Surface plasmon polariton resonant sensors [1] have found wide-ranging applications, particularly for nanoscale biosensing [2], where they allow label-free optical detection of binding events between molecules (e.g.,  antibodies and antigens [3]), protein interactions [4] or exosomes [5]. The original prism-based geometries such as the Kretschmann [6] and Otto [7] configurations are versatile and precise, but rely on free-space optics and are thus relatively bulky. Surface plasmons lend themselves to extreme confinement of light, which could be implemented in very small footprint devices, motivating extensive research investigating strategies for integrating surface plasmon sensors with chip-based [8,9] and fiber-based [10,11] circuitry.