## 1 Introduction  
Nanophotonic structures can increase the performance of plasmonic sensors by controlling light propagation and interaction with the environment at subwavelength resolution [1; 2]. More precisely, they can also promote the excitation of surface plasmon resonances (SPR), hybrid Fano responses, guiding modes, and grating coupling. These phenomena generate a wide variety of spectral responses that improve the sensitivity, \(S_{B}\), and figure of merit (FOM) of the sensors [3; 4; 5]. The spectral line shape and the spatial and spectral location of the resonances depend on: (i) the dimensions of the nanostructure as well as, (ii) the optical properties of the chosen materials. Therefore, depending on the application, we can adjust the geometry and material choice to optimize sensitivity and FOM. The design can be tailored to work as filters, perfect absorbers, or perfect mirrors [6; 7; 8; 9; 10; 11].  
Plasmonic resonances are widely employed to enhance the performance of refractometric sensors; they are usually revealed by monitoring the spectral response (spectral interrogation), or by modifying the angle of incidence (angular interrogation) [12; 13]. Kretschmann and Otto configurations--well-known techniques--require reflectance to be measured angularly under oblique incidence conditions [14; 15; 16]. These configurations employ a dielectric prism to direct the radiation towards a metallic/dielectric interface to generate SPR, assuming that the wave vector matching condition is fulfilled [17; 18]. A plasmonic sensor incorporating nanophotonic structures provides tuneability for applications in biosensing, gas and liquids refractometers, and so forth [19; 20; 21]. It is also possible to considers normal incidence excitation through the substrate, while maintaining the sensor's performance competitive with some other previous strategies [3; 4; 12]. Furthermore, this could allow the integration of the sensor at the tip of an optical fiber. However, this convenient set-up still needs of spectral measurement techniques.  
In this contribution, we model and simulate an alternative measurement technique that exploits the bolometer effect, which is triggered by the absorption of the electromagnetic radiation [22; 23]. This way, we avoid costly spectral measurement systems because the output signal is electrical by definition [24]. In fact, the bolometric effect is responsible for the variation in resistivity caused by a change in temperature. This mechanism has been widely used in antenna-coupled detectors in the infrared [22; 23; 25; 26]. In a nutshell, our device works under normal incidence conditions and a spectrometer is unnecessary because we measure an electrical signal directly related with the sensed property. A monochromatic laser source eases integration with off-the-shelf driving and signal acquisition electronics, both for illumination and signal retrieval. Our sensor is based on a dielectric grating that couples radiations into a dielectric/metal interface where the SPR are excited. The shape of the resonance is described best as a hybrid-Fano resonance between the pure SPR and a wider response generated by the grating [4]. The resonance strongly depends on the index of refraction of the analyte, which is in contact with the metal layer. At the resonant wavelength, the metal absorbs most of the incident radiation and works almost as a perfect absorber. This absorption increases the temperature in the metal layer, that consequently changes its electric resistance through the bolometric effect. This mechanism is parameterized by the Temperature Coefficient of Resistance (TCR) of the material. This optically induced variation in resistance can be measured as a voltage change by an external circuit biased with a voltage or current source (see Figure 1a).  
To simulate the device we have used a multiphysics package, Comsol Multiphysics, that evaluates the interaction between electromagnetic and thermal domains.