Dynamics of the CO2 lower laser levels as measured with a tunable diode laser

A tunable diode laser operating in the 4.3 m region is used to probe a conventional cw CO₂ laser discharge. Vibrational populations in the 10⁰0, 02⁰0, 02²0, and 01¹0 levels of CO₂ are measured under lasing conditions, i.e., in the presence of intense 10.4 and 9.4 m fields. The tunable diode laser is also used to monitor the energy transfer processes between the four levels after the passage of an intense 10.4 m pulse. The detailed information provided by the tunable probe laser enables us to determineseparately all the vibration-vibration (V-V) and vibration-translation (V-T) rate constants of importance in the relaxation of the lower laser levels in CO₂. The V-V rate constants are found to vary from a low value of 4.5×10⁴ s^–1 Torr^–1 for the coupling of 01¹0 to 10⁰0 to a high value of 8.0×10⁵ s^–1 Torr^–1 for the coupling of 01¹0 to 02²0.This work was supported by the National Science and Engineering Research Council of Canada and the Provincial Government of Ontario     

Multiple harmonic conversion of pulsed CO2 laser radiation in Tl3AsSe3

The nonlinear material Tl₃AsSe₃ is used to convert pulsed 9.6 m CO₂ laser radiation into its second, third, fourth and fifth harmonic. Internal (external) conversion efficiencies of 28, 14(6.7), 3.6(3.1), and 0.5(0.3)% from the pump fundamental are achieved in 4.8, 3.2, 2.4, and 1.9 m generation respectively.     

A high-power transverse-flow cw CO2 laser-output and small-signal gain measurements

A high-power CO₂ laser for investigations on materials processing is described. Measurements of the output characteristics of the laser are discussed on the basis of an adequate model.     

Detailed vibrational population distributions in a CO2 laser discharge as measured with a tunable diode laser

A tunable diode laser (TDL) operating in the 2150–2350 cm^–1 wavenumber region is used to probe a conventional cw CO₂ laser discharge. Absorption lines in more than 25 different vibrational bands are observed, enabling us to determine absolute vibrational populations inall levels of concern to the dynamics of the 10 m CO₂ laser. Levels in thev ₃ mode of CO₂ as high as 00°9 are monitored, and it is found that anharmonic effects play a significant role in the populations of such levels. Thev ₁ andv ₂ mode populations are also investigated in detail, and it is found that these modes are strongly coupled and maintain a common vibrational temperature under all discharge conditions. The use of a TDL is shown to be a powerful technique for investigating the dynamics of infrared molecular lasers.This work was supported in part by the National Science and Engineering Research Council, Canada     

Crystallographic and magnetic properties of (Nd,Dy)3Fe27.5(Ti,Mo)1.5 compounds

A systematic study of the formation, structure and magnetic properties of (Nd,Dy)₃Fe_27.5(Ti,Mo)_1.5 compounds has been performed. Rietveld analyses of the X-ray patterns of the samples indicate that the concentrations of Ti and Mo affect the formation and structural properties slightly, whereas different rare-earth (Nd and Dy) contents influence them significantly. It is found that high Dy contents make it difficult to form the 3:29-type structures. The Curie temperatures of Nd_2.1Dy_0.9Fe_27.5Ti_1.5−xMo_x decrease monotonically as more Ti was replaced by Mo but their saturation magnetizations remain almost unchanged; in contrast, for Nd_3−yDy_yFe_27.5TiMo_0.5, their saturation magnetizations decrease monotonically with increasing Dy contents while their Curie temperatures are constant.     

On the origin of intrinsic donors in ZnO

As-grown undoped zinc oxide (ZnO) films have been annealed in zinc-rich, oxygen-rich and vacuum ambient, and the electron concentration varied greatly after the annealing process. It decreased nearly two orders of magnitude after the sample was annealed in oxygen, while increased nearly three times after annealed in metallic zinc ambient, and increased slightly after annealed in vacuum. It was found that the variation trend of the electron concentration is always the same with the expected variation of oxygen vacancy (V_O) under the three investigated conditions, it is thus speculated that V_O may be the dominant donor source in ZnO. By supplying more oxygen during the growth process to suppress V_O, ZnO films with lower electron concentration were obtained, which verifies the above speculation.     

First-principles calculations on the origins of the gap bowing in InAs1-xPx alloys

We perform self-consistent ab-initio calculations to study the structural, electronic and thermodynamic properties of InAs_1-xP_x alloy. The full potential-linearized augmented plane wave (FP-LAPW) method was employed within density functional theory (DFT). The ground-state properties are determined for the bulk materials (InAs and InP) as well as for the different concentration of their alloys. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The gap bowing for the alloy of interest was found to be mainly caused by the charge-exchange contributions. In addition, the thermodynamic stability of InAs_1-xP_x alloy was investigated by calculating the excess enthalpy of mixing ΔH_m and the calculated phase diagram showed a broad miscibility gap with a high critical temperature.     

Why is nacre strong? Elastic theory and fracture mechanics for biocomposites with stratified structures

Nacre, stratified ceramic layers surrounded by organic matrix, is a tough material found inside certain seashells. We construct a coarse-grained elastic energy for such an anisotropic system and present an analytic solution for a notch crack normal to the stratified sheets. This analysis proves the reduction in stress concentration which was announced in our earlier work (P. G. de Gennes and K. Okumura, C. R. Acad. Sci. Paris 1, Ser. IV, 257 (2000)) and the related increase in toughness. Received 18 March 2000     

Nonlinear polarizabilities of atoms from their ground-state densities

Using density based perturbation theory [M.K. Harbola, A. Banerjee, Phys. Lett. A 222, 315 (1996)], we calculate the static hyperpolarizabilty for spherical atoms and ions from their ground-state densities. Since densities are being employed, calculations are performed using approximate functionals for the kinetic and the exchange-correlation energies. Use of densities - instead of the wavefunctions or Kohn-Sham orbitals - reduces the computational effort substantially. The results obtained are within of those calculated from the corresponding orbital-based calculations. Received: 23 June 1997 / Revised: 25 September 1997 / Accepted: 24 December 1997     

Charge transfers in complex transition metal alloys ()

We introduce a new Non-Orthogonal Tight-Binding model, for complex alloys, in which electronic structure is characterized by charge transfers. We give the analytic calculation of a charge transfer, in which overlapping two-center terms are rigorously taken into account. Then, we apply numerically this result to an approximant phase of a quasicrystal of alloy. This model is more particularly adapted to transition metals, and gives realistic densities of states. Received: 7 July 1997 / Revised: 10 March 1998 / Accepted: 3 April 1998