Ki-yeon Yang

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.

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.

30nm full field quartz template replicated from Si master for FLASH active layer NIL

38nm half pitch pattern was replicated from Si master pattern to quartz blank template. It is a novel approach different from typical quartz to quartz replication. This replication concept is expected to alleviate the burden not only in cost but also resolution for NIL template fabrication. In this study, full field Si master fabricated by ArF immersion lithography, UV-transparent hard mask for quartz blank template and core-out quartz blank template were applied to prove the concept. And the replica template was evaluated with NIL and subsequent etching.