Rare-Earth Iron Garnets – Enabling Integrated Isolators in Photonics and Novel Material Platforms in Magnonics

Abstract

Rare-earth iron garnets are a unique class of materials that has opened the doors for designing non-reciprocal passive devices in photonics and are emerging as a suitable material platform for studying novel spin-wave phenomenon in magnonics. In photonics, garnets deposited as claddings on waveguides can be utilized in designing optical isolators through non-reciprocal phase shift (NRPS) or non-reciprocal mode conversion (NRMC). In magnonics, the ferrimagnetic ordering and an absence of parasitic effects from conduction electrons produce very low spin damping in garnets and allow for the observation of different magnon interaction processes.Here, seedlayer-free cerium doped terbium iron garnet (CeTbIG) thin films are developed and optimized for high Faraday rotation through modifications to annealing temperature and concentration of cerium substitution. Also, adjustments in characteristic fluctuations in the bias voltage at the dopant sputtering target has resulted in Faraday rotations > -3000°/cm. The high gyrotropy CeTbIG is integrated with a silicon-on-insulator NRMC isolator that matches the thickness (500 nm) and mode (transverse electric, TE) of integrated lasers to provide an isolation ratio of 30 dB in a 1D footprint of 7.35 mm x 1 μm (L x W). Subsequently, garnet-specific two-step annealing processes are introduced to achieve 100% crystallinity in CeTbIG and cerium doped yttrium iron garnet (CeYIG) claddings integrated with NRPS isolators. CeYIG/YIG is also used as the defect layer in a one-dimensional magnetophotonic crystal with Bragg mirrors made of periodic dielectric media to obtain a resonant transmission of 60% in a 200 nm wide optical bandgap. The need for an external magnetic field for optical isolation is mitigated by engineering stoichiometric CeTbIG thin films on silicon with favorable in-plane effective anisotropy, a large remnant ratio, and large coercivity. Prototype magnet-free NRPS and NRMC devices with latched garnet claddings are simulated and found to be magnetostatically stable and immune to stray fields. Further simplification to the garnet integration process is demonstrated through a strain-enhanced diffusion driven mechanical exfoliation of CeTbIG thin films from silicon substrates. Exfoliation is enabled by a vacancy diffusion from a modified thermal treatment that follows the Nabarro-Herring model. The exfoliated garnet nanosheets have magnetic and gyrotropic properties comparable to their thin film counterparts. On the magnonics front, sub-micron yttrium iron garnet (YIG) thin films are investigated for non-linear magnetic susceptibility at different microwave powers and pump frequencies due to a three-magnon scattering mechanism. Preliminary results confirm the presence of a frequency threshold with a characteristic dependence on the thickness of the film. Lastly, highly epitaxial single crystalline and polycrystalline thin films of gadolinium iron garnet (GdIG) with a compensation temperature around 290 K are investigated for structural and magnetic inhomogeneities that are detrimental to the recently observed magnon-phonon interactions. This thesis shows that through innovations in materials processing and integration techniques of rare-earth iron garnets, integrated isolators can be realized at a chip-scale. At the same time, the development and characterization of new types of garnet thin films provide a deeper perspective on emergent physical phenomenon in nanoscale ferrimagnetic systems.