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arXiv 2108.03397physics.opticsphysics.app-ph
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量化铌酸锂受限纳米结构中二次谐波显微术的相干相互作用长度

Quantifying the Coherent Interaction Length of Second-Harmonic Microscopy in Lithium Niobate Confined Nanostructures

Zeeshan Hussain Amber, Benjamin Kirbus, Lukas M. Eng, Michael Rüsing

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AI总结：

本研究针对薄膜铌酸锂（TFLN）中二次谐波显微术受界面效应影响的问题，通过楔形样品调控厚度，实验结合数值模拟揭示其背反射下主导信号为共传播相位匹配分量，且相干作用长度依赖泵浦波长与物镜数值孔径。

摘要：

x切或z切的绝缘体上铌酸锂（LNOI）形式的薄膜铌酸锂（TFLN）近来成为开发集成光电子（纳米）器件与探索基础研究的极具前景的新型平台。本文研究了此类样品中光学二次谐波（SH）显微术的相干相互作用长度$l_{c}$；样品被特意制备成楔形，以便将几何限制从体厚度平滑调控至约50 nm。SH显微术是一种功能强大的非侵入式工具，可用于研究生物科学与固态科学中的结构特性，尤其适用于铁电畴与畴壁的可视化和分析。然而，与体铌酸锂（LN）不同，TFLN中的SH显微术会受到界面反射与共振增强的显著影响，这两种效应均取决于薄膜厚度与衬底材料。本文表明，背反射模式下测得的主要二次谐波产生（SHG）贡献是共传播相位匹配的SH信号，而非体LN样品中那样的反向传播SH分量。此外，$l_{c}$不仅显著依赖于入射泵浦激光波长（样品色散），更依赖于所用聚焦物镜的数值孔径。我们在x切与z切TFLN上获得的这些实验结果，得到了先进数值模拟的极佳支持。

英文摘要：

Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator (LNOI) has recently popped up as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length $l_{c}$ of optical second-harmonic (SH) microscopy in such samples, that are purposely prepared into a wedge shape, in order to elegantly tune the geometrical confinement from bulk thicknesses down to $\approx$ 50 nm. SH microscopy is a very powerful and non-invasive tool for the investigation of structural properties in the biological and solid-state sciences, especially also for visualizing and analyzing ferroelectric domains and domain walls. However, unlike bulk LN, SH microscopy in TFLN is largely affected by interfacial reflections and resonant enhancement that both rely on film thickness and substrate material. In this paper we show that the dominant SHG contribution measured in back-reflection, is the co-propagating phase-matched SH signal and \textit{not} the counter-propagating SH portion as is the case for bulk LN samples. Moreover, $l_{c}$ dramatically depends also on the incident pump laser wavelength (sample dispersion) but even more on the numerical aperture of the focussing objective in use. These experimental findings on x- and z-cut TFLN are excellently backed up by our advanced numerical simulations.

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