Phonon resonant spectroscopic recognition of 4 nm silicon nitride particles by infrared near field microscopy

Silicon nitride polar dielectric nanoparticles, grown by high-temperature high vacuum reaction of Si(100) with a nitrogen plasma, are imaged using optical near-field microscopy in the infrared. Phonon resonant particles as small as 4 nm in size are detected, indicating a spatial resolution of $\sim \quad \lambda $/700, where $\lambda $ is the wavelength of light. Locally excited silicon nitride phonon polariton resonances occur around 934 cm$^{-1}$ and increase in intensity with the size of the nanoparticles. Experimental results are in very good agreement with dielectric function model calculations for thin films in the extended dipole approximation.