a) SNOM PL spectra collected in the point of maximum intensity of the mode in each with fixed wall thickness w = 0.5a and varying lattice constants a = 340 nm, 360 nm, 380 nm. b) Summary plot showing resonance wavelength versus length scale a for the three different wall thicknesses. The inset shows a SNOM PL map of the studied mode detected in a structure with a = 360 nm and w = 0.415a.