Fig. 4: High quality factor in large polystyrene spheres.

a Two laser wavelength series of EELS spectra, taken at the same electron probe position over an increasingly small wavelength range with step sizes of 50 pm and 2 pm, respectively. b EEGS spectrum constructed from the rightmost EELS series in (a), along with Lorentzian fittings revealing three resonances (1–3) of quality factors 7430, 10541, and 9178, respectively. The uncertainties for the measured quality factor are <1 for the three resonances.