Effects of Photoinduced Elastic Responses on Debye-Waller Temperature Measurements

The performance and robustness of smaller, faster, and more efficient electronic architectures and devices becomes limited as thermal effects begin to dominate at small scales. The push to shrink dimensionality, and the subsequent impact of thermal effects, has led to the development of methods capable of being used to quantify nanoscale thermal transport and brought about insights into (for example) thermal conductivity, thermoelasticity, and thermal boundary conductance [1]. Owing to the combined nanoscale dimensions and ultrafast timescales associated with thermal-energy carriers, it is useful to extend metrology techniques into these domains in order to directly resolve discrete carrier dynamics. To this end, we are exploring ultrafast electron microscopy (UEM) as a possible method to facilitate access to this parameter space. A typical UEM experiment consists of a pump-probe approach, wherein a specimen is excited (pumped) in situ with a femtosecond laser pulse and subsequently probed with a discrete packet of electrons generated from a photocathode. In this way, the ultrafast structural dynamics of nanoscale and nanostructured specimens can be resolved using imaging and diffraction in the same manner as is done with conventional (static) transmission electron microscopy. Typically, for general ultrafast electron and X-ray scattering experiments, transient temperatures are determined using the Debye-Waller (DW) effect, in which the attenuation of Bragg scattering is related to atomic thermal energies. This approach is appealing, because it provides a direct probe of the lattice response requiring only detection of a scattered electron or X-ray photon, and it can in principle be performed with combined high spatial and temporal resolutions. It has been shown, however, that other factors, in addition to mean atomic displacements, can affect (and even dominate) the intensity of Bragg reflections, most notably thermally-induced specimen tilting and translation [2-5]. Therefore, effects that obfuscate intensity attenuation due to thermal vibrations must be identified, quantified, and deconvoluted in order to accurately and precisely determine nanoscale transient temperatures in this way.