Contents lists available at ScienceDirect Physica E: Low-dimensional Systems and Nanostructures journal homepage: www.elsevier.com/locate/physe Highly sensitive nano-scale plasmonic biosensor utilizing Fano resonance metasurface in THz range: Numerical study Ali Farmani a,∗ , Ali Mir a , Maryam Bazgir b , Ferdows B. Zarrabi c a School of Electrical and Computer Engineering, Lorestan University, Khoramabad, Iran b Department of Electrical and Computer Engineering, Islamic Azad University, Arak, Iran c Department of Electrical and Computer Engineering, Iran University of Science and Technology, Tehran, Iran ARTICLE INFO Keywords: Plasmonic sensor Fano resonance Terahertz Nano-scale Metasurface ABSTRACT We report a numerical study of the tunable-enhanced sensitivity of a nano-scale plasmonic biosensor in THz range. In the structure, gold Metasurface is utilized to excite of Fano resonance modes that their dispersion properties can be harnessed with different geometrical parameters. Here, the coupling of the incident beam to the surface modes of the structure is used to improve the performance parameters including figure of merit, sensitivity, and footprint. The Fano resonance, which is strongly rely on any change in refractive index of the material, is excited in the structure by changing geometrical parameters. The structure is numerically simulated by the finite difference time domain method. In the optimum design of the proposed sensor, the maximum value of sensitivity is achieved as high as S = 1700 nm/refractive index unit with a large value of figure of merit (FoM = 283.3 1/refractive index unit) and a narrow linewidth of Δλ = 6 nm. Moreover, the structure has a nano-scale footprint of 500 nm × 500 nm × 190 nm. It is also shown that Fano resonance can be controlled through manipulating the external parameters such as incident angle and various bio-materials. Therefore, we expect that this theoretical result leads to remarkable applications in plasmonic integrated circuits, e.g. optical biosensors. 1. Introduction Plasmonic nano-scale devices have received remarkable attention owing to their extraordinary properties in plenty emerging applications such as modern medicine [1], imaging field [2], terahertz device [3], nano-scale sensing device [4–6], nano-scale logic gate [7–10], and nano-scale switching device [11–13]. Amongst the plasmonic devices, sensing systems exhibit superior potential and have various platforms including biosensors [14–17], gas sensors [18–20], DNA sensors [21–25], temperature sensors [26,27], electrochemical sensors [28], and novel nano sensors [29,30]. To design and enhance the perfor- mance of sensors, a narrowband transmission spectrum (i.e, high figure of merit) [31], high sensitivity [32,33], and low footprint [34,35] are significantly desired. Due to the ohmic loss of metals in the optical regime, the waist of transmission spectrum is relatively broad; hence, it is rather challenging to provide a sensor with appropriate properties. To this purpose, advanced phenomena such as the Fano resonance [36,37], electromagnetically-induced-transparency [38–40], and plasmon-in- duced-transparency [41–44] have been introduced. According to these phenomena, the excitation of surface wave can occurs in plasmonic systems considering the interference between the two scattering path- ways. In this regard, a propagated surface waves at the interface of metal-dielectric in plasmonic systems can be either bright mode or dark mode relying on how strong an incident light beam from prism or denser optical medium can be strongly coupled into the surface waves of the structure. The dark mode normally has a significantly large quality factor and a low scattering cross section, which is restricted via the ohmic loss of the metal. While, the bright mode has a larger scat- tering cross section and a lower quality factor because of the radiation coupling. These modes with slightly different resonant frequencies lead to a narrow transparency window with a high figure of merit and high sensitivity could be easily obtained by bright-dark coupling modes between waveguide and resonator [45]. Owing to the wide potential applications in both on-chip and off- chip nanodevices including nanoscale sensor [46–48], Fano and Fano- like resonances has attracted remarkable attention in recent years. Moreover, the sensing devices based on Fano and Fano-like resonances have been proposed in various wavelength ranges including visible [49], infrared [50], terahertz [51], and microwave [52]. Likewise, several configurations theoretically and experimentally have been https://doi.org/10.1016/j.physe.2018.07.039 Received 19 May 2018; Accepted 31 July 2018 ∗ Corresponding author. E-mail addresses: Farmani.a@lu.ac.ir (A. Farmani), Mir.a@lu.ac.ir (A. Mir), maribazgir@yahoo.com (M. Bazgir), ferdows.zarrabi@yahoo.com (F.B. Zarrabi). Physica E: Low-dimensional Systems and Nanostructures 104 (2018) 233–240 Available online 04 August 2018 1386-9477/ © 2018 Elsevier B.V. All rights reserved. T