Lithium isotope geochemistry and origin of Canadian shield brines.

Hypersaline calcium/chloride shield brines are ubiquitous in Canada and areas of northern Europe. The major questions relating to these fluids are the origin of the solutes and the concentration mechanism that led to their extreme salinity. Many chemical and isotopic tracers are used to solve these questions. For example, lithium isotope systematics have been used recently to support a marine origin for the Yellowknife shield brine (Northwest Territories). While having important chemical similarities to the Yellowknife brine, shield brines from the Sudbury/Elliot Lake (Ontario) and Thompson/Snow Lake (Manitoba) regions, which are the focus of this study, exhibit contrasting lithium behavior. Brine from the Sudbury Victor mine has lithium concentrations that closely follow the sea water lithium-bromine concentration trajectory, as well as delta6Li values of approximately -28/1000. This indicates that the lithium in this brine is predominantly marine in origin with a relatively minor component of crustal lithium leached from the host rocks. In contrast, the Thompson/Snow Lake brine has anomalously low lithium concentrations, indicating that it has largely been removed from solution by alteration minerals. Furthermore, brine and nonbrine mine waters at the Thompson mine have large delta6Li variations of approximately 30/1000, which primarily reflects mixing between deep brine with delta6Li of -35 +/- 2/1000 and near surface mine water that has derived higher delta6Li values through interactions with their host rocks. The contrary behavior of lithium in these two brines shows that, in systems where it has behaved conservatively, lithium isotopes can distinguish brines derived from marine sources.

Second-harmonic generation from chalcopyrite-structure semiconductor thin films.

We propose that thin films of tetragonal chalcopyrite-structure semiconductor media are promising nonlinearoptical (NLO) materials for the following reasons: Phase matching is possible with them in principle, they can be utilized in a waveguide geometry, and they are compatible with III-V substrates. We investigate the second-order NLO properties of chalcopyrite-structure CuInSe(2) and CuGaSe(2) thin films grown on GaAs(001), using optical second-harmonic (SH) generation at a fundamental wavelength of 790 nm. The SH intensities produced by approximately stoichiometric CuInSe(2) and CuGaSe(2) thin films are similar to that of GaAs. The second-order optical nonlinearities are strongly curtailed in nonstoichiometric thin films.

Reflectance anisotropy from the low-index faces of cubic media.

The phenomenological theory of ref lectance anisotropy (RA) from the low-index faces of cubic media is revised. This revision is necessary as the result of recent studies of Svirko and Zheludev [Opt. Lett. 20, 1809 (1995)], who showed that the interaction of an electromagnetic wave with a noncentrosymmetric medium does not have to obey time-reversal symmetry. For noncentrosymmetric media a hitherto neglected bulk quadrupolar RA contribution therefore exists that typically can be expected to be comparable with the surface RA response. This contribution complicates the interpretation of RA data from all faces of cubic noncentrosymmetric media and can be measured in isolation on the (111) face.

Exact separation of surface and bulk contributions to anisotropic second-harmonic generation from cubic centrosymmetric media.

We solve this long-standing problem theoretically by recognizing that the ranks of the tensors that describe the surface and bulk second-order nonlinear susceptibilities differ in this class of media. We show that this implies that the phenomenologies of the anisotropic optical second-harmonic (SH) responses of the two sources differ for all possible crystal facial orientations except (111). To demonstrate the result, we apply the theory to separate the surface and bulk contributions to SH generation from an oxidized vicinal Si(001) wafer for p-polarized 775-nm fundamental and s-polarized SH radiation. It is shown that knowledge of the phase of the SH field is necessary to achieve a unique separation.