All-dielectric metasurfaces have been attracting much attention. Low optical losses and a huge variety of optical modes provide unique possibilities for light manipulation at the nanoscale. Recent studies showed that the magneto-optical effects in such metasurfaces are enormously enhanced. Moreover, it is possible to observe novel magneto-optical effects that are absent in smooth films. Excitation of particular photonic resonances makes it possible to design the magneto-optical interaction by the metasurface design. This opens up broad opportunities for magneto-photonic metasurface applications, including optomagnetism, light modulation, sensing, magnetometry, etc.

We report a study of surface reconstruction effects in yttrium iron garnets using density functional theory. This work responds to the need to explain the physical basis of recent experimental results showing a significant enhancement in Faraday rotation in iron garnets. These materials are extensively used in the telecom industry for nonreciprocal device applications. Understanding the physical basis of the heightened Faraday response at the surface is important for the development of ultrathin nonreciprocal devices. Our results show that the bandgap near the surface is significantly reduced compared to that of the bulk, and that spin-orbit coupling effects become more important near the surface. We find that the decrease in band gap results in an enhancement and change in direction in the Faraday rotation in the visible range all the way to the deep red. Electronic transition matrix elements for surface-sensitive ultra-thin layers are calculated and compared to those of bulk samples, leading to the conclusion that the octahedrally-oxygen-coordinated iron sublattice in these ferrimagnetic materials is more strongly affected by surface reconstruction than the other, anti-ferromagnetically coupled, tetrahedral sublattice. We explain how these changes contribute to the enhancement in magneto-optic response near the surface as compared to the bulk.

Bismuth-substituted lutetium iron garnets have exhibited a remarkable enhancement in Faraday rotation (FR) for films thinner than 50 nm. A sevenfold amplification in the magneto-optic gyrotropy was found to occur within 2 nm of the air-surface interface at 532 nm wavelength. The present study delves into the underlying physical mechanisms contributing to such amplification. Near-surface changes in band structure in these materials and their connection to the magneto-optic response are explored. Density functional theory is employed to investigate the changes in density of states and overall band structure reconfiguration of surface atoms. The transition dipole matrix (TDM) model is then applied to both bulk and surface states, correctly predicting a Faraday rotation enhancement at the surface as a result of overall surface band structure reconfiguration and resulting bandgap reduction. Surface versus bulk FR spectral response is extended beyond prior studies over the full visible and the near-infrared spectral ranges, predicting significant amplification across the telecom band. Experimental analysis through X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopy reveal a reduction in bandgap as films are thinned down from 200 nm to 40 nm. By providing a deeper physical understanding of the origin of enhanced Faraday rotation at the surface, this work opens up avenues for more efficient miniaturized Faraday rotation applications. Knowledge of the band structure information thus uncovered may be used to demonstrate novel and more advanced applications.

Surface reconstruction and electronic transitions are examined. XMCD through total fluorescence yield and total electron yield show evidence of density of state differences between bulk and surface. Surface reconstruction was examined via STEM.

This dissertation presents the results of a study investigating the physical mechanisms underlying an unexpectedly large increase in magneto-optic efficiency observed in iron garnet. Such materials are technologically important for telecommunications due to their nonreciprocal optical action. In the past, our group had found evidence of an enhanced Faraday rotation in bismuth-substituted iron garnet films less than 50 nm thick. Subsequent investigation revealed that this enhancement could be traced to surface effects. This is significant because understanding these phenomena could be used to formulate engineering solutions for device miniaturization. In this dissertation, we present the result of a research project investigating the physical mechanisms underlying this unexpectedly large increase in Faraday response. Both experimental and theoretical techniques were brought to bear, including spectroscopic analysis and density-functional theory calculations, to understand how surface reconstruction effects impact the quantum origins of these curious phenomena. High-energy X-ray magnetic circular dichroism (XMCD) and X-ray absorption (XAS) measurements were used to analyze electronic transitions, while scanning transmission electron microscopy yielded concomitant structural analysis. The XAS and XMCD allowed us to produce a comparative analysis of surface and bulk densities of state impacting the 3d Fe orbitals responsible for Faraday rotation. This investigation was supplemented with surface-sensitive XPS analysis and ultraviolet and visible light absorption studies of the electronic distribution near the Fermi-level bands. These results were used to build and verify a density functional theory (DFT) model for iron garnet materials. Vienna ab-initio simulation (VASP) techniques yielded density-of-state (DOS) and band structure information for these systems. These allowed us to compare differences in bulk and surface band structures and DOS and to trace the surface-induced electronic origin of changes in Faraday rotation. DFT calculations based on these results showed that, indeed, the Faraday effect is enhanced in the visible and near-infrared regimes. Transition dipole matrix analysis was used to predict an accurate Faraday rotation response based on DFT calculations. Surface symmetry-breaking and surface reconstruction were thus found to be responsible for the effects under investigation. The combined evidence from this comprehensive study shows that a decrease in band gap at the surface plays a decisive role in the enhanced Faraday rotation.