Ultrafast Ultraviolet Transient Grating—Tabletop Access to Nanoscale Thermal Transport in High-Bandgap Materials

Nanostructuring on length scales comparable to the fundamental length scales of phonons in materials such as silicon can provide control over thermal transport, making it possible to engineer thermal properties [1]. Finding effective thermal management strategies is critical for a host of applications, as temperature often determines the speed and efficiency, <i>e.g. </i>the clock-speed of modern processors is limited due to a heat buildup [2]. However, as devices become increasingly complex, thermal management solutions are elusive due to a lack of predictive models that span from micro-to-nano-to-atomic scale lengths. To advance our understanding of nanoscale heat flow, more precise measurements of the thermal properties of general, complex, nanostructured, and high-bandgap materials are needed, which can be used to validate predictive theories.<br/>Specifically, recent research has shown that transient thermal grating experiments can reveal non-diffusive thermal transport [3], as well as exotic phonon behaviors such as second sound [4]. However, due to the fundamental diffraction limit of visible light, such visible-laser-based techniques cannot access the nanoscale length scales (&lt;0.5 µm) relevant to advanced devices. Moreover, visible lasers cannot directly excite high-bandgap materials such as ceramic materials for advanced battery applications, or diamond for quantum systems applications. Our recent work has demonstrated that tabletop, coherent short-wavelength laser-like beams can provide unique insight into the thermal transport properties of nanosystems—uncovering surprising new behaviors. These include exploring and explaining new behaviors for the cooling of periodic nanostructured heaters where close-packed hot spots cool faster than widely-spaced ones [5-7], validating novel theories using a hydrodynamic-like transport equation to predict phonon flow in general semiconductors [7], and a universal scaling of nanoscale heat flow in highly-confined geometries [8].<br/>Here, we combine the versatility of tabletop transient thermal grating experiments with the sensitivity and precision of short-wavelength light by demonstrating a deep-ultraviolet (UV) transient grating metrology using ultrafast UV lasers with wavelengths &lt;200 nm and with 100s of femtosecond pulse durations. We first upconvert ultrafast, infrared pulses to the deep-UV using nonlinear crystals [9], and then split and recombine the pulses to create a transient, sinusoidal interference pattern [10] of deep-UV light on the sample. The relaxation of the resulting sinusoidal thermal excitation is then observed using an ultrafast probe beam with wavelengths from the visible to UV. The short-wavelength of the light allows us to probe transport distances on the 100s of nanometer scale in both visibly-transparent and opaque materials. We then use this technique to observe non-diffusive nanoscale thermal transport in diamond on scale lengths less than dominant phonon mean free paths. We use microscopic theories and mesoscopic models to benchmark our results. This work demonstrates that tabletop sources of ultrafast, deep-UV light can reveal new insight into phonon transport at its intrinsic scales in a general set of materials allowing for the development and validation of advanced theories and for nondestructive, noncontact characterization of thermal properties of energy and quantum materials.<br/>[1] Phys. Rev. Lett. 112, 055505 (2014)<br/>[2] Nature 530, 144 (2016).<br/>[3] Phys. Rev. Lett. 110, 025901 (2013)<br/>[4] Science 364, 375 (2019)<br/>[5] Phys. Rev. Appl. 11, 024042 (2019)<br/>[6] PNAS 118, e2109056118 (2021)<br/>[7] ACS Nano 15, 13019 (2021)<br/>[8] arXiv:2209.11743<br/>[9] Opt. Lett. 18, 2035 (1993)<br/>[10] J. Appl. Phys. 111, 023503 (2012)