Terahertz Photoacoustic Generation Using Ultrathin Nickel Nanofilms

Abstract

Picosecond ultrasonic laser sonar (PULSE), one of the noninvasive technologies with a high thickness sensitivity, is capable of probing multilayer films directly on the production wafers. To reach high spatial resolution in the current PULSE system, generation of terahertz photoacoustic pulses through an ultrathin metal film has been the focus of recent studies. In this study, we propose and demonstrate the high potential of ultrathin nickel (Ni) films for THz photoacoustic generation. After femtosecond laser excitation, the photoacoustic frequency launched by the ultrathin Ni nanofilms could be up to 1.4 THz, corresponding to a record-breaking spatial resolution in the single metallic layer system. We discuss the detailed thickness-dependent mechanism with the two-temperature model, which indicates that the ultrahigh-frequency acoustic signals were generated due to the significant contribution of hot electron pressure in Ni, very different from the case for gold. By looking into the ultrafast hot electron relaxation mechanism, our results provide a guideline to further break the technical limit of the metallic terahertz photoacoustic frequency generation in the near future.

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

“…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].…”

“…Various metal films, including Al [9][10][11], Au [12,13], Pt [14][15][16], Ni [17,18], and Cr [18,19], have been proven suitable for PU to generate and detect longitudinal acoustic phonons (LAP), which travels and repeat reflections along the film's thickness direction. Based on the film thickness and the measured round trip time of LAP, we can determine the Coatings 2023, 13, 438 2 of 9 sound velocity related to the out-of-plane elastic modulus [20].…”

“…Among the above metals, the Pt, Ni, and Cr [18] have strong electron-phonon coupling constants and their thermal diffusion is typically confined to the optical absorption depth, resulting in ultrashort pulses of LAP; therefore, such metals are considered candidates for high-frequency photoacoustic transducers. Previous studies have shown that Pt [15] and Ni [17] can generate LAP with central frequency up to hundreds of gigahertz (GHz). For thin metal films, the out-ofplane elastic modulus can also be determined by the breathing mode central frequency of non-propagating LAP trapped in the film [16,20].…”

Effect of surface plasmon on optical detection of picosecond ultrasonic pulses generated in aluminum nanofilms

“…Heat transfer and lattice dynamics had been studied numerically and theoretically [44] . The energy coupling with the lattice caused a thermal expansion pulse after photoexcitation, and the thermal stress was mainly caused by pressure from hot electrons and lattice expansion [45] , [46] . Macroscopic continuum elasticity can explain the longitudinal photoacoustic effect in homogeneous materials [47] .…”

Modeling of femtosecond photoacoustic non-destructive testing in AlCu nanofilms

“…Femtosecond photoacoustic detection employs ultrashort laser pulses as the driving source, allowing it to attain higher acoustic frequencies, reaching up to hundreds of GHz. This also reduces the corresponding acoustic wavelengths to nanometers, thereby enhancing measurement accuracy at the nanoscale [1]- [6] . Another notable feature of photoacoustic detection is that it can use laser pulses with relatively low fluence to avoid damaging metal films, making it suitable for non-contact and non-destructive testing (NDT) [7] [8] .…”