  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.
