Past research activities
During the last decade, our group have investigated the interaction between electromagnetic waves and artificial media. Specifically, the past research activities of our group can be categorized into three distinct, but coherent topics.
(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 the 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.
: 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
(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 fashioned in a two-dimensional platform (i.e., metasurfaces), a natural choice of materials for the active control is the two-dimensional materials, such as graphene and TMDCs. With this picture in mind, we proposed a structure consisting of a two-dimensional array of 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).
: 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
(3) Tunable metasurface + Temporal boundary: Rapidly time-variant metaphotonics
Illustration of frequency conversion mechanism by temporal boundary. Image adapted from our paper published in Nature Photonics (2018)
When a photon is transmitted through the temporal boundary, the photon energy is changed while its momentum being conserved. This phenomenon is based on the principle that time-translation invariance breaking results in the energy conversion in a physical system. This change of energy (or the frequency) does not rely on the nonlinearity of the medium and therefore, the conversion efficiency remains invariant with respect to the intensity of light. For realization of the temporal boundary, we proposed a metadevice platform and demonstrated the linear frequency conversion of light occurring at the temporal boundary.
: 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