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The aim of this article is to provide an introduction to picosecond laser ultrasonics, a means by which gigahertz-terahertz ultrasonic waves can be generated and detected by ultrashort light pulses. This method can be used to characterize materials with nanometer spatial resolution. With reference to key experiments, we first review the theoretical background for normal-incidence optical detection of longitudinal acoustic waves in opaque single-layer isotropic thin films. The theory is extended to handle isotropic multilayer samples, and is again compared to experiment. We then review applications to anisotropic samples, including oblique-incidence optical probing, and treat the generation and detection of shear waves. Solids including metals and semiconductors are mainly discussed, although liquids are briefly mentioned.

Using an optical technique we generate and detect picosecond shear and quasishear coherent acoustic phonon pulses in the time domain. Thermoelastic and piezoelectric generation are directly achieved by breaking the sample lateral symmetry using crystalline anisotropy. We demonstrate efficient detection in isotropic and anisotropic media with various optical incidence geometries. DOI: 10.1103/PhysRevLett.93.095501 PACS numbers: 63.20.Dj, 43.35.+d, 78.20.Hp, 78.47.+p By shaking atoms one may assess interatomic bond strengths and the integrity of crystal lattices. In particular this may be achieved by high-frequency phonon excitation and detection, providing a wealth of information on the elastic properties of solids on nanometer and atomic length scales owing to the enhancement in scattering when the phonon wavelength is of the same order as the structure under investigation. This field of research, initially driven by terahertz phonon measurements involving superconducting tunnel junctions, heat pulses, phonon-induced fluorescence, and Raman or Brillouin scattering [1,2], has more recently been supplemented with ultrafast optical techniques in the time domain. In particular, such impulsive optical generation and delayedtime optical probe detection at surfaces permits the use of propagating GHz-THz phonon pulses to acoustically inspect the interior of nanostructures [3][4][5][6][7][8][9][10]. Acoustic phonon generation with ultrashort optical pulses is enabled by a variety of mechanisms, such as themoelasticity [3][4][5][6][7][8][9], deformation potential coupling [10,11], or screening of electric fields combined with piezoelectricity [12,13]. The respective excitation of thermal phonons, carriers, or (rapid changes in) screening potential in an opaque material produce an initial stressed near-surface region whose size in the lateral direction ( * 1 m) depends on the optical spot diameter and in the depth direction ( & 100 nm) on optical absorption, carrier diffusion or builtin electric field localization. Phonon detection is achieved through the photoelastic effect or surface displacement when the phonon pulse returns to the same point on the surface after scattering within a short distance. In this case, with isotropic media or symmetrically cut crystals, the constraints of symmetry imply that one only excites longitudinal acoustic phonons in the depth direction.Such longitudinal acoustic phonon experiments have lead to picosecond time-scale studies involving as diverse a range of subjects as ultrashort time-scale carrier diffusion in metals and semiconductors [5,9,10], highfrequency ultrasonic attenuation in crystals and glasses [14,15], phonon generation and detection in semiconductor quantum wells and superlattices [6,12,16], and soliton propagation and their coupling to two-level systems in ruby [7,17]. In spite of these successes, these experiments only address one of the three acoustic polarizations. To match the impressive capabilities of Brillouin and Raman scattering techniques one would naturally w...

The newly popular topic of "phonon diodes" is discussed in the context of a broader issue of reciprocity in reflection/transmission (R-T) of waves. We first review a theorem well known in electromagnetism and optics but underappreciated in acoustics and phonon physics, stating that the matrix of R-T coefficients for properly normalized amplitudes is symmetric for linear systems that conform to power conservation and time reversibility for wave fields. It is shown that linear structures proposed for "acoustic diodes" in fact do obey R-T reciprocity, and thus should not strictly be called diodes or isolators. We also review examples of nonlinear designs violating reciprocity, and conclude that an efficient acoustic isolator has not yet been demonstrated. Finally, we consider the relationship between acoustic isolators and "thermal diodes", and show that ballistic phonon transport through a linear structure, whether an acoustic diode or not, is unlikely to form the basis for a thermal diode.

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 ...