Nonlinear ultrafast acoustics at the nano scale.

Received 12 November 2009; revised manuscript received 23 March 2010; published 13 April 2010We study one-dimensional nonlinear propagation of high-amplitude acoustic waves in sapphire, for varioussample temperatures, sample thicknesses, and pump ﬂuences. Strain waves are generated in a 100-nm-thickchromium ﬁlm and launched into the sapphire. For temperatures 60 K, damping can be neglected andpropagation is dominated by the nonlinear and dispersive properties of the sapphire substrate. An interfero-metric technique is used to detect the wave on an epitaxially grown 20-nm-thick Cr ﬁlm at the opposite sideof the sample. At the lowest temperature of 18 K, a train of up to seven solitons is detected in sapphire for apump ﬂuence of 11 mJ/cm

/pdf/time-resolved-interferometric-detection-of-ultrashort-strain-2abbw55gav.pdf

Time-resolved interferometric detection of ultrashort strain solitons in sapphire

We monitor how destructive interference of undesired phonon frequency components shapes a quasi-monochromatic hypersound wavepacket spectrum during its local real-time preparation by a nanometric transducer and follow the subsequent decay by nonlinear coupling. We prove each frequency component of an optical supercontinuum probe to be sensitive to one particular phonon wavevector in bulk material and cross-check this by ultrafast x-ray diffraction experiments with direct access to the lattice dynamics. Establishing reliable experimental techniques with direct access to the transient spectrum of the excitation is crucial for the interpretation in strongly nonlinear regimes, such as soliton formation.

Brillouin scattering of visible and hard X-ray photons from optically synthesized phonon wavepackets.

For broadband and sensitive detection of acoustic waves, the surface plasmon resonance (SPR) can be used, which responds to variations of dielectric properties in close proximity to a metal film supporting surface plasmon waves. When an acoustic wave is incident onto a receiving plate positioned within the penetration depth of the surface plasmons, it creates displacements of the surface of the plate and thus modulates the dielectric properties, affecting the SPR and the reflection of the incident light. Here we study characteristics and determine the optimal configuration of such an acousto-optical transducer with surface plasmons for efficient conversion of an acoustic signal into an optical one. We simulate the properties of this transducer and present estimates showing that it can have a large frequency bandwidth and high sensitivity.

Optical detection of acoustic waves with surface plasmons.

The surface plasmon resonance (SPR) is a sensitive technique for the detection of changes in dielectric parameters in close proximity to a metal film supporting surface plasmon waves. Here we study the application of the SPR effect to an efficient conversion of an acoustic signal into an optical one. Such a transducer potentially has a large bandwidth and good sensitivity. When an acoustic wave is incident onto a receiving plate positioned within the penetration depth of the surface plasmons, it creates displacements of the surface of the plate and, thus, modulates the dielectric properties in the proximity of the gold film. This modulation, in turn, modifies the light reflection under surface plasmon resonance conditions. We simulate characteristics of this acousto-optical transducer with surface plasmons and provide sets of parameters at the optical wavelength of 800 nm and 633 nm for its realization.

Acousto-optical Transducer with Surface Plasmons

We excite high-amplitude longitudinal coherent acoustic strain waves in a chromium film deposited on a thin sapphire slab, by 100-fs pulses from a 1-kHz Ti:sapphire regenerative amplifier. The bipolar strain pulse that is launched into the crystal, gradually transforms into a shock wave when attenuation by thermal phonons is weaker than the nonlinear action. We detect such shock waves at the opposite side of the slab by probing the reflection of a second chromium film in a standard reflection geometry, with a common-path interferometer to resolve both the induced amplitude and phase changes. We first discuss the interferometer and its standard performance. We then present the measurements on propagating shock waves, and analyze the results on a qualitative level.

https://iopscience.iop.org/article/10.1088/1742-6596/92/1/012092/pdf

G van der Star

Papers

Interferometric detection of acoustic shock waves