Picosecond ultrasonics

This time-resolved method for generation and photoelastic detection of coherent picosecond acoustic phonon pulses was proposed by Christian Thomsen and coworkers in a collaboration between Brown University and Bell Laboratories in 1984.

[1] Initial development took place in Humphrey Maris’s group at Brown University and elsewhere in the late 1980s.

[2][3] In the early 1990s the method was extended in scope at Nippon Steel Corp. by direct sensing of the picosecond surface vibrations of the film caused by the returning strain pulses, resulting in improved detection sensitivity in many cases.

The picosecond ultrasonics has also been applied to measure the acoustic velocity inside nanomaterials or to study phonon physics.

The absorption of an incident optical pump pulse sets up a local thermal stress near the surface of the sample.

The exact depth for the stress generation depends, in particular, on the material involved and the optical pump wavelength.

In metals and semiconductors, for example, ultrashort-timescale thermal and carrier diffusion tends to increase the depth that is initially heated within the first ~1 ps.

When the optical spot diameter D, for example D~10 μm, at the surface of an elastically isotropic and flat sample is much greater than the initially heated depth, one can approximate the acoustic field propagating into the solid by a one-dimensional problem, provided that one does not work with strain propagation depths that are too large (~D²/Λ=Rayleigh length, where Λ is the acoustic wavelength).

For small spot sizes approaching the optical diffraction limit, for example D~1 μm, it may be necessary to consider the three-dimensional nature of the problem.

Shear waves may also be generated by the use of elastically anisotropic solids cut at oblique angles to the crystal axes.

These so-called acoustic solitons have been demonstrated at low temperatures over propagation distances of a few millimeters.

Strain pulses returning to the surface from buried interfaces or other sub-surface acoustically inhomogeneous regions are detected as a series of echoes.

In this case the echo shape when measured through the optical phase variation is proportional to a spatial integral of the strain distribution (see below).

(n the refractive index and κ the extinction coefficient) is the complex refractive index for the probe light in the sample, k is the wave number of the probe light in vacuum, η(z, t) is the spatiotemporal longitudinal strain variation,

The theory of optical detection in multilayer samples, including both interface motion and the photoelastic effect, is now well-developed.

[16][18] The control of the polarization state and angle of incidence of the probe light has been shown to be useful for detecting shear acoustic waves.

[6][19] Picosecond ultrasonics has been applied successfully to analyze a variety of materials, both solid and liquid.

It is increasingly being applied to nanostructures, including sub-micrometre films, multilayers, quantum wells, semiconductor heterostructures and nano-cavities.

Generation and detection of picosecond strain pulses in an opaque thin film with ultrashort optical pulses. In this example, the optical probe pulse arrives at the film surface at the same time as the returning strain pulse. In general, measurements are made by varying the arrival time of the optical probe pulse. The thermal expansion of the surface is omitted. For example, in the case of an aluminum film, the strain pulse will have a typical frequency and bandwidth both ~ 100 GHz, a duration of ~ 10 ps, a wavelength of ~100 nm, and a strain amplitude of ~ 10 −4 when using optical pulses of duration ~ 100 fs and energy ~ 1 nJ focused to a ~ 50 μm spot on the sample surface.