(a) Far-field spectra of high-order harmonic radiation generated by driving fields ω₁ (2 × 10¹⁴ W/cm², 800 nm) and ω₂ (0.2 × 10¹⁴ W/cm², 400 nm) with 100-μm beam waist, 30-fs pulse length, and at an angle of 17.5 mrad in a slice of argon at focus. (b) Dipole phase coefficient α (dots) retrieved from the spectra in panel (a) and calculated by the saddle point analysis (solid line), in units of 10⁻¹⁴ cm²/W.