Fig. 3: Theoretical and simulation analysis of the generation of radiation with a selectable frequency.
a Frequency dependence on the grating period and electron energy. The solid lines, crosses, and hollow circles denote the theoretical, numerical simulation, and experimental results of the radiation frequency, respectively. The change in p is represented by different colors. The measured tunable range could cover the frequency range of 3.2–14 THz. b Simulated spectra of the THz radiation when p is fixed to 5 μm and E = 1.4 keV (deep red line), 2.0 keV (red line), and 2.6 keV (light red line). The black line is the normalized spectrum of the broadband CR in the HMM, and the colored lines are spectra coupled into free space. c Simulated spectra of THz radiation when E is fixed at 2 keV and p = 5 μm (red line), 4 μm (violet line), and 3 μm (yellow line). The black line is the normalized spectrum of the wideband CR in the HMM, and the colored lines are the spectra coupled into free space. d Simulated CR field contour (normalized |Ez| components) in the HMM. The first row corresponds to E = 2 keV, with f = 3 THz, 8 THz, and 13 THz, which shows that a wideband CR field exists in the HMM. The second row corresponds to f = 8 THz, with E = 1.8 keV, 2.2 keV, and 2.6 keV. The CR field distribution changes with increasing electron velocity, which means that the period of the extraction grating needs to correspondingly change.
