Min-Kwan Kim

Superresolution imaging with optical fluctuation using speckle patterns illumination

Superresolution fluorescence microscopy possesses an important role for the study of processes in biological cells with subdiffraction resolution. Recently, superresolution methods employing the emission properties of fluorophores have rapidly evolved due to their technical simplicity and direct applicability to existing microscopes. However, the application of these methods has been limited to samples labeled with fluorophores that can exhibit intrinsic emission properties at a restricted timescale, especially stochastic blinking. Here, we present a superresolution method that can be performed using general fluorophores, regardless of this intrinsic property. Utilizing speckle patterns illumination, temporal emission fluctuation of fluorophores is induced and controlled, from which a superresolution image can be obtained exploiting its statistical property. Using this method, we demonstrate, theoretically and experimentally, the capability to produce subdiffraction resolution images. A spatial resolution of 500 nm, 300 nm and 140 nm with 0.4, 0.5 and 1.4 NA objective lenses respectively was achieved in various samples with an enhancement factor of 1.6 compared to conventional fluorescence microscopy.

Electrically Driven Diffraction Grating Designed for Visible-Wavelength Region

We report on an electrically driven diffraction grating designed for visible light, where the refractive index of a liquid crystal (LC) was modulated periodically at an interval of 700 nm by applying an external dc bias to a metallic nanograting (NG). The LC-NG structure exhibited a maximum refractive index variation (Δn) of 0.088 and a diffraction efficiency (η) change of 0%-16% with a large diffraction angle of 64° for incident light of 633-nm wavelength. This approach, with the help of faster electronics, provides an opportunity of developing active holograms for real 3-D displays.

Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis

The V-pits and potential fluctuations in InGaN/GaN multiple quantum wells (MQWs) are key factors for understanding the performance of InGaN/GaN-based light-emitting diodes (LEDs). However, photoluminescence (PL) measurements using conventional optical microscopy only provide ensemble information due to the spatial resolution limit, known as the diffraction barrier, which hinders the analysis of dislocations and potential fluctuations. Here, in order to investigate the influence of the V-pits and potential fluctuations on local optical properties, we performed nanoscopic luminescence mapping for standard and V-pit InGaN/GaN MQWs samples with different sized V-pits using near-field scanning optical microscopy (NSOM) with illumination mode (I-mode) at various laser excitation powers. From the nanoscopic PL mapping data, we could clearly observe luminescence features associated with dislocations and potential fluctuations in the InGaN/GaN MQWs. We also employed correlation analysis to quantitatively analyze the nanoscopic PL mapping data for the different MQWs samples. Based on the results of NSOM PL with I-mode and correlation analysis, we could demonstrate that carrier transfer in the MQWs sample with large sized V-pits is suppressed by deeper potential fluctuations and higher energy barriers compared to the standard sample.

Resonant vortex-core reversal in magnetic nano-spheres as robust mechanism of efficient energy absorption and emission.

We report on novel vortex-core reversal dynamics in nano-spheres of single-vortex spin configuration as revealed by micromagnetic simulations combined with analytical derivations. When the frequency of an AC magnetic field is tuned to the frequency of the vortex-core precession around the direction of a given static field, oscillatory vortex-core reversals occur, and additionally, the frequency is found to change with both the strength of the applied AC field and the particle size. Such resonant vortex-core reversals in nano-spheres may provide a new and efficient means of energy absorption by, and emission from, magnetic nanoparticles, which system can be effectively implemented in bio-applications such as magnetic hyperthermia.

Semiconductor Photonic Nanocavity on a Paper Substrate

Direct integration of semiconductor photonic nanocavities with paper substrates is demonstrated for the first time. 1D photonic crystal nanocavities successfully show lasing action on paper substrates. The device has great synergy as a sensor because paper has good wicking ability while a photonic crystal cavity has high figure of merit. The research provides a platform for eco-friendly and sustainable devices.

Single-crystalline Gd-doped BiFeO3 nanowires: R3c-to-Pn21a phase transition and enhancement in high-coercivity ferromagnetism

We fabricated single-crystalline, Gd-doped BiFeO3 (BFO) nanowires using a hydrothermal technique. X-ray diffraction (XRD) data combined with their Rietveld refinements and high-resolution transmission electron microscopy (HRTEM) revealed pure single-phase crystalline Bi1−xGdxFeO3 (x = 0, 0.05, 0.10) nanowires of 40–60 nm diameter and their structural transformation from the rhombohedral R3c (for x = 0 and 0.05) to the orthorhombic Pn21a crystal structure (for x = 0.10). The addition of Gd3+ ions to the pure-phase BFO leads to remarkable changes in the structural and magnetic properties, and these effects are caused by differences in the ionic-radii and magnetic moment between the Bi3+ and Gd3+ ions. According to the observed magnetization-field (M–H) and magnetization-temperature (M–T) curves, with increasing Gd3+ concentration, the saturation magnetization (MS), squareness (Mr/MS), coercivity (HC), exchange-bias field (HEB) and magnetocrystalline anisotropy (K) increased markedly, by MS = 1.26 emu g−1 (640%), Mr/MS = 0.19 (20.5%), HC = 7788 Oe (4560%), HEB = 501 Oe (880%) and K = 1.62 × 105 erg cm−3 (3500%), for x = 0.10 relative to the data for x = 0. In such Gd-doped BFO nanowire samples, spin-canted Dzyaloshinskii–Moriya interaction, remarkable enhancements in the magnetocrystalline anisotropy as well as uncompensated surface ferromagnetic spin states in the antiferromagnetic core regions were also found. Such remarkable enhancements in Gd-doped BFO nanowires might offer a variety of spintronic applications.

Stress-induced magnetic properties of PLD-grown high-quality ultrathin YIG films

Yttrium iron garnet (YIG:Y3Fe5O12) thin films were grown on (111) gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates using pulsed-laser deposition under several different deposition and annealing conditions. X-ray diffraction measurements revealed that the crystallographical orientation of the YIG films is pseudomorphic to and the same as that of the GGG substrate, with a slight rhombohedral distortion along the surface normal. Furthermore, X-ray reciprocal space mapping evidenced that in-situ annealed YIG films during film growth are under compressive strain, whereas ex-situ annealed films have two different regions under compressive and tensile strain. The saturation magnetization ( 4 π M S) of the films was found to vary, according to the deposition conditions, within the range of 1350 to 1740 G, with a very low coercivity of H C < 5 Oe. From ferromagnetic resonance (FMR) measurements, we estimated the effective saturation magnetization ( 4 π M e f f) to be 1810 to 2530 G, which are larger than that of single crystalline bulk YIG (∼1750 G). Such high values of 4 π M e f f are attributable to the negative anisotropy field ( H U) that increases in size with increasing compressive in-plane strain induced in YIG films. The damping constant ( α G) of the grown YIG films was found to be quite sensitive to the strain employed. The lowest value of α G obtained was 2.8 × 10−4 for the case of negligible strain. These results suggest a means of tailoring H U and α G in the grown YIG films by the engineering of strain for applications in spintronics and magneto-optical devices.

Real-time monitoring and visualization of the multi-dimensional motion of an anisotropic nanoparticle

As interest in anisotropic particles has increased in various research fields, methods of tracking such particles have become increasingly desirable. Here, we present a new and intuitive method to monitor the Brownian motion of a nanowire, which can construct and visualize multi-dimensional motion of a nanowire confined in an optical trap, using a dual particle tracking system. We measured the isolated angular fluctuations and translational motion of the nanowire in the optical trap, and determined its physical properties, such as stiffness and torque constants, depending on laser power and polarization direction. This has wide implications in nanoscience and nanotechnology with levitated anisotropic nanoparticles.