14:15 〜 14:30
▼ [15p-F206-2] Heat guiding and focusing using phononic nanostructures
キーワード：Phonons, Heat conduction, Ballistic transport
In the past few years, ballistic heat transport has been demonstrated experimentally in the bulk, thin-films and various nanostructures. Yet, a practical use of this phenomenon remains challenging, because thermal phonons tend to travel in almost random directions. We used µTDTR experiments and Monte-Carlo (MC) simulations to show the in-plane ballistic heat transport in silicon phononic crystals (PnCs) and a possibility to use such PnCs for directional thermal emission and heat focusing.
First, we fabricated and measured PnCs with aligned (Fig. 1a) and staggered (Fig. 1b) lattices of five different periods, and demonstrated that significant difference in thermal decay times (t) appears in the structures with small period (Fig. 1c). This difference depended on the neck size (n) and strengthened by factor of two at 4 K (not shown). We attributed the faster heat dissipation in the samples with aligned lattice to the presence of ballistic heat transport in the passages between the holes.
To prove this hypothesis, we fabricated samples of PnCs coupled with nanowires (NWs) (Fig. 1d), in “coupled” (Fig. 1e) and “uncoupled” (Fig. 1f) configurations. The type of the heat transport in NWs is length dependent, being ballistic in short and diffusive in long NWs. Indeed, we observed a significant difference (Δ) between t of coupled and uncoupled samples with short NWs at 4 K, due to the enhancement of ballistic transport in the coupled configuration. This implies that the PnC acted a source of ballistic phonons for the NWs. Remarkably, as the NWs lengthen and/or the temperature is increased, the effect gradually disappears (Fig. 1g and 1e), because heat transport becomes diffusive.
Finally, we showed that this effect can be used to create thermal lens nanostructures that can focus thermal energy in the focal point. We demonstrate the formation of a hot spot of 115 nm (Fig. 1i) using MC simulations, and show experimental evidence of the heat focusing. These results motivate the concept of ray-like heat manipulations at the nanoscale.
First, we fabricated and measured PnCs with aligned (Fig. 1a) and staggered (Fig. 1b) lattices of five different periods, and demonstrated that significant difference in thermal decay times (t) appears in the structures with small period (Fig. 1c). This difference depended on the neck size (n) and strengthened by factor of two at 4 K (not shown). We attributed the faster heat dissipation in the samples with aligned lattice to the presence of ballistic heat transport in the passages between the holes.
To prove this hypothesis, we fabricated samples of PnCs coupled with nanowires (NWs) (Fig. 1d), in “coupled” (Fig. 1e) and “uncoupled” (Fig. 1f) configurations. The type of the heat transport in NWs is length dependent, being ballistic in short and diffusive in long NWs. Indeed, we observed a significant difference (Δ) between t of coupled and uncoupled samples with short NWs at 4 K, due to the enhancement of ballistic transport in the coupled configuration. This implies that the PnC acted a source of ballistic phonons for the NWs. Remarkably, as the NWs lengthen and/or the temperature is increased, the effect gradually disappears (Fig. 1g and 1e), because heat transport becomes diffusive.
Finally, we showed that this effect can be used to create thermal lens nanostructures that can focus thermal energy in the focal point. We demonstrate the formation of a hot spot of 115 nm (Fig. 1i) using MC simulations, and show experimental evidence of the heat focusing. These results motivate the concept of ray-like heat manipulations at the nanoscale.