Efficient Sub-wavelength Plasmonic Superconducting Photodetectors in Ultra-Thin Planar Waveguides and in Optical Fibers

Abstract

By using a finite-element method we investigate the efficiency of sub-wavelength plasmonic superconducting photodetectors in ultra-thin planar waveguides (the incident wave is P-polarized) and in optical fibers (a traveling wave). The unit cell of a periodic sub-wavelength planar waveguide is made by a gold strip that is encapsulated in an YBCO layer and further enclosed above by an air layer and beyond by a thick gold plate substrate. For a planar structure with a single unit cell, the power absorption in YBCO layer is very large (between 0.929 and 0.975) for a fixed wavelength λ=1.55 μm when the distance between the strips and gold substrate is varied between 17 nm and 32 nm. When the wavelength is varied between 1.1 μm and 1.9 μm, the power absorption in YBCO layer is between 0.944 and 0.965 with a maximum of 0.976 at λ=1.40 μm. The superconducting traveling wave photodetector is made by an optical fiber, which includes a gold core surrounded by a number of smaller gold cylinders at a small distance from the central gold region and that are encapsulated in an active YBCO layer and further enclosed by an air layer. In our structure of the fiber with 18 smaller gold cylinders, the imaginary parts of the effective index of plasmon modes (two nondegenerate modes and eight twofold degenerate modes) and the power absorption (between 0.904 and 0.976) in active YBCO layer are very large. The first nine modes are highly confined in the YBCO layer in the space between the neighboring gold cylinders and the other nine modes are highly confined in the YBCO layer but in the space between the small gold cylinders and the gold core. Although in our structure, the thickness of the YBCO layer is 10 times smaller in comparison with that of a recently published model with 32 small gold cylinders, the losses in active YBCO layer are also very large.