基于微球透镜-纳米探针全尺度高效光场耦合原理的超高灵敏度探针增强光谱方法

Tip-enhanced optical spectroscopy (TEOS) is the technique combining scanning probe microscope and spectroscopy to realize spectroscopic imaging with nanoscale spatial resolution due to local enhancement of electromagnetic field underneath the apex of the silver or gold nanoprobe. Typical TEOS techniques are tip-enhanced Raman spectroscopy (TERS), tip-enhanced fluorescence, and etc. Strong optical coupling between the tip and the substrates is the necessary condition for reasonable sensitivity of TEOS. That’s why most of the demonstrated application examples of TEOS are samples on metallic substrates which support strong near-field coupling between the gold or silver probe and metallic substrates. In the contrast, TEOS is rarely applied for characterization of nonmetallic film samples or samples on a nonmetallic substrate. Here, we find that in a simple TEOS system, the optical coupling in multiple length scale ranging from incident focused spot in several wavelength, nanoprobe in around 20 nm, to a molecule in sub-nanometer is inefficient. The reason is that the nanoprobe, acted as an optical nano-antenna is general featured in small receiving and transmitting cross-section, and there is lack of methods to efficiently couple the incident focused spot as well as the nanoprobe in all length scales. In this proposal, we raise a concept that a hierarchical structure with compound subwavelength and deep-subwavelength optical antenna is possible to support highly efficient optical coupling. To take TERS as an example, We invent a compound probe with a microsphere lens hyphened with a nanoprobe fabricated onto a cantilever of an atomic force microscope. We further develop a method to systematically improve the spatial resolution by high-order difference of the near-field TERS signals. With the guidance of the new working principle, and with the assistance of the new techniques, we are going to develop a new generation of TEOS methods with ultrahigh spatial resolution down to 5 nm on general nonmetallic substrates. The technique set will substantially expand the comprehensive applications of TEOS in materials, biologic systems and semiconductor industries.

探针增强光谱(TEOS)是将扫描探针显微术和光谱相结合,借助金或银等探针逼近金属衬底时激发纳腔等离激元而与光有强的耦合作用,实现纳米分辨的光谱成像技术,如针尖增强拉曼光谱(TERS)、荧光光谱等.然而长期以来TEOS因灵敏度过低很难用于研究广泛存在的非金属衬底体系,其物理根源是传统探针作为单臂光学天线与光场耦合效率很低.本项目以TERS为例提出“亚波长-深亚波长级联型光学天线”可实现从数个波长到数纳米全尺度高效光场耦合的原理,发明“微球透镜-纳米探针”新型探针,提出径向偏振光激发微球显著增强针尖轴向光电场的策略,进一步基于自适应准静态精确控制探针-样品距离的新技术,提出近场光谱对探针-样品距离的高阶差分以系统提高TERS空间分辨的原理和实施方法,有望打破TERS难以应用于非金属衬底体系的瓶颈,实现超灵敏且好于5纳米分辨的TERS新方法,有力推动TEOS在材料、生物和半导体工业中的应用研究.

多尺度光学天线之间的光场级联耦合是提高光与微纳结构耦合效率，实现光从数个波长尺度到数纳米、亚纳米尺寸的关键途径，实现超灵敏表面及针尖增强光谱的重要途径。每个尺度光学天线的波段区间、匹配波矢和相位的匹配性，是产生高效率、宽波段（即低色差）的光场级联耦合的关键。本项目围绕如何基于光场级联耦合原理提高等离激元增强拉曼和红外光谱灵敏度的难题，开展相关微纳光学理论设计和实验研究。（1）基于严格的光学互易定理，推导了可准确计算辐射增强因子的半解析计算方法；检验了使用近40年的平面波近似计算两个经典等离激元光学天线（POA）-发射体复合体系（纳米粒子-金属膜耦合的SERS体系，扫描探针-衬底耦合的TERS体系）的平均辐射增强因子与精确值的偏差；将该计算方法从电偶极子发射体推广到磁偶极子、电四极子等发射体。（2）微纳光学设计了全内反射棱镜级联耦合纳米颗粒的构型，用于级联增强金属、非金属表面拉曼光谱。（3）设计并构筑了一种同时支持红外多波段共振的纳米桥联的菱形天线，以及该天线与F-P腔级联耦合的结构用于获得具有单层分子灵敏度的表面增强红外光谱。（4）制备了含有纳米隆起结构的石墨烯包覆MnO的微米线级联结构，用于超灵敏表面增强红外光谱的检测；发明了石墨烯包覆介质针尖，用于超灵敏纳米分辨红外光谱和红外近场光学显微镜。

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