SeokJae Yoo

Effective permittivity for resonant plasmonic nanoparticle systems via dressed polarizability

We present an effective medium theory for resonant plasmonic nanoparticle systems. By utilizing the notion of dressed polarizability to describe dipolar particle interactions, we show that even highly concentrated, resonant plasmonic particles can be correctly described by the effective medium theory. The effective permittivity tensor of a nanoparticle monolayer is found explicitly and the resulting absorbance spectrum is shown to agree with rigorous numerical results from the FDTD model. The effective theory based on dressed polarizability provides a powerful tool to tailor resonant optical behaviors and design diverse plasmonic devices.

Enhancement of Chiroptical Signals by Circular Differential Mie Scattering of Nanoparticles

We enhance the weak optical signals of small chiral molecules via circular differential Mie scattering (CDMS) of nanoparticles immersed in them. CDMS is the preferential Mie scattering of left- and right-handed circularly polarized light by nanoparticles whose sizes are about the same as the wavelength of light. Solving the Mie scattering theory for chiral media, we find that the CDMS signal of the particle is linearly proportional to the chirality parameter κ of the molecules. This linear amplitude enhancement by CDMS of the particle holds, even for large particles, which have a retardation effect. We also demonstrate that the CDMS of a nanoparticle is sensitive to changes of molecular concentration, and that the nanoparticle can be utilized as a chiroptical biosensor detecting the concentration of analyte. We expect that the enhancement of molecular chiroptical signals by CDMS will pave the way for novel chiroptical spectroscopy using nanostructures.

Plasmon Enhanced Direct Bandgap Emissions in Cu7S4@Au2S@Au Nanorings

Nanostructured copper sulfides, promising earth-abundant p-type semiconductors, have found applications in a wide range of fields due to their versatility, tunable low bandgap, and environmental sustainability. The synthesis of hexagonal Cu7 S4 @Au2 S@Au nanorings exhibiting plasmon enhanced emissions at the direct bandgap is reported. The synthesized Cu7 S4 @Au2 S@Au nanorings show greatly enhanced absorption and emission by local plasmons compared to pure copper sulfide nanoparticles.

Strong visible magnetic resonance of size-controlled silicon-nanoblock metasurfaces

To extend the operating window of all-dielectric metamaterials into the visible regime, obtaining controllable magnetic resonance is essential. We experimentally demonstrated strong magnetic resonance at 595 nm using an array of amorphous silicon (a-Si) nanoblocks. The results of both theoretical calculations and experiments show that magnetic resonance can be tuned continuously by appropriately varying the size and thickness of a-Si nanoblocks. We also experimentally achieved a magnetic resonance Q-factor of ~10, which is a higher value than that yielded by a metallic split-ring resonator in the visible regime.

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.

Optically Patternable Metamaterial Below Diffraction Limit

We report an optically patternable metamaterial (OPM) for ultraviolet nanolithography below the diffraction limit. The OPM features monolayered silver nanoislands embedded within a photosensitive polymer by using spin-coating of an ultrathin polymer, oblique angle deposition, and solid-state embedment of silver nanoislands. This unique configuration simultaneously exhibits both negative effective permittivity and high image contrast in the ultraviolet range, which enables the surface plasmon excitation for the clear photolithographic definition of minimum feature size of 70 nm (≲ λ/5) beyond the near-field zone. This new metamaterial provides a new class of photoresist for ultraviolet nanolithography below the diffraction limit.

Measuring the optical permittivity of two-dimensional materials without a priori knowledge of electronic transitions

Abstract We propose a deterministic method to measure the optical permittivity of two-dimensional (2D) materials without a priori knowledge of the electronic transitions over the spectral window of interest. Using the thin-film approximation, we show that the ratio of reflection coefficients for s and p polarization can give a unique solution to the permittivity of 2D materials within the measured spectral window. The uniqueness and completeness of our permittivity measurement method do not require a priori knowledge of the electronic transitions of a given material. We experimentally demonstrate that the permittivity of monolayers of MoS2, WS2, and WSe2 in the visible frequency range can be accurately obtained by our method. We believe that our method can provide fast and reliable measurement of the optical permittivity of newly discovered 2D materials.

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.

Chiral light-matter interactions in resonant metamaterials

We present a theory of the chiral Purcell effect to describe the cavity-modified differential decay rate of chiral molecules to left and right circular-polarized modes. We also show negative-index metamaterials support the chiral Purcell effect.

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.