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Plasmonics and dense optical integration Surface plasmon polaritons (SPPs) are quasiparticles comprised of collective electron oscillations coupled to electromagnetic waves at the interface between two materials with positive and negative permittivity; typically, a dielectric and a metal. The use of a plasmonic waveguide allows for stronger optical confinement and denser integration of photonic integrated circuits (PICs). The integration of different optical components on one substrate has many advantages. Similar to electronic devices, an integrated optical circuit will have a lower cost and better functionality. In the case of electronic circuits, the small size of semiconductor transistors means that millions of transistors can be integrated into one electrical circuit. Integrated electronic devices have complex functionalities and low cost because of this dense integration of components. However, the size of optical components is a problem for dense integration of optical circuits. The length of an optical component is limited by the wavelength of light, the relatively long length of interaction between light and matter (e.g., the magnetooptical [MO] or electro-optical interaction), and the large minimal bending radius of an optical waveguide. Because of these limitations, only a few optical components can be integrated into one chip. It is possible to substantially reduce the size of optical components when an optical waveguide with strong optical

confinement, such as a Si nanowire waveguide or a plasmonic waveguide, is used. The optical confinement in a Si nanowire waveguide is strong, due to a high-refractive-index contrast between Si and SiO2. Because of the strong optical confinement, a Si nanowire waveguide is narrow, with a typical width of 450 nm, and it can be sharply bent with a radius as small as 1 μm. Even though the length of a Si nanowire device is relatively long, the optical device can be packed into a small area of a few square micrometers by the bending of the waveguides. Optical confinement in a plasmonic waveguide is even stronger than in a Si nanowire waveguide, because a plasmon is confined by a metal–dielectric interface, and the refractive index step between a metal and dielectric is large. Therefore, the size of optical components made of a plasmonic waveguide is even smaller than the size of optical components made of a Si nanowire waveguide.

Plasmonic isolator The optical isolator is an important element of optical networks.1 It protects optical elements from unwanted back reflection. The integration of optical elements into PICs is important—it reduces cost and improves performance of high-speed optical data-processing circuits for high-speed optical networks. In achieving denser integration, the optical isolator is an indispensable component, because the problem of unwanted back reflection increases as integration becomes denser.

Hiromasa Shimizu, Department of Electrical and Electronic Engineering, Tokyo University of Agriculture and Technology, Japan; [email protected] Vadym