David H. Hurley

Scanning ultrafast Sagnac interferometry for imaging two-dimensional surface wave propagation

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.

Measurement of the Kapitza resistance across a bicrystal interface

The Kapitza resistance across a Si bicrystal interface was measured using a pump probe optical technique. This approach, termed time resolved thermal wave microscopy (TRTWM), uses ultrafast laser pulses to image lateral thermal transport in bare semiconductors. The sample geometry is that of a Si bicrystal with the vertically oriented boundary intersecting the sample surface. High resolution transmission electron microscopy of the boundary region revealed a thin SiO2 layer at the interface. By comparing experimental results with a continuum thermal transport model the Kapitza resistance between the Si and SiO2 was estimated to be 2.3 × 10−9 m2K/W.

Kapitza resistance of Si/SiO2 interface

A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance.

Line source representation for laser-generated ultrasound in an elastic transversely isotropic half-space

Theoretical and experimental results are presented for a laser line source in an elastic, transversely isotropic half-space. The thermoelastic source (laser source) is represented as an appropriately weighted shear stress dipole applied at the sample surface. The plane of isotropy coincides with the half-space boundary. Analytical expressions representing the out-of-plane displacements for the surface wave and for the epicentral cases are given for all crystal classes that exhibit elastic transverse isotropy. In addition, quasianalytical results are given for observation points off the epicentral axis. Theoretical wave forms for all of the source/observation geometries considered are compared with experimental wave forms generated in single crystal zinc samples. The close comparison between experiment and theory confirms, for this particular line source orientation and crystal symmetry, that a laser line source is accurately modeled using an equivalent boundary stress.

In situ laser-based resonant ultrasound measurements of microstructure mediated mechanical property evolution

In situ laser-based resonant ultrasound spectroscopy is used to characterize the development of a recrystallized microstructure in a high purity copper sample. The modal shapes, used for mode identification, of several resonant modes are determined before and after annealing by raster scanning the laser interferometric probe. This information is used to isolate the motion of individual modes during high temperature annealing. The evolution of a particular mode during annealing is examined in detail. During recrystallization, the center frequency of this mode shifts by approximately 20% of the original value. Using electron backscatter data it is shown that the majority of this shift is due to changes in the polycrystal average elastic stiffness tensor, driven by changes in texture, and that changes in dislocation density and pinning length are secondary influences.

EPICENTRAL AND NEAR EPICENTER SURFACE DISPLACEMENTS ON PULSED LASER IRRADIATED METALLIC SURFACES

The elastic deformations associated with the pulsed laser, photothermal heating of an aluminum alloy surface have been measured within the irradiated region using a Michelson‐type interferometer. The displacements within this region have been measured and have been compared to calculations based on temperature‐rate dependent thermoelasticity theory. The quantitative agreement between calculation and experiment shows the contributions of the local thermal expansion and the propagating acoustic modes on the overall displacements at the epicenter and near the epicenter of the irradiated region. Nonlinear effects on the propagation of the near‐field surface acoustic wave may be present.

Point-source representation for laser-generated ultrasound in an elastic, transversely isotropic half-space

A point-source representation for laser-generated ultrasound in an elastic, transversely isotropic half-space is developed. This representation is comprised of a set of boundary conditions that approximates a thermoelastic point source located on the bounding surface of the half-space. An analytical expression representing the displacements for wave propagation along the symmetry axis is given for zinc where the bounding surface is the plane of transverse isotropy. The displacements obtained from theory are compared to experimental wave forms generated in a sample of single crystal zinc.

Coherent control of gigahertz surface acoustic and bulk phonons using ultrafast optical pulses

We demonstrate the coherent generation and control of gigahertz acoustic phonons with ultrafast optical pulses. Two distinct acoustic phonon modes, a surface acoustic phonon mode and a longitudinal acoustic phonon mode, are generated simultaneously by irradiating nanolithographic absorption gratings on semiconductor substrates. Two material systems are examined: suboptical wavelength aluminum absorption gratings on Si and GaAs substrates. Constructive and complete destructive interference conditions are demonstrated using two pump pulses derived from a single Michelson interferometer.

Measurement of thermal transport using time-resolved thermal wave microscopy

A theoretical and experimental analysis of a time resolved thermal wave microscopy (TRTWM) technique used for thermal transport measurements is presented. TRTWM utilizes elements of frequency and time domain laser based thermoreflectance techniques and is well suited to measure both lateral and cross plane thermal transport. A primary advantage of this method is that the pump and probe spot sizes do not have to be known accurately. Implementation of TRTWM to measure thermal transport in oxide substrates coated with thin metal films is demonstrated.

Application of laser-based resonant ultrasound spectroscopy to study texture in copper

Two copper specimens with distinct grain microstructures are investigated using laser resonant ultrasound spectroscopy (LRUS). One consists of randomly oriented crystallites and exhibits isotropic elastic behavior (two elastic constants), and the other has been highly textured by rolling and exhibits anisotropic elastic behavior (three elastic constants). The elastic constants are measured using electron backscatter diffraction, LRUS, and time domain laser ultrasound (LU). The elastic constants of the isotropic sample obtained via electron backscatter diffraction (EBSD), LU, and LRUS agree closely. However, for the anisotropic sample, there is considerable disagreement between results obtained using LRUS and results obtained using LU and EBSD. Analysis reveals that increasing the dimensionality of the modulus space leads to a questions of whether the LRUS results are unique to within experimental error. The consequence is that for anisotropic materials, small measurement uncertainties can lead to large un...