Photoluminescence (PL) mapping was utilized to investigate damage in β-Ga2O3 epilayers induced by 1064 nm laser pulses. The intensity and position of the intrinsic UV band were determined and plotted as a false-color image. Two types of damage were identified: circular damage and damage cracks. Circular damage shows lower UV PL intensity than the surrounding material with color centers in a “halo” around the damaged region. Damage cracks are aligned with the a and c axes and show higher PL intensity than undamaged material. Defects in the as-grown material were revealed by shifts in the UV band energy.
The usefulness of GYGAG(Ce) transparent polycrystalline ceramic garnet scintillators as the conversion medium in alpha and beta-fueled radioisotope batteries was explored through 0.5 to 3.5 MeV helium ion, alpha, and 0.5-2 MeV electron irradiations. Absorption spectra and light yields were measured before and after irradiations. Within experimental error no degradation in light yield was observed for the electron-irradiated samples as measured via beta or gamma excitation. A small increase in optical absorption near the emission wavelength was observed following the largest dose electron irradiation. Significant reduction in light yield was observed following helium ion irradiation. Partial recovery of the light yield was observed following annealing in oxygen above 400oC for helium ion irradiated samples. These results suggest that GYGAG(Ce) may prove useful for beta-fueled scintillation-based radioisotope batteries by allowing for higher energy beta emitters, increased power densities, and long service lifetimes.
Power handling capabilities of broad-area high-power diode lasers are limited by the heat extraction capabilities of the device packaging. Traditional methods of heat extraction rely on conductive heat extraction from the diode chip and an emitting facet in contact with either quiescent or naturally convecting air. This leads to a thermal profile in the lasing direction of the cavity and a hot emitting facet. A hot facet accelerates material degradation, reducing the mean time to failure and limiting the safe operating power. Direct contact between the facet and a liquid coolant could enable higher levels of heat extraction compared to traditional cooling pathways. An innovative approach to cooling high-power, broad-area diode lasers via total immersion in liquid coolant is proposed and tested. In this study, we demonstrate that single emitters can operate with the emitting facet in direct contact with static coolant, with no negative change to device power or efficiency. Thermal analysis and models show that immersed diodes operate with improved thermal pathways, yielding lower total thermal resistance with the greatest improvement to thermal resistance at the facet-fluid interface.
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