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 / cm 2 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.
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机译：远离其平衡结构的游览可以通过集体转变来带来晶体固体，包括转变为可能是瞬态或长期的新阶段。远程均衡重排的直接光谱观察为化学和结构转变和实际应用的潜在路线提供了基本的机械洞察，包括超快光学控制材料结构和性能。然而，在许多情况下，光诱导的过渡是不可逆转的或仅缓慢可逆，或者所需的光线量超过材料损伤阈值。该要求排除了传统的超快光谱，其中光学激发和探针脉冲多次照射样品，每次测量在激励之后只提供一个探针延迟时间的样本响应的信息，每次测量都以高重复率和样本充分测量在测量之间恢复其初始状态。使用单次实时测量方法，我们能够观察从结晶铋的半金属，低对称相位到高对称相位的光诱导相转变，其基于更早地预测了高电子励磁密度的存在性的高对称阶段在损伤阈值以下的中等激励密度下测量。我们的观察结果表明，在散装铋10 mJ / cm 2以上的入射率为10mJ / cm 2中的相干晶格振动运动在散列铋中将晶格结构直接带入几种皮秒的高对称配置，之后载体松弛和扩散恢复平衡晶格配置。
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