Optical generation of surface acoustic waves guided at the linear boundary between two thin films

Abstract

Laser-induced transient grating measurements and ultrafast optical pump-probe imaging of surface acoustic waves near a linear boundary between copper and silica films on a silicon substrate indicate the presence of a boundary-localized mode with a phase velocity slightly below the Rayleigh wave velocity on the copper film. We analyze in detail the dispersion of this localized mode in comparison with that of the Rayleigh waves in the surrounding materials. The existence of the localized mode is ascribed to the nonuniformity of the copper film thickness near its edge resulting from polishing during fabrication.

Imaging arbitrary acoustic whispering-gallery modes in the gigahertz range with ultrashort light pulses

“…It consists of a polycrystalline copper disk of radius r 0 18.75 μm embedded in a silicon oxide layer (thickness 370 nm) lying on a silicon nitride layer (thickness 100 nm), another silicon oxide layer (thickness 550 nm), and a Si (100) substrate. Polishing the sample to remove the excess deposited copper results in a concave disk surface [3,13]; atomic-force microscopy measurements indicate that the center of the disk is recessed with respect to the surface of the silicon oxide layer by ∼150 nm. show two SAW images corresponding to the real part X of the complex signal Z X iY at 1.96 and 4.40 ns after the pump pulse arrival, respectively [3], where X (Y ) is the in-phase (quadrature) component of the measured optical phase difference.…”

Acoustic whispering-gallery modes generated and dynamically imaged with ultrashort optical pulses

“…35 An interface wave may also exist if one of the media is inhomogeneous near the interface. 32 Indeed, experiments on straight edges with the same dished structure as in the present investigation did reveal guided modes with a velocity slightly less than that of the SAWs in the surrounding Cu, 31 which leads us to conclude that we are detecting a WG-like mode associated with the Cu film thickness nonuniformity. The mode is guided because the thicker copper near the edge provides a waveguide channel with a lower SAW velocity compared to both the interior of the disk and the silica layer outside.…”

“…2͑a͒, the SAWs propagating in the −y direction dominate the in-phase response, with a phase velocity 3000Ͻ v Ͻ 4000 ms −1 . The numerically determined dispersion curve, [29][30][31] assuming an average Cu thickness of 275 nm ͑dashed line͒, shows good agreement. In Fig.…”

Tuning to a particular acoustic whispering-gallery mode in the GHz range

“…It consists of a polycrystalline copper disk of radius r 0 =18.75 μm embedded in a silicon oxide layer (thickness h=370nm) lying on a silicon nitride layer (h=100 nm), another silicon oxide layer (h=550 nm), and a Si (100) substrate. Polishing the sample to remove the excess deposited copper results in a concave disk surface [3,13]; atomic-force microscopy measurements indicate that the center of the disk is recessed with respect to the surface of the silicon oxide layer by ∼150 nm.…”