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. 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, low-symmetry 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/cm2 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.Comment: 22 pages, 4 figure

Carrier confinement and bond softening in photoexcited bismuth films

Femtosecond pump-probe spectroscopy of bismuth thin films has revealed strong dependencies of reflectivity and phonon frequency on film thickness in the range of 25−40 nm. The reflectivity variations are ascribed to distinct electronic structures originating from strongly varying electronic temperatures and proximity of the film thickness to the optical penetration depth of visible light. The phonon frequency is redshifted by an amount that increases with decreasing film thickness under the same excitation fluence, indicating carrier density-dependent bond softening that increases due to suppressed diffusion of carriers away from the photoexcited region in thin films. The results have significant implications for nonthermal melting of bismuth as well as lattice heating due to inelastic electron-phonon scattering.United States. Office of Naval Research (Grant N00014-12-1-0530)National Science Foundation (U.S.) (Grant CHE-1111557

Long-lived photoinduced response observed under extreme photoexcitation densities in a one-dimensional Peierls insulator

One-dimensional metal-halide compounds provide model systems to investigate the manner in which coupling between elementary degrees of freedom—here, electronic and vibrational—result in instabilities that give rise to both chemical and structural rearrangements. Here, we employ “single-shot” pump-probe spectroscopy to examine a one-dimensional platinum iodide compound (PtI(en)) under far-from-equilibrium conditions where repeated photoexcitation results in sample damage. It presents evidence for a distinct collective excited state lasting more than 100 ps upon self-trapped exciton generation at high densities, as measured by electronic signal amplitudes and phonon properties.National Science Foundation (U.S.) (Grant CHE-1111557)United States. Office of Naval Research (Grant N00014-12-1-0530