Enhanced Faraday rotation in garnet films and multilayers
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Enhanced Faraday rotation in garnet films and multilayers S. Kahl and A. M. Grishin Condensed Matter Physics, Royal Institute of Technology, S-16440 Stockholm, Sweden ABSTRACT Films were prepared by pulsed laser deposition. We investigated or measured crystallinity, morphology, film-substrate interface, cracks, roughness, composition, magnetic coercivity, refractive index and extinction coefficient, and magneto-optical Faraday rotation (FR) and ellipticity. The investigations were partly performed on selected samples, and partly on two series of films on different substrates and of different thicknesses. BIG films were successfully tested for the application of magneto-optical visualization. The effect of annealing in oxygen atmosphere was also investigated - very careful annealing can increase FR by up to 20%. Periodical BIG-YIG multilayers with up to 25 single layers were designed and prepared with the purpose to enhance FR at a selected wavelength. A central BIG layer was introduced as defect layer into this one-dimensional magneto-optical photonic crystal (MOPC) and generated resonances in optical transmittance and FR at a chosen design wavelength. In a 17-layer structure, at the wavelength of 748 nm, FR was increased from −2.6 deg/µm to −6.3 deg/µm at a small reduction in transmittance from 69% to 58% as compared to a single-layer BIG film of equivalent thickness. In contrast to thick BIG films, the MOPCs did not crack. We were first to report preparation of all-garnet MOPCs and second to experimentally demonstrate the MOPC principle in magneto-optical garnets. INTRODUCTION It was realized at the end of the 1960s that bismuth doping strongly enhanced the Faraday rotation (FR) in iron garnets [1]. Since then, iron garnets with higher and higher bismuth contents have been prepared. Thin films made by liquid phase epitaxy could be prepared with higher bismuth content than single crystals [2], but the complete bismuth substitution with three bismuth atoms per chemical formula unit was reported for the first time at the end of the 1980s [3]. Bismuth iron garnet (BIG), Bi3 Fe5 O12 , is not stable in bulk form and has so far only been prepared by vapour deposition techniques. FR is a nonreciprocal effect and can therefore be enhanced strongly at a chosen wavelength by placing magneto-optical material inside an optical cavity [4]. This concept was also proposed for iron garnet films where the mirrors of the cavity are highly reflective multilayers composed of two materials with a difference in refractive index [5]. Single layers have quarter-wave thickness in the respective material. The magneto-optical material inside the cavity is an iron garnet film of different thickness: a defect layer inside this one-dimensional magneto-optical photonic crystal (MOPC). More advanced concepts with several defect layers have been proposed as well, in order to improve transmittance [6]. Simple examples of MOPCs have been realized experimentally in two different ways, either by using polycrystalline garnet films sandwiched between dielectric quar
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