Figure 3: Correlation between modulation frequency and transit-phase.

The transit-phase θ is numerically calculated for thousands of sample electrons, varying the initial phase ϕ of the AC electric field and the modulation frequency f normalized by ω_o = 1.05 × 10¹⁰ (rad/s) for different normalized DC-bias field . The horizontal width of strips corresponds to the bunch-length and the normalized current density is scaled by color. When and , the diodes fail above an upper frequency barrier or at the discrete ETP regions. Otherwise, electrons behave as a well-confined bunch regardless of the modulation frequency, which implies generation of ultrashort electron micro-bunch trains.