Myoung-Kyu Oh

Morphological and SERS Properties of Silver Nanorod Array Films Fabricated by Oblique Thermal Evaporation at Various Substrate Temperatures

Aligned silver nanorod (AgNR) array films were fabricated by oblique thermal evaporation. The substrate temperature during evaporation was varied from 10 to 100 °C using a home-built water cooling system. Deposition angle and substrate temperature were found to be the most important parameters for the morphology of fabricated films. Especially, it was found that there exists a critical temperature at ~90 °C for the formation of the AgNR array. The highest enhancement factor of the surface-enhanced Raman scattering (SERS), observed in the Ag films coated with benzenethiol monolayer, was ~6 × 107. Hot spots, excited in narrow gaps between nanorods, were attributed to the huge enhancement factor by our finite-difference time-domain (FDTD) simulation reflecting the real morphology.

Application of Pulsed Buffer Gas Jets for the Signal Enhancement of Laser-Induced Breakdown Spectroscopy

We report a new simple method for the signal enhancement of laser-induced breakdown spectroscopy using a pulsed buffer gas jet. The signal is enhanced up to more than 10 fold by using argon gas jets, which are injected through a pulsed nozzle onto the sample area to be analyzed. By synchronizing the buffer gas pulse with the laser pulse and optimizing the spatial arrangements between the gas jet and the sample surface, we have successfully exploited the useful properties of the buffer gas in open atmosphere. The signal-enhancement mechanism in our buffer gas jet has been discussed. Also, applications to various samples (metal, glass, and paper) have been demonstrated.

Continuous tuning of a narrow-band terahertz wave in periodically poled stoichiometric LiTaO3 crystal with a fan-out grating structure

Continuous tuning of terahertz waves was demonstrated with seamless change in grating period in a periodically poled stoichiometric lithium tantalite (SLT) crystal. The periodically poled fan-out structure allows wide tunability such as 610 GHz with the bandwidth as narrow as 17 GHz at a carrier frequency of 1.00 THz. Temperature-dependent measurements show a gradual intensity increase of the THz pulses as the temperature decreases. Furthermore, absorption and refractive index of SLT in the THz range were estimated and compared with those of LiNbO3 (LN). The absorption coefficient of the LT crystal at ordinary wave was almost half of that in the LN crystal. SLT could be one of the powerful crystals for high-power THz generation with large optical aperture because of the fewer defects inside the crystal.

Label-free imaging and quantitative chemical analysis of Alzheimer’s disease brain samples with multimodal multiphoton nonlinear optical microspectroscopy

Abstract. We developed multimodal multiphoton microspectroscopy using a small-diameter probe with gradient-index lenses and applied it to unstained Alzheimer’s disease (AD) brain samples. Our system maintained the image quality and spatial resolution of images obtained using an objective lens of similar numerical aperture. Multicolor images of AD brain samples were obtained simultaneously by integrating two-photon excited fluorescence and second-harmonic generation on a coherent anti-Stokes Raman scattering (CARS) microendoscope platform. Measurements of two hippocampal regions, the cornus ammonis-1 and dentate gyrus, revealed more lipids, amyloid fibers, and collagen in the AD samples than in the normal samples. Normal and AD brains were clearly distinguished by a large spectral difference and quantitative analysis of the CH mode using CARS microendoscope spectroscopy. We expect this system to be an important diagnosis tool in AD research.

Rapid diagnosis of liver fibrosis using multimodal multiphoton nonlinear optical microspectroscopy imaging

Abstract. A multimodal multiphoton nonlinear optical (NLO) microspectroscopy imaging system was developed using a femtosecond laser and a photonic crystal fiber. Coherent anti-Stokes Raman scattering (CARS) microspectroscopy was combined with two-photon excitation fluorescence and second-harmonic generation microscopy in one platform and the system was applied to diagnose liver fibrosis. Normal and liver fibrosis tissues were clearly distinguished with the great difference from CARS spectra as well as multimodal multiphoton NLO images. We expect the system to be a rapid diagnosis tool for liver fibrosis at tissue level with label-free imaging of significant biochemical components.

Coherent Absorption Spectroscopy with Supercontinuum for Semiconductor Quantum Well Structure

We suggest that supercontinuum can be used for absorption spectroscopy to observe the exciton levels of a semiconductor nano-structure. Exciton absorption spectrum of a GaAs/AlGaAs quantum well was observed using supercontinuum generated by a microstructrured fiber pumped by a femtosecond (fs) pulsed laser. Significantly narrower peaks were observed in the absorption spectrum from 11 K up to room temperature than photoluminescence (PL) spectrum peaks. Because supercontinuum is coherent light and can readily provide high enough intensity, this method can provide a coherent ultra-broad band light source to identify exciton levels in semiconductors, and be applicable to coherent nonlinear spectroscopy such as electromagnetically induced transparency (EIT), lasing without inversion (LWI) and coherent photon control in semiconductor quantum structures.

Adsorption behaviors and structural transitions of organic cations on an anionic lipid monolayer at the air–water interface

We studied the adsorption behaviors of organic cations, malachite green (MG), on an anionic dipalmitoylphosphatidylglycerol (DPPG) monolayer at the air–water interface for different thermodynamic phases of the lipid monolayer. Simultaneous monitoring of both the surface pressure and the optical second harmonic generation intensity, combined with the fluorescence microscopy and X-ray reflectivity measurement, gave detailed structural information on the adsorbed MG, showing three distinct types of adsorption behaviors depending on the initial phase of the monolayer. We provided plausible molecular models for the vertical position and orientation of MG in each phase: penetration into the DPPG monolayer with a vertical one direction at a fluid phase; vertically but oppositely paired orientations at a more condensed phase; parallel adsorption at the interface near the lipid headgroup region at a highly packed condensed phase. This result can be usefully expanded for the understanding of the penetration mechanism and conformational change of biological adsorbates at the cellular membrane.

High-Speed Imaging of Broadband Multiplex Coherent Anti-Stokes Raman Scattering Microscopy Using a Supercontinuum Source

We combined broadband multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopy with CARS microscopy using a femtosecond laser and a photonic crystal fiber. Broadband multiplex CARS microspectroscopy, which simultaneously measures broadband range (including significant Raman vibrational modes) from 500 to 3500 cm-1 at a high spectral resolution was subsequently performed. Also, via multiplex CARS microscopy using a pair of galvanometer mirrors we successfully demonstrated the high-speed imaging of polymer beads and a lily pollen grain for the study of cell biology dynamics. The imaging speed was 37 s per image, which is 30–40 times faster than that previously reported in broadband CARS microscopy.

Ultimate sensing resolution of water temperature by remote Raman spectroscopy

The limit of sensing resolution of water temperature by remote Raman spectroscopy was investigated experimentally. A remote Raman spectrometer, which employed a telescope of 20 cm in pupil size and the second harmonic generation (SHG) of a Q-switched Nd:YAG laser, was used for the measurement. By analyzing the broad O-H stretching Raman band located near 3500  cm⁻¹, a parameter which is in second-order polynomial relation with water temperature from 13°C to 50°C could be obtained. The resolution of our remote Raman temperature sensor was better than ±0.2°C with measurement time shorter than 10 s. The influence of the Raman signals signal-to-noise ratio on the resolution and salinity effect on the accuracy of temperature sensing were also investigated.

Comparative Study of Breast Normal and Cancer Cells Using Coherent Anti-Stokes Raman Scattering Microspectroscopy Imaging

A coherent anti-Stokes Raman scattering (CARS) microspectroscopy imaging system was developed using a femtosecond laser and a photonic crystal fiber (PCF). We separated resonant and non-resonant CARS signals in the time domain by the chirp of the PCF, and applied this system to compare live human breast normal and cancer cells. The CARS image and spectrum at C–H stretch vibration in lipid droplets could subsequently be used to differentiate cancer cells from normal cells, thereby confirming the potential of the CARS microspectroscopy imaging system as a diagnostic tool that allows the high-sensitivity, high-resolution, and fast detection of breast cancer.