Taik Jin Lee

Tunable wide blue photoluminescence with europium decorated graphene

The current paper describes europium decorated graphene (EuG) which provides high and wide blue emission at 400 nm and 458 nm. The chemical and structural properties of the products are characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy. Fourier transform infrared (FT-IR) and UV–Vis spectrometery are employed to analyze the optical properties. The photoluminescence features are investigated using the excitation/emission spectra and fluorescence microscopy images. The photoluminescence intensity of EuG with the bright fluorescent nature of europium is higher than that of reduced graphene oxide. The transition of trivalent europium (Eu3+) that leads to the radiation of light with a 590 nm wavelength can be turned into a 4f–4f transition of divalent (Eu2+) europium upon heating in the presence of the graphene sheet, which assists the reduction of the europium ion. The enhancement of the blue emission at 458 nm with quenching in the red at 590 nm is affected by the modification of properties (by → via) the europium–graphene composite concentration and external thermal energy. The result suggests a new possibility for the fluorescence characteristics of the lanthanide–graphene nanocomposite that can be applied to the display, optoelectronic devices, and bio-imaging fields. The temperature-tunable photoluminescence characteristics can be used as a non-contact thermal sensor.

Imaging of single retinal ganglion cell with differential interference contrast microscopy (Conference Presentation)

Glaucoma is a progressive optic neuropathy, characterized by the selective loss of retinal ganglion cells (RGCs). Therefore, monitoring the change of number or morphology of RGC is essential for the early detection as well as investigation of pathophysiology of glaucoma. Since RGC layer is transparent and hyporeflective, the direct optical visualization of RGCs has not been successful so far. Therefore, glaucoma evaluation mostly depends on indirect diagnostic methods such as the evaluation of optic disc morphology or retinal nerve fiber layer thickness measurement by optical coherence tomography. We have previously demonstrated single photoreceptor cell imaging with differential interference contrast (DIC) microscopy. Herein, we successfully visualized single RGC using DIC microscopy. Since RGC layer is much less reflective than photoreceptor layer, various techniques including the control of light wavelength and bandwidth using a tunable band pass filter were adopted to reduce the chromatic aberration in z-axis for higher and clearer resolution. To verify that the imaged cells were the RGCs, the flat-mounted retina of Sprague-Dawley rat, in which the RGCs were retrogradely labeled with fluorescence, was observed by both fluorescence and DIC microscopies for direct comparison. We have confirmed that the cell images obtained by fluorescence microscopy were perfectly matched with cell images by DIC microscopy. As conclusion, we have visualized single RGC with DIC microscopy, and confirmed with fluorescence microscopy.

Detection of retinitis pigmentosa by differential interference contrast microscopy.

Differential interference contrast microscopy is designed to image unstained and transparent specimens by enhancing the contrast resulting from the Nomarski prism-effected optical path difference. Retinitis pigmentosa, one of the most common inherited retinal diseases, is characterized by progressive loss of photoreceptors. In this study, Differential interference contrast microscopy was evaluated as a new and simple application for observation of the retinal photoreceptor layer and retinitis pigmentosa diagnostics and monitoring. Retinal tissues of Royal College of Surgeons rats and retinal-degeneration mice, both well-established animal models for the disease, were prepared as flatmounts and histological sections representing different elapsed times since the occurrence of the disease. Under the microscopy, the retinal flatmounts showed that the mosaic pattern of the photoreceptor layer was irregular and partly collapsed at the early stage of retinitis pigmentosa, and, by the advanced stage, amorphous. The histological sections, similarly, showed thinning of the photoreceptor layer at the early stage and loss of the outer nuclear layer by the advanced stage. To count and compare the number of photoreceptors in the normal and early-retinitis pigmentosa-stage tissues, an automated cell-counting program designed with MATLAB, a numerical computing language, using a morphological reconstruction method, was applied to the differential interference contrast microscopic images. The number of cells significantly decreased, on average, from 282 to 143 cells for the Royal College of Surgeons rats and from 255 to 170 for the retinal-degeneration mouse. We successfully demonstrated the potential of the differential interference contrast microscopy technique’s application to the diagnosis and monitoring of RP.

Two Spin-1/2 Particles Interacting with a Common Environment

We know that the quantum system evolves to different physical phenomena according to initial states. However, the situation is very different in the open system. We consider the system and the environment governed by the Markovian process. The density matrix (the reduced density matrix) of the system satisfies the Kossakowski-Lindblad equation. We calculate the expectation value of the z-direction-magnetization in the system for various types of 1) initial states and 2) interactions between the system and the environment. In all the cases, the values of the magnetization approached asymptotically to the same point even though the different initial states and different interaction types between the system and the environment are applied. The paths to asymptotically approach the final values are different according to the initial states and the interactions. These facts show that the physical results of expectation value are independent of the initial states.

Asbestos concentration measurement using Differential Interference Contrast microscopy

We designed a new method for imaging and counting the concentration of asbestos fibers. In current research, we combined the principle of Differential Interference Contrast (DIC) microscopy with imaging program for counting their concentration automatically.

Analytic solution of the nonlinear equation

We analytically solve the nonlinear wave equation of the beam, which travels through the nonlinear Kerr medium. The tanh function method, a powerful method solving the traveling wave equation, is applied to the self-guiding light.