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 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 a time-division interferometer based on the Sagnac geometry for monitoring ultrafast changes in the real and the imaginary components of the refractive index as well as phase changes that are due to surface displacement. Particular advantages of this interferometer are its simple common-path design and operation at normal incidence with a microscope objective for both pumping and probing. Operation is demonstrated by detection of temperature changes and coherent phonon generation in a gold film.

Thermal transport is a key performance metric for thorium dioxide in many applications where defect-generating radiation fields are present. An understanding of the effect of nanoscale lattice defects on thermal transport in this material is currently unavailable due to the lack of a single crystal material from which unit processes may be investigated. In this work, a series of high-quality thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600 °C to create microscale regions with varying densities and types of point and extended defects. Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects. Together, this combination of methods provides important initial insight into defect formation, recombination, and clustering in thorium dioxide and the effect of those defects on thermal transport. These methods also provide a promising pathway for the quantification of the smallest-scale defects that cannot be captured using traditional microscopy techniques and play an outsized role in degrading thermal performance.

Measurements on a film of silica on crystalline zinc using picosecond laser acoustics are theoretically analyzed to quantitatively explain the generation and detection of picosecond shear and longitudinal-acoustic waves. The theory encompasses the scattering of obliquely incident probe light of arbitrary polarization by a depth-dependent anisotropic permittivity modulation in a multilayer, including terms arising from the photoelastic effect, interface displacements, and local rotations. Sound velocities, ultrasonic attenuation, and photoelastic constants are experimentally derived.