Quantum Treatment of Two-Color-Induced THz Field Generation

Terahertz (THz) pulse generation from gases ionized by two-color laser pulses [1] is commonly described as a Brunel zeroth-order harmonic [2] induced by the photocurrent <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\partial_{t}\mathbf{j}_{B}(t)\propto \rho(t)E_{L}(t)$</tex>, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$E_{L}(t)$</tex> is a time-asymmetric optical field and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\rho(t)$</tex> is the photo-induced electron density. Modeling this process needs an accurate description of the electron rate equation, which, so far, has employed standard ionization probabilities, e.g., the quasi-static Ammosov, Delone and Krainov (ADK) rate [3], in principle valid for a single-frequency pump.