Study of Nano-scale Non-equilibrium Thermal Transport Using Far-field and Near-field Ultrafast Optical Microscopy

As modern technology advances keep pushing the size limit of electronic devices down to nanoscale, the densities of elementary units on integrated circuits (ICs) are drastically increased. One of the major deleterious effects is the huge density of heat generated that may lead to device malfunction or break-down. Therefore, nanoscale thermal management has become an important research topic, which requires progress in both fundamental understanding of nanoscale thermal physics on the material level and novel engineering strategies on the device end. On the other hand, recent advances in material synthesis, processing, and nanofabrication have made available rich families of new materials with unprecedented properties that elevate them to potential candidates for next-generation electronic devices. Characterizing such materials with structure that varies on the length scale of a few nanometers, such as nanowires, 2D materials and Van de Waals heterostructures, necessitates new methodologies and experimental strategies that can unveil unexplored thermal physics mechanisms. In this thesis, we describe our recent work on developing a novel and comprehensive optical diagnostic platform with ultra-high sensitivity to conduct thermal measurements for a wild range of nanomaterials and nanostructures. Specifically, femto-second laser based optical pump-probe scheme is utilized to enable ultra-high temporal resolution (~300 fs), which offers access to investigating ultrafast dynamic processes that fundamental thermal carriers undergo in extreme non-equilibrium circumstances. Based on the ultra-fast optical pump-probe microscopy technique, we have extended the time-domain thermo-reflectance (TDTR) technique to frequency domain and constructed the time-resolved frequency domain thermo-reflectance (Tr-FDTR) measurement to characterize the transient material thermal properties across different time scales, from 100 fs to 10 ns upon ultrafast thermal excitation. Unlike TDTR, this newly developed methodology takes full advantage of the ultra-high time resolution embedded in ultra-fast pump-probe technique, for measuring dynamic thermal transport processes. Next, we have integrated the developed ultra-fast optical pump-probe microscopy technique with an atomic force microscopy (AFM) instrument in the configuration of scattering near-field scanning optical microscopy (s-NSOM). The combination of ultra-high temporal resolution and ultra-high spatial resolution (~10 nm) makes this comprehensive optical diagnostics platform an unprecedented tool for studying nanoscale transient heat transfer and energy transport processes. With this powerful tool, we have successfully demonstrated the ability to capture the two-dimensional ‘snapshots’ of the ultra-fast photoexcitation process in silicon nanowires, and the ability to extract the near-field signals for studying material dynamics at a level that is not achievable with any of the current far-field techniques./nThe developed instrumentation has the potential to contribute a new technology for investigating ultra-fast nanoscale thermal transport phenomena in both equilibrium and non-equilibrium regimes, which are currently completely unattainable with conventional thermal measurement techniques. The first major contribution will be the ability to directly investigate the ultrafast non-equilibrium thermal transport processes that various material systems undergo in response to an abrupt release of thermal energy. Key to the success of this effort is to utilize the femto-second time resolution to characterize the different behaviors of thermal energy carriers (electrons and phonons) and their interactions across all the stages (from sub-picosecond to nanosecond time range). This technique will enable us to experimentally verify new thermal transport mechanisms that emerge in unconventional material systems. The second major contribution is that the developed ultrafast thermal measurement technique can be extended down to nanoscale, thus offering the opportunity to interrogate fundamental nanoscale heat transfer problems. Therefore, we anticipate with confidence that this work will not only push the boundaries of our understandings of the nanoscale heat transfer and energy transport, but also shed light on unconventional mechanisms of thermal transport in newly discovered material systems that may be potential candidates for the next-generation electronics, and suggest new strategies for engineering nanomaterials for nanoscale thermal management in the next-generation electronics.

Advisor: Grigoropoulos, Costas P.