Enhanced high-order harmonic generation for tunable comb-like THz spectra in PPLN

We demonstrate the efficient generation of multicolor, comb-like terahertz (THz) spectra via high-order harmonic quasi-phase-matching (QPM). The emission is achieved by optical rectification of NIR laser pulses in periodically poled lithium niobate (PPLN) with a 400-μm poling period. Varying crystal temperature (over 15-350 K) and pump pulse duration (over 50 fs to 3 ps) provide precise control over central THz frequencies and harmonic intensities. The THz emission spans odd harmonics from 299 GHz (1st harmonic) to 2.41 THz (9th harmonic) when the PPLN is at cryogenic temperature and pumped using the shortest laser pulses. In addition, due to the changes in the THz refractive index, the temperature variation modifies the phase matching, resulting in up to ~10% frequency tunability in the harmonic lines. This synchronized multicolor THz source, with multioctave comb-like spectra, promises advanced applications in THz spectroscopy and metrology.