Studies of magneto- and electro-optical effects at cryogenic temperatures.

摘要

This dissertation describes two projects that combine the techniques of modern atomic physics, and low-temperature physics. The first project is an investigation of the electro-optical Kerr effect, induced by a slowly-varying electric field in liquid helium at temperatures below the superfluid transition. The Kerr constant of liquid helium is measured to be (1.43 +/- 0.02 (stat) +/- 0.04(sys)) x 10-20 (cm/V)2 at T = 1.5 K. Within the experimental uncertainty, the Kerr constant is independent of temperature in the range T = 1.5 K to 2.17 K, which implies that the Kerr constant of the superfluid component of liquid helium is equal to that of normal liquid helium. It is deduced that hyperpolarizabilities of clusters of two or more helium atoms in the liquid phase account for approximately 25% of the measured Kerr constant. Kerr effect, can be used as a icon-contact technique for measuring the magnitude and mapping out the distribution of electric fields inside cryogenic insulants, such as liquid helium and liquid nitrogen.; The second project explores the possibility of obtaining ultra-narrow magneto-optical resonance lines with a vapor of paramagnetic atoms in a high-density cryogenic-temperature buffer gas. Laser ablation of lithium and rubidium bulk targets is used to achieve atomic vapor densities of 1011 cm -3 in helium buffer gas at temperatures between 30 K and 295 K. Atomic vapor loss, however, limits the lifetime of such vapor to about 4 ms. Ablation of sub-millimeter diameter silver and gold wires is demonstrated to produce similar atomic vapor densities, 1011 cm-3, but with much longer lifetimes, up to 110 ms. Diffusion coefficients for silver and gold atoms in helium gas are extracted from the vapor lifetime data: DAgHe ≃ 0.35 cm2/s, and D AuHe ≃ 0.48 cm2/s at 295 K and atmospheric pressure. The possible mechanisms responsible for measured atomic vapor loss are: diffusion to cell walls, buffer-gas convection currents, dimer formation, and attachment to clusters produced during the ablation process. It is unclear which of these mechanisms dominates at high buffer gas density (> 10 18 cm-3). Even if no further improvement of atomic vapor density lifetimes can be achieved, the techniques of non-linear magneto-optical rotation can now be extended to the cryogenic environments, unaccessible to vapor cell-based experiments.