Abstract
Magnetooptical (MO) glasses and, in particular, Faraday rotators are becoming key components in lasers and optical information processing, light switching, coding, filtering, and sensing. The common design of such Faraday rotator materials follows a simple path: high Faraday rotation is achieved by maximizing the concentration of paramagnetic ion species in a given matrix material. However, this approach has reached its limits in terms of MO performance; hence, glass‐based materials can presently not be used efficiently in thin film MO applications. Here, a novel strategy which overcomes this limitation is demonstrated. Using vitreous films of xFeO·(100 − x)SiO2, unusually large Faraday rotation has been obtained, beating the performance of any other glassy material by up to two orders of magnitude. It is shown that this is due to the incorporation of small, ferromagnetic clusters of atomic iron which are generated in line during laser deposition and rapid condensation of the thin film, generating superparamagnetism. The size of these clusters underbids the present record of metallic Fe incorporation and experimental verification in glass matrices.
Highlights
Introduction neither YIG norBIG single-crystals are suitable for applications in wavelength regimes other than the infrared, in particular, in Magnetooptical (MO) effects play a key role in optical inforthe visible (Vis) or even UV spectral ranges
The magnitude of magnetooptical activity of a material is often expressed through the Verdet constant V, which provides a straightforward measure for the Faraday rotation angle θ as a function of the applied magnetic field with strength B, and the geometrical path length d of linearly polarized light passing through the rotator material, θ = VBd
We demonstrated record Faraday rotation efficiency in pulsed laser deposition (PLD)-derived vitreous films of FeO-SiO2, beating the performance of conventional MO glasses by up to two orders of magnitude. The origin of this is the incorporation of nano- and subnanoscopic ferromagnetic clusters of Fe0n. The formation of these clusters is facilitated by the rapid deposition kinetics occurring during the PLD process and hindering further segregation and growth of precipitates
Summary
XPS and XANES analyses were employed as complementary methods to elucidate the atomic state in which the iron species is incorporated into the present material and, to provide further evidence for the chemical origin of the magnitude of Faraday rotation. This assumes that iron is the key element for producing the observed increase in MO performance. The sum of integrated band areas for x = 14.8 is close to that of staurolite, where divalent iron ions occupy tetrahedral lattice sites This value decreases with increasing iron fraction x, providing evidence for an increase in iron coordination number (with fivefold coordination VFe2+, in the grandidierite mineral reference). As with XPS, this evidence remains ambiguous, here due to eventual photoreduction by high-energy X-ray irradiation.[29]
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