Evaluating domain uniformity in PPLN via microscopy and terahertz waveform analysis

We quantitatively analyzed domain-width deviations due to duty-cycle errors (DCEs) in periodically poled lithium niobate (PPLN) using optical microscopy and terahertz (THz) waveform analysis. Microscopy revealed significant domain-width differences between top and bottom surfaces due to electric-field poling asymmetry. Internal domain structures were complementarily assessed using THz waveform analysis, showing that microscopy can overestimate irregularities, particularly at shorter quasi-phase-matching (QPM) periods. Measured domain-width deviations systematically increased from approximately 3.8% for the largest QPM period (400 µm) to 8.8% for the smallest period (75 µm), aligning closely with known fabrication challenges at shorter periods. Theoretical modeling indicates observed DCE causes a modest (∼5%) efficiency reduction for first-order QPM. However, this loss grows exponentially with QPM order, reaching ∼30% at the third order, and concurrently introduces undesirable spectral pedestals. Combining microscopy and THz analysis provides reliable quantitative benchmarks that are essential for optimizing domain engineering and nonlinear optical device performance.