Near-field second harmonic generation by using uncoated silicon tips

The authors present a concept of a scanning near-field optical microscope for second harmonic imaging. The microscope is based on uncoated silicon atomic force microscope tips, which provide high intensity transmission in the midinfrared spectral range. An approximately three times larger contrast was found for the second harmonic compared to the linear signal. By using a lithographically designed sample of an array of gold nanoparticles the capability of imaging structures much smaller than the fundamental and second harmonic wavelength is demonstrated.