(a) Normalized cavity mode electric field intensity |E|2 superimposed on a scanning electron microscope image of the centre of one of the cavities. Scale bar represents 200 nm. (b) Experimental central wavelength of 50 circular grating cavities with varying period and central radius, plotted as a function of the simulated central wavelength. When only one peak is observed in the spectrum, black squares are used to denote the peak wavelength. When two peaks are observed, red circles and blue triangles are used. Such two-peak behaviour is also seen in simulations depending on the device parameters, and is due to coupling to a second cavity mode. Top inset: Atomic Force Microscope image of a circular grating cavity and a line cut (along the dashed line) showing the etch depth of the trenches. Bottom inset: examples of photoluminescence spectra of circular grating cavities, measured from a high-QD density region.