## 3 Results and Discussion
### Variations of PNA Signals with Bias
Without lifting up the probe, but tuning the interferometer, the protocol of capture of interferometric was repeated (bottom black and red curve in Figure S3a, t\({}_{0}\) + 829 s) followed by time profiles with step bias of \(-\)1 V, \(-\)2 V and \(-\)3 V and f\({}_{\rm{OP}}>\) f\({}_{0}\) (black/pink, grey/red and pale grey/pale red amplitude/phase profiles in Figure S3c). In Figure S3b,c, at \(-\)1 V, negligible echo of the step bias could be observed in the amplitude time profiles, but only in the phase ones. This suggests that several mechanisms can be probed upon bias variations, being more sensitively detected by the amplitude or the phase signals. In Figure S3c, however, at \(-\)3 V, an artefact occurring at \(-\)20 s in the time profiles is simultaneously detected by both amplitude and phase signals, also indicating possible crosstalk between the signals. In Figure S3b,c, profiles differ from f\({}_{\rm{OP}}\) and their relative value compared to f\({}_{0}\) and the fringe amplitude of the interferometric profiles. Very high fringe amplitude (Figure S3c) is likely to exhibit high variations upon small perturbations like that of DC bias. Both phase and amplitude signals are then expected to significantly vary in the very narrow spectral range with f\({}_{\rm{OP}}\approx\) f\({}_{0}\), thus leading to as observed cross-talking between the signals. Conversely, lower fringe amplitude even f\({}_{\rm{OP}}\approx\) f\({}_{0}\) will limit the impact of its variations with bias to the only PNA phase, thus allowing other mechanisms to be detected upon bias in PNA amplitude. Interestingly, consistent amplitude variations between Figure 2b and Figure S2b and other profiles (not shown) can be explained by a frequency shift towards lower frequency upon negative DC tip bias. Meanwhile, consistency between Figure 2b, Figures S2b and S3b could also be observed between phase signals, considering variations towards a \(-\)\(\pi\) phase shift at f\({}_{0}\). The results reported by [PERSON] et al. on benzothiophene derivative exposed to 9 GHz in a dielectric resonator also discriminated between a frequency shift and Q-factor variations of the S\({}_{11}\) upon bias at the resonance frequency of the cavity, and attributing them to variations of the complex permittivity of the material under test, i.e., the dielectric constant and the loss (thus the conductivity), respectively [13]. Here, the variations of the 180\({}^{\circ}\) phase shift at f\({}_{0}\) upon bias can be correlated to a more pronounced imbalance between the two branches of the interferometer initially set to compensate the pathway probing with the reference one. In general, the effect of electrical perturbations on interferences and resonances cannot be so easily  
Figure 3: Interferometric PNA amplitude (**a**) and phase (**b**) profiles highlighting their respective sensitivity to negative DC tip bias in operating conditions f\({}_{\rm{OP}}<\) f\({}_{0}\). The bias induces in (**a**) a displacement of the interferometric fringe towards lower frequencies and, in this case, a deepening of the interferometric amplitude fringe, while in (**b**) a steepening of the phase profiles is observed. Hence, upon bias a decrease in the amplitude and an increase in the phase signals are observed at f\({}_{\rm{OP}}\), respectively. In insets, the different amplitude and phase spectra across a wider range including f\({}_{0}\). For each profile at t \(>\) t\({}_{0}\) the frequency shift (\(\sim\)\(\pi\)7 Hz s\({}^{-1}\)) was artificially compensated to allow better observation of the variations of both PNA signals with negative DC tip bias. The highlighted spectral range of the profiles corresponds to the green area in their respective insets. correlated. Resonances result from multiple interferences. They can be modeled by an equivalent RLC circuit. Therefore, their frequency shift can be related to a variation in capacitance or inductance and the Q-factor is related to the resistance or more generally to energy loss phenomena.