Fig. 4. Approach curves recorded with a Au film as sample-under-test with the 180 nm detector. The curves were recorded at different demodulation frequencies: at the fundamental tip oscillation frequency 1Ω and at different harmonics n from 2Ω to 7Ω. The dotted line at 100 nm serves as a guide to the eye to facilitate visual distinction of the signal rise upon approach of null distance.