arXiv subjects

Sang Jun Kim

Publications and source records attributed to Sang Jun Kim.

3 recordsLinked to original sources

arXiv2020

Optical-Field Driven Charge-Transfer Modulations near Composite Nanostructures

Optical activation of material properties illustrates the potentials held by tuning light-matter interactions with impacts ranging from basic science to technological applications. Here, we demonstrate for the first time that composite nanostructures providing nonlocal environments can be engineered to optically trigger photoinduced charge transfer dynamic (CTD) modulations in the solid state. The nanostructures herein explored lead to unprecedented out-of-phase behaviour between charge separation and recombination dynamics, along with linear CTD variations with the optical-field amplitude. Using transient absorption spectroscopy, up to 270 % increase in charge separation rate is obtained in organic semiconductor thin films. We provide evidences that composite nanostructures allow for surface photovoltages to be created, which kinetics vary with the composite architecture and last beyond optical pulse temporal characteristics. Furthermore, by generalizing Marcus theory framework, we explain why CTD modulations can only be unveiled when optic field effects are enhanced by nonlocal image dipole interactions. Demonstrating that composite nanostructures can be designed to use optical fields as CTD remote actuators opens the path for their use in practical and original applications ranging from photochemistry to optoelectronics.

cond-mat.mtrl-sci↗

arXiv2019

Natural-mixing guided design of refractory high-entropy alloys with as-cast tensile ductility

Multi-principal-element metallic alloys have created a growing interest that is unprecedented in metallurgical history, in exploring the property limits of metals and the governing physical mechanisms. Refractory high-entropy alloys (RHEAs) have drawn particular attention due to their (i) high melting points and excellent softening-resistance, which are the two key requirements for high-temperature applications; and (ii) compositional space, which is immense even after considering cost and recyclability restrictions. However, RHEAs also exhibit intrinsic brittleness and oxidation-susceptibility, which remain as significant challenges for their processing and application. Here, utilizing natural-mixing characteristics amongst refractory elements, we designed a Ti38V15Nb23Hf24 RHEA that exhibits >20% tensile ductility already at the as-cast state, and physicochemical stability at high-temperatures. Exploring the underlying deformation mechanisms across multiple length-scales, we observe that a rare beta prime precipitation strengthening mechanism governs its intriguing mechanical response. These results also reveal the effectiveness of natural-mixing tendencies in expediting HEA discovery.

cond-mat.mtrl-sci↗

arXiv2018

Hyperbolic Dispersion Dominant Regime Identified through Spontaneous Emission Variations near Metamaterial Interfaces

Surface plasmon polariton, hyberbolic dispersion of energy and momentum, and emission interference provide opportunities to control photoluminescence properties. However, the interplays between these regimes need to be understood to take advantage of them in optoelectronic applications. Here, we investigate broadband variations induced by hyperbolic metamaterial (HMM) multilayer nanostructures on the spontaneous emission of selected organic chromophores. Experimental and calculated spontaneous emission lifetimes are shown to vary non-monotonously near HMM interfaces. With the SPP and interference dominant regimes. With the HMM number of pairs used as the analysis parameter, the lifetime is shown to be independent of the number of pairs in the surface plasmon polaritons, and emission interference dominant regimes, while it decreases in the Hyperbolic Dispersion dominant regime. We also show that the spontaneous emission lifetime is similarly affected by transverse positive and transverse negative HMMs. This work has broad implications on the rational design of functional photonic surfaces to control the luminescence of semiconductor chromophores.

cond-mat.mtrl-sci↗