Wonwoo Lee

Tunable middle infrared downconversion in GaSe and AgGaS2

We report on the generation of middle infrared radiation, tunable over the 3.5-8.5 mm spectral range. This was achieved . by difference frequency generation DFG in GaSe and AgGaS crystals, using the new combination of two solid state 2 . q )) . lasers; the alexandrite 0.73-0.75 mm laser and the alexandrite-pumped tunable LiF:F 0.8-1.2 mm color center laser. 2

Novel laser breakdown spectrometer for environmental monitoring

A novel experimental set-up using laser-induced breakdown spectroscopy (LIBS) for environmental analyses of heavy metals is described in this paper. It is based on state-of-the-art spectroscopic equipment, advanced detectors, and laser atomizers: a 0.75 m spectrometer ARC-750, intensified TE- cooled 256 X 1024 CCD camera, probe with fiber optic guide for signal transportation, and Nd:YAG laser plasma atomizers with two different methods for sample delivery. In the first method the liquid solution containing the atoms to be investigated is drawn into the chamber of the nebulizer. The mixture passes through the nozzle, accompanied by argon gas along with formed aerosol, and enters the plasma plume, which is generated by the laser spark in argon. The second method is based on direct generating of the plasma in the water jet of a continuously circulating sample. LIBS testing of samples containing Al, Cd, Cu, Fe, Pb, Zn, and Cr ions was compared with results using atomic absorption spectrophotometry. Initial indications showed good agreement between these two methods. Detection levels of less than 100 ppb were observed for copper and chromium. The described spectroscopic system exhibits high sensitivity, accumulation of luminescence spectrum in real time; and high dynamic range for concentrations detection from 100 ppb to 1000 ppm.

Deep Level Point Defects in Semi-Insulating SiC

The high resistivity of SiC required for many device applications is achieved by compensating residual donors or acceptors with vanadium or intrinsic defects. This work addresses the defect levels of substitutional vanadium and the positively charged carbon vacancy (VC +) in semiinsulating (SI) SiC. After reviewing the earlier studies related to both defects, the paper focuses on temperature-dependent Hall measurements and photo-induced electron paramagnetic resonance (EPR) experiments of 4H and 6H SI SiC. In vanadium-doped samples, a V3+/4+ level near Ec-1.1 eV (4H) and Ec-0.85 eV (6H) is estimated by a comparison of dark EPR spectra and the activation energy determined from the Hall data, assuming that vanadium controls the Fermi level. In high purity semiinsulating substrates, analysis of time-dependent and steady-state photo-EPR data suggests that the plus-to-neutral transition of the carbon vacancy involves a structural relaxation of about 0.6 eV.

All solid state laser system, continuously tunable over 0.2-10 micron spectral range

An efficient room temperature all-solid-state laser system continuously tunable in the 0.2 - 10 micrometers spectral range has been developed. It is based on the alexandrite laser pumped LiF:F2+** color center laser system. The alexandrite - LiF:F2+** color center laser combination system has been shown to be a suitable drive source for a number of efficient nonlinear processes, including harmonic, sum-frequency and difference-frequency generation.

The Optical Admittance Spectroscopy of the Vanadium Donor and Acceptor Levels in Semi-Insulating 4H-SiC and 6H-SiC

The purpose of this study is to determine the vanadium defect levels in semi-insulating 4H-SiC and 6H-SiC using optical admittance spectroscopy (OAS). OAS data show several distinct peaks for the vanadium-doped SI 4H-SiC and 6H-SiC. Comparison of the data for the two polytypes suggests that peaks at 0.67 ± 0.02 eV and 0.70 ± 0.02 eV in 6H substrates and 0.75 ± 0.02 eV in 4H substrates are related to V3+/4+ levels at the cubic sites. A peak at 0.87 ± 0.02 eV in the 6H sample is assigned to the same defect level at the hexagonal site and the associated transition in 4H was observed at 0.94 ± 0.02 eV in our spectra. The donor levels are thought to be related to peaks at 1.94 ± 0.05 eV and 1.87 ± 0.05 eV in 4H and 6H samples, respectively. The differences between the values obtained from the optical admittance measurements and those reported in the literature are attributed to thermal relaxation and/or contributions from defect complexes.

A study of V 3+ and the Vanadium acceptor level in semi-insulating 6H-SiC

Infrared absorption (IR) and electron paramagnetic resonance (EPR) spectroscopies are used to study the V 3+ impurity and the vanadium acceptor level in 6H semi-insulating SiC. IR and EPR data obtained from samples cut from the same wafer support the assignment of the 0.60 and 0.62 eV IR absorption lines to substitutional V 3+ . Photo-induced EPR measurements reveal identical photo-thresholds for V 3+ and V 4+ ions. A peak at 0.8 eV, where the intensity of the three plus charge state decreases and the four plus charge state increases by an equal amount, is thought to represent excitation of an electron from V 3+ to the conduction band edge. The 0.8 eV peak is therefore attributed to the V 3+/4+ level. The difference between the optically measured value reported here and that measured previously using temperature dependent techniques is attributed thermal relaxation.