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The magnetooptical control of light implies different directions of polarization plane rotation, linear-to-circular and other polarization transformations. These opportunities can be opened using polarization-sensitive resonances, for example, Bloch surface waves (BSWs) and waveguide modes (WGMs) in magnetophotonic crystals (MPCs) [1]. Magneto-optical phenomena, such as Faraday effect, can be significantly increased near spectrally narrow optical resonances [2]. In this case, the Faraday rotation angle is determined both by the magnetic properties of the material and by the Q-factor of the resonances, which define their spectral width. The resonance of the BSW is shown to be extremely narrow. The proper choice of the MPC parameters gives the ways to observe the s-polarized BSW and p-polarized WGM of the MPC in the same spectral region. Here, we experimentally demonstrate how a fundamental property of magneto-optical effects to couple two linearly polarized modes allows one to control and modify the values of Faraday rotation angles. The interplay of the BSW and WGM results in an enhancement of the Faraday rotation angle, change of lineshape of Faraday rotation spectra and direction of the polarization rotation.
Reflectance spectra of the one-dimensional magnetophotonic crystal and the corresponding Faraday rotation spectra were experimentally measured using the Kretschmann attenuated total internal reflection configuration and numerically calculated using the transfer matrix approach [3]. The studied magnetophotonic crystal sample consists of 15 alternating layers of fused quartz and Bi-substituted yttrium-iron-garnet on a sGGG substrate.
The BSW excitation corresponds to a narrow resonance in the reflectance spectra of the s-polarized light. Wide dips in the reflectance spectra of p-polarized light correspond to the WGM resonances. As the incident angle increases, both the resonances shift to short wavelengths, but the WGM resonance shifts faster; thus, the spectral distance between the BSW and WGM resonances decreases. The spectral dependence of the Faraday rotation angle of s-polarized light has a feature coinciding in the BSW wavelength and caused by the BSW excitation. The feature in the Faraday rotation spectrum has a Fano resonance shape and changes from an asymmetric shape to a symmetric one, while the incident angle increases and the BSW and the WGM resonances approach each other. This behavior is observed both in the experiment and calculations. Thus, it can be argued that the spectral dependence of the Faraday rotation angle depends not only on the BSW resonance in the structure but also on the coupling of the BSW with the WGM mode that is not excited in the s-polarization of the incident light. Besides, the Faraday rotation changes direction while BSW and the WGM resonances spectral position approach each other that makes these resonance in MPCs promising for the future photonics devices.
[1] M. N. Romodina, I. V. Soboleva, A. I. Musorin, Y. Nakamura, M. Inoue, A. A.
[2] M. Inoue, M. Levy, A.V. Baryshev, Magnetophotonics: From Theory to Appli- cations, Springer Series in Materials Science, 2013.
[3] D.W. Berreman, J. Opt. Soc. Am. 62, 502–510 (1972).
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