Apertureless SNOM imaging of higher density samples

Various efforts in optical microscopy have been devoted to overcome the resolution limit imposed by classical light diffraction. Apertureless NSOM (ANSOM) techniques circumvent this limit by placing a sharp atomic force microscope (AFM) tip in the focus of a laser. In our scheme, a green He-Ne laser beam ($\lambda $=543 nm) was introduced into an inverted microscope equipped with a high numerical-aperture objective. A mask was used to produce an axially-polarized evanescent excitation field confined to a near diffraction-limited focus spot. The AFM tip will locally enhance the incident optical intensity. This enhanced field locally excites fluorescence on the sample. High-density quantum dot (QD) samples were dried onto a clean coverslip and then imaged using ANSOM. The fluorescence signal was modulated by a lock-in amplifier to suppress unwanted background from the excitation laser. The technique yielded spatial resolution near 10 nm with an optimized peak-peak oscillation amplitude of $\sim $30 nm. The signal-to-noise ratio (\textit{SNR}) of single QD within high-density ensembles was measured and individual particles were still easily resolved (\textit{SNR} $>$ 5) at a density of 14 QDs/$\mu $m$^{2}$.