(a) Experimental field intensity (black dots) of the HPP mode compared with different tip-sample separations, t, from its exiting point. The optical mode height of FWHMz=53 nm is deep sub-wavelength for λ=633 nm, HPP strip dimensions H=230 nm, W=174 nm. Note, the NSOM operates in tapping-mode having an average separation of t=10 nm (black line), and the simulated mode profile (t=2 and 5 nm, red dotted and black dashed line, respectively) at this distance agrees well with the NSOM result. (b) Summary of experimental HPP mode dimensions yielding the smallest measured mode area of 53×63 nm2, at λ=633 nm. The good agreement between experimental data (mode width and height, black squares and red triangles, respectively) and theoretical simulations (dashed lines, colours, respectively) confirms that the optical HPP mode is indeed squeezed into the low dielectric constant gap: the mode height is independent on the strip width (triangles), whereas the mode width is scaling with the strip width (squares). (c, d) Line scans of the mode height and width for wavelengths of the illumination beam of 633, 808 and 1,427 nm featuring broadband, deep sub-wavelength operation of HPP-based devices. The FWHM (solid line) are Gaussian fits to binned data, yielding measured mode areas of λ2/120, λ2/59 and λ2/157, which are deep sub-wavelength modes for all three wavelengths. As the strip height, H, is optimized for 633 nm, a slightly larger mode height for longer excitation wavelengths is expected.