Picosecond Ultrasonics

When an ultrashort light pulse (duration: ~ 100 fs, energy: <1 nJ) is absorbed at the surface of a thin metal film, the resulting thermal expansion of the surface results in the generation of a strain pulse (composed of longitudinal acoustic phonons) that propagates into the film and the substrate. For a 15 nm Al film grown on a Si substrate this pulse will have a peak frequency and bandwidth both ~ 100 GHz, and a strain amplitude of about 1x10^-4. Reflections from buried interfaces will return to the surface, where they can be detected by means of a time delayed optical probe pulse. This phenomena was first studied in depth by researchers in Humphrey Maris’ ^[1]group at Brown University in the late 1980’s^[2], and is currently used in a thin film metrology technique (with nm resolution) that sees widespread use in the microprocessor industry.^[3]

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Picosecond Ultrasonics: Generation, Applications, and Challenges

Since the end of the 1980s, picosecond longitudinal acoustic pulses have been applied successfully to analyze nanometer-sized structures. Femtosecond laser pulses have been used to perform generation and detection of longitudinal acoustic waves in submicrometric films, multilayers structures, and other nanostructures. The absorption of the light pump pulse sets up a local thermal stress near the surface. This stress generates elastic strain pulses propagating into the sample and inducing changes in the optical reflectivity of the structure. These acoustic perturbations are probed by transient optical reflectometric or interferometric detection. In the acoustic near field of the laser-induced ultrasonic source, in an elastically isotropic solid, most of the laser energy couples to acoustic waves with a temporal duration approximately equal to the acoustic transit time through the optical absorption depth. When a femtosecond pulse duration is used, the source directivity is limited by the size of the spot, which has a minimum value of about the optical wavelength, according to far field diffraction limit. Thus, the laser generation of picosecond acoustic waves in opaque thin films was until now considered as a one dimensional problem because the ratio of the laser spot size at the sample surface (tens of microns with classical lenses) to the film thickness (from a few nanometers to a few microns) is much greater than unity. As a result of this one-dimensional character, acoustic diffraction in the thin film and reflection with conversion of the polarization of the acoustic wave at the interfaces were neglected. Thus these experiments only addressed one of the three acoustic polarizations, the longitudinal polarization. However, generation and detection of both longitudinal and shear waves would greatly extend the field of picosecond ultrasonics. For instance, for an isotropic film where the two shear modes are degenerate, one would have access to both the shear and tensile stiffness coefficients. Moreover, the shear wave velocity is lower, leading to a shorter wavelength for a given frequency that is advantageous for nanoscale probing. Recent studies have presented experimental results demonstrating the generation and the detection of a quasishear acoustic wave in picosecond laser ultrasonics. A quasishear wave was already generated by refraction of a longitudinal pulse at normal incidence at the interface between an isotropic polycrystalline metallic film and an anisotropic transparent crystal cut along a nonsymmetry direction, or via the laser induced electrostrictive effect. Recently, thermoelastic and piezoelectric generation have been directly achieved by breaking the sample lateral symmetry using crystalline anisotropy.

The effects of the lateral size of the optoacoustic source on the generation of shear acoustic waves are investigated. The changes in the acoustic waveforms induced by decreasing the width of the source are discussed, dealing with calculated and experimentally recorded signals for isotropic submicrometric aluminum plates. The lateral size of the acoustic source, a, is defined as the width of a Gaussian function describing the focalization of the laser spot intensity on the surface of the sample. Reducing the width of the laser spot on the surface, the optoacoustic source is less directive and shear waves can be diffracted by the source and detected in the far field of the acoustic source. Moreover, another result of diffraction, the waves associated with shear modes have a nonzero component in the direction normal to the film surface, and thus their normal amplitude can be measured by interferometry. Furthermore, longitudinal and shear waves are reflected at the interfaces of the plate with or without mode conversion. Successive echoes of waves propagating back and forth through the plate provide observation of the acoustic field at increasing travel distances from the source. From the near field to the far field, the amplitude of the shear wave increases. This increase competes with acoustic attenuation and with leakage due to transmission through the film substrate interface.^[4],[5]

See also

• Acoustics
• Ultrasound
• Phonons
• Soliton
• Waves
• Light

References