Full Spatiotemporal Control of Laser-Excited Periodic Surface Deformations

Abstract

We demonstrate full control of acoustic and thermal periodic deformations at solid surfaces down to sub-nanosecond time scales and few-micrometer length scales via independent variation of the temporal and spatial phase of two optical transient grating (TG) excitations. For this purpose, we introduce an experimental setup that exerts control of the spatial phase of subsequent time-delayed TG excitations depending on their polarization state. Specific exemplary coherent control cases are discussed theoretically and corresponding experimental data are presented in which time-resolved x-ray reflectivity measures the spatiotemporal surface distortion of nanolayered heterostructures. Finally, we discuss examples where the application of our method may enable the control of functional material properties via tailored spatiotemporal strain fields.

A new concept for temporal gating of synchrotron X-ray pulses

“…Only recently thermal TGs were investigated by diffracting hard X-ray pulses under grazing-incidence geometry (Sander et al, 2017a). Measurement of the transient surface excursion yields insights into the dynamics of coherent surface acoustic waves (Sander et al, 2017b) and of the lateral and perpendicular thermal diffusion (Pudell et al, 2019 Experimental data. (a) Schematic of the optical generation and TRXRR probe of thermal TGs.…”

“…Laser pulses are subsequently coupled into the TG setup which consists of a transmission phase mask and a system of lenses that images the phase mask onto the sample. The TG setup is discussed in detail elsewhere (Pudell et al, 2019).…”

“…the TG is almost diffused within the bunch spacing dt = 196 ns. However, for shorter bunch spacing the thermal grating can be removed with a second TG excitation as described elsewhere (Pudell et al, 2019). The specular reflection from the sample was blocked and photons diffracted into higher orders were captured with a hybrid pixel area detector (Pennicard et al, 2013) with a pixel size of 55 mm  55 mm (X-Spectrum, Lambda 750k).…”

“…Experimental setup for NanoGate characterization measurements performed at P23 beamline at PETRA III, DESY. A Q-switched laser is synchronized to the PETRA III bunch clock and delivers optical excitation pulses with a duration of 7 ns to the TG setup (Pudell et al, 2019) at a repetition rate of almost 1 kHz. The synchronization unit is also used to tune the pumpprobe delay.…”

“…A grazing-incidence X-ray pulse is diffracted from the thermal transient grating (TG) away from the specular reflection and can be separated with an aperture or an analyzer crystal. The dynamics of the thermal grating can be controlled by the optical excitation (Pudell et al, 2019), and temporal gating with opening times of 50 ps were demonstrated (Sander et al, 2017a) with 1 ps optical pulses for the excitation of the TG. The theoretical limit of the diffraction efficiency is 33% (Sander et al, 2017b).…”