Comparison of Electron-Atom Collision Parameters for S to P Transitions under Reversal of Energy Transfer

Inelastic and superelastic electron scattering from the optically prepared 32P3/2 state of sodium has enabled atomic collision parameters to be deduced for the 4S-3P deexcitation and the 3S-3P excitation processes. These data are compared with convergent close coupling and second order distorted wave Born calculations. For excitation, both theories agree with experiment, whereas for deexcitation the close coupling theory is in better agreement. A long-standing proposal relating to the sign of the transferred angular momentum is not supported

Comparison of Electron-Atom Collision Parameters for S to P Transitions under Reversal of Energy Transfer

Inelastic and superelastic electron scattering from the optically prepared 32P3/2 state of sodium has enabled atomic collision parameters to be deduced for the 4S-3P deexcitation and the 3S-3P excitation processes. These data are compared with convergent close coupling and second order distorted wave Born calculations. For excitation, both theories agree with experiment, whereas for deexcitation the close coupling theory is in better agreement. A long-standing proposal relating to the sign of the transferred angular momentum is not supported

Third order nonlinearities in Ge-As-Se-based glasses for telecommunications applications

We have studied the third order optical nonlinearities of Ge-As-Se-based glasses. The glasses have high melting and glass transition temperatures that offer the potential for integration with traditional compound oxide glasses into highly nonlinear, high-index-contrast fibers. We used z-scan and femtosecond pump-probe techniques to measure the nonlinear refractive index and two-photon absorption coefficient of the glasses at telecommunication wavelengths. Nonlinearities as high as ϳ900ϫ that of silica were measured at 1540 nm in High capacity optical systems require devices such as cross connects, add-drop filters, repeaters, and wavelength converters. While some of these functions are currently being performed electronically, it is expected that they may eventually be replaced with optical devices in which nonlinear materials will play an important role. A key property of such materials is the optical Kerr effect that produces a change in the index of refraction proportional to the optical intensity I and the nonlinear index coefficient n 2 , ⌬n = n 2 I. The Kerr effect has an ultrafast time response and could be the basis for ultrafast optical switches with low switching energy. Two-photon absorption also occurs when the photon energy is above half-gap in the material and limits the maximum phase shift achievable. A figure of merit n 2 / ␤, 1 where n 2 is the nonlinear refractive index and ␤ the two-photon absorption coefficient, can be defined to assess the material properties relevant for efficient optical switching. To achieve a nonlinear optical phase shift of , necessary for a MachZender optical switch, with a nonlinear transmission loss of 20%, a figure of merit of ϳ2 is required. 2 Chalcogenide glasses have large values of nonlinearity at 1.55 m, several orders of magnitude larger than the value for conventional silica glass. 2 Many such glasses have been previously studied. 3-5 Among these, the Ge-As-Se system is of interest due to high nonlinearity, high refractive index (2.4-2.65), suitable optical transmission at 1.55 m and a relatively broad glass formation region. In this paper, we focus on glasses with particular promise for fabrication into high-index-contrast highly nonlinear fiber for 1.55 m applications. Glasses from Ge-As-Se family have glass transition temperatures in the range of 150-390°C making them suitable for integration with low refractive index compoundoxide glasses into high-index-contrast solid-core fiber. Highly nonlinear fiber can be used for applications including supercontinuum generation, 6 frequency metrology, 7 and wavelength conversion. 11 However, the chalcogenide glasses used in the fiber core must have a glass transition and softening temperature compatible with that of lower index glasses used for the cladding. We have investigated several chalcogenide glasses with glass transition temperatures from 292 to 380°C: Ge 33 As 12 Se 55 (commercially available as AMTIR-1, from Amorphous Materials), Ge 35 As 15 Se 50 , Ge 25 As 10 Se 65 , and Ge 22 As 20 Se 58 (commercially available as GASIR1, from Umicore). The glasses are found to have nonlinearities between 200ϫ −900ϫ that of silica, and figures of merit n 2 / ␤ as high as 3.2. The samples of Ge 33 As 12 Se 55 , Ge 35 As 15 Se 50 , and Ge 25 As 10 Se 65 were prepared as follows. For each glass composition, 5N (99.999%) purity amorphous selenium shot, 7.5N (99.999995%) purity crystalline lump arsenic, and 6N (99.9999%) purity single crystal germanium were batched into a fused quartz looped tube along with a magnesium metal strip (4N purity). The tube was placed into a two-level furnace, with the looped portion of the tube, in the hotter furnace zone. Over ϳ12 h, the As and Se components melted and were distilled from the loop into the lower part of the tube containing the Ge. After distillation, the lower portion of the tube was sealed, creating the melt vessel, and the loop containing impurities was discarded. 12 The melt vessel was placed into a rocking furnace at 900°C for 12 h, homogenizing the glass melt. The melt was then placed into a second furnace at the expected glass transition temperature. This furnace was switched off, allowing the glass to cool slowly to room temperature. The glass boules were cut into flat disks of about 3 mm thickness and the facets were ground parallel and polished to optical quality. Samples of Ge 33 As 12 Se 55 prepared in this manner were found to have similar n 2 , ␤, and bandgap energy to commercial samples a) Electroni