Sketch of the experimental setup.
Plot of the background signals [(a) and (c)], (b), and (d) vs the sample position.
(Color online) Dependence of the BKG modulation amplitudes [(a)–(c) respectively] on the tuning-fork current (bottom axes) and on the tip oscillation amplitude (top axes). The linear behavior of is pointed out by a linear fit [(b), red line)]. A power fit (, ) of the second harmonic BKG amplitude [(c), red line)] evidences the quadratic dependence on .
(Color online) Plot of the -harmonic-to-dc-BKG ratio as a function of the tip oscillation amplitude for first (a) and second harmonic (b) demodulations. The red lines reproduce the linear (a) and quadratic (b) fits of the experimental data.
(Color online) (black symbols) Plot of the dc background observed when removing the tip from the optical path as a function of the sample position. (red line) Sinusoidal fit of the experimental data [, ].
(Color online) Plot of the background modulation amplitudes at the various harmonics [dc (black), first (red), second (blue), and third (green)] as a function of the ratio (bottom axis) and of assuming (top axis).
(Color online) Values of the tip oscillation amplitude corresponding to the first zeros of the Bessel functions for various wavelengths .
(Color online) Approach curves predicted for the dc background (a) and the modulus of the first three harmonics [(b)–(d), respectively] base on Eqs. (5)–(7), for and . The colored circles and the arrows indicate how the optical signal changes as a function of an increased sample topography.
(Color online) [(b) and (f)] dc, [(c) and (g)] first harmonic, and [(d) and (h)] second harmonic BKG maps simulated for a topography [(a) and (e)] consisting of pillars on a horizontal substrate, having heights of 30 and , respectively.
[(a)–(h)] Line profiles drawn in correspondence of the red lines marked in Fig. 9. The optical profiles reproduce the topography only for the high pillars.
(Color online) [(b) and (f)] dc, [(c) and (g)] first harmonic, and [(d) and (h)] second harmonic BKG maps simulated a topography [(a) and (e)] consisting of pillars on an wedgelike substrate (inclination 10°), having heights of 30 and , respectively.
Theoretical values of the ratios , corresponding to the zeros of the Bessel functions, that yield complete background suppression at each harmonic . The values indicate the precision needed on to have a BKG rejection better than at every harmonic.
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