THz-assisted population inversion in strong-field excitation

Strong-field excitation provides new methods and means for coherent population of Rydberg states. In this study, we numerically solve the time-dependent Schrödinger equation to investigate the strong-field excitation and radiation dynamics under the cooperative interaction of terahertz (THz) and femtosecond strong laser pulses. In the radiation spectrum, the Rydberg states excited by the strong field are manifested as resonance peaks with energy below the ionization threshold. By using THz fields to control the electron motion on sub-optical cycles, we achieve modulation of Rydberg state population and high-order harmonic radiation under the influence of a strong laser field. We analyze the variation of the excited state population with THz intensity, frequency, and the angle between the infrared field polarization and the THz field. Our results show that, with the assistance of the THz field, population inversion occurs between the Rydberg states. This study provides a feasible scheme for THz amplification based on Rydberg atoms and holds significant implications for THz amplification, high-sensitivity detection, and quantum information processing.