## 3 Results and Discussion
### Terahertz Measurements
Figure 7 gives the maximum values of the measured photoresponse signal under excitation at 0.15 THz (Figure 7a) and 0.3 THz (Figure 7b) for both configurations of the illumination (front and back). In the case of back illumination, the photoresponse signal is enhanced by a factor of two when the excitation frequency is shifted from 0.15 THz to 0.3 THz. This is mainly due to the power of the source (6 mW at 0.3 THz and 3 W at 0.15 THz). For the front illumination configuration and under excitation at 0.15 THz, four lobes were observed at \(\sim\) 30\({}^{\circ}\), 120\({}^{\circ}\), 190\({}^{\circ}\), and 310\({}^{\circ}\) (blue cross marks in Figure 7a). However, for back illumination only two lobes were observed along with a lower intensity of the signal (red circles in Figure 7a). This lower intensity found in measurements under back illumination  
Figure 5: Photoresponse signal vs. gate voltage under excitation of (**a**) 0.15 THz and (**b**) 0.3 THz and for both front and back illumination configurations. points toward a substantial contribution of the bonding wires to the coupling of the incoming terahertz to the channel [25]. Accordingly, under excitation at 0.15 THz, the response measured under front illumination exceeds the one obtained under back illumination for any orientation of the THz beam; this can be explained as follows: for back illumination both the electron channel and the electrodes' metal pads screen the incoming EM radiation, the coupling through the bonding wires is blocked and the overall photoresponse is considerably weakened. Since the four lobes are mainly determined by the layout of the four bonding wires they are not observed in back illumination. For excitation at 0.3 THz measurements reveal that both device's photoresponses under both front and back illumination start to exhibit a similar dependence with respect to the THz beam orientation (Figure 7b). This is in good agreement with the weakening of the ability of the bonding wires to act as an antenna as the frequency of the THz increases [25], accordingly, since at 0.3 THz the influence of the bonding wires is lowered, both responses under front and back illumination become qualitatively similar. For the front illumination, we could observe a local maximum an important value of the measured signal for the angles around the one obtained for 330\({}^{\circ}\) that could be attributed to a residual coupling mediated by the bonding wires or the contacts pads. Nevertheless, this latter value is quite reduced as compared to the one observed at the angle of 240\({}^{\circ}\). Moreover, a clear asymmetry of the photoresponse dependence can be observed for the angles 240\({}^{\circ}\) and 60\({}^{\circ}\) on Figure 7b under  
Figure 6: Spatial distribution of the module of the normalized induced electric field under front, (**a**), and back, (**b**), illumination at 0.3 THz at the plane of the transistor channel (it is found in simulations that the induced electric field is mainly developed in the channel of the FET). The maximum values of the normalized electric field in the plotted plane are 18.47 dB and 29.58 dB for front and back illumination, respectively. Black solid lines indicate the contour of the top side transistor electrodes and are a guide to the eye. Figure 7: Photoresponse signal as a function of the rotation angle under excitation of (**a**) 0.15 THz and (**b**) 0.3 THz and for both configurations front and back illumination. front illumination configuration. While it is true that ideally both values should be almost equal, this asymmetric dependence could be a combined effect of a better coupling with the pads and/or the gate fingers at the angle 240\({}^{\circ}\) and a slight misalignment between the device mounted on the rotational axis in the plane perpendicular to the beam propagation and the THz beam during measurements.