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The 23-fs pulses were produced from a mode-locked Ti:Sapphire oscillator (KMLabs Griffin) and amplified in a multipass amplifier (KMLabs Dragon). These pulses were split into two arms: the pump arm and the probe arm. The pump arm contained a delay stage to control the timing between the pump and probe. The pump light was focused onto the sample near normal incidence, with a full width at half maximum (FWHM) of 650 μm. The intensity of the pump beam was 100 mW, making the fluence of the pump used in the experiments 8.4 mJ/cm2.

To produce the EUV light through the HHG process, the probe arm was focused through a glass capillary filled with 44.5 torr of argon. Most of the IR light was rejected via a pair of super-polished silicon mirrors set near Brewster’s angle. The residual IR light was attenuated by a 100-nm aluminum filter. A single harmonic at 28.9 nm was selected by steering the beam off of a pair of narrow-pass multilayer mirrors set at 45°. The EUV beam was then focused using a 5°, off-axis ellipsoidal mirror to a spot size of 7-μm FWHM, incident at 60° from the normal of the sample plane. To attenuate the IR pump light, a 195-nm-thick aluminum filter (Luxel) was placed before the CCD sensor (PI-MTE), which was placed 36.5 mm away from the sample plane. In this overall geometry, the highest theoretical resolution attainable due to the numeric aperture of the system is 38.1 nm.

To characterize the long-term stability of the probing 28.9-nm harmonic light, the direct beam was measured on the detector every 1.5 s for an hour. The standard deviation of the centroid of the beam on the camera was 1.9 and 0.11% of the beam diameter in the vertical and horizontal, respectively. This was a sufficient level of stability as each ptychographic scan took 20 min to acquire. However, the stability of the system extended well beyond the 1 hour of dedicated stability measurements, which was confirmed by acquiring ptychographic scans 3 days apart and reconstructing nearly identical probes.

Stroboscopic imaging was conducted at room temperature in reflection geometry, with the background pressure in the experimental vacuum chamber being ~10−7 mbar. For a single movie frame, the ptychographic CDI scan consists of 82 diffraction patterns recorded in a Fermat spiral pattern (32) with separations of 2 μm between adjacent positions. The data were collected with 2 × 2 on-chip binning, averaging over three accumulations, with a 200-kHz readout rate and a 0.15-s exposure time. Each image was created from a total of 49.2-s exposure from ~6 × 109 photons/s. The stroboscopic movie frames recorded with ptychographic CDI consisted of 14 different pump-probe delays. At each time delay, a differential measurement with and without the laser-driven excitation was carried out to account for fluctuations in the system. In addition, 10 images of the EUV beam reflected off of the bare silicon substrate were recorded for each measurement to apply the modulus-enforced probe constraint technique (14). We note that because the beam was reflected off of silicon, the diffraction patterns needed to be renormalized based on the reflectivity of silicon. The result is 28 images of the sample: 14 at different pump probe delays with respect to the pump pulse (t = 0) and 14 as reference images without the laser-driven excitation. The experimental movie of the nanostructure was reconstructed from >15 GB of raw data collected over a 14.5-hour period, which is a large amount of data for a tabletop-scale stroboscopic microscope.

For our experiment, we selected time delays such that the predominant dynamics observed in this experiment are the surface acoustic waves propagating within the nanostructure. We remark that this range can be extended at will to access the nanosecond-long thermal decay. Moreover, finer time steps can be taken to access high-frequency longitudinal acoustic waves propagating in the nanostructure and substrate, which have been extensively studied with spectroscopic techniques (2, 28, 33).

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