In Situ Ultra-fast TEM Observation of Acoustic Excitation of 128° Y-X LiNbO3

Le résumé fourni par la source

During in situ electron microscopy observations, a variety of stimuli are controllably applied to a specimen during observations. Key examples of prior work in this area includes heating, cooling, electric and magnetic fields, liquids, gases, and applied strain [1]. For high-speed time-resolved imaging, the stimulus must also be extremely fast, or applied at high frequency, and prior examples include heating with a laser, or applied electric fields [2]. Here, we report on a new type of in situ stimulus technique involving a time-varying strain that arrives at the specimen in the form of an acoustic wave. This is achieved by employing interdigitated transducers (IDTs) to convert an electrical input signal to acoustic wave stimuli on a 128° Y-X LiNbO3 single crystal. Fig. 1A describes the synchronized pulsed beam and specimen excitation allowing for time-resolved data acquisition using a phase shifter. Fig. 1B illustrates the specimen construction. In our demonstration, a 100 MHz RF excitation was selected based on the fundamental frequency of the device calculated from f0=υg/λ⁠, where the group velocity, υg is ∼3600 m/s for 128° Y-X LiNbO3, and λ = 36 um as given by the pitch of the IDT fingers. Fig. 2A is a micrograph showing a LiNbO3 lamella with bend contours fluctuating due to the motion of the 100 MHz acoustic wave. Fig. 2B shows a difference image between zero phase and a phase shift of π/3. This reveals the time-evolution of real-space dynamics of acoustic waves in LiNbO3 in and around engineered nanomechanical structures. This approach can be extended to other piezoelectric materials exhibiting strong electromechanical coupling. This geometry also shows potential to be further developed as a platform for mechanical actuation of a variety of other materials for ultrafast TEM studies. Any material that could be applied to LiNbO3 before or after thinning would be amenable to such studies. (A) Schematic of pulsed electron beam coupled to the phase-delayed specimen drive. (B) Schematic showing TEM lamella and IDT specimen support. While RF excitation of the IDTs results in an electric field in the drive region, the lamella experiences no electric field and the stimuli at the region of interest is an acoustic wave. (A) Reference bend contour image of lamella at t0 (B) Difference image of lamella demonstrating strain dynamics. Scale bar = 500 nm.

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