Photocurrent Enhancement in Si-Ge Photodetectors by Utilizing Surface Plasmons Mehdi Afshari Bavil 1 & Zhi Liu 1 & Wenzhou Wu 1 & Chuanbo Li 1 & Buwen Cheng 1 Received: 8 August 2016 /Accepted: 9 November 2016 # Springer Science+Business Media New York 2016 Abstract A circular slit-groove surface plasmon polaritons (SPPs) launcher surrounding a photodetector is employed the- oretically to enhance the photocurrent of atypical Si-Ge pho- todetectors. The slit and grooves are designed such that the SPPs are focused at the center of the absorption layer of the photodetector to result in additional electric current. Fabry– Perot resonance condition accurately calculates the period of the groove, slit-groove distance, photodetector radius, and slit- photodetector distance. The manipulation leads to construc- tive interference between the incident light impinging from the top and the SPPs propagating toward the photodetector. Simulation result shows that photocurrent increases by ap- proximately 13-fold when the SPPs are introduced. Keywords Surface plasmons . Fabry-Perot resonance . Photodetectors . Silicon Introduction Photodetectors, which convert optical signals into electrical sig- nals, are highly important in optical communication [1]. Detectors with ability of detecting extremely low-intensity light signals, such as P-N junction detectors [2–4], metal–semiconduc- tor–metal detectors [5, 6], avalanche photodetectors [7–9], and separate absorption, charge, and multiplication structures [10–12], have been developed. Si has been recognized as an excellent photodetector material compared with III–V-based de- vices because of its lower multiplication noise [13, 14]. However, another material should be introduced to adjust the absorption at telecommunication wavelengths. Ge is a feasible candidate be- cause of its compatibility with complementary metal oxide semi- conductor (CMOS) despite the 4% mismatch in the lattice con- stants between Ge and Si [15, 16]. Surface plasmon polaritons (SPPs) are electromagnetic waves that propagate at the interface between a metal and a dielectric layer beyond the diffraction limit [17, 18]. Nanoplasmonics ex- ploits surface plasmons that can control and manipulate optical signals to provide a platform for developing and optimizing var- ious nanophotonic components, especially photodetectors [19–22]. Potential geometries to excite SPPs are direct- coupling configurations, such as sub-wavelength nanohole ar- rays and nanoslits drilled on the metal contact of the light- radiating surface capable of extraordinary transmission [23–25]. However, the high reflectivity of the metallic layer on the radia- tion side and very low light transmission through the holes and slits remarkably limit these geometries, resulting in very low photodetector sensitivity. Side-coupling of SPPs via grooves [26], patches [27], and circular grating [28, 29], which the SPPs can control and unidirectionally propagate, is another ap- proach to efficiently excite SPPs. In this paper, we propose, design, and demonstrate a novel plasmonic-based photodetector with operating wavelength of 1550 nm to enhance the photocurrent. Our proposed structure has a circular configuration and includes sub-wavelength slit and grooves in a metallic layer surrounding a Si-Ge photode- tector at the center. SPPs can be steered to predominantly propagate toward the center by tailoring the distance between the slit and grooves and the separation between the core and slit. The most important feature of this structure is that the top electrode ring can be as narrow as possible to avoid light reflection. Geometrical parameters are obtained theoretically * Chuanbo Li cbli@semi.ac.cn 1 State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China Plasmonics DOI 10.1007/s11468-016-0437-5