Sungrae Lee

Violet and UV luminescence emitted from ZnO thin films grown on sapphire by pulsed laser deposition

Abstract Visible violet photoluminescence (PL) has been achieved at room temperature (RT) from ZnO films grown on sapphire (001) substrate by pulsed laser deposition (PLD). Substrate temperatures of 200, 300, and 400°C have been used in an oxygen pressure of 1 mTorr during the PLD. As the oxygen pressure for the thin film deposition increases over 20 mTorr at a substrate temperature of 400°C, the violet luminescence vanishes. Instead ultra-violet (UV) and green–yellow luminescence appear. The most intense UV and green–yellow luminescence is obtained from a sample grown in an oxygen pressure of 200 mTorr at 400°C. It is concluded that the intensity of the UV luminescence strongly depends on the stoichiometry of the film as well as the crystalline quality, while the violet PL is due to a defect level in the grain boundaries of the ZnO x crystals.

Effects of native defects on optical and electrical properties of ZnO prepared by pulsed laser deposition

ZnO thin film has been deposited on a sapphire (001) at a temperature of 400°C using a pulsed laser deposition (PLD) with oxygen pressures of 50, 200, 300 and 500 mTorr. As the oxygen pressure for the thin film deposition increases, the crystallinity of the samples degrades as measured by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In contrast, the photoluminescence (PL) intensity of ultra-violet (UV) luminescence increases as the oxygen pressure increases up to 300 mTorr. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. According to the results from Hall measurements, the oxygen vacancy as a native donor defect in the ZnO decreases in concentration as the pressure increases. It is concluded that the UV luminescence intensity strongly depends on the stoichiometry in the ZnO film rather than the micro-structural quality of the crystal.

Objective-free optical-resolution photoacoustic microscopy

Abstract. Optical-resolution photoacoustic microscopy (OR-PAM) becomes a premier microscopic imaging tool in biomedicine because it provides agent-free optical absorption information in tissues. By tightly focusing light to a spot, a significantly improved lateral resolution can be achieved in OR-PAM. The focal spot size is typically determined by the numerical aperture of the used objective lens. Here, we demonstrate objective-free OR-PAM using a fiber optic Bessel beam generator. In this approach, no objective lens is required and, beneficially, the complexities of conventional OR-PAM systems can be greatly relieved. We have obtained photoacoustic images of a carbon fiber with a diameter of ∼6  μm, whose lateral resolution was measured to be better than 6 to 7 μm.

Comparison of various subjective video quality assessment methods

In this paper, we present comparison of three subjective testing methods: the double stimulus continuous quality scale (DSCQS) method, the single stimulus continuous quality evaluation (SSCQE) method and the absolute category rating (ACR) method. The DSCQS method was used for validate objective models in the VQEG Phase II FRTV test. The SSCQE method is chosen to be used in the VQEG RRTV test. The ACR method is chosen to be used in the VQEG Multimedia test. Since a different subjective test method is used in each test, analyses of the three methods will provide helpful information in understanding human perception of video quality.

Dispersion control in square lattice photonic crystal fiber using hollow ring defects

We propose a new dispersion control scheme by introducing hollow ring defects having a central air hole and a GeO2-or F-doped silica ring with in a square lattice photonic crystal fiber. We confirmed the flexible dispersion controllability in the proposed structure in two aspects of dispersion managements: ultra-flattened near-zero dispersion in the 530 nm-bandwidth over all communication bands and dispersion compensation in C, L, and U band with a high compensation ratio of 0.96~1.0 in reference to the standard single mode fiber. The proposed SLPCFs were also estimated to have an inherently low splice loss due to the index contrast between the doped-ring and silica that kept a good guidance even along with collapsed air holes, which cannot be achieved in conventional PCFs.

Crossed fiber optic Bessel beams for curvilinear optofluidic transport of dielectric particles

Due to its unique non-diffracting and self-reconstructing nature, Bessel beams have been successfully adopted to trap multiple particles along the beams axial direction. However, prior bulk-optic based Bessel beams have a fundamental form-factor limitation for in situ, in-vitro, and in-vivo applications. Here we present a novel implementation of Fourier optics along a single strand of hybrid optical fiber in a monolithic manner that can generate pseudo Bessel beam arrays in two-dimensional space. We successfully demonstrate unique optofluidic transport of the trapped dielectric particles along a curvilinear optical route by multiplexing the fiber optic pseudo Bessel beams. The proposed technique can form a new building block to realize reconfigurable optofluidic transportation of particulates that can break the limitations of both prior bulk-optic Bessel beam generation techniques and conventional microfluidic channels.

Columnar deformation of human red blood cell by highly localized fiber optic Bessel beam stretcher

A single human red blood cell was optically stretched along two counter-propagating fiber-optic Bessel-like beams in an integrated lab-on-a-chip structure. The beam enabled highly localized stretching of RBC, and it induced a nonlinear mechanical deformation to finally reach an irreversible columnar shape that has not been reported. We characterized and systematically quantified this optically induced mechanical deformation by the geometrical aspect ratio of stretched RBC and the irreversible stretching time. The proposed RBC mechanism can realize a versatile and compact opto-mechanical platform for optical diagnosis of biological substances in the single cell level.

Polarized light-emitting backlight unit using a retardation film for improving light efficiency in a twisted nematic mode liquid crystal module

We propose and demonstrate a new polarized backlight unit (BLU) configuration to enhance light transmission in a twisted nematic (TN) mode liquid crystal (LC) panel. By adding an optimized phase retardation film (PRF) to the BLU, we efficiently controlled the polarization state of the light from the light guide plate (LGP), enabling it to be aligned along the transmission axis of the bottom polarizer of the LC panel. We designed and fabricated a 7 in. edge-lit BLU with a PRF. To obtain a uniform positional distribution in both light intensity and degree of polarization (DOP), the LGP was optimized with linear groove patterns. To maximize BLU light transmission through the bottom polarizer in a TN-mode LC panel, the slow axis of the PRF was aligned at 22.5° with respect to the LED array axis. We improved the transmittance from 45 to 72%.

The influence of inhomogeneous birefringent medium on the polarization properties of the LCD backlight unit

The degree of polarization (DOP) of a LCD backlight unit was investigated by rotating a polarizer on it. The reduction of its DOP was mostly influenced by the inhomogeneous birefringence in the prism film.

The transient ytterbium doped fiber amplifier phase noise measurement using heterodyne detection

We have demonstrated phase noise measurements of ytterbium doped fiber amplifier on transient pumping process based on active phase locking using optical heterodyne detection from all-fiber Mach-Zehnder interferometer. The intrinsic and extrinsic factors during amplifying process worsen coherence and lead to randomly fluctuated phase error. Root mean square, power spectral density, and phase drift velocity were employed as quantitative values for evaluating phase error. The results implied that main frequency bandwidth of phase noise was characterized by type of pumping process, output power, and fiber amplifier components. This platform can be utilized to evaluate phase noise error directly under the different amplifier configuration and condition, and it helps find better condition of amplifier for assuring rigid beam combining performance henceforth.