Fig. 4

Propagation length measurement for different thicknesses of additional layer with the aid of MH-SNOM. a MH-SNOM image demonstrating BSW propagation in the near-field. b. Exponential fit of the field amplitude along the direction of propagation to measure the propagation length. c Near-field measurement of propagation lengths with the error margin for BML (0 nm TiO₂), 15 nm of TiO₂, 30 nm of TiO₂ and 60 nm of TiO₂ thicknesses