Kwang-Geol Lee

Optical magnetic field mapping using a subwavelength aperture.

Local distribution of the optical magnetic field is a critical parameter in developing materials with artificially engineered optical properties. Optical magnetic field characterization in nano-scale remains a challenge, because of the weak matter-optical magnetic field interactions. Here, we demonstrate an experimental visualization of the optical magnetic field profiles by raster scanning circular apertures in metal film or in a conical probe. Optical magnetic fields of surface plasmon polaritons and radially polarized beam are visualized by measuring the transmission through metallic apertures, in excellent agreements with theoretical predictions. Our results show that Bethe-Bouwkamp aperture can be used in visualizing optical magnetic field profiles.

Parity-time-symmetry breaking in double-slab surface-plasmon-polariton waveguides.

We theoretically demonstrate spontaneous PT-symmetry breaking behavior of surface-plasmon polaritons (SPP) in coupled double-slab (DS) waveguides. By virtue of a flat-top field at critical wavelength, the imaginary index of a DS-SPP mode can be controlled via changing the core thickness, while the real index is kept constant. Therefore, a waveguide coupler that consists of a pair of DS-SPP waveguides with different core thicknesses can represent a passive PT-symmetric system, which always maintains symmetry under a real potential. This set-up also represents a good opportunity to investigate the underlying physics of PT-symmetry breaking in non-Hermitian Hamiltonian systems.

Experimental observation of electroluminescence enhancement on green LEDs mediated by surface plasmons

We experimentally demonstrate the 1.5-fold enhancement of the electroluminescence (EL) of surface-plasmon (SP)-mediated green LEDs. On the p-clad surface of InGaN/GaN multi-quantum well LEDs, a 2-dimensional, second-order grating structure is textured and coated with an Ag electrode. With this setup, a larger EL enhancement factor is obtained at a higher injected current, which suggests that SP-LEDs can be a possible solution to efficiency droop, which is one of the main problems in developing high-power LEDs. Details regarding the implementation of our device are discussed.

Enhancing the radiative emission rate of single molecules by a plasmonic nanoantenna weakly coupled with a dielectric substrate

Enhancing the spontaneous emission of single emitters has been an important subject in nano optics in the past decades. For this purpose, plasmonic nanoantennas have been proposed with enhancement factors typically larger than those achievable with optical cavities. However, the intrinsic ohmic losses of plasmonic structures also introduce an additional nonradiative decay channel, reducing the quantum yield. Here we report on experimental studies of a weakly coupled dielectric substrate and a plasmonic nanoantenna for enhancing the radiative decay rate of single terrylene molecules embedded in an ultrathin organic film. We systematically investigate how the refractive index of the dielectric substrate affects the lifetime and the quantum efficiency and show that the coupled structure could moderately enhance the radiative decay rate while maintaining a high quantum efficiency.

Fluorescent impurity emitter in toluene and its photon emission properties

Single fluorescent emitters like colloidal quantum dots or single molecules are usually prepared in solutions and spin-coated onto cover glasses for studying. Toluene has been a widely used solvent in such studies. Here, we report on a fluorescent impurity emitter contained in toluene and its optical properties. The emission spectra of the single emitters show multiple peaks with the main peak varying from 2.03 eV (610 nm) to 2.14 eV (580 nm) and a red-shifted side peak with an average separation of 167 meV from the main peak. The emitted photons show a strong anti-bunching with a fluorescence lifetime of a few nanoseconds. They show very fast blinking behavior which cannot be properly detected by time-trajectory of photoluminescence intensity. An analysis based on the second-order correlation functions reveals that a three-level model can explain our measurements well and that the blinking transition time ranges only a few tens of microseconds. This single emitter in toluene is clearly distinguished from the fluorescent centers in the cover glass by their respective emission spectra. The single emitters in the cover glass also exhibit fast blinking behavior. These background emitters should be carefully identified and distinguished while studying the single fluorescent emitters.

Coherence Studies of Photons Emitted from a Single Terrylene Molecule Using Michelson and Young’s Interferometers

Coherence length (time) is a key parameter in many classical and quantum optical applications. Two interferometers – Michelson and Young’s double-slit – are used to characterize the temporal coherence of single photons emitted from single terrylene molecules. For quantitative analysis, a dispersion-related distortion in the interference pattern of a Michelson interferometer is carefully corrected by a simple dispersion compensation. Additionally, it has been demonstrated that Young’s interferometer can be used in temporal coherence studies at the single photon level with high accuracy. The pros and cons of the two systems are discussed. The measured coherence lengths in the two systems are consistent with one another under the self-interference interpretations.

A deep subwavelength cavity formed by total external reflection of surface plasmon polariton

We numerically analyze the characteristics of a nanocavity in surface plasmon polariton (SPP) modes confined by total external reflection (TER) at deep subwavelength scales. This SPP-TER cavity consists of a low-index dielectric channel on a flat metal surface covered by a high-index gain medium. Compared to other types of nanocavities formed by total internal reflection such as a metallic channel and a high-index dielectric channel, an SPP-TER nanocavity provides superior functionality on mode area, confinement factor in the gain medium, Q-factor, and threshold gain. From this result, we suggest the SPP-TER nanocavity as a promising high-quality deep-subwavelength scale resonator, which is an essential ingredient in nanophotonics.