We experimentally demonstrate critical coupling of whispering gallery mode (WGM) disk resonators implemented on a Bloch surface wave platform using scanning near-field optical microscopy. The studied structure is a 60 nm thick TiO 2 TiO2 WGM disk cavity (radius of 100 μm) operating within the C-band telecommunication wavelength. An extinction ratio of 26 dB and a quality factor of 2200 are measured. Such a high extinction ratio verifies the critical coupling of the WGM resonator. This result paves the way to planar optical signal processing devices based on the proposed geometry, for which a critical coupling condition is a guarantee of optimum performance.