Near-infrared photoluminescence in germanium oxide enclosed germanium nano- and micro-crystals

We have studied the near-infrared photoluminescence properties of free-standing germanium nano-crystals (20 nm on average) and micro-crystals (60 µm on average) at 80–300 K. Two peaks were observed at ~1.0 and ~1.4 eV from both the nano- and micro-crystals. The integrated PL (IPL) intensity of the nano-crystals is about an order of magnitude stronger than that of the micro-crystals and the IPL is also enhanced by ageing in air for both crystals. The ~1.0 eV peak position does not change with either the crystal size or temperature. We suggest that the deep traps located at the interfacial region between the surface GeO2 layer and the bulk crystal Ge is responsible for the near-infrared PL.

Source:IOPscience

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Crystal growth and optical characterization of an organic single crystal for frequency conversion applications

Organic nonlinear optical 2-methylquinolinium L-malate single crystals have been grown by slow evaporation solution growth technique from a mixture of 2-methylqiunoline and L-malic acid in ethanol solution at ambient temperature. Single crystal X-ray diffraction analysis reveals that grown crystal in monoclinic system with non-centrosymmetric space group P21 and the lattice parameters are a = 7.35 Ǻ, b = 26.51 Ǻ, c = 10.83 Á, α = γ = 90° β = 102.95º and V = 2057.4 Ǻ3. UV-vis spectrum indicates that the crystal is transparent (75%) in the entire visible region with a cut-off wavelength of 437 nm and optical energy band gap Eg is found to be 2.71 eV. Microhardness measurement reveals the mechanical strength of the grown crystal. The photoluminescence spectrum shows the blue emission of the crystal. Laser damage threshold studies was carried out to ascertain the suitability of grown crystal for laser applications.The relative second harmonic generation efficiency of 2-methlquinolinium L-malate crystal was found to be two times greater than that of KDP.

Source:IOPscience

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On the optical properties of undoped and rare-earth-doped yttrium aluminium garnet single crystals

Optical absorption and photoluminescence (PL) measurements were performed on single crystals of undoped Y3Al5O12 (YAG) and a number of rare-earth-doped YAG to study the effect of dopant type and concentration, growth atmosphere, post-growth annealing and UV irradiation on the optical properties of YAG crystals. The presence of hydrogen in the growth atmosphere was found to be essential for enhancing the incorporation of Ce ions in the Ce3+ state in Ce-doped YAG (Ce : YAG). Annealing in air was shown to have no effect on the PL emission of Ce : YAG crystals. An absorption peak around 256 nm was observed in the undoped YAG and Ce : YAG crystals after air anneal at 1200 °C. Optical absorption and annealing experiments support the association of the 256 nm peak with Fe impurities and oxygen ions. UV irradiation modifies the valency of impurities and generates electronic defects leading to an increase in the optical density of YAG crystals. Optimizing the growth and annealing conditions is critical in order to develop Ce : YAG single crystals as efficient scintillators.

Source:IOPscience

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