Seojoo Lee

Robust numerical evaluation of circular dichroism from chiral medium/nanostructure coupled systems using the finite-element method

It has been demonstrated that circular dichroism (CD) signals from chiral molecules can be boosted by plasmonic nanostructures inducing strong local electromagnetic fields. To optimize nanostructures to improve CD enhancement, numerical simulations such as the finite element method (FEM) have been widely adopted. However, FEM calculations for CD have been frequently hampered by unwanted numerical artifacts due to improperly discretizing problem spaces. Here, we introduce a new meshing rule for FEM that provides CD simulations with superior numerical accuracy. We show that unwanted numerical artifacts can be suppressed by implementing the mirror-symmetric mesh configuration that generates identical numerical artifacts in the two-opposite circularly polarized waves, which cancel each other out in the final CD result. By applying our meshing scheme, we demonstrate a nanostructure/chiral molecule coupled system from which the CD signal is significantly enhanced. Since our meshing scheme addresses the previously unresolved issue of discriminating between very small CD signals and numerical errors, it can be directly applied to numerical simulations featuring natural chiral molecules which have intrinsically weak chiroptical responses.

Revealing local origin of surface-enhanced circular dichroism

We present a theory of the microscopic origins of the surface-enhanced circular dichroism (CD) with nanostructures. Recently, nanostructures and metamaterials have been used for the enhancement of CD signals of chiral molecules. However, the complete description of microscopic origins of the surface-enhanced circular dichroism (CD) has never been achieved. We find the total CD signals of the nanostructure coupled to chiral molecules can be decomposed into two factors: the induced and inherent CD. The inherent CD comes from the molecular absorption which can be enhanced by the strongly localized optical helicity density of resonant near-fields near the nanostructure. The induced CD is originated from the asymmetric absorption of light inside the nanostructure perturbed by nearby chiral molecules upon two opposite circularly polarized light. The recent surge of interest in the surface-enhanced CD spectroscopy has been inspired by the inherent CD enhancement, but our work shows that the induced CD can significantly contribute to the total CD signal of the chiral molecule/nanostructure coupled system. Using an example of gold nanodisk arrays, we demonstrate that the inherent and induced CD can compete with each other in plasmonic nanostructures. In this presentation, we also provide design principles for CD sensor using nanostructures and metamaterials.

Giant phase retardation of terahertz waves by resonant hyperbolic metasurface

Due to the relatively weak birefringence of natural materials in terahertz regime, metasurfaces have been proposed for compact terahertz phase modulators since they show effectively strong birefringence only with ultrathin structures. However, previous designs of metasurface show limited phase modulation reaching only up to the quarter-wavelength phase, and there has been no single metasurface design that works for a terahertz half-waveplate. Here, we present a metasurface that modulates the phase variably up to 180 degrees. The phase modulation is achieved by a hyperbolic metasurface composed of periodically arrayed rectangular metal rings with different periods for horizontal and vertical axis. By controlling each period, we show that our hyperbolic metasurface can possess large positive and negative permittivity values for horizontal and vertical axis and the phase shift can reach up to the 180 degrees. To check the validity of our design, we fabricate reconfigurable metasurface films and demonstrate the phase modulation 90 to 180 degrees. All results show good agreement with numerical simulation results.

Effects of Fano Resonance on Optical Chirality of Planar Plasmonic Nanodevices

The effects of Fano resonance on the optical chirality of planar plasmonic nanodevices in the visible wavelength range are experimentally observed and theoretically explained. The nanodevice consists of a nanodisk at the center with six gold nanorods with an orientation angle to exhibit optical chirality under dark-field illumination. The chiral response induced by the gold nanorods are affected by the presence of the nanodisk with different diameters which causes Fano resonance. An intriguing change to the opposite selection preference of different handedness of the circularly polarized light has been clearly observed experimentally. This change of the preference is understood based on the extended coupled oscillator model. Moreover, electrostatic analysis and the time-dependent simulations provide a further understanding of the phenomenon. The observed and understood effects of Fano resonance on optical chirality enables effective manipulation of chiral characteristics of planar subwavelength nanodevices.

Anomalous Wavelength Scaling of Tightly Coupled Terahertz Metasurfaces

We theoretically and experimentally demonstrate the drastic changes in the wavelength scaling of tightly coupled metasurfaces caused by deep subwavelength variations in the distance between the unit resonators but no change in the length scale of the units themselves. This coupling-dependent wavelength scaling is elucidated by our model metasurfaces of ring resonators arranged with deep subwavelength lattice spacing g, and we show that narrower g results in rapider changes in wavelength scaling. Also, by using terahertz time-domain spectroscopy, we experimentally observed a significant shift of the spectral response arising from very small variations in lattice spacing, confirming our theoretical predictions.

Analysis on the surface-enhanced circular dichroism spectroscopy

Circular dichroism (CD), the difference in absorption of two opposite circularly polarized light by chiral molecules. Based on the generalized Poyntings theorem for chiral media, we reveal the origin of the CD in the chiral molecule/ nanostructure system. We find that CD enhancement arises through two distinct CD channels — inherent and induced. The inherent CD channel is direct molecular CD enhanced by strongly localized optical helicity density near the nanostructure. The induced CD channel, previously ignored, is asymmetric excitation and absorption of electromagnetic fields inside nanostructures surrounded by chiral molecules. We demonstrate that the induced CD channel can be significant in CD signals of entire system.

Microscopic origin of metal-to-insulator transition in tightly-coupled metamaterials

We present a microscopic origin of metal-to-insulator transition (MIT) in tightly-coupled metamaterials. Like Mott transition in crystal solid, MIT in metamaterial is shown to arise when the distance between unit resonators passes the critical gap-size.

Fano resonant chiral electromagnetic fields by metasurfaces

We demonstrate for the first time that chiral electromagnetic fields display Fano resonance using nano-hole metasurfaces. We show the nano-hole structure can be utilized as biosensors to detect chiral molecules.