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Enhancement of Every Magneto-Optical Effect with All-Dielectric Guided-Mode Resonant Gratings Based on a Magnetic Sol-Gel Nanocomposite

Abstract : Magneto-optical (MO) photonic devices are currently highly desirable because of their ability to improve the sensitivity of biosensors or their sensitivity to the magnetic field. However, MO effects being rather small through classical magnetic films, it is relevant to find ways to enhance such effects which can manifest as light polarization rotation or intensity modification under magnetic field. The proposed device in this work to enhance MO effects is an all-dielectric planar structure formed by a 1D photoresist (PR) grating deposited on top of a MO film itself deposited on a glass substrate. Under coupling conditions through the grating, guided-modes (TE and TM) with narrow resonances are excited in the MO film by the incident light, increasing hence the light-matter interaction. Such coupling results as a dip (peak) in the transmittance (reflectance) spectrum. The MO film is a composite formed by magnetic nanoparticles (CoFe2 04) embedded in a silica matrix and obtained through sol-gel process. This nano-structurable composite can be easily deposited on common substrates with low annealing temperature (90°C), which is not the case of the most MO materials used within integrated optics platforms. Large enhancements of the different non-reciprocal polarization rotation effects (such as Faraday and longitudinal MO Kerr) were achieved experimentally and numerically through the all-dielectric resonant structure. The main results of this work concern the transverse MO Kerr effect (TMOKE). This effect induces a non-reciprocal spectral shift of the transmittance (reflectance) resonance upon magnetization reversal, resulting in an intensity modulation effect. TMOKE values up to 9.5% and 18.5% were measured respectively in transmission with T = 80% and in reflection with R = 5%. These large TMOKE values are mainly due to the high quality factor of TM transmittance (reflectance) resonances. The TMOKE signal for a single MO film is around 0.01%, hence an enhancement with three orders of magnitude was achieved through the fabricated structure. The reached measured TMOKE values are highly competitive with the literature where, to our knowledge, maximum values of 1.5% and 15% were experimentally demonstrated respectively through all-dielectric and magneto-plasmonic structures. Moreover, unexpected reciprocal magnetic effects were experimentally evidenced. Finally, the proposed all-dielectric structure is a low-cost device, which can be fabricated on large scale substrate, and able to enhance all the MO effects. Hence, it is a promising structure for non-destructive testing, magnetic field sensing and even biosensing.
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Laure Bsawmaii. Enhancement of Every Magneto-Optical Effect with All-Dielectric Guided-Mode Resonant Gratings Based on a Magnetic Sol-Gel Nanocomposite. Optics / Photonic. Université de Lyon, 2020. English. ⟨NNT : 2020LYSES028⟩. ⟨tel-03207393⟩

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