Watching a phase transition unfold
The layered material 1 T-polytype of tantalum disulfide has several intricate charge-ordered phases. How exactly one phase transitions into another is tricky to observe directly with current technologies. Danz et al. used pump-probe ultrafast dark-field electron microscopy to follow such a transition with fine spatial and temporal resolution (see the Perspective by Kogar). To achieve that goal, they shaped the electron beam used as the probe to bring out the features peculiar to the transition.
层状TaS2材料拥有复杂的电荷有序相。如何观察从一个相变过程具有非常大挑战性。Danz 等人使用泵浦-探测的超快暗场电子显微镜，在一定空间和时间分辨率下观察到了相变过程。需要把电子束作为探测光来观察到相变特有的特征。
Abstract
Understanding microscopic processes in materials and devices that can be switched by light requires experimental access to dynamics on nanometer length and femtosecond time scales. Here, we introduce ultrafast dark-field electron microscopy to map the order parameter across a structural phase transition. We use ultrashort laser pulses to locally excite a 1T-TaS2 (1T-polytype of tantalum disulfide) thin film and image the transient state of the specimen by ultrashort electron pulses. A tailored dark-field aperture array allows us to track the evolution of charge-density wave domains in the material with simultaneous femtosecond temporal and 5-nanometer spatial resolution, elucidating relaxation pathways and domain wall dynamics. The distinctive benefits of selective contrast enhancement will inspire future beam-shaping technology in ultrafast transmission electron microscopy.
理解光引发的材料和器件中的微观过程需要试验上能改在纳米空间尺度和飞秒时间尺度上观察的实验设备。这里介绍了超快暗场电子显微镜技术能够为结构相变过程提供信息。首先使用超快激光局域激发1T-TaS2薄膜，然后对样品用超快电子束对样品的瞬态过程进行成像。定制的暗场孔径阵列能够追踪电荷密度波的演化，在fs和5nm尺度激发下，理解弛豫的过程和畴壁动力学。选择对比加强的独特优势能够对于启发未来超快电子显微镜中的光束整形技术。
Snapshots of a light-triggered transition Anshul Kogar, 对于pump-probe技术有一个科普性的介绍，对于此项技术的应用领域有一个简要的介绍。
In an iconic 1964 photo of a bullet piercing an apple, Harold “Doc” Edgerton captured a snapshot that took a mere millionth of a second (1). The same working principle behind his highspeed images have been adopted today to study the evolution of ordered states of matter after excitation by extremely short light pulses. To trace the time evolution of these ordered states after excitation, in which the atoms move on the femto- to pico second time scale (10−12 to 10−15 s), images need to be taken with both extraordinary temporal and spatial resolution. On page 371 of this issue, Danz et al. (2) show that this goal can be accomplished using time-resolved dark-field electron microscopy by obtaining real-space snapshots of a charge density wave phase transition triggered with pulses of light.
The famous high-speed photos taken by Doc Edgerton help us to understand how these experiments were carried out (see the figure). This image is captured after a drop lands in an otherwise quiescent pool of milk (3). Taking these kinds of photos, which Edgerton started pursuing in the 1930s, required ingenuity, because shutter speeds even on most modern cameras are only in the millisecond range. Edgerton, an early innovator in electronic flashtube technology, simply turned off the lights in the room and used a microsecond flash to capture the images at a particular instant. Despite the long camera exposure, light is only captured at the instant of the flash. As long as the perturbing drop (the “pump”) is appropriately synchronized with the flash (the “probe”), the result is a perfectly timed snapshot.
To synchronize the events, the drop falling from a pipette triggers an electronic circuit that is used to delay the flash precisely to the moment that the drop hits the pool. By electronically delaying the time between the pump and probe events, a sequence of images can be captured to illustrate the splash evolving in time . Using this method, each photo in a sequence is actually captured with different milk drops. Thus, a sequence of images patched together is not a “movie” in the truest sense, but so long as the event is repeatable, a movie can be meaningfully constructed. Capturing the evolution of the milk drop disturbing the pool away from its quiescent equilibrium state inspires those today who use the pump-probe technique.
Before the advent of pump-probe spectroscopy, states of matter were studied at, or very close to, thermal equilibrium. Studies of glass provide a counterpoint, but the time evolution for glasses is extremely slow. In the past two decades, building on advances in laser technology, the first glimpses have begun into how ordered states evolve on femtosecond time scales after being driven far from equilibrium with intense light pulses. Major goals of this enterprise are to use the light pulses to manipulate and control existing states of matter and to discover different ones that do not exist under equilibrium conditions. For example, prior studies have hinted that light pulses can substantially raise transition temperatures in molecular superconductors (4, 5) and that pulses can create quantum mechanical hybrid photon-electron states, so-called Floquet-Bloch states, in insulating materials (6, 7).
Capturing fast physics in a flash. This famous image from Harold Edgerton (below) was captured on the microsecond time scale using a synchronized electronic flashtube. Analogously, the evolution of a charge density wave transition is captured on the picosecond scale (right) with time-resolved dark-field imaging in an electron microscope.PHOTO (LEFT): H. EDGERTON/SCIENCE SOURCE; (SERIES ON RIGHT) DANZ ET AL./SCIENCE
Because of the extreme conditions to which matter is subjected in these experiments, some of the most basic and prized experimental probes are unusable. Even measurements of the electrical resistivity, perhaps the most run-of-the-mill among probes of near-equilibrium matter, become monumentally difficult to perform on the femtosecond time scale. Thus, attention has shifted to expanding the experimental repertoire to better capture the far-from-equilibrium dynamics of ordered matter in pursuit of what happens on extremely short time scales. Only in the past 10 to 15 years have methods such as time- and angle-resolved photoemission spectroscopy (7, 8) and ultrafast electron scattering (9–11) matured to present us with meaningful insights into condensed phases of matter.
Danz et al. substantially boost our experimental arsenal by capturing real-space images of the spatiotemporal order parameter dynamics during a light-triggered phase transition. They take snapshots, in the manner of Doc Edgerton, of charge density wave material 1T-TaS2 (see the figure). A charge density wave is an ordered state that gives rise, among other phenomena, to a periodic lattice distortion. In the authors' experiment, a femtosecond light pulse melts this density wave locally, and over time, the sample returns to its distorted state. Images in their scheme are captured by using electron pulses in a conventional transmission electron microscope, but the authors spearhead two advances. They use an inhomogeneous light excitation to melt the density wave in specific regions within their field of view. And instead of exposing the camera to all scattered electrons from the sample, they design a sample-specific, tailor-made mask to collect only diffraction peaks from the density wave. Imaging just these diffraction peaks—so-called dark-field imaging—allows the authors to watch how the vaporized regions interact with each other, shrink, and eventually recondense into the charge density wave state.
These never-before-seen images will inspire others to come up with schemes that do not require the use of the tailor-made mask, which currently prohibits the technique from being adopted more widely. The future will no doubt see strategies for masking “dynamically” and applying the method more systematically across materials. Had Doc Edgerton been alive to watch the kinds of movies directed today by those pursuing studies of far-from-equilibrium physics, we imagine that they would evoke in him both awe and a nostalgic flashback.
References and Notes
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