Chang Seung Lee

Low-visibility patterning of transparent conductive silver-nanowire films

A partial etching mechanism is proposed to meet the requirement for low-visibility patterning of silver nanowire (AgNW)-based transparent conductive electrodes (TCEs) by reducing the difference in optical properties between conductive and nonconductive regions of the pattern. Using the finite difference time domain (FDTD) method, etched geometries that provide the smallest difference in transmittance after etching are theoretically determined. A sodium hypochlorite-based etchant capable that allows the etched geometry to be varied by controlling the pH is used to create a low-visibility pattern with a transmittance and haze difference of 0.07 and 0.04%, respectively. To the best of our knowledge, this is the first time that a partial etching mechanism such as this has been studied in relation to AgNW-based TCEs.

Period reduction lithography in normal UV range with surface plasmon polaritons interference and hyperbolic metamaterial multilayer structure

A plasmonic lithography method is proposed to generate a large-area, period-reduced, subwavelength one-dimensional grating pattern. It is demonstrated with a surface plasmon polaritons interference effect and a hyperbolic metamaterial multilayer structure by UV radiation at 405 nm wavelength. Photoresist patterns of 1.5 × 1.5 cm2 area with 350 nm period and 100 nm linewidth were fabricated on polyethylene terephthalate films by using a 700-nm-period Al grating mask and exposed to a transverse magnetic polarized laser. The proposed lithography approach provides a cost-effective and less laborious pathway for the fabrication of large-area periodic nano-patterns.

Nanofabrication of low extinction coefficient and high-aspect-ratio Si structures for metaphotonic applications

We investigated forming of high refractive index (n), low extinction coefficient (k) of Si dielectrics in visible wavelength ranges. To decrease k, pulsed green laser annealing (GLA) with line beam of a 532-nm wavelength was applied in this study for homogeneous melting. By AFM, XRD and TEM analysis, we examined the defect reduction in various conditions during poly-crystallization. We achieved dielectric nanostructures having optical properties of n>4.2, k<0.06 at 550 nm wavelength and fine pitches down to 40 nm (aspect ratio 3:1) and 130 nm (aspect ratio 7:1) with ±5% size accuracy. Finally, we realized optical metasurfaces for optical band filters, flat lens and beam deflectors.

Tension assisted metal transfer of graphene for Schottky diodes onto wafer scale substrates.

We developed an effective graphene transfer method for graphene/silicon Schottky diodes on a wafer as large as 6 inches. Graphene grown on a large scale substrate was passivated and sealed with a gold layer, protecting graphene from any possible contaminant and keeping good electrical contact. The Au/graphene was transferred by the tension-assisted transfer process without polymer residues. The gold film itself was used directly as the electrodes of a Schottky diode. We demonstrated wafer-scale integration of graphene/silicon Schottky diode using the proposed transfer process. The transmission electron microscopy analysis and relatively low ideality factor of the diodes indicated fewer defects on the interface than those obtained using the conventional poly(methyl methacrylate)-assisted transfer method. We further demonstrated gas sensors as an application of graphene Schottky diodes.

Low diffuse reflection of silver nanowire/ruthenium oxide nanosheet hybrid films for high-performance transparent flexible electrodes

Transparent conducting electrodes (TCEs) based on silver nanowire (AgNW) networks possess high conductance, transmittance, and mechanical flexibility. However, due to the relatively high diffuse reflection of incident light on AgNWs, they cannot be practically implemented in displays requiring low pattern visibility. One promising strategy for solving this problem is to place an optical stack with high refractive index underneath the AgNW layer. In this work, AgNW-RuO2 nanosheet hybrid TCEs with low diffuse reflections are fabricated using metallic RuO2 nanosheets as undercoats. As predicted by theoretical simulations, RuO2 nanosheets with high refractive indices reduce the diffuse reflections of AgNWs by almost 8%. Moreover, after the partial etching of AgNWs, the difference in the diffuse reflections of their etched and non-etched regions becomes equal to about 0.003, leading to the formation of an invisible pattern. The film consisting of micro-sized RuO2 nanosheets is not damaged during etching, but instead forms a current path between different AgNWs broken by cyclic bending, resulting in a tenfold decrease in the resistance of the AgNW TCE after 170 000 cycles. Further, RuO2 nanosheets suppress the diffusion of humid air from the outside, thus improving the environmental stability of the AgNW-RuO2 nanosheet hybrid TCEs.

Time-asymmetric loop around an exceptional point over the full optical communications band

Topological operations around exceptional points1–8—time-varying system configurations associated with non-Hermitian singularities—have been proposed as a robust approach to achieving far-reaching open-system dynamics, as demonstrated in highly dissipative microwave transmission3 and cryogenic optomechanical oscillator4 experiments. In stark contrast to conventional systems based on closed-system Hermitian dynamics, environmental interferences at exceptional points are dynamically engaged with their internal coupling properties to create rotational stimuli in fictitious-parameter domains, resulting in chiral systems that exhibit various anomalous physical phenomena9–16. To achieve new wave properties and concomitant device architectures to control them, realizations of such systems in application-abundant technological areas, including communications and signal processing systems, are the next step. However, it is currently unclear whether non-Hermitian interaction schemes can be configured in robust technological platforms for further device engineering. Here we experimentally demonstrate a robust silicon photonic structure with photonic modes that transmit through time-asymmetric loops around an exceptional point in the optical domain. The proposed structure consists of two coupled silicon-channel waveguides and a slab-waveguide leakage-radiation sink that precisely control the required non-Hermitian Hamiltonian experienced by the photonic modes. The fabricated devices generate time-asymmetric light transmission over an extremely broad spectral band covering the entire optical telecommunications window (wavelengths between 1.26 and 1.675 micrometres). Thus, we take a step towards broadband on-chip optical devices based on non-Hermitian topological dynamics by using a semiconductor platform with controllable optoelectronic properties, and towards several potential practical applications, such as on-chip optical isolators and non-reciprocal mode converters. Our results further suggest the technological relevance of non-Hermitian wave dynamics in various other branches of physics, such as acoustics, condensed-matter physics and quantum mechanics.Time-asymmetric light transmission over the entire optical communications band is achieved using a silicon photonic structure with photonic modes that dynamically encircle an exceptional point in the optical domain.

Epitaxial integration and properties of SrRuO3 on silicon

We report the integration of SrRuO3, one of the most widely used oxide electrode materials in functional oxide heterostructures, with silicon using molecular-beam epitaxy and an SrTiO3 buffer layer. The resulting SrRuO3 film has a rocking curve full width at half maximum of 0.01°, a resistivity at room temperature of 250 μΩ cm, a residual resistivity ratio (ρ300 Kρ4 K) of 11, and a paramagnetic-to-ferromagnetic transition temperature of ∼160 K. These structural, electrical, and magnetic properties compare favorably to the best reported values for SrRuO3 films on silicon and rival those of epitaxial SrRuO3 films produced directly on SrTiO3 single crystals by thin film growth techniques other than molecular-beam epitaxy. These high quality SrRuO3 films with metallic conductivity on silicon are relevant to integrating multi-functional oxides with the workhorse of semiconductor technology, silicon.We report the integration of SrRuO3, one of the most widely used oxide electrode materials in functional oxide heterostructures, with silicon using molecular-beam epitaxy and an SrTiO3 buffer layer. The resulting SrRuO3 film has a rocking curve full width at half maximum of 0.01°, a resistivity at room temperature of 250 μΩ cm, a residual resistivity ratio (ρ300 Kρ4 K) of 11, and a paramagnetic-to-ferromagnetic transition temperature of ∼160 K. These structural, electrical, and magnetic properties compare favorably to the best reported values for SrRuO3 films on silicon and rival those of epitaxial SrRuO3 films produced directly on SrTiO3 single crystals by thin film growth techniques other than molecular-beam epitaxy. These high quality SrRuO3 films with metallic conductivity on silicon are relevant to integrating multi-functional oxides with the workhorse of semiconductor technolog...

All dielectric metasurface nano-fabrication based on TiO2 phase shifters

All dielectric metasurface of low loss TiO2 in visible wavelengths was devised forming subwavelength-scale nanostructures. DC-magnetron sputtering of oxygen-reduced TiOx (x<;2) target with reactive oxygen gas made dense amorphous TiO2 layers of 1.35nm Ra. A 380nm-thick TiO2 has low extinction coefficient (k) under 1x10-5, transparency of 98.33% and high refractive index (n) of 2.55 at 485nm wavelength. Highly precise TiO2 meta-atoms were successfully defined with 100nm-300nm feature size. Phase shift properties of TiO2 metasurface were measured. Finally, we constructed dielectric metaphotonic platform for various optical devices such as band pass filters, flat lens and beam deflectors in visible ranges.