Nonlinear optical spectroscopies such as 4-wave mixing are very powerful tools for materials characterization. However, the spatial lengths over which information can be obtained are limited by the wavelength of light. By extending nonlinear spectroscopy to EUV wavelengths, ultrafast thermal and electronic responses on nanometer length scales can be examined. Measurement of correlation length scales associated with multiple structural relaxation time scales in polymers, or investigation of small structural features in nanoporous materials or in patterned thin film structures fabricated through EUV lithography, will be possible.

In this experiment in collaboration with Keith Nelson's group at MIT, a transient interference pattern is generated by interfering two short EUV pulses on a surface, in order to excite high-frequency acoustic modes with the same period as the interference pattern. These acoustic excitations are then probed using a time-delayed third EUV pulse. Using EUV light, much shorter wavelengths can be used, making it possible to probe acoustic responses in these materials on nanoscale spatial scales over a dramatically larger fraction of "k-space".

In recent work, we illuminated a thin-film surface with EUV light from high-harmonic generation, and then heated a section of the surface with a laser pulse. This launched an acoustic wave that propagated both into the film, and along the surface. Although we estimate the resulting surface motion to be just the width of a single atom, we could observe this motion in the reflected light. This corresponds to the first Gabor holograms taken with femtosecond time resolution and atomic-scale resolution (although these first images only have this high resolution in the "vertical" dimension).

Fig. 2. A holographic image of the surface deformation created by the laser. Applied Physics Letters 85, 564 (2004); Applied Physics Letters 89, 091108 (2006); Optics Letters 32, 286 (2007).