Complementary, down-shifted THz lines in PPLN via ordinary-polarized emission

Narrowband terahertz (THz) emission from periodically poled lithium niobate (PPLN) is typically realized in the d₃₃ (extraordinary, z-axis) geometry due to the large nonlinearity of this tensor element. Here, as an alternative, we instead engage the d₂₂ tensor channel in PPLN and show that ordinarily polarized (y-axis), d₂₂-driven quasi-phase-matched (QPM) emission preserves narrowband THz output while shifting matched resonances to lower frequency (typically f₃₃/f₂₂ ≈ 1.5), thereby providing complementary THz lines from the same device. Furthermore, we use an in situ EO-sampling cross-calibration between the two tensor-selective channels to retrieve the relative spectral intensities of the extraordinary and ordinary emission and extract an effective d₂₂/d₃₃ ratio of 0.11 ± 0.03, in agreement with phonon dispersion models. Despite the weaker nonlinearity, d₂₂ excitation still delivers around 10 kV/cm peak fields for 1 mJ pumping. These results establish d₂₂ as a practical secondary channel for extending spectral coverage and enable polarization-selective narrowband THz spectroscopy in PPLN.