# Bloch-Surface-Polariton-Based Hybrid Nanowire Structure for Subwavelength, Low-Loss Waveguiding
## 1 Introduction  
Plasmonic devices possessing the capability to confine light far beyond the fundamental diffraction limit have attracted considerable attention in recent years [1]. By exciting surface plasmon polaritons (SPPs) at the interface between metal and dielectrics, a versatile array of waveguides and nanophotonic devices have been reported and employed as building blocks for high-density photonic integrated circuits [2; 3; 4; 5; 6; 7; 8; 9]. Among the many reported SPP guiding configurations, metal nanowire structures that can support strongly confined SPPs have received ever-increasing attention in recent years [4; 5; 10; 11]. The nanowire plasmonic waveguides can localize the optical mode size far beyond the diffraction limit, especially when the geometric size of the nanowire is in the sub-wavelength order. This advantage has been explored to enable numerous high-performance compact nanophotonic devices. Many intriguing applications in all-optical logic gates [12], nanolasers [13], and resonators [11] have been intensively investigated. However, despite their admirable feature of the tight confinement of guided modes, the propagation loss induced by the ohmic loss in metal is still a major limitation to overcome forlong-range light transmission. To address this issue, a number of modified nanowire plasmonic structures, with special attention paid to promote light transmission distance, have been proposed to improve the guiding performance of the preceding tight mode confinement counterparts in recent years [14; 15; 16; 17]. Representative examples are schemes based on the dielectric mediated guiding of nanowire plasmonic mode, such as single-dielectric-layer-mediated plasmonic waveguides and similar configurations incorporating additional dielectric interlayers [18; 19; 20]. Nonetheless, despite increasing the propagation distance to a certain extent, the guiding mechanism of all these waveguides still abides by the coupling between the dielectric mode and SPP mode, which restricts the effective balance between the propagation loss and mode confinement. On the other hand, a Bloch surface wave (BSW) excited in a truncated periodic multilayer dielectric stack such as a one-dimensional photonic crystal (1D PC) is another kind surface electromagnetic mode [21; 22]. The excited BSW exists in the photonic band gap of the truncated periodic multilayer and propagates along the interface between the multilayer dielectric and the adjacent dielectric medium [23; 24]. The electric field bounded at the interface will be enhanced significantly and confined dramatically, similar to the SPP [25; 26]. In contrast, unlike the SPP, which can only be excited by p-polarization, the BSW can be excited by either p- or s-polarized incident light with a much wider optical wavelength under appropriate designed multilayer structures. In particular, due to the absence of metal, the light transmitting in the all-dielectric structure enables a much lower propagation loss compared to that of SPP, resulting in long-range optical surface polaritons. For a typical 1D PC multilayer structure, the supported BSW exhibits rather low propagation loss with a millimeter-range propagation constant. This feature has been utilized in many applications, including waveguides [27; 28; 29], optical sensing [30; 31; 32; 33; 34; 35; 36; 37], optical switch [38], Raman scattering, and fluorescence enhancement [39; 40; 41; 42; 43]. A drawback associated with such a low propagation loss mode is the relatively weak mode confinement, which results in a very large mode area. Despite significant efforts devoted to modified guiding structures, including dielectric-ridge-loaded 1D PCs with small propagation loss [28], the optical performance of this waveguide is still restricted by the weak mode confinement owing to the distribution of the electric field in the multilayer region in the all-dielectric structure. This represents a great challenge limiting the implementation of this waveguide in high-performance photonic integrated devices.