  High-order harmonic generation by femtosecond laser pulse in the presence of
a moderately strong terahertz (THz) field is studied under the strong field
approximation, showing a simple proportionality of near-cutoff even-order
harmonic (NCEH) amplitude to the THz electric field. The formation of the THz
induced-NCEHs is analytically shown for both continuous wave and Gaussian
pulse. The perturbation analysis with regard to the frequency ratio of the THz
field to the femtosecond pulse shows the THz-induced NCEHs originates from its
first-order correction, and the available parametric conditions for the
phenomenon is also clarified. As the complete characterization of the
time-domain waveform of broadband THz field is essential for a wide variety of
applications, the work provides an alternative time-resolved field-detection
technique, allowing for a robust broadband characterization of pulses in THz
spectral range.
