Fig. 2: Measurement results of the THz radiation frequency and spectrum of the chip.

a Measured spectral peaks spanning 3.2–14 THz for different grating periods under different electron energies. The central frequency and the sample numbers are shown at the top and bottom of the figure, respectively. The corresponding values of E and p for different samples are listed in Supplementary Information Section S5. b Wavevector matching for the CR in the HMM and radiation coupled into free space. The deep red, red, and light red solid lines represent the projections of the k_x-k_z hyperbolic curve (the red hyperbolic curve indicated in Fig. 1b) onto the ω-k_z plane at different frequencies. The black lines are the dispersion lines of the evanescent fields surrounding free electrons (namely, ω = u₀·|k_z|) with different u₀ (E). The black lines and colored lines intersect at the deep red, red, and light red points, which indicate the end points of the k_z of the excited CR in the HMM and correspond to the same k_z. To extract the CR into free space, k_z needs to be matched by the wavevector introduced by the grating (2πn/p) to change it to the free space radiation region (|k_z| < ω/c). According to this figure, a higher electron velocity (energy) results in a higher radiation frequency of the chip. c The black line (ω = u₀·|k_z|) intersects with the red, violet, and yellow solid lines (projections of the k_x-k_z hyperbolic curve onto the ω-k_z plane) at the red, violet, and yellow points, which indicate the end points of the k_z of the excited CR and correspond to different k_z values. To extract the CR into free space, k_z needs to be matched by the wavevector introduced by the gratings (2πn/p) of different grating periods p, and a smaller p corresponds to a higher radiation frequency. d Measured radiation spectra at E = 1.4 keV (deep red line), 2.0 keV (red line), and 2.6 keV (light red line). The grating period of the chip is fixed at 5 μm. e Measured radiation spectra at p = 5 μm (red line), 4 μm (violet line), and 3 μm (yellow line). The electron energy is fixed at 2 keV. Each spectrum in this figure is normalized by its own maximum peak.