Carrier-envelope-phase-dependent below-threshold harmonic generation in few-cycle mid-infrared laser fields

Area of Science:

• Atomic physics
• Quantum optics
• Nonlinear optics

Background:

• Few-cycle laser pulses offer unique control over light-matter interactions.
• Carrier-envelope phase (CEP) is a critical parameter in ultrafast laser science.
• Below-threshold harmonic generation (BTHG) provides insights into electron dynamics in atoms.

Purpose of the Study:

• Investigate the dependence of atomic BTHG on the CEP of few-cycle mid-infrared laser pulses.
• Elucidate the physical mechanisms governing CEP effects in BTHG.
• Determine the influence of laser intensity on CEP sensitivity.

Main Methods:

• Theoretical study using the three-dimensional time-dependent Schrödinger equation.
• Numerical calculation via the time-dependent generalized pseudospectral method.
• Analysis of BTHG spectra as a function of laser-field CEP.
• Application of synchrosqueezing time-frequency transform and semiclassical analysis.

Main Results:

• BTHG spectra exhibit CEP-dependent enhancement or suppression at low laser intensities.
• This CEP dependence arises from resonant effects involving multiple quantum trajectories.
• At high laser intensities, BTHG becomes insensitive to the CEP.
• Physical mechanisms are explained through time-frequency analysis and semiclassical models.

Conclusions:

• The CEP of few-cycle mid-infrared pulses significantly influences atomic BTHG at low intensities.
• Quantum trajectory interference dictates the CEP dependence of BTHG.
• Laser intensity is a key factor determining the CEP sensitivity of BTHG.