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...

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 ...

We describe an improved two-dimensional optical scanning technique combined with an ultrafast Sagnac interferometer for delayed-probe imaging of surface wave propagation. We demonstrate the operation of this system, which involves the use of a single focusing objective, by monitoring surface acoustic wave propagation on opaque substrates with picosecond temporal and micron lateral resolutions. An improvement in the lateral resolution by a factor of 3 is achieved in comparison with previous setups for similar samples.

We describe an experimental investigation of the generation and detection of picosecond acoustic-phonon pulses in a thin slab of GaAs using ultrashort optical pulses. Comparison of the optical phase variation with a simple theory for ambipolar diffusion indicates that carrier diffusion has a significant effect on the shape of the phonon pulses generated. The phonon pulse duration is measured to be ϳ25 ps, four times longer than that expected from optical-absorption considerations alone, indicating that hot carriers penetrate more than 100 nm into the sample during the phonon pulse generation process. DOI: 10.1103/PhysRevB.64.081202 PACS number͑s͒: 73.50.Ϫh, 43.35.ϩd, 62.65.ϩk Ultrafast carrier diffusion in semiconductors has been studied by a variety of experimental techniques. Ultrashort pulse optical pump and probe methods based on the measurement of optical reflectivity, electro-optic sampling or luminescence, including near-field methods, have been applied to the measurement of diffusion of hot carriers in bulk semiconductors and quantum nanostructures in the lateral or through-thickness directions.1-4 The detection principle in this case involves the coupling of the carriers to the electric field of the optical wave. Optical pump and probe methods can also exploit the coupling of the carriers to strain, and can be used to monitor carrier diffusion from the shape of the acoustic-phonon pulses generated, a technique that involves the pulse-echo methods of laser picosecond acoustics. 5,6 The penetration of hot carriers perpendicular to metal surfaces when excited with an ultrashort optical pulse has been shown to broaden the phonon pulses generated.7 Laser acoustics studies with nanosecond or sub-nanosecond temporal resolution in crystalline Ge and Cd x S 1Ϫx Se have demonstrated that carrier diffusion similarly affects acoustic generation in semiconductors. 8,9 However, there have been no studies of the effect of carrier diffusion on acoustic generation in semiconductors with picosecond time resolution. This is unfortunate in view of the pressing need for such studies to support industrial development in quantitative nondestructive evaluation of integrated circuits or semiconductor nanostructures. There are also possible applications in the field of GHz-THz acousto-optic modulation in ultrahigh speed semiconductor devices. Moreover, probing carrier diffusion on ultrashort timescales from acoustic measurements provides an interesting perspective on the time-and space-dependent nonequilibrium carrier distribution, because it allows depth profiling of the carrier penetration into the bulk.7 Despite this fundamental and practical interest previous laser picosecond acoustics experiments with semiconductor thin films involved complex GaAs multilayer geometries, and were not designed for the investigation of carrier diffusion. [10][11][12][13] There are several challenging experimental problems related to such studies of carrier diffusion in crystalline semiconductors on picosecond timescales. First one must ove...

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We derive analytical formulas for the modulation of the reflectance and transmittance of light normally incident on a multilayer thin-film structure whose refractive indices are perturbed by an ultrashort optical pulse. The formulas, expressed in compact form, should prove useful for analysis of a wide range of ultrashort timescale experiments on multilayers as well as longer time-scale photoacoustic and photothermal experiments based on optical probing. We demonstrate our method by the analysis of the modulated reflectance variation of a SiO 2 /Cr structure in which picosecond acoustic pulses have been optically excited.

We investigate the vibrational modes of gold nanorings on a silica substrate with an ultrafast optical technique. By comparison with numerical simulations, we identify several resonances in the gigahertz range associated with axially symmetric deformations of the nanoring and substrate. We elucidate the corresponding mode shapes and find that the substrate plays an important role in determining the mode damping. This study demonstrates the need for a plasmonic nano-optics approach to understand the optical excitation and detection mechanisms for the vibrations of plasmonic nanostructures.