AFRL-OSR-VA-TR-2015-0163 Magneto-electric Energy Conversion of Optical Energy to Electricity

Reporting Period: April 1, 2014-March 31, 2015) In the third and final year of research on magneto-electric conversion of optical energy to electricity, several major project objectives were achieved in the areas of theory and experiment. This was chiefly the result of a breakthrough in understanding the reported enhancement of magneto-electric scattering and switching to computer codes based on Matlab to improve both the speed of computations and the time required to acquire data. On the theoretical front, a dressed state approach was successfully developed to analyze optical magnetization. The introduction of Matlab software resulted in significant speedup. Computational timescales for large matrix diagonalizations went from days to hours. Applied to atomistic models, these computations of induced dipole moments revealed that the family of nonlinearities of interest in this work arise from dynamic symmetry-breaking by the optical magnetic field. However they fell short of explaining the high signal intensities observed in laboratory measurements. Subsequently a molecular model was devised to exploit the idea that magnetic torque can enhance high frequency magnetic moments by transferring orbital angular momentum to rotational motion. A key advance was the realization that one can increase magnetic moments by switching from atoms to molecules and exploiting magnetic torque to increase the area enclosed by librating charges. Progress on a semi-classical version of the theory showed that this concept had all the key features needed to explain experimental intensities, but development of a fully-quantized version of the theory proved to be very challenging. The exact version of the theory was not completed before the end of the award. Nevertheless work during this grant period established both the universality of magneto-electric (M-E) nonlinearities and the key role played by rotational dynamics. On the laboratory front, a major effort was made to improve signal-to-noise by automating data acquisition. Software was developed to control the drivers for mechanical rotation mounts and other optics involved in recording radiation patterns from scattering experiments. The first observations of cross-polarized signals and unpolarized background scattering in C6H6, C6D6, ethylene glycol, a viscous hybrid material (phenylsilsesquioxane: dimethylsiloxane), analine, benzonitrile, YAG and GGG were made subsequently. A comparison of deuterated and un-deuterated benzene permitted us to reach a milestone by verifying the importance of rotational frequencies in enhancing magnetic scattering. The ratio of magnetization slopes of these compounds agreed with the ratio of their (squared) moments of inertia. This provided theoretical confirmation that molecules as opposed to atoms have unique advantages for energy conversion applications. 1. Review of Research Objectives This program undertook an initial investigation of processes relevant to optical energy conversion based on magneto-electric rather than photovoltaic processes. The research built on