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picosecond ultrasonics
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.
==Introduction==
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.〔(C. Thomsen, J. Strait, Z. Vardeny, H. J. Maris, J. Tauc and J. J. Hauser,", "Coherent Phonon Generation and Detection by Picosecond Light Pulses", Phys. Rev. Lett. 53, 989 (1984). )〕
Initial development took place in Humphrey Maris’s group at Brown University and elsewhere in the late 1980s.〔(C. Thomsen, H. Grahn, H. J. Maris and J. Tauc,", "Surface Generation and Detection of Phonons by Picosecond Light Pulses", Phys. Rev. B. 34, 4129 (1986). )〕〔(G. L. Eesley, B. M. Clemens, and C. A. Paddock, "Generation and Detection of Picosecond Acoustic Pulses in Thin Metal Films", Appl. Phys. Lett. 50, 717 (1987). )〕
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.〔( O. B. Wright and K. Kawashima,"Coherent Phonon Detection from Ultrafast Surface Vibrations" , Phys. Rev. Lett. 69, 1668 (1992). )〕 Advances after the year 2000 include the generation of picosecond acoustic solitons by the use of millimeter propagation distances〔(H.-Y. Hao and H. J. Maris, "Experiments with acoustic solitons in crystalline solids", Phys. Rev. B 64, 064302 (2001). )〕 and the generation of picosecond shear waves by the use of anisotropic materials〔( O. Matsuda, O. B. Wright, D. H. Hurley, V. E. Gusev, and K. Shimizu, "Coherent Shear Phonon Generation and Detection with Ultrashort Optical Pulses", Phys. Rev. Lett. 93, 095501 (2004). )〕 or small (~1 μm) optical spot sizes.〔( C. Rossignol, J. M. Rampnoux, M. Perton, B. Audoin, S. Dilhaire, "Generation and Detection of Shear Acoustic Waves in Metal Submicrometric Films with Ultrashort Laser Pulses", Phys. Rev. Lett. 94, 166106 (2005). )〕 Acoustic frequencies up to the terahertz range in solids〔( M. F. Pascual Winter, G. Rozas, A. Fainstein, B. Jusserand, B. Perrin, A. Huynh, P. O. Vaccaro, and S. Saravanan, "Selective Optical Generation of Coherent Acoustic Nanocavity Modes", Phys. Rev. Lett. 98, 265501 (2007). )〕〔(C.-K. Sun, J.-C. Liang, and X.-Y. Yu, "Coherent Acoustic Phonon Oscillations in Semiconductor Multiple Quantum Wells with Piezoelectric Fields", Phys. Rev. Lett. 84, 179 (2000). )〕 and up to ~ 10 GHz in liquids〔( O. B. Wright, B. Perrin, O. Matsuda and V. E. Gusev, "Optical Excitation and Detection of Picosecond Acoustic Pulses in Liquid Mercury", Phys. Rev. B 78, 024303 (2008). )〕 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.
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