry configuration for several picoseconds, after which carrier relaxation and diffusion restore the equilibrium lattice configuration.