Magneto-optic Effects In Subwavelength Microstructures

Subwavelength microstructures, including photonic crystals, surface plasmonstructures, negative-refraction materials etc., are periodic or nonperiodic structures with thecharacteristic length scale comparable to or smaller than the working wavelength. Thesenew materials are especially important for applications due to their novel properties ofreflectivity, transmittivity, polarization and spectrum features, which are totally differentfrom those of conventional optical devices. The studies of subwavelength microstructureshave sparked renewed interest on a series of well-known optical phenomena, such asmagneto-optic(MO) effects. Traditionally, MO effects were studied in homogeneous media.But now with the rapid development in nanotechnologies, researchers found some specialMO effects in subwavelengh microstructures, and the new trend of combiningnanophotonics and nanomagnetics is emerging. This not only offers deep insight into theunderlying physics of the interaction between light and magnetism, but also paves newways to applications on nanophotonic devices and integrated optical circuits.In the early stage, the main subject was how to increase traditional MO effects withsubwavelength microstructures. Then it was found that many novel features and functionsby combining MO materials and subwavelength microstructures. Particularly, byintroducing MO materials into these structures one can achieve additional tenability andnon-reciprocality, greatly extending possible applications.Here we propose two new types of subwavelength microstructures based on MOmaterials. Magnetic manipulation is realized on the defect modes and the waveguidemodes in photonic crystals (PC). PCs are the most common type of subwavelengthmicrostructures with photonic bandgaps similar to those in semiconductors. Light orelectromagnetic waves with frequency within the gaps is prohibited to propagate in PCs.But the applications of PCs are not restricted to these Bragg gaps. On one hand, byintroducing proper defects into the periodic structures, one can obtain the defect modes atcertain frequencies, which will affect the optical properties of PCs. On the other hand, if the components of PCs are special materials such as semiconductors with surface plasmonresonance, the resulting band structures may contain special bands and gaps totallydifferent from Brag gaps. In our work, we focus on the two types of subwavelengthmicrostructures, trying to find effective methods to magnetically manipulated their opticalproperties by introducing MO materials.First, we find the transmittivity and absorptivity of the defect mode can bemagnetically manipulated by adding MO semiconductors to an one-dimensional PC withan impurity layer. The optical properties of this structure are calculated with thetransformation method for different parameters. The results show that, by choosing properstructural and material parameters, the magnetic tunability of this structure can rise up to80%, without changing the positions of the absorption peak or the transmission peak. Therole of applied magnetic fields is similar to an adjusting switch for a certain frequency.Secondly, we propose a new design of multi-channel nonreciprocal waveguides basedon two-dimensional PCs composed of typical gyromagnetic materials YIG. The one-wayguide modes result from the combination of magnetic surface plasmon resonance (MSPR)and the broken time-reversal symmetry (TRS), and all the output ports can be open in thisstructure. The optical characteristics are calculated by using the finite element method.There are two aspects of magnetic manipulation: one is the dramatic suppression of thebackward mode in each channel of the waveguide due to the broken TRS. The incidentwave can only propagate along the permitted direction determined by the magneticconfiguration. Even the existence of obstacles will not cause backscattering, which greatlylowers the energy lost during propagation. The other aspect is that the working frequencywill shift with applied magnetic field since the underlying mechanism is MSPR of YIGrods. The combination of the non-reciprocality and tunability contribute to the obviousadvantage of these structures in applications.