The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip–sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 μm as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ∼70 nm full width at half-maximum.
© 2000 Optical Society of America
June-Hyoung Park, Myong R. Kim, and Wonho Jhe, "Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals," Opt. Lett. 25, 628-630 (2000)