Fig. 4

Detection under off-centered illumination for increasingly long antenna arrays. a Experimental configuration for detecting a diverging THz beam test. The device is initially placed on the THz focal plane, where the THz spot mainly illuminates the initial pairs of antennas. The device is then moved out of the focal plane on the z-axis. Time-domain acquisitions have been carried out at positions x = 0, 5, and 15 mm, away from the focal plane. Three different detector types have been used, featuring b 3, f 6, and m 9 antennas per array. Time traces are reported in panel (c, g, n) for the above-mentioned devices, whereas (d, h, p) shows the corresponding Fourier-Transform calculated spectra. The general trend among all cases is that the THz transients acquire more cycles as the detectors get farther from the focal plane, due to the broadening THz illumination, which covers an increasing number of antennas. Besides, all spectra reveal that at the intermediate z-position of 5 mm, the peak frequency exhibits a higher amplitude, due to an augmented contribution from more antennas, despite the decrease in electric field strength of the diffracting THz beam, compared to the focal plane. These considerations are summarized in plots (e, l, q), which show the trends of the frequency peak and its linewidth as a function of the longitudinal shift. In panels (c, g, n), curves are vertically shifted for clarity