We theoretically investigate the effects of the excitation frequency on theplateau of high-order terahertz sideband generation (HSG) in semiconductorsdriven by intense terahertz (THz) fields. We find that the plateau of thesideband spectrum strongly depends on the detuning between the NIR laser fieldand the band gap. We use the quantum trajectory theory (three-step model) tounderstand the HSG. In the three-step model, an electron-hole pair is firstexcited by a weak laser, then driven by the strong THz field, and finallyrecombine to emit a photon with energy gain. When the laser is tuned below theband gap (negative detuning), the electron-hole generation is a virtual processthat requires quantum tunneling to occur. When the energy gained by theelectron-hole pair from the THz field is less than 3.2 times the ponderomotiveenergy, the electron and the hole can be driven to the same position andrecombine without quantum tunneling, so the HSG will have large probabilityamplitude. This leads to a plateau feature of the HSG spectrum with ahigh-frequency cutoff at about 3.2 times the ponderomotive energy above theband gap. Such a plateau feature is similar to the case of high-order harmonicsgeneration in atoms where electrons have to overcome the binding energy toescape the atomic core. A particularly interesting excitation condition in HSGis that the laser can be tuned above the band gap (positive detuning),corresponding to the unphysical "negative" binding energy in atoms forhigh-order harmonic generation. Now the electron-hole pair is generation byreal excitation, but the recombination process can be real or virtual dependingon the energy gained from the THz field, which determines the plateau featurein HSG.
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