Sun-Kyun Lee

Large room-temperature intergrain magnetoresistance in double perovskite SrFe1−x(Mo or Re)xO3

We report significant intergrain magnetoresistance (IMR) in polycrystalline double perovskites of SrFe1−x(Mo or Re)xO3 at room temperature. Systematics of the temperature dependence of IMR indicates that the observed large room-temperature IMR in SrFe1/2Mo1/2O3 originates from the ferrimagnetic nature of insulating grain boundaries as well as the half-metallicity of this perovskite. Our results indicate that a new avenue for spin-polarized tunneling junctions is to utilize insulating interface layers with ferromagnetic or ferrimagnetic coupling.

Carrier loss and luminescence degradation in green-light-emitting InGaN quantum wells with micron-scale indium clusters

Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

UV photovoltaic cells based on conjugated ZnO quantum dot/multiwalled carbon nanotube heterostructures

In situ growth of ZnO quantum dots (QDs) on the surface of multiwalled carbon nanotubes (MWCNTs) was realized via a mild solution-process method, which resulted in an improvement in photoinduced charge separation and transport of carriers to the collecting electrode. The charge transfer efficiency was significantly increased by more than 90% due to the conjugation of ZnO QDs with MWCNTs, as confirmed by photoluminescence measurements. Ultraviolet photovoltaic cells based on the charge transfer at the ZnO QD-MWCNT heterostructures were fabricated, and their power conversion efficiency was measured to be above 1%.

Optical investigation of p-type ZnO epilayers doped with different phosphorus concentrations by radio-frequency magnetron sputtering

Optical properties of p-type ZnO epilayers doped with different amounts of phosphorus by radio-frequency magnetron sputtering are investigated by x-ray diffraction, temperature dependent photoluminescence (PL), and time-resolved PL techniques. Bound exciton, free electrons-to-acceptors, donor-to-acceptor pair, and deep-level emissions are observed at about 3.356, 3.32, 3.24, and 2.4eV at 10K for p-type ZnO, respectively. The crystal quality and luminescence efficiency are improved with increasing phosphorus doping concentration. These results show that phosphorus doping plays an important role both in reducing native defects and in generating shallow acceptors in ZnO, leading to a good p-type conductivity in ZnO.

Long lifetime of free excitons in ZnO tetrapod structures

Time-resolved photoluminescence (PL) is employed to characterize optical quality of ZnO tetrapods. PL decay of free excitons (FE) is concluded to contain two components with time constants of 1 and ...

Infrared to visible up-conversion in thulium and holmium doped lutetium aluminum garnet

We report on the up-converted emissions at 367, 458 and 483 nm in thulium doped lutetium aluminum garnet (LuAG) under infrared excitation in the range of 730-800 nm. We also observe blue and strong green emission in thulium and holmium codoped LuAG with near infrared excitation. Excitation spectra, power dependence, temporal behavior and temperature dependence of up-converted emissions were measured to explain the up-conversion mechanisms. Several cross-relaxation mechanisms and excited state absorption are found to be responsible for the observed up-conversion processes.

1.5μm emission characteristics of Er3+-doped stoichiometric LiNbO3

The 1.5μm emission characteristics of stoichiometric LiNbO3 crystals with different Er3+ doping levels of 0.2, 0.5, 1.0, and 2.0mol% were investigated. All of the emission decays could be fitted by a single exponential curve. The lifetimes of the 1.5μm transition (I13∕24→I15∕24) were measured to be 3.0–3.2ms, which are about 25% longer than those reported for conventional Er-doped congruent LiNbO3. Our analysis showed that the longer lifetime in stoichiometric LiNbO3 is likely to originate from the reduced density of intrinsic defects.

Photoluminescence of terbium ions in near-stoichiometric lithium niobate

We investigated the photoluminescence properties of near-stoichiometric LiNbO3 crystals doped with Tb only and double doped with Tb and Fe. Excitation spectra for the emission from the 5D4 state in both crystals have been studied in the range 200-500 nm. We observed two broad ultraviolet absorption bands and discussed their relations to Tb ions. Temperature dependence of the 5D4 emission intensity and lifetime showed strong quenching below room temperature.

Optical memory using photoluminescence change in silver-doped glasses

We report formation of nanoparticles and colors in the silver doped glasses produced by femtosecond laser irradiation and feasibility of multilayer three-dimensional optical recording using the photoluminescence change.

Three-dimensional memory using photoreduction of Eu ions

In this work, we report three-dimensional memory by recording optical bits with irradiation of near-infrared femtosecond laser pulses and by reading photoluminescence change in the blue due to permanent reduction of Eu3+ to Eu2+ in sodium borate glasses. We produced a multilayered micro-bit pattern which was read out by detecting the blue emission from the 405 nm excitation with a high S/N ratio. The readout was performed by using a scanning reflection-type confocal microscope.