Ultrafast laser-induced strain waves in thin ruthenium layers

Abstract

We report on the time-dependent optical diffraction from ultra-high frequency laser-induced acoustic waves in thin layers of ruthenium deposited on glass substrates. We show that the thermo-optic and strain-optic effects dominate the optical response of Ru layers to a traveling longitudinal strain wave. In addition, we show the generation and detection of acoustic waves with a central frequency ranging from 130 GHz to 750 GHz on ultra-thin layers with thicknesses in the range of 1.2 - 20 nm. For these ultra-thin layers we measure a strong dependency of the speed of sound on the layer thickness and, thus, the frequency. This frequency-dependent speed of sound results in a frequency-dependent acoustic impedance mismatch between the ruthenium and the glass substrate, leading to a faster decay of the measured signals for increasing frequency. Furthermore, for these extremely high-frequency oscillations, we find that the frequency and phase remain constant for times longer than about 2 ps after optical excitation. Back extrapolation of the acquired acoustic signals to t = 0 gives a starting phase of −π/2. As this seems unlikely, we interpret this as an indication of possible dynamic changes in the phase/frequency of the acoustic wave in the first 2 ps after excitation.

Picosecond Ultrasonics for Studying Elastic Modulus of Polycrystalline Chromium Nanofilms: Thickness Dependence and Stiffness Enhancement

“…where d and f are the film thickness and breathing mode frequency [16,22]. It has been shown that thin Cr films on the Si substrate in Figure 2c have broader peaks than the SiO 2 substrate in Figure 2d, which is in accordance with the fact that the acoustic impedance mismatch of Cr-Si is larger than that of Cr-SiO 2 .…”

“…Therefore, the result of vaporized Cr films on the Si substrate are not shown in Figure 3 and reported in this study. Figure 3c show that the out-of-plane elastic modulus C33 decrease with the decrease of the film thickness due to the thin-film softening effect, which has been reported in thin films of Au [12], Ni [17], and Ru [22]. For comparison, we calculated the elastic modulus along the <110> directions using the known Cij of monocrystal Cr by 𝐶 = (𝐶 + 𝐶 + 2𝐶 )/2 [26].…”

“…For the analysis of the measured acoustic signals from the Cr films, it is important to separate the slow decaying thermal signal caused by the thermal diffusion. In this study, we removed the thermal signals by subtracting an exponentially decaying function from the data [22]. By solving Maxwell's equations inside the metal film [7], the acoustic signals in the reflectivity change can be expressed as…”

Optical Modification of 2D Materials: Methods and Applications

“…For example, laser‐induced acoustic waves can be used to introduce time‐(in)dependent modification for various applications, such as 2D material time crystals. [ 187 ] Additionally, light polariton propagation in 2D materials can be explored to produce self‐induced waveguide writing. [ 188 ]…”

“…For example, laser-induced acoustic waves can be used to introduce time-(in)dependent modification for various applications, such as 2D material time crystals. [187] Additionally, light polariton propagation in 2D materials can be explored to produce selfinduced waveguide writing. [188] Further, combining optical modification with other photonic methods, such as near-field optics, micro-and nanoresonators, photonic crystal cavities, and plasmonics, can further boost the performance and tuning of 2D devices, potentially enabling even higher resolutions beyond the diffraction limit, and faster operating or patterning speeds.…”