Alexei Maznev

Optical heterodyne detection of laser-induced gratings

A novel optical arrangement for heterodyne detection of laser-induced gratings based on the use of a phase mask for both excitation and probe beams provides phase stability and control without the need for an active stabilization scheme. The arrangement greatly simplifies the laser-induced grating experiment. The performance of the technique in both transmission and reflection geometries is illustrated through measurements of bulk and surface acoustic waves generated by picosecond laser pulses.

How to make femtosecond pulses overlap

Normally, femtosecond light pulses that cross at a nonzero angle overlap over only a small region in space. This limitation can be overcome by the use of diffraction orders of a grating. We consider an arrangement in which, on diffraction of a femtosecond pulse by a grating, two beams that correspond to the first-order diffraction maxima are recombined at the image plane by a system of two confocal lenses. In this arrangement the beams overlap over the their full aperture, with the short duration of the pulses being preserved. We demonstrate the use of this setup as a simple autocorrelator and discuss a possible application to time-resolved vibrational spectroscopy.

Measuring Phonon Mean Free Path Distributions by Probing Quasiballistic Phonon Transport in Grating Nanostructures

Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.

High-precision film thickness determination using a laser-based ultrasonic technique

A noncontact and nondestructive laser-based acoustic technique called impulsive stimulated thermal scattering (ISTS) is used to measure thicknesses of metal films including Cu, Ta, W, Al, Ti, and others in single-layer and multilayer assemblies on silicon substrates. Other opaque film materials and substrates have also been examined. Thicknesses are determined with a repeatability of a few angstroms with data acquisition times of about 1 s. ISTS and conventional measurements (scanning electron microscopy, profilometry, and four-point electrical sheet resistance) are made on the same samples and the results are found to compare favorably.

Interaction of laser-generated surface acoustic pulses with fine particles: Surface cleaning and adhesion studies

The mechanical forces associated with the surface acceleration in high-amplitude surface acoustic waves (SAWs) detach the particles from the surface. The removal of micron sized particles with a nanosecond SAW pulse excited by a focused laser beam in a silicon wafer was quantitatively investigated. Both vertical and horizontal particle displacements have been observed. It is shown that for nanosecond SAW pulses the limit of the surface acceleration of about 1010 m/s2 is set by the fracture of the material and corresponds to the removal of particles larger than about 0.05 μm. In addition, the nonlinear transformation of the excited SAW pulses results in an increase of the surface acceleration and contributes to the cleaning process extending it to even smaller particle dimensions. The technique is applicable in vacuum and improves the energetic effectiveness of the cleaning due to the removal of particles not only in the irradiated region, but also in the wider area covered by the SAW pulse propagation. It...

Phase-controlled, heterodyne laser-induced transient grating measurements of thermal transport properties in opaque material

The methodology for a heterodyned laser-induced transient thermal grating technique for non-contact, non-destructive measurements of thermal transport in opaque material is presented. Phase-controlled heterodyne detection allows us to isolate pure phase or amplitude transient grating signal contributions by varying the relative phase between reference and probe beams. The phase grating signal includes components associated with both transient reflectivity and surface displacement whereas the amplitude grating contribution is governed by transient reflectivity alone. By analyzing the latter with the two-dimensional thermal diffusion model, we extract the in-plane thermal diffusivity of the sample. Measurements on a 5 μm thick single crystal PbTe film yielded excellent agreement with the model over a range of grating periods from 1.6 to 2.8 μm. The measured thermal diffusivity of 1.3 × 10−6 m2/s was found to be slightly lower than the bulk value.

The elastodynamic response of a semi-infinite anisotropic solid to sudden surface loading

Integral expressions are derived for the displacement response tensor of a semi-infinite anisotropic elastic continuum of unrestricted symmetry to a concentrated force suddenly applied to its surface. The surface response is reduced to a one-dimensional integral for numerical evaluation, while the interior response is left as a two-dimensional integral. Calculated surface response functions for Cu(001) are presented. These display multiple Rayleigh wave arrival singularities as well as bulk wave arrivals. Calculated interior response functions for Zn(0001) are presented. These display bulk and head wave arrivals. In followup papers these methods will be used in the interpretation of capillary fracture generated waveforms measured in a number of different materials.

Transient grating measurements of film thickness in multilayer metal films

A transient grating technique has been used to measure film thicknesses in five multilayer film structures. Through-plane acoustic responses are observed through time-resolved diffraction that reveals their return to the free surface after partial reflections within the structure. Optical heterodyne detection with a controlled reference field amplitude permits optimization of the signal/noise ratio relative to detection of the signal against the far stronger incident probe field or against a dark background (homodyne detection). In-plane acoustic waves are also generated and observed.

Transient grating measurements of picosecond acoustic pulses in metal films

A transient grating technique is used to detect picosecond acoustic pulses in supported metal films. Crossed femtosecond laser pulses generate acoustic responses with longitudinal components propagating normal to the film plane and surface acoustic wave components propagating in the film plane. Surface “ripple” associated with both components is detected through the diffraction of a probe beam. The measurements yield enhanced information content for characterization of film thickness and mechanical properties.

Secluded supersonic surface waves in germanium

Abstract A secluded supersonic SAW is known to exist in an isolated direction on the (001) surface of certain cubic crystals. This wave consists of two evanescent partial wave components and is not polarized in a symmetry plane. Calculations are reported on this mode, using germanium as an example, and it is found that this wave continues to exist for some surface orientations different from (001). A one dimensional family of nonsymmetrical supersonic SAW geometries is identified and it is found to be joined to the family of supersonic SAW polarized in the (1 1 0) symmetry plane.