Post Date
Sep 19 2021

Some Explorations in Magneto-Optics and Opto-Magnetics

Dr. Sabieh Anwar
Ali Akbar
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The main focus of this research is to explore magneto-optic phenomena which have been extensively used as a tool for non-destructive, remote characterization of magnetic properties of materials and also demonstrate immense potential in device manufacturing and magnetic recording media. Magneto-optic effects are a manifestation of light-matter interaction in the presence of magnetic field and can be effectively described by the dielectric tensor. The emergence of different asymmetries in different elements of the tensor is dependent 0upon the direction of the applied magnetic field relative to the wavevector of light. The current work is an amalgamation of theoretical and experimental investigations of these asymmetries—the various forms of birefringence, while both the classical and quantum nature of light is utilized.

The theoretical pursuit involves the description and analysis of magnetic circular birefringence (the Faraday effect), magnetic circular dichroism (the Kerr effect), magnetic linear birefringence (the Voigt effect) and magnetic linear dichroism. The intermixing of these effects present a comprehensive picture of these phenomena. Magnetic birefringence effects are miniscule and require phase sensitive detection. Several experimental techniques have been devised, designed and implemented in order to quantify these asymmetries. The experimental methods employed in this work encompass diverse kinds of modulation techniques, adapted according to the challenges imposed by the experiment. For example, for measuring the Faraday rotation, modulation is achieved through an ac magnetic field generated by the Helmholtz coils, hence circumventing the need for a large dc field. The Verdet constant for terbium gallium garnet (TGG) crystal is measured by a home-built setup. The analysis for the Kerr effect is based on Jones calculus and modulation is realized through photoelastic modulator. To test the functionality of the setup, ferromagnetic thin films are deposited by magnetron sputtering and subsequently the absolute Kerr rotation is measured.

The rotation of polarization plane under transverse magnetic fields (the Voigt effect) for TGG crystal is then studied for a wide range of temperatures (8–100 K) by Stokes polarimetry where an optical chopper modulates the polarized beam of light. This particular method utilizes the discrete Fourier transform of polarized light intensity described in terms of the Stokes parameters. The magnitude of rotation and ellipticity are quantified and the Curie-Weiss constant is estimated from the analysis of Voigt coefficients. We consider it the first reported instance where the Curie-Weiss constant is derived from magnetooptic measurements based on the Voigt effect. The Voigt effect, due to its small value, requires ultra-sensitive measurement techniques, and therefore is, otherwise, rarely studied.

With technological advancement in femtosecond laser technology, the conventional role of magneto-optics has been widened from probing to controlling the magnetization of magnetic system. This is dubbed as opto-magnetics. In this context, the response of rare earth-transition metal (RE-TM) alloys and magnetic nano-structures in the form of bilayers, core shell and alloys is simulated, when excited with femtosecond laser pulses. The simulation environment is based on Heisenberg’s spin Hamiltonian which then utilizes Monte-Carlo algorithm and Landau-Lifshitz-Gilbert (LLG) in conjunction with two temperature model for investigation of different magnetic properties of the materials.

The simulation results demonstrate the correct estimation of Curie temperatures for well known rare earth and transition metals. Furthermore, all-optical deterministic switching of magnetization is observed for rare earth-transition metal (RE-TM) ferrimagnetic alloys. This switching mechanism operates in the femtosecond timescale. The control of magnetization switching time can be achieved by varying certain parameters, i.e., doping concentration of particular elements in host alloys and variation in laser fluence. These effects are also studied in this work.

The last part of this work reproduces measurements of the earlier chapters 2 using quantum light comprising single photons, generated from a heralded down-conversion nonlinear optical process. The Faraday rotation for quantum light is demonstrated using single photons. The synthesis of polarized single photon states are realized through spontaneous parametric down-conversion and state estimation is performed by quantum state tomography. The tomographic results are then analyzed and various kinds of minimization algorithms are adopted to extract Faraday rotation angles. The extracted Faraday rotation angles from the estimated state are corroborated with the previous experimental findings. Furthermore, our tomographic data is analyzed to assess the ellipticity acquired by single photons while establishing a correspondence with single qubit operations described on the Bloch sphere.

During the course of my doctoral research, I was also part of a collaborative team working on magneto-optic and active optical design of nano-structured devices. This work resulted in some publications which are listed below, but are not a part of this thesis.

List of Publications:

(Principal Author)

1.        Akbar, A., Khalid, M. W., and Anwar, M. S. ( 2017 ). ``Low temperature Voigt effect in the terbium gallium garnet crystal".  Optics Express,  25 ( 24 ),  30550 -- 30559 .

2.        Akbar, A., Saleem, M., Atiq, S., and Anwar, M. S. ( 2018 ). ``Magnetic Dynamics and All-Optical Switching in  5  nm Dy–Fe Nanostructures",  IEEE Transactions on Magnetics,  54 ( 7 ),  1 -- 6 .

(Collaborating Author)

3.       Mahmood, N., Kim, I., Mehmood, M.Q., Jeong, H., Akbar, A., Lee, D., Saleem, M., Zubair, M., Anwar, M.S., Tahir, F.A. and Rho, J., ( 2018 ). ``Polarisation insensitive multifunctional metasurfaces based on all-dielectric nanowaveguides", Nanoscale,  10 ( 38 ), pp. 18323 -- 18330 .

4.       Fatima, T., Akbar, A., Anwar, M.S. and Tahir, M.N., 2019. ``Six new dinuclear Schiff base complexes of Cu (II)/Ln (III) system: Synthesis, characterization and magnetic studies",  Journal of Molecular Structure,  1184 , pp. 462 -- 467 .

5.       Mahmood, N., Jeong, H., Kim, I., Mehmood, M.Q., Zubair, M., Akbar, A., Saleem, M., Anwar, M.S., Tahir, F.A. and Rho, J., 2019. ``Twisted non-diffracting beams through all dielectric meta-axicons",  Nanoscale,  11 ( 43 ), pp. 20571 -- 20578 .

               6. Abbas, S.K., Akbar, A., Atiq, S., Saleem, M., Iftikhar, M., Rafiq, H.M., Naseem, S. and Anwar, M.S., 2019. ``Optimization of DyFe nanostructures using E-beam lithography for magneto-optical applications", Journal of Magnetism and Magnetic Materials,  469 , pp. 196 -- 202 .