page_title: Picosecond ultrasonics
section_title: Introduction
hierarchical_section_title: Picosecond ultrasonics / Introduction
caption_attribution_description: English: Principle of generation and detection in picosecond ultrasonics. Image created by the Applied Solid State Physics Laboratory, Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo, Japan. Dec. 18, 2008 [1]
context_page_description: Picosecond ultrasonics is a type of ultrasonics that uses ultra-high frequency ultrasound generated by ultrashort light pulses. It is a non-destructive technique in which picosecond acoustic pulses penetrate into thin films or nanostructures to reveal internal features such as film thickness as well as cracks, delaminations and voids. It can also be used to probe liquids. The technique is also referred to as picosecond laser ultrasonics or laser picosecond acoustics.
context_section_description: When an ultrashort light pulse, known as the pump pulse, is focused onto a thin opaque film on a substrate, the optical absorption results in a thermal expansion that launches an elastic strain pulse. This strain pulse mainly consists of longitudinal acoustic phonons that propagate directly into the film as a coherent pulse.
  After acoustic reflection from the film-substrate interface, the strain pulse returns to the film surface, where it can be detected by a delayed optical probe pulse through optical reflectance or (for films that are thin enough) transmittance changes. 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.
  Initial development took place in Humphrey Maris’s group at Brown University and elsewhere in the late 1980s.
  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. Advances after the year 2000 include the generation of picosecond acoustic solitons by the use of millimeter propagation distances and the generation of picosecond shear waves by the use of anisotropic materials or small (~1 μm) optical spot sizes. Acoustic frequencies up to the terahertz range in solids and up to ~ 10 GHz in liquids have been reported.
  Apart from thermal expansion, generation through the deformation potential or through  piezoelectricity is possible.  Picosecond ultrasonics is currently used as a thin film metrology technique for probing films of sub-micrometer thicknesses with nanometer resolution in depth, that sees widespread use in the semiconductor processing industry.
caption: 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. Thermal expansion of the surface is omitted. For example, in the case of an aluminium 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.
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