Experimental evidence for terahertz emission of continuum electrons in the dual-color laser field

Abstract

Terahertz (THz) wave generation (TWG) in a dual-color laser is investigated with joint measurements between THz and third-harmonic generation, where the relative phase delay of dual-color fields is determined in situ in sub-wavelength accuracy, allowing for the clarification of the TWG mechanism in a direct comparison with various theoretical predictions. The delay- and polarization-dependent experiment validates that the continuum–continuum transition within the escaped electron wavepacket in the single atom gives birth to THz emission, while the bound energetic level does not contribute to TWG. TWG from atoms and molecules would provide an all-optical, vacuum-free, and ultrafast tool to record the spatiotemporal evolution of tunneling electron wavepackets.

Electron trajectory backanalysis for spectral profile in two-color terahertz generation

“…Compared against the photocurrent model from macroscopic viewpoint, the strong-field approximation (SFA) is a successful workhorse of strong-field physics under quantum mechanical treatment. The continuum-continuum transition in the SFA dipole moment [14] has precisely reproduced the phase-and polarization-dependent measurement of TWGs [15,16], proving the feasibility of SFA theory. The semiclassical method of SFA theory, where the electron tunnelling is treated quantum mechanically and its subsequent motion is envisioned as classical Newtonian particle trajectory, can connect the TWGs and photoelectron momentum distributions (PEMDs) [17][18][19][20], providing new insight into the microscopic mechanism of TWG.…”

“…The trajectory method derived from SFA in the semiclassical limit is introduced to explain the TWG. Since the contributions of the rescattering electron have been excluded [15], only the 'direct' electron is taken into account. The bound electron is released at the ionization instant t i and subsequently driven only by the strong laser field without interaction with its parent ion.…”

Terahertz field induced near-cutoff even-order harmonics in femtosecond laser

“…The laser induced gas plasma involves rather complicated processes dominated by the strong field photoionization. In this sense, the two-color field induced THz wave generation (TWG) has been interpretated under the framework of strong field theory as the continuum-continuum transition [17][18][19], complementary to the continuum-bound transition mechanism for highorder harmonic generation (HHG) [20], i.e., the emssion of high energy photon when the released electron recollides with the parent ion after the ionization.…”

Terahertz Emisssion from Quantum Interference of Electron Trajectories

“…Simultaneously, much effort has been made to explain the TWG within the strong-field theoretical framework, including the strong-field approximation (SFA) model [13] and numerical solving time-dependent Schrödinger equation [14]. Recently, our experimental and theoretical works indicated that the TWG originates from the continuum electron under single active electron approximation, and established the link between quantum mechanical SFA and classical PC models [15,16]. However, which kind of electron behaviors in convoluted tunneling dynamics produces the TWG still remains indistinct.…”

“…The S 3ω (τ ), shown in Fig. 2(a2), are jointly measured with S THz (τ ) for experimentally in situ calibration of the phase delay τ [15]. The experimental detail is referred in Supplemental Material S. .…”