LiNbO 3 Crystals Reduced in Vacuum Show a Photorefractive Response Time in the Order of 100 ms
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and its dependence on crystallographic orientation and surface roughness on the atomic scale. They also reveal how repeated contact between surfaces leads to nucleation and progression of damage at the nanoscopic dimensions, and why real crystals exhibit unusally high local strength for defect nucleation beneath a free surface when subjected to nanoscale contact. With such insights gained from the bubble model, the researchers formulated a mechanistic theory for defect nucleation at surfaces during nanoindentation. The researchers have since used the bubble system to explore how defects form for a variety of surface conditions. They have experimentally simulated the effects of atomic-level surface roughness on defect nucleation at surfaces. Although soap bubbles have long been used to study deformation of bulk metals, this work attempts the quantitative simulation of nanoscale contact deformation and defect nucleation at surfaces. By monitoring the defect nucleation characteristics in the bubble experiments as a function of surface asperity dimensions and the radius of the indenter tip, Suresh and his colleagues were also able to identify the conditions governing the nucleation of defects either at surfaces or in the interior for different local contact geometries. With the information obtained on homogeneous defect nucleation beneath the surface when the asperity dimension is comparable to or larger than the indenter tip radius, they were then able to rationalize why many metals exhibit unusually high local strengths near surfaces prior to the onset of defect nucleation during nanoindentation when the surface is penetrated by an indenter to a depth of only a few tens of nanometers. “Our ultimate goal is to use them to predict how defects will form on the nanolevel, because such defects can affect the performance of these surfaces and nanoscale devices,” said Suresh.
LiNbO3 Crystals Reduced in Vacuum Show a Photorefractive Response Time in the Order of 100 ms Photorefractive crystals have different applications in optics including optical storage, coherent optical amplification, and phase conjugation. In many photorefractive materials, it is possible to write a holographic grating with a response time of less than a second. However, in the case of lithium niobate, the response time is slow, of the order of several minutes, in contrast with predictions from theoretical calculations. A group of researchers from Nankai University in China has demonstrated a method that MRS BULLETIN/AUGUST 2001
improves this condition in LiNbO 3 . According to their latest results published in the July 1 issue of Optics Letters, the application of a reducing treatment on a near-stoichiometric crystal significantly decreases its response time in the order of 100 ms. A high-purity LiNbO3 single crystal grown by the Czochralski method and with 49.6 mol% Li2O was reduced in vacuum at 950°C for 5 h. The resultant, nearstoichiometric sample had blue-shifted absorption edge as compared with the as-grown crystal. Holographic gratings were wr
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