Research On Fabrication And Magneto-optical Effects Of Periodic Nanostructured Films

With the development of Nanostructures preparation technology, the magneto-optical (MO) activity in the multi-layer systems of metal and dielectric film perforated with different shapes, such as circle, stripe, disk etc., has become the research hotspot. This periodic structure of the film can produce the surface plasmon resonance (SPR), which can be mediated by the structure parameters and the direction of incidence. The magneto-plasmonics combined plasmonic properties with MO effects has aroused great interest, for its enabling sophisticated control of magneto-optical material properties, which can be used in many fields, such as magneto-optical modulator, magneto-optical isolators, magneto-optical switching, magneto-optical sensors etc.. In this work, we apply the Finite Difference Time Domain (FDTD) method into the MO calculation, which enriches the MO computing methods. The reliability of this method is verified by comparing it with other theoretical and experimental results. The main research work of this paper includes the following aspects:1. The optical transmission and MO activity in the bilayer systems of a noble metal film perforated with nanohole arrays and a uniform dielectric film magnetized perpendicular to its plane are investigated by using FDTD method. The shapes of holes main include a square hole and circle annular hole. We study the relationship between the transmission spectra and Faraday spectra by changing the period of arrays and the refractive index of infilling medium. The best conditions for enhanced optical and magneto-optical enhancement can be obtained in our nanostructure. For square hole system, we calculated the reflectance and Faraday rotation of nanostructure with hole size 250 nm and period increasing from 350 nm to 450 nm. A new peak corresponding to magneto-optics enhancement in the visible region is found, and the new peak shows obvious redshift with the period increasing. Moreover, the peak position is sensitive to the refractive index of the medium filling, which can be applied in MO sensors fields. By using structure search method, we constructed a bilayer nanostructure consisting of a ferromagnetic dielectric film and a noble-metal film perforated with circle annular arrays (CAA) (outer diameter of 150 nm, the inner diameter of 75 nm). The nanostructure has a larger transmission and magneto-optical enhancement. We compared it with the nanostructure with circular hole which radius is also 150 nm, and found that, although the area ratio of CAA is less than a circle hole, but the transmission of CAA is larger. Take the 450 nm period as an example, the area ration of circle is 35%, the transmission is 53%, the enhancement coefficient (G) is 1.5; the area ration of CAA is 26%, the transmission is 65%, the enhancement coefficient (G) reach to 2.48, almost twice the hole. In addition, the large Faraday and Kerr rotation angle are also found at the position of enhanced transmission. The transmission reach to 63%, corresponding to the Faraday angle is 0.74 degrees in the vicinity of 892 nm position. The Faraday angle of flat film with the same thickness, is only 0.03 degrees. Therefore, the enhance the effect of the CAA nanostructure is very obvious. In addition, these two structures are sensitive to infilling medium, the Faraday angle and transmission peaks appear redshift with the refractive index increasing. Therefore, our calculations have important potential implications for the magneto-optical sensors and magneto-optical isolator design.2. The enhancement of transverse magneto-optical Kerr effect (TMOKE) of one-dimensional array structure is studied. Firstly, one-dimensional periodic array of films is prepared by using interference lithography method. The reflectivity of Ag/Co/Ag film has an obvious dip than Ag/Co film which indicate the Ag can induce the SPP excitation easily. At the same time, the two systems both induce the obvious TMOKE peak which produce redshift with the incident angle increasing. the metal-insulator-metal (MIM) is constructed by adding the dielectric layer. New peak caused by the waveguide plasmon resonance is found from the reflectance and TMOKE spectrum. Thus, the optical and magneto-optical information is more rich, which is feasible for realizing magneto-optical multiple channel modulation. Theoretical analysis shows that, magnetic field changes the non-diagonal elements of dielectric constant which changes the SPP wave vector. This change alters the intensity of the reflected light and produces the enhanced TMOKE.3. Study on the optical properties and magneto-optical effect of the two-dimensional periodic array. The reflectance of the chessboard structure and dot array structure is measured. For the chessboard structure, the SPP characteristics of the Ag/Co/Ag film is obvious than Ag/Co/SiO2 film, and the corresponding TMOKE is also obvious in a wide range of incident angles and azimuth, which has more regulation parameters and broaden the measurement range. For the Ag/Co/Si02 systems, although the TMOKE is not observed, but the polar-MOKE angle is larger than pure Co flat film. The Co film has larger optical loss and its SPP character is not obvious, thus the LSPR resulting from the particle leads to the large Polar Kerr rotation. For the Ag/Co/Ag film with dot array structure, the dip of reflectance is not obvious, thus the TMOKE is very weak. However, the polar Kerr rotations is very great. In this case, the LSPR plays an important role.