A Highly Efficient On-chip 3D Plasmonic Nanofocusing Structure
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A Highly Efficient On-chip 3D Plasmonic Nanofocusing Structure
Myung-Ki Kim1 and Hyuck Choo1 1 The Moore Laboratory, Electrical Engineering, California Institute of Technology, Pasadena, CA 91125, U.S.A.
ABSTRACT We demonstrate and analyze a highly efficient on-chip 3D metal-insulator-metal (MIM) nanofocusing structure. Here, we show the in-depth theoretical design, analysis and discussion to provide a detailed picture of the highly efficient, on-chip nanofocusing process which is linearly tapered in 3D.
INTRODUCTION Extreme field enhancement using an efficient nanofocusing process is essential for maximizing the performance of nanoscale optical/photonic devices, especially on-chip communication nano-devices [1-3]. However, due to the diffraction limits of the conventional dielectric devices, achieving extreme light-concentrations in the deep sub-wavelength spatial scale has been challenging. The utilization of surface plasmon polaritons (SPPs) has been proposed as a feasible mechanism for overcoming the diffraction limits of conventional focusing devices. However, due to the challenges involved in various loss mechanisms and on-chip nanofabrication, most studies have been limited to the 2D-nanofocusing processes on a chip [47]. To the further increase the field enhancement and localization, the 3D nanofocusing process is necessarily required. In recent, we have successfully demonstrated, experimentally, the highly efficient on-chip 3D metal-insulator-metal (MIM) nanofocusing structures [8]. We realized those structures on a chip by employing electron-beam-induced deposition (EBID)/ focused-ion beam (FIB) and demonstrated highly localized light confinement using the two-photon photoluminescence techniques. From the experimental measurements, we have observed the intensity enhancement of 400 within a 14-by-80 nm2 cross-sectional area from a 200-by-500-nm2 area and the coupling efficiency of −1.3 dB (74% transmittance). In this work, we combine our previous fabrication/experimental work with in-depth theoretical design, analysis, and discussion to provide a detailed picture of the highly efficient, on-chip nanofocusing process which is linearly tapered in 3D. We highlight three major distinctive advantages of our MIM gap-plasmon nanofocusing device which we have theoretically understood and experimentally observed. (1) Hotspot size: Because we utilize the fundamental MIM anti-symmetric mode with no theoretical cutoff, it is possible to reduce the field profile down to the size of the MIM cross-section even in the deep sub-wavelength spaces. (2) Loss optimization: The precisely engineered, linear tapering geometry of our device effectively overcomes the major loss mechanisms present in the large wavevector region where the most power loss occurs during the nanofocusing process. (3) On-chip nanofabrication: Our
process involves the use of EBID/FIB and standard IC processes that are highly controllable and compatible with other on-chip device fabrication techniques. THEORY Fundamental Metal-Insulator-Metal (MIM) Wav
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