Nanoscale mechanical contacts probed with ultrashort acoustic and thermal waves

Using an ultrafast optical technique we measure coherent phonon-pulse reflection from—and heat flow across—a mechanical contact of nanoscale thickness between a thin metal film and a spherical dielectric indenter. Picosecond phonon wave packets at $\sim 50\text{GHz}$ returning from this interface probe the pressure distribution, the contact area, and the indentation profile to subnanometer resolution, revealing the film deformation in situ. These measurements and simultaneous thermal-wave imaging at $\gtrsim 1\text{MHz}$ are consistent with significant enhancement of phonon transport across the near-contact nanogap.

Figure 1

(Color online) (a) Setup. (b) AFM traces. (c) $\left|\delta R\left(t\right)/R_0\right|$ at $F=4.2\text{N}$. The theory out of contact is shown displaced. Inset: normalized experimental echoes.

Figure 2

(Color online) (a) Acoustic reflection coefficient $\left|r_{fi}\right|$ vs $r$ at $F=2.1\text{N}$. (b) Contact radius from $\left|r_{fi}\right|$ (dots) and Hertz theory for ${\text{Al}}_2{\text{O}}_3{\text{-Si}}_3{\text{N}}_4$ (line). (c) $\left|r_{fi}\left(r=0\right)\right|$ against $p_0$ (dots) with fit (line). (d) Pressure $p$ vs $r$ (solid lines) and Hertz theory (dashed lines). (e) Film indentation $\Delta h$. Dots: Hertz contact radii. Dashed line: $-p\left(r\right)$ for $F=4.2\text{N}$. (f) $F$ vs $\left|\Delta h\right|$ at $r=0$ for loading (◼) and unloading (▲) compared with theory (dashes). Arrows: permanent deformation.

Figure 3

(Color online) (a) $\left|\delta R/R_0\right|$ images of the contact. (b) Measured air gap (▲, ●) and Hertz theory (lines) for two loads. (c) Normalized amplitude and phase for $t=-10\text{ps}$. Arrows: contact radii. (d) Normalized amplitude and phase difference for 1.1 N and 3D thermal-diffusion theory (dashed lines).