## 3 Results and Discussion  
### Measurement Artifacts  
Figure 2a shows the PNA phase (pale red) and amplitude (grey) interferometric profiles recorded prior to bias variations, while Figure 2b shows the variations for a fixed frequency of operation of both the PNA Amplitude and Phase signals upon DC step bias between 0 V and \(-\)2.5 V applied at the tip at a frequency of 100 mHz (10 s period) and in a time frame of 100 s (10 cycles). The interferometric frequency f\({}_{0}\) was initially measured at 10.469247 GHz (the light grey profile at t\({}_{0}\)), while the operating one f\({}_{\mathrm{OP}}\) was set to 10.469249 GHz, i.e., 2 kHz higher (the right side of the interferometric profile). A low drift, estimated in this case to be around 23 Hz s\({}^{-1}\) and inherent to the interferometric detection unit, induced a slow and continuous increase in f\({}_{0}\) with time. The time profiles displayed in Figure 2b were captured sufficiently later to be in the situation f\({}_{0}>\) f\({}_{\mathrm{OP}}\) (between t\({}_{1}\) and t\({}_{1}+\Delta t\), with \(\Delta t=100\) s, and t\({}_{1}-t_{0}=442\) s). Hence the phase values between 170\({}^{\circ}\) and 180\({}^{\circ}\) and both the PNA phase and amplitude signals increasing with time as the drift further shifts the interferometric towards higher frequencies. Figure 2a displays in black and dark gray the reconstructed amplitude, and in red and paler red the reconstructed phase interferometric profiles at the beginning (t\({}_{1}\)) and the end of the time profile (t\({}_{1}+\Delta t\)) assuming the sole drift as disturbing artifacts. At f\({}_{\mathrm{OP}}\), now on the left side of the interferometric profile, the two profiles barely differ in phase while the bias average PNA amplitude increases within a 2 dB range as observed in Figure 2b. Along with the frequency shift, additional minor distortions of the interferometric profiles are also observed. These distortions are exemplified in Figure S1 (S stands for supplementary). This figure shows the superimposition of the PNA amplitude interferometric profile of Figure 2a (t\({}_{0}\)) with the corresponding one recorded after the capture of the time profiles (t\({}_{0}+711\) s \(>\) t\({}_{1}+\Delta t\)). The drift is compensated to stress that the perturbations are limited in the close vicinity of f\({}_{0}\) (grey zone in Figure S1, i.e., in a spectral range \(\pm 5\) kHz around f\({}_{0}\) thus lying away of the f\({}_{\mathrm{OP}}\) ranges. These perturbations can therefore be disregarded in the following analysis.  
Figure 1: (a) Schematic of the Keysight SMM set-up augmented with the Mach-Zehnder interferometer for 50 \(\Omega\) free detection system. The sample Metal/Insulator/Semiconductor (MIS) architecture is also depicted along of the chemical structure of the reference material studied (P3 HT). (b,c) Typical interferometric performance network analyzer (PNA) Amplitude and Phase profiles recorded across the samples described in (a) in broad and narrow bandwidth at the interferometric frequency, respectively. f\({}_{0}\) and f\({}_{\mathrm{OP}}\) stand for the interferometric minimum amplitude and operating frequencies, respectively. The profiles in (c) are shown in semi-Log scale, and in linear scale in the inset.