Recovering of the apertureless scanning near-field optical microscopy signal through a lock-in detection

Abstract : To enhance the signal-to-noise ratio and to remove the spatially slowly varying signals, a lock-in amplifier is often used in apertureless scanning near-field optical microscopy (ASNOM). The detected signals are therefore the Fourier harmonics of the signal along a vertical vibration of the probe and depend drastically on the vibration amplitude. All the Fourier harmonics contain contributions of the ‘near field’ and the ‘far field’ that are mixed in the near-field zone (i.e. at very short distance). Therefore, physical interpretation of contrast in high-harmonics records may be questionable. A more accurate approach consists in using all the harmonics of lock-in demodulation to reconstruct the ASNOM signal above nanoparticles. We show that the reconstructed and the detected signals can strongly differ and that the reconstructed signal must be computed from the lock-in harmonics and the dc term in order to give a physical interpretation of the phenomenon.