## 1 Introduction  
The electromagnetic surface waves propagating at the interface between dielectric multilayer platform and external medium are called Bloch Surface Waves (BSWs) [1; 2]. A dielectric multilayer platform, sustaining BSWs, has been studied extensively for many applications, such as sensing and in-plane optics [3; 4; 5]. The modes propagating on the multilayer interface decay exponentially inside the multilayers due to the presence of the photonics band gap [1]. At the same time, modes are not allowed to propagate in the external medium, which is air in this study, because of total internal reflection. This salient propagation mechanism keeps the modes bound close to the multilayer surface. Absorption into the material, surface scattering, and leakage into the multilayer due to prism coupling are the main sources of the decay of the surface mode amplitude along the propagation direction [5; 6; 7]. The present design of the multilayer, whose details are provided in the 'Materials and Methods' section, is based on the optimized design for longer propagation lengths [8]. The BSW platform provides the possibility of ample choice of constituent material, provided that the material is transparent at the operating wavelengths. In addition, the maximum field amplitude of the BSW mode can be tuned at the surface of the dielectric multilayer [9; 10]. This ability to tune the local field confinement is attractive for sensing applications [11; 12]. The aforementioned advantages and features make the BSW platform an interesting candidate for the development of 2D optical systems [13; 14; 15]. To this end, several 2D optical componentsbased on BSWs have been studied theoretically and experimentally. They include waveguides [16; 17], lenses [5; 18], disk resonators [19; 20], BSW reflectors [21], Bessel-like beams [22; 23; 24; 25], subwavelength focusers [26], phase-shifted Bragg gratings [27] and grating couplers [28; 29]. Moreover, tunable planar optical components on BSW platforms have been demonstrated [30; 31]. However, to envision BSW-based 2D integrated optical systems, detectors and light sources are inevitable elements to be studied. In this paper, we focus on in-plane detectors for BSW-based multilayer platforms, whereby the absorption mechanism of the surface waves is an important subject of study. To this end, we exploit the absorption properties of the graphene layer deposited on the top of the multilayer platform. Graphene is a two-dimensional single layer of carbon atoms with sp\({}^{2}\) hybridization forming a hexagonal lattice. Graphene exhibits exceptional electrical and optical properties. Thanks to its zero band gap semiconductor structure providing unique electronic properties, graphene has attracted much attention from researchers. Several high-speed electronic devices have already been proposed, thanks to the ultrafast response of the graphene [32]. Similarly to the electronic properties of graphene, its optical properties show an equal interest due to the strong interaction of graphene with light over a broadband range of wavelengths. The optical conductivity of graphene (from visible to infrared wavelengths) leads to an absorption of 2.3% of light in the case of a monolayer [33]. Indeed, graphene has become an attractive photonic material for a wide range of applications, such as photovoltaic devices, optical modulators and photodetectors, etc. [34]. Another significant advantage of graphene is the possibility of tuning its optical properties by applying external fields (electrical doping) or nonlinear optical effects [35]. With the advantage of tunable optical absorption and ultrafast response, graphene has become a promising candidate for on-chip integrated silicon photonics. For example, graphene-based photodetectors integrated on Si waveguides have been studied by [PERSON] et al. [34]. The spatial confinement and the propagation length of surface waves in the graphene layer has also been studied [36].