Giant Infrared Sensitivity of Surface Plasmon Resonance-Based Refractive Index Sensor Akhilesh Kumar Mishra 1 & Satyendra Kumar Mishra 2 & Amar Pal Singh 3 Received: 2 January 2017 /Accepted: 17 May 2017 # Springer Science+Business Media New York 2017 Abstract Surface plasmons (SPs), the coherent charge densi- ty oscillations of the electrons bound to the metal-dielectric interface, are dominating the research field of optics. One of the ubiquitous applications of SPs is in sensing. In the present work, we have theoretically studied a couple of surface plas- mon resonance (SPR)-based fiber-coupled ultra-sensitive re- fractive index sensors working in the infrared (IR) region. Either of the copper (Cu) and aluminum (Al) is used as surface plasmon exciting layers in these sensing probes. On the top of the metal layer, field-enhancing graphene and silicon layers are considered. The probes are characterized in terms of sen- sitivity and detection accuracy (DA). The sensitivities of Cu- and Al-based optimized probes are obtained respectively to be 23.50 and 24 μm/refractive index unit (RIU). To ensure the probes’ compatibility with bio-samples, an extra bio- recognition layer of graphene has been considered over the silicon layer which resulted into the respective sensitivities of 20 and 19.50 μm/RIU for Cu- and Al-based probes with ap- preciably good DAs. Keywords Surface plasmon . Sensitivity . Sensor . Graphene . Silicon . Fiber Introduction The signatory characteristic of smaller wavelength of the SPs has brought forward widespread applications encompassing imaging [1, 2], sensing [3], nano-antenna [4], metamaterials [5–7], topological insulators [8], and recently ultra-fast pro- cessing [9, 10]. Direct light lacks momentum to excite these coherent charge density oscillations and therefore extra geo- metrical arrangements such as high refractive index prism, optical fiber/waveguide, grating, and rough surfaces are re- quired to fetch the direct light the requisite momentum [3]. Conventionally, noble metals are used as SP exciting metallic layer. But these metals come with their own limitations such as thin film of gold (Au) agglomerates and forms island and therefore lacks uniformity; silver (Ag) and copper (Cu), on the other hand, are chemically unstable. To circumvent these is- sues of noble metals, conducting metal oxides (CMOs) are usually resorted on [11, 12]. These oxides not only show good conductivity as those of noble metals but can also form very thin uniform film [13]. An alternative route of taking advan- tage of remarkable properties of noble metals (Ag, Cu, etc.) bypassing their chemical unstableness is to cover the metal layers with thin protecting layer of some other almost non- permeable high permittivity materials such as graphene and silicon [14]. These covering materials not only protect the underlying metal from oxidation but also enhance the field within and therefore the device performance [13, 15, 16]. One of the ubiquitous applications of SPs is in refractive index sensing [17–20]. SPs excite at the interface of metal and dielectric and its field decay evanescently in transverse direc- tions. The evanescently decaying tail of SPs in dielectric me- dium is very sensitive to the complex permittivity of the di- electric material, and therefore, the phenomenon has found enormous applications in the field of refractive index sensing. To realize SP-based sensors, two configurations have been * Akhilesh Kumar Mishra iitd.akhilesh@gmail.com 1 Andrew and Erna Viterbi Faculty of Electrical Engineering, Technion—Israel Institute of Technology, 32000 Haifa, Israel 2 Electronic Engineering Department, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong 3 Faculty of Biology, Technion—Israel Institute of Technology, 32000 Haifa, Israel Plasmonics DOI 10.1007/s11468-017-0619-9