Ppp nchpt

In our experiment, a metal-coated silicon tip (Nanosensors, PPP-NCHPt, $f_0$ = 297.6 kHz, Q = 12,524) is approached to the sample surface. A measured normalized frequency shift ( $\mathrm{\Delta }f/\text{mean}\left(\mathrm{\Delta }f\right)$ ) is shown in Fig. 2C. The tr-AFM signal can be recorded in multiple modes, like any other spectroscopy technique in AFM. The measurement shown in Fig. 2A is recorded with a very slow z feedback to correct for slow drift, but the feedback is not fast enough to compensate for the frequency shift change due to the pulse overlap. For the measurement in Fig. 2C, the AFM tip is held at a constant height above the sample, while the beam delay is swept. The tip is occasionally approached to the surface to correct for any drift in the tip–sample distance. Fig. 2D shows an optical intensity autocorrelation trace recorded simultaneously outside of the UHV chamber (Fig. 1A). The envelopes of both curves are fit to a Gaussian profile, and this yields a pulse width of $106.5\pm 0.2$ fs for the optical intensity autocorrelation. The envelope measured by FM-AFM shows a pulse width of $108.6\pm 3.8$ fs. The observed pulse broadening can be accounted for by the dispersion that arises from the UHV window and the lens used to focus the beam into the UHV chamber. LiNbO₃ as a sample can therefore be used to characterize the pulse shape at the tip apex.