Interplay effects of carrier-envelope phase and plasmon resonances in terahertz generation by ionizing ultrashort optical pulses

摘要

The direct conversion of femtosecond ionizing la- ser pulses into low-frequency radiation attracts con- siderable attention due to associated possibilities for creating sources of powerful broadband terahertz (THz) pulses. Previous studies of this phenomenon were mainly related to two limiting cases: (i) few- cycle (extremely short) ionizing pulses with several optical field cycles and (ii) multicycle (quasimono- chromatic) pulses with a large number of field cycles. In the first case, the amplitude and energy of THz oscillations strongly depend on the carrier-envelope phase (CEP) in the ionizing pulse [1–3], whereas for multicycle pulses the CEP dependence is unnoticeable [4–6]. The spectral and mode structures of generated radiation also differ in these two cases. For the few- cycle pulses, the plasmonic oscillations corresponding to the dipole geometric resonance are excited and emitted from the laser-produced plasma filament to the surrounding space. For multicycle pulses, the ra- diation is produced by a relaxing current directed along the plasma filament, and the spectrum width and central frequency are determined by the collision frequency of electrons with heavy particles. Thus, the shape of low-frequency spectrum strongly depends on the pump pulse duration, and there is a range of dura- tions where transition between different spectral shapes occurs. In this range of durations, the spectrum shape also significantly depends on the CEP, as was apparently observed in experiment [7]. Here, we study this intermediate case based on the universal model that considers the motion of the plasma electrons un- der forces both linear and quadratic with respect to the optical field strength.