No abstract

We have developed an efficient iterative algorithm for electromagnetic scattering of arbitrary but relatively smooth dielectric objects. The algorithm iteratively adapts the equivalent surface currents until the electromagnetic fields inside and outside the dielectric objects match the boundary conditions. Theoretical convergence is analyzed for two examples that solve scattering of plane waves incident upon air/dielectric slabs of semi-infinite and finite thicknesses. We applied the iterative algorithm for simulation of sinusoidally-perturbed dielectric slab on one side and the method converged for such unsmooth surfaces. We next simulated the shift in radiation pattern of a 6-inch dielectric lens for different offsets of the feed antenna on the focal plane. The result is compared to that of the Geometrical Optics (GO).

Magnon scattering enables non-linear microwave devices, such as frequency selective limiters and signal to noise enhancers. It may also impact information transfer within spintronic devices. Here, a quantitative understanding of magnon processes in thin films is developed using micromagnetic simulations, in combination with newly developed analytic theory and experimental data. A technique for calculating the number of magnons at each frequency and wavevector as a function of external input such as power and frequency is identified. It is shown that, near the nonlinear threshold, the dominant parametrically excited magnon pairs are those with minimal group velocity and the correct energy. These results complement Brillouin Light Scattering experiments and indicate a path for wavevector-modulated magnon production based only on simulated results and/or analytic theory, a desirable goal for information transfer and communication.

We investigate the electron spin dynamics of n-type c-axis oriented bulk zinc oxide (ZnO) by using time-resolved Kerr rotation and resonant spin amplification measurements. Calculating resonant spin amplification using an anisotropic spin dephasing model reveals that there are two species involved in the spin dynamics, which we attribute to conduction and impurity-bound electron spins, respectively. We find that the impurity-bound electron spin dephasing mechanism is strongly anisotropic due to anisotropic exchange interactions. The identification of the two spin species and their dephasing mechanisms is further supported by the temperature, power, and wavelength dependence of the spin coherence measurements.

With minimal Joule loss, magnetic insulator-based quantized spin-waves or magnons are becoming increasingly popular for device applications including logic-circuits and signal processing. The parametric excitation-based nonlinear behavior that plays an important role in such applications is also interesting from a physics perspective. In this work, we demonstrate quantitative prediction of the threshold microwave-field needed for initiating nonlinear behavior in the presence of a secondary microwave frequency. This would allow the in situ control of non-linearity and, hence, prove to be useful for a wide range of applications, especially those involving microwave devices. The fine structures, appearing in the threshold-field upon variation in the frequency of the secondary frequency, have been demonstrated using simulations and explained analytically. The impact of the magnon phase relative to the pump is also quantitatively determined.