Professor John Gregg

Abstract:

The effective magnetic field induced by a femtosecond pulse of circularly polarized light, via the inverse Faraday effect, is shown to excite a magnetic-dipole forbidden exchange spin resonance in a lutetium iron garnet. An external magnetic field cannot excite this mode, as the iron sublattices have the same gyromagnetic ratio and no net torque can be applied between them. However, since the sublattices have different magneto-optical susceptibilities, the inverse Faraday effect induces different effective fields on different iron sites, allowing excitation.

Abstract:

Pump-probe experiments on the rare-earth paramagnet Dy3Al 5O12 demonstrate that coupling between light and magnetism on the subpicosecond time scale cannot be adequately described by the thermodynamic model of the inverse Faraday effect but instead must be described microscopically by stimulated magneto-Raman scattering. Light-induced paramagnetic resonance, predicted by the thermodynamic theory, is not observed in Dy3Al5O12, however, the formation of a coherent superposition between other magnetic sublevels of the Dy³⁺ -ion's ground-state multiplet is measured. It is shown that coherence can only be induced between magnetic levels which are connected by the Raman selection rules. © 2010 The American Physical Society.

Abstract:

The time reversal of pulsed signals or propagating wave packets has long been recognized to have profound scientific and technological significance. Until now, all experimentally verified time-reversal mechanisms have been reliant upon nonlinear phenomena such as four-wave mixing. In this paper, we report the experimental realization of all-linear time reversal. The time-reversal mechanism we propose is based on the dynamic control of an artificial crystal structure, and is demonstrated in a spin-wave system using a dynamic magnonic crystal. The crystal is switched from an homogeneous state to one in which its properties vary with spatial period a, while a propagating wave packet is inside. As a result, a linear coupling between wave components with wave vectors k≈π/a and k'=k-2π/a≈-π/a is produced, which leads to spectral inversion, and thus to the formation of a time-reversed wave packet. The reversal mechanism is entirely general and so applicable to artificial crystal systems of any physical nature. © 2010 Macmillan Publishers Limited. All rights reserved.

Abstract:

An acoustic oscillator arrangement includes an acoustic system having at least one acoustic transmission path through it, and at least one mode. The acoustic transmission path is of variable length. A controller is provided with an amplifier and a feedback network which together provide a positive …