Fabrication technology of low-propagation-loss plasmonic waveguide containing a ferromagnetic metal

Abstract

The optical isolator is an important element of optical networks. It protects optical elements from an unwanted back reflection. The integration of optical elements into Photonic Integrated Circuits (PIC) is an important task, because it reduces the cost and improves the performance of high speed optical data processing circuits of the optical networks. Figure 1 (a)shows the proposed design of the plasmonic isolator 1, 2. It consists of a Si nanowire waveguide, a part of which (about 2–16 mm) is etched out, and a ferromagnetic metal is deposited in the gap. The ferromagnetic metal is not transparent and the direct light propagation from the input Si waveguide to the output Si waveguide is blocked by the Co. However, a surface plasmon is excited at the Co / TiO 2 / SiO 2 interface. As a result, light reaches the output fiber. The Co is a magneto-optical material. When its magnetization is perpendicular to the light propagation direction and is in the film plane, its optical constants are different for two opposite light propagation directions. The plasmonic waveguide is optimized so that a plasmon is excited only in one direction, but a plasmon can not be excited in the opposite direction. Therefore, light can pass from input to output only in forward direction, but light is blocked in the opposite direction. A device, which is transparent only in one direction, is called the optical isolator. Figure 1(b)shows a top view of a Co / TiO 2 / SiO 2 plasmonic waveguides of different lengths integrated with a Si nanowire waveguides. Figure 1(c)shows the fiber-to-fiber transmission as function of wavelength for different lengths of the Co / TiO 2 / SiO 2 bridge-type plasmonic waveguide integrated with a Si nanowire waveguide. The measured propagation loss is 0.7 dB/mm and the measured coupling loss between plasmonic and Si nanowire waveguides is 4 dB per a facet. A metal is an essential material of a plasmonic waveguide. Any metal significantly absorbs light. Therefore, some optical loss is unavoidable in a plasmonic waveguide. In case if this loss is too large, all light is absorbed in plasmonic waveguide. Even if such plasmonic waveguide might have a unique property, it has no any practical use. Therefore, any practical plasmonic waveguide should have a reasonably-low propagation loss. It is known 3that the propagation loss of the surface plasmons in a structure made of a ferromagnetic metal like Fe, Co or Ni is at least an order of magnitude larger than that of plasmons in structure made of Au, Ag and Cu, which are the conventional metals for the plasmonic devices. We have found that the in-plane and out-plane optical confinement are critically important in order to fabricate a low-optical loss plasmonic waveguide. When both the in-plane and out-plane optical confinement are well optimized, the propagation loss of a plasmon in ferromagnetic metal becomes even smaller (See Fig.1(c)) than the propagation loss in a conventional plasmonic waveguide, which is made of Au or Ag or Cu. The out-of-plane confinement is important because it minimizes the loss due to the absorption of light in the bulk of a metal. Since a metal absorbs light, the less light is inside of the metal and the more light is inside of the dielectric, the smaller propagation loss is. In a double-dielectric or multi-dielectric plasmonic structure, the thickness of one dielectric can be optimized so that the amount of light in the metal becomes smaller and the amount of light in the dielectric becomes larger. Additionally, in such structure a substantial enhancement of MO effect is observed 1, 2, 4, 5. The in-plane confinement is critically important because it minimizes the loss due to the scattering at a side edge of a plasmonic waveguide. Often a metal stripe is used for in-plane confinement of a plasmon (Fig.2 (a)). In this case a surface plasmon propagates just under the metallic stripe. Even though the fabrication of a stripe-type plasmonic waveguide is very simple, the propagation loss of a plasmon in such structure is very high. We have studied 3 types of in-plane confinement for a surface plasmon: groove-type 6, wedge-type and bridge-type (Fig. 2 (b)- (d)). All these types of the in-plane confinement are effective for the reduction of the propagation loss of a plasmon. The reduction is at least in 10 times comparing to the stripe confinement. The reason of the reduction of the propagation loss is that light is removed from the place of the metal edge. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 16H04346) from JSPS.