Experimental research on ultracompact silicon hybrid plasmonic nanofocusing device

Abstract

Silicon-based optoelectronics, using the mature silicon-based microelectronic complementary metal oxide semiconductor (CMOS) manufacturing process, is a large-scale optoelectronic integration platform that has attracted much attention. Surface plasmonic devices have also received extensive attention in the past decades, and especially the silicon-based surface plasmonic nanofocusing devices have become a research hotspot. Typical nanofocusing structures include chirped surface gratings, plasmonic Fresnel zone plate, nano-slit array, tapered metal tips. However, there occur some inevitable problems in these devices, such as the fine structure being too complex to be fabricated and too large transmission loss of metal slot waveguide. In this work, an ultra-compact hybrid surface plasmon nanofocusing device is designed and fabricated by the silicon-based optoelectronic technology, and the nanofocusing performance of the device is also experimentally verified. The hybrid surface plasmon nanofocusing devices are fabricated on a silicon-on-insulator (SOI) wafer by electron beam lithography (EBL) system. The silicon wire waveguides, tapers and the thin silicon strips in the middle of nanofocusing regions are patterned in only one step EBL. The gold layer is formed by a deposition and lift-off process, and then a partially etching process is introduced to make the thickness of the middle thin silicon strips the same as that of the gold layer. With a 1.23-μm-long tapered structure, our nanofocusing devices focus the light field of a silicon strip waveguide into a hybrid surface plasmon waveguide, making non-resonant optical field increase 20 times in the 1550 nm near-infrared band experimentally. The entire insertion loss is about 4.6 dB, and the mode area of the nanofocusing area is about <inline-formula><tex-math id="M1">\begin{document}${\left( {\lambda /n} \right)^2}/640$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20212340_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20212340_M1.png"/></alternatives></inline-formula> which is over 300 times smaller than that of the input silicon waveguide. When the middle slot silicon waveguide width <i>W</i>_Si = 120 nm, the insertion loss reaches a minimum value of 2.8 dB. In our design, we adopt the design of silicon-based hybrid plasmonic waveguides. In this design, a layer of material with low refractive index is inserted between the metal layer and the silicon layer to act as a “container” of light field, which makes this silicon-based hybrid plasmonic waveguides have less loss than the traditional metal plasmonic waveguides, and can still maintain high optical field localization. Such silicon-based hybrid surface plasmon nanofocusing devices with simple structures and excellent performances are promising alternatives for future applications in optical field manipulation, optical sensing, nonlinear optical devices, and optical phase-change storage.