We present a new method for imaging surface phonon focusing and dispersion at frequencies up to 1 GHz that makes use of ultrafast optical excitation and detection. Animations of coherent surface phonon wave packets emanating from a point source on isotropic and anisotropic solids are obtained with micron lateral resolution. We resolve rounded-square shaped wave fronts on the (100) plane of LiF and discover isolated pockets of pseudosurface wave propagation with exceptionally high group velocity in the (001) plane of TeO 2 . Surface phonon refraction and concentration in a minute gold pyramid is also revealed. DOI: 10.1103/PhysRevLett.88.185504 PACS numbers: 63.20.Dj, 62.65. +k, 68.35.Iv, 77.65.Dq Sound waves in crystals, dependent on the fourth-order elastic constant tensor, display a rich array of anisotropic propagation phenomena. Despite a crystal being homogeneous, a point acoustic source in the bulk can lead to singularities in acoustic flux in certain directions owing to the angular dependence of the phase and group velocities of the three acoustic polarizations [1]. This phonon focusing effect was first discovered in the bulk [2], but surface phonons were predicted to produce equally intriguing focusing patterns [3]. In the 10 MHz -1 GHz range, where acoustic wavelengths are typically 3 300 mm, various methods have been suggested for two-dimensional surface phonon imaging, such as stroboscopic probing, the sprinkling of powder on the surface, or detection by immersed point-focus transducers [4][5][6]. However, despite the growing interest in the field of surface acoustic wave devices, no technique has been successful in imaging surface phonon focusing in real time. Such imaging allows direct access to the dispersion characteristics of the wave propagation and the possibility of following the temporal evolution of cuspidal structures. In this Letter we image the propagation of coherent surface phonons at frequencies up to 1 GHz in real time, allowing animations of pointexcited surface phonon wave packets to be made with picosecond temporal and micron spatial resolutions.We use an ultrafast optical pump and probe technique with a common-path interferometer [7]. Surface phonon wave packets are thermoelastically excited in thin metal films on transparent substrates with optical pulses of wavelength 415 nm, repetition rate 80 MHz (one pulse every 12.5 ns), duration ϳ1 ps, and pulse energy ϳ0.3 nJ, producing a maximum transient temperature rise ϳ100 K. This pump light is focused at normal incidence through the substrate to a circular spot of diameter D ഠ 2 mm (full width at half maximum intensity; see Fig. 1). Out-ofplane (z) surface motion is detected interferometrically with ϳ1 pm resolution by the use of two probe pulses at an interval of t 510 ps, focused at normal incidence to a single spot of diameter ϳD on the front surface of the film. These pulses, of wavelength 830 nm, are derived from the same laser as the pump. In a simple modification of the apparatus of Ref.[7], we divide the output beam from ...
Based on the finite-difference time-domain ͑FDTD͒ method, we study theoretically the wave-front images of acoustic waves propagating on the ͑100͒ and ͑001͒ surface of a highly anisotropic tetragonal TeO 2 crystal for which imaging experiments have recently been conducted. The theoretical images well reproduce characteristic features observed experimentally. The group-velocity calculations of both surface and pseudosurface acoustic waves account for the shapes and locations of the major wave fronts obtained with the FDTD method. Additional weak wave-front structures that disappear rather quickly in time are attributed to bulk acoustic waves.
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