Investigating wavelength dependence of near-threshold harmonics with different atoms and laser intensities: from analysis of an expanded quantum trajectory Monte Carlo model

Abstract

We present an investigation of the wavelength dependence of near-threshold harmonics (NTH) in different atoms and laser intensities. Similar to the previous research [Phys. Rev. Lett. 112, 233001 (2014)10.1103/PhysRevLett.112.233001], using the three-dimensional time-dependent Schrödinger equation, two distinct types of NTH can be identified: one exhibiting a noticeable energy shift correlating with the laser center wavelength(frequency) (NTH 1), and another with a fixed photon energy (NTH 2). To gain deeper insights, we develop an expanded quantum trajectory Monte Carlo model, including the Rydberg state contributions, and identify three quantum trajectories: TRA 1, 2, and 3. TRA 1 and TRA 3 generate NTH 1 and NTH 2, respectively, while TRA 2 serves as a transitional trajectory capable of generating both types simultaneously. In varying the atomic species and laser intensities, TRA 1 behaves similarly to continuous states, while TRA 3 resembles Rydberg states, revealing the connections between the quantum trajectories and excited states. This research significantly enhances our understanding of NTH generation and offers valuable insights for investigating the structure and ultrafast dynamics of Rydberg states using NTH in future experiments.