氧化鎳擴散與高電場製作週期性極化反轉鈮酸鋰

Abstract

本研究使用一種利用氧化鎳(NiO)的擴散製程，於週期性極化反轉鈮酸鋰(PPLN)中實現準相位匹配(QPM)結構的垂直空間調制。此方法於PPLN晶體 ±Z軸表面濺鍍數十奈米氧化鎳薄膜，並在950°C的條件下進行退火，用來產生折射率差異與淺層反轉。之後進行高脈衝電壓導致極化反轉製程，從而對準相位匹配結構所需補償的晶格動量進行有效的縱向空間調制，以最大化地利用鈮酸鋰的高非線性係數。 在實驗中，主要以Z-切0.75mm厚度的鈮酸鋰晶體為基板。結構從大週期往較小週期推進，並確認穩定度。結果為在週期29.5µm，50%佔空比以順向製程便能夠成功做出，而週期16µm，50%佔空比需搭配逆向製程較能做出，而週期11µm在順向製程下，則需要使用25%佔空比才能做出，75%佔空比效果反而不佳。 最後，在週期11µm使用25%佔空比較能將區域內面積完整反轉，雖然藉由改變週期比例降低了轉換效率，但在此製程下能較完整地做出較小的週期。

This study demonstrates a nickel oxide (NiO) diffusion process for achieving vertical spatial modulation of quasi-phase-matching (QPM) structures in periodically poled lithium niobate (PPLN). In this method, a nanometer-scale NiO thin film is sputtered onto the ±Z faces of the PPLN crystal, followed by annealing at 950°C. This process induces a refractive index change and shallow domain inversion. A subsequent polarization inversion process is then applied by pulsed electric poling, enabling longitudinal modulation of the nonlinear coefficient thus creating a probing vector necessary for QPM compensation, thereby leveraging the high nonlinear coefficient of lithium niobate. The technique was demonstrated on 0.75 mm thick Z-cut lithium niobate substrates. Fabrication was performed from larger to smaller QPM periods to assess structural stability. Our results show that a 29.5 µm period and a 50% duty cycle grating can be successfully achieved with a forward poling process, whereas a 16 µm period and a 50% duty cycle required a reverse poling process for effective patterning. For finer QPM structures, such as an 11 µm period with a forward poling process, a 25% duty cycle was found to be essential, as a 75% duty cycle resulted in poor outcomes. In conclusion, employing a 25% duty cycle for the 11 µm period structure results in more complete domain inversion within the poled regions. Although this reduced duty cycle inherently decreases the theoretical conversion efficiency, it enables the reliable fabrication of smaller-period QPM structures under the given processing conditions.