Direct mapping of surface plasmon polaritons (SPPs) by rare-earth-doped (RE-doped) upconversion nanoparticles (UCNPs). (a) SEM image of Au patterns (75 nm in thickness) on SiO2/Si substrate (SiO2 thickness: 300 nm). (b) Collaged real-color PL image showing interference fringes supported on Au-UCNP patterns with various morphologies (squares, rectangles, triangles, disks, and rings), sizes, and orientations. (c–f) PL images showing interference fringes supported on (c) Au triangle, (d) square, (e) disk, and (f) ring. (g–j) PL images acquired from (g) blue, (h) green, (i) red, and (j) merged channels showing interference fringes probed by UCNPs with different emission bands. The incident light for panel (b–j) is a TM mode 980 nm laser with an incident angle of 60°. Scale bar: (a, b) 200 μm and (c–j) 25 μm.