## 1 Introduction  
Electromagnetic modes arising from the coupling between photons and free-electron oscillations at a conducting surface, also known as surface plasmons-polaritons, introduced a revolution in the field of optical sensing. The engineering of surface plasmon resonance (SPR)-based devices has increased dramatically the levels of resolution and sensitivity typically provided by conventional optical sensors [1]. A wide variety of configurations have been proposed to reach the SPR condition: optical fibers with cladding partially removed for the deposition of a conducting layer as tapered fibers, grating-based fibers, and D-shaped fibers [2, 3, 4]. Notably, photonic crystal fibers (PCFs) are frequently employed to excite SPR modes due to their unique features, such as fine control of the evanescent field penetration in the conducting medium and high mode confinement within a large mode area [5, 6, 7].  
The characteristics of an SPR sensor rely on the dependence of the optical fields associated with the plasmon mode on the geometry and nature of the conducting layer as well as on the refractive index of the surrounding media (also known as analyte). Small variations in the refractive index of the analyte change significantly the mode phase and losses of the surface plasmon mode. This dependence can be tailored with the application of optical metamaterials, artificial structured media designed to achieve a set of properties not typically found in nature. The lattice of such materials has dimensions many orders of magnitude smaller than the optical wavelength, and thus the light propagation through the entire medium can be described by effective constitutive parameters. For instance, field enhancement can be favored in a large extent when metamaterials with metallic inclusions are used in the construction of a compacted optical sensor [8; 9; 10], thus providing an alternative to plasmonic sensors composed of metallic films [4; 5; 6; 7; 8; 9; 10; 11] or individual metallic nanostructures on optical fibers, the latter typically harder to simulate and fabricate [12].  
This paper reports the design of an SPR sensor based on a D-shaped PCF with a metamaterial film for detection of the refractive index. The metamaterial layer is composed of an array of silver nanorods embedded in an alumina matrix. This type of metamaterial, generally called wire metamaterial, resembles a crystalline lattice with the optical axis along the axial direction of the nanorods [10]. The bulk optical modes result from the coupling between surface plasmons locally excited at a unitary cell when the electric field is polarized along the optical axis. The sensitivity is taken from the spectral shifts of the resonance absorption peaks upon changes in the refractive index of the analyte. We address the dependence of the resonance wavelengths on the amount of silver nanorods embedded in alumina, showing that the SPR spectral channels can be tuned by the filling ratio of the metamaterial. As a consequence, we demonstrate that a linear relationship between the SPR spectral response and the volume ratio of silver in the metamaterial can be accurately regarded to characterize effective optical properties of the metamaterial. In the following sections, we detail the modeling of the SPR sensor based on a D-shaped PCF and discuss the application of the tunable spectral sensing response.