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:

Spontaneous excitation of the dominant mode in a spin-wave active ring-a self-exciting positive-feedback system incorporating a spin-wave transmission structure-occurs at a certain threshold value of external gain. In general, the wavenumber of the dominant mode is extremely sensitive to the properties and environment of the spin-wave transmission medium, and is almost impossible to predict. In this letter, we report on a backward volume magnetostatic spin-wave active ring system incorporating a magnonic crystal. When mode enhancement conditions-readily predicted by a theoretical model-are satisfied, the ring geometry permits highly robust and consistent forced dominant wavenumber selection. © 2010 American Institute of Physics.

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:

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 …