Research

Ultrafast imaging
Figure 1. Ultrafast photography| Multiple exposure photograph of a supersonically colliding (393 m/s) and rebounding (29 m/s) aluminum alloy sphere (approx. 20 um diameter)

High-rate Dynamics in the Micro-scale: The mechanical properties of a material cannot be extrapolated from its static mechanical properties when the deformation speed  is comparable to the speed of sound of the material, the ultimate speed of mechanical interactions among local mass elements. Since such macroscopic characteristics are believed to be originating from its nano-scale morphology, it is critical to study the nano-scale morphology at extreme dynamic conditions. Moreover, the mechanical properties of either natural or synthetic nano-materials have attracted great attention due to their potential for the future mechanical materials. However, most of their characterizations have been carried out at the quasi-static conditions. Our group has a unique experimental method for investigating the high-strain-rate and high-strain characteristics of nano-materials, known as Laser Induced Projectile Impact Test (LIPIT), In the LIPIT, a micrometer-scale projectile is accelerated up to ~3 km/s by the use of laser ablation, and impacts a very localized area of a nano-material to induce a HSR deformation. Using the LIPIT, we are quantitatively studying the dynamic responses of nano-structured materials, and attempting to extend the use of LIPIT to fluidic systems including biological substances.

 

Figure 2. Concept of metamaterials| Lee, J.-H. et al. Adv. Mater. 24, 4782 (2012).

Metamaterial: Generally, "material" and "device" are conceptually distinguished well. However, material having a designed regular structure in the nano-scale has a character somewhat in between them. This is a new class of materials called "metamaterials" as illustrated in Fig. 2. For example, the optical properties of the woodpile-like structure made of silicon can drastically be modified from its intrinsic properties of  silicon when the scale of the structure is comparable or less than the wavelength of light. Our group is greatly interested in the opportunity to create unprecedented material characteristics via nano-structuring, which is also extendable for other properties such as acoustic, thermal, and mechanical characteristics.

 

S7R-40X-3D
Figure 3. Hybrid plasmonic structure| This structure can engineer various characteristics of mid-infrared (2.5 - 10 um) thermal emission.

Heat Transport Engineering via Nano-structures:  The introduction of designed nano-structures can provide various novel ways to control heat transport. Unlike the classical prediction based on the Stefan-Boltzmann law, the radiative heat flux is dominantly governed by the quantum tunneling of photons at the nano-scale. Moreover, for far-field radiative transport, the surface of a material can be further engineered for non-Plankian thermal radiation. Our group is pursuing various functional micro- and nano-structures to control heat transport for energy, defense, and bio applications.