Phononic crystals and acoustic metamaterials enable the precise control of elastic properties, even in ranges inaccessible to traditional materials, making them useful for applications ranging from acoustic waveguiding to thermoelectrics. In particular, surface phononic crystals (SPCs) consisting of periodic nanolines on a semi-infinite substrate can be used to generate narrow bandwidth pseudosurface acoustic waves with exquisite sensitivity to the elastic properties of the underlying substrate. Tuning the period of the surface phononic crystal tunes the penetration depth of the pseudosurface wave, and thus selectively probes different depths of layered substrates. In our experiments, we use ultrafast near infrared laser pulses to excite these waves in the hypersonic frequency range by illuminating absorbing metallic nanolines fabricated on top of complex substrates. We probe the nanoscale dynamics launched by our SPCs via pump-probe spectroscopy where we monitor the diffraction of ultrafast pulses of extreme ultraviolet light generated via tabletop high harmonic generation. We then extract the mechanical properties of the substrate by comparing our measurements to quantitative finite element analysis. Utilizing this technique, we characterize the effective elastic and thermal transport properties of 3D periodic semiconductor metalattices.
Using a tabletop coherent extreme ultraviolet source, we extend current nanoscale metrology capabilities with applications spanning from new models of nanoscale transport and materials, to nanoscale device fabrication. We measure the ultrafast dynamics of acoustic waves in materials; by analyzing the material’s response, we can extract elastic properties of films as thin as 11nm. We extend this capability to a spatially resolved imaging modality by using coherent diffractive imaging to image the acoustic waves in nanostructures as they propagate. This will allow for spatially resolved characterization of the elastic properties of non-isotropic materials.
Coherent extreme ultraviolet beams from tabletop high harmonic generation offer revolutionary capabilities for observing nanoscale systems on their intrinsic length and time scales. By launching and monitoring acoustic waves in such systems, we fully characterize sub-10nm films and find that the Poisson’s ratio of low-k dielectric materials does not stay constant as often assumed, but increases when bond coordination is bellow a critical value. Within the same measurement, by following the heat dissipation dynamics from nano-gratings of width 20-1000nm and different periodicities, we confirm the effects of the newly identified collectively-diffusive regime, where close-spaced nanowires cool faster than widely-spaced ones.
Coherent extreme ultraviolet beams from tabletop high harmonic generation offer several revolutionary capabilities for
observing nanoscale systems on their intrinsic length and time scales. By launching and monitoring hypersonic acoustic
waves in such systems, we characterize the mechanical properties of sub-10nm layers and find that the material densities
remain close to their bulk values while their elastic properties are significantly modified. Moreover, within the same
measurement, by following the heat dissipation dynamics from 30-750nm-wide nanowires, we uncover a new thermal
transport regime in which closely-spaced nanoscale heat sources can surprisingly cool more efficiently than widelyspaced
heat sources of the same size.
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