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temperature for 1 h. It was then ground and polished to 0.5 mm thick. SHG was obtained by pulses of 3.6 ns at 1.064 µm (beam ω) and 3 ns at 532 nm (beam 2ω) from a Q-switched Nd:YAG laser. The samples were prepared with both beam ω and beam 2ω incident. The samples were probed with beam 2ω blocked. Behind the sample, two heatabsorbing filters and a 532-nm bandpass filter allowed a photomultiplier to detect only the SHG signal with beam 2ω blocked. The signal was averaged by an oscilloscope. The intensity of SHG, normalized by the maximum intensity of second-harmonic waves from 1-mm-thick Y-cut quartz, increased rapidly and saturated after 50 min. The maximum intensity was seven times larger than that of the quartz and four orders of magnitude larger than that of 15Nb2O5-85TeO2 under the same optical poling conditions. As-made samples showed no SHG. The signal from the chalcogenide glass was stable, unlike other glasses. The signal from Bi2O8-based glasses decreased to 80% after 10 min. For 15Nb2O5-85TeO2, the signal decreased to 5% after 10 min. The researchers suggest that the nonlinear coherent field of ω + 2ω creates free electrons that are trapped by active sites in the glass, which breaks the inversion symmetry of the microstructure and allows SHG. Selecting the proper glass composition could increase the conversion efficiency and stability of the SHG, the researchers said. They believe that with their technique “it is possible to achieve practically useful nonlinear optical frequency-conversion devices with a chalcogenide glass fiber.” ELIZABETH A. SHACK

Nonvolatile Memory Observed in Chromium-Doped SrTiO3 Single Crystals Materials that exhibit reversible resistive switching are potential candidates for random-access memory (RAM). Various metal-insulator-metal-oxide structures display switching and show memory retention of more than 18 months. Y. Watanabe and colleagues, members of the IBM research team at Zurich Research Laboratory, examined single crystals of one such oxide, Cr-doped SrTiO3. Using these single crystals as a model system, the researchers determined that microstructural defects in the thin films are not a significant contribution to the switching effect. Instead, the researchers conclude that a bulk electronic change is necessary for memory switching to occur. As reported in the June 4 issue of 498

Applied Physics Letters, at 4 K the crystal has an initial resistance of 1 GΩ up to 100 V. Sweeping to 200 V causes a drop in resistance leading to a hysteretic currentvoltage characteristic. Stressing the crystal with pulsed or dc voltage causes an additional resistance drop by several orders of magnitude. This creates a conductive state which allows for memory switching. By applying a positive voltage pulse, the oxide is switched to a low-impedance state with a resistance of 500 Ω. Applying a negative voltage erases the “information” written to the device and switches the oxide back to a high-impedance state with a resistance of 5000 Ω. This effect is also seen at room temperature. Once the con