AI 中文总结
本研究利用cAFM光刻制备LaAlO$_3$/SrTiO$_3$纳米结，实现电场诱导二次谐波产生，验证其可作为纳米非线性光学可重构平台，兼具亚波长光源与近场探测功能。
AI 中文摘要
在纳米尺度实现电可调非线性光学响应仍具挑战性，因为传统非线性材料难以同时具备大极化率、纳米尺度限域和原位可重构性。本研究报道了通过导电原子力显微镜（cAFM）光刻在LaAlO$_3$/SrTiO$_3$界面定义的纳米级隧道结产生的电场诱导二次谐波（EFISH）。在该界面写入的导电通道被纳米级绝缘间隙中断，施加在间隙两侧的直流偏置产生超过$10^7$ V/m的局部电场。二次谐波（SHG）信号空间定位于结处，呈现二次偏置依赖关系，可由$I(2\boldsymbol{\u03c9}) \u221d |\boldsymbol{\u03c7}^{(2)}_\text{0} + \boldsymbol{\u03c7}^{(3)} E_\text{DC}|^2$描述，且无磁滞现象；在$|V_\text{DC}| = 1$ V时，调制深度超过380%；输入偏振模式呈与结轴对齐的双瓣结构，与中心对称基质的EFISH特征一致。以BBO参考晶体校准后，得到6 K下$|\boldsymbol{\u03c7}^{(3)}| \u2248 1\u00d710^{-19}$ m$^2$/V$^2$。这些结果证实，cAFM写入的氧化物纳米结可作为纳米非线性光学的可重构平台，其中结几何决定响应对称性，SrTiO$_3$的大场诱导$\boldsymbol{\u03c7}^{(2)}$提供光学非线性；由于非线性在同一纳米间隙内产生并读出，该结可同时作为亚波长光源和光学非线性的近场探测器。
英文摘要
Electrically tunable nonlinear optical responses at the nanoscale remain challenging to achieve because conventional nonlinear materials lack the combination of large susceptibility, nanoscale confinement, and in situ reconfigurability. Here we report electric-field-induced second harmonic (EFISH) generation from a nanoscale tunnel junction defined by conductive atomic force microscope lithography at the LaAlO$_3$/SrTiO$_3$ interface. A conducting channel written at the interface is interrupted by a nanoscale insulating gap, across which applied DC bias produces local electric fields exceeding $10^7$ V/m. The SHG signal is spatially localized at the junction, exhibits a quadratic bias dependence described by $I(2ω) \propto |χ^{(2)}_\mathrm{0} + χ^{(3)} E_\mathrm{DC}|^2$ with no hysteresis, a modulation depth exceeding 380% at $|V_\mathrm{DC}| = 1$ V, and shows a two-lobed input-polarization pattern aligned with the junction axis, consistent with EFISH from a centrosymmetric host. Calibration against a BBO reference crystal gives $|χ^{(3)}| \approx 1\times10^{-19}$ m$^2$/V$^2$ at 6 K. These results establish cAFM-written oxide nanojunctions as a reconfigurable platform for nanoscale nonlinear optics in which the junction geometry sets the symmetry of the response and the large field-induced $χ^{(2)}$ of SrTiO$_3$ provides the optical nonlinearity. Because the nonlinearity is both generated and read out within the same nanoscale gap, the junction operates simultaneously as a subwavelength source and a near-field detector of optical nonlinearity.
Comments12 pages, 7 figures
