Optical and Electro-Mechanical Devices Group [National Leading Research Laboratory]
During the last decade, we have investigated interaction between electromagnetic waves and spacetime periodic media. More specifically, past achievement of our group in this vibrant and emerging field can be categorized into three distinct, but coherent topics as shown below.

(1) Strongly-coupled meta-atoms for extreme optical properties

(Left) Unnaturally high refractive index metamaterial operating at THz frequencies (Right) Capacitively coupled meta-atom structure designed for extremely large effective permittivity. Images adapted from Nature news and views on our paper published in Nature (2011)

Seen from the perspective of conventional optics, the collection of artificially-structured atoms has enabled the observation of a vast variety of unexpected physical phenomena. Research activities of our group were focused on experimentally demonstrating extreme effective material (optical) parameters, such as large refractive index and strong chirality. For this purpose, we made use of meta-atom or meta-molecule structures that maximally utilize capacitive coupling between nearest neighboring meta-units. This approach made it possible for us to demonstrate record-high values of effective refractive index and chirality that have not been observed previously. It is noteworthy that the approach emphasized the role of interaction between neighboring meta-atoms or meta-molecules in addition to the structural design of individual meta-units.

Key achievements: Proposal on the utilization of strong capacitive coupling between adjacent meta-atoms for realizing extreme effective material parameters (M. Choi et al., Nature, 2011, Y. Kim et al., Nature Photonics, 2016),

(2) 2D strongly-coupled meta-atoms + 2D natural material for slowly time-variant tunable metasurfaces

Electrically controllable graphene hybridized metasurface. Image adapted from Science (Perspective) on our paper published in Nature Materials (2012)

As most metamaterials are configured in a two-dimensional array (i.e., metasurfaces), a natural choice of materials for the active control is two-dimensional materials, such as graphene and TMDCs. With this picture in mind, We proposed a structure consisting of a two-dimensional array of strong capacitively-coupled meta-atoms hybridized with gated graphene. The exotic electrical and optical properties of graphene, when enhanced by the strong capacitive response of meta-atoms, lead to a very strong interaction between massless Dirac fermions and photons such that persistent switching and fast linear modulation of low-energy photons are made possible in the extreme subwavelength-scale thickness (below λ/1,000,000).

Key achievements: Proposal on the hybridization of strong capacitively-coupled meta-atom structures with gated-graphene and its application as an electrically-controllable metadevice (S. H. Lee et al., Nature Materials, 2012, W. Y. Kim et al., Nature Communications, 2016, T. -T. Kim, Science Advances, 2017),

(3) Tunable metasurface + Temporal boundary: Rapidly time-variant metaphotonics

Illustration of frequency conversion mechanism by temporal boundary. Image adapted from Nature Photonics (2018)

Benefitting from the research experience on slowly time-variant metasurfaces, We started to work on a rapidly time-variant metasurface that allows the realization of a temporal boundary for photons. When photons are passing through the temporal boundary, the energy of a photon is changed while its momentum conserved. This phenomenon is based on the fact that time-translation invariance breaking results in the energy conversion in a physical system (Noether's theorem). This change of energy (or the frequency of light) does not rely on the nonlinearity of the medium and therefore, the conversion efficiency remains invariant with respect to the intensity of light. Furthermore, the frequency of a converted wave and its efficiency are tailorable to a large degree as the conversion process does not require energy conservation between participating waves due to the presence of a temporal boundary. For realizing the temporal boundary, We proposed a metadevice platform and demonstrated the linear frequency conversion of light occurring at the temporal boundary. This research on the temporal boundary led me to investigate more interesting spatiotemporal effect in photonics, which will be briefly described below in the future research plan.

Key achievements: Proposal on the use of a temporal boundary in rapidly time-variant metasurfaces for linear frequency conversion of light (K. Lee et al., Nature Photonics, 2018).
KAIST Department of Mechanical Engineering
Optical and Electro-Mechanical Devices Group, B. Min Group @KAIST