Local non-linear spectroscopy using electromagnetic enhancement effect of a metal tip apex

当激光束照射在扫描漏斗显微镜（STM）的针尖-样品间隙处时，在针尖尖端正下方的电磁场被强烈地增强。利用这一现象，人们可以实现具有超过光学衍射极限的非常高的特殊分辨率的光谱学。在这项研究中，我们构建了一个尖端增强非线性光谱系统，包括超高真空低温STM，高灵敏度的光子探测系统，和可调谐激光系统。作为第一个实验，我们测量了沉积在Au（111）表面的单壁碳纳米管（SWNT）在785 nm激发光下的针尖增强拉曼光谱。在这个实验中，我们使用Au尖端，这是预期显示大的电磁增强。Au探针是在饱和NaCl水溶液中通过电化学腐蚀制备的。通过调节电解液的温度和pH值，我们成功地生产出曲率半径小于50 nm的Au针尖。我们用Au针尖观察样品的STM图像，直到发现单个单壁碳纳米管束，然后将STM针尖固定在目标单壁碳纳米管束上，随后测量拉曼光谱作为针尖-样品距离的函数。当<-1>针尖-样品距离增加到1590 cm-1时，单壁碳纳米管G带的拉曼强度迅速下降。该结果清楚地显示了尖端增强散射效应。此外，我们还观察了在NaCl（110）面基片上的Au纳米线阵列的光学二次谐波产生，线宽从40到60 nm，以研究Au纳米结构附近的一般局域场。我们已经发现，偏振的入射场的振幅垂直于纳米线轴的纳米线中诱导的去极化场显着降低。

When a laser beam is shined at the tip-sample gap of a scanning funneling microscope (STM), the electromagnetic field is strongly enhanced immediately below the sharp tip apex. Using this phenomenon one can realize optical spectroscopy with very high special resolution beyond the optical diffraction limit. In this study we have constructed a tip-enhanced nonlinear spectroscopy system that consists of an ultra-high-vacuum low-temperature STM, a highly sensitive photon detection system, and a tunable laser system. As a first experiment, we have measured the tip-enhanced Raman spectra of single wall carbon nanotubes (SWNT) deposited on the Au(111) surface with excitation light of 785 nm. In this experiment we used Au tips, which are expected to show large electromagnetic enhancement. The Au tips were fabricated by electrochemical etching in a saturated aqueous solution of NaCl. By adjusting the temperature and pH of the electrolyte, we have succeeded in producing Au tips with the radii of curvature less than 50 nm. Using an Au tip we observed STM images of the sample until an individual SWNT bundle was found. Then the STM tip was fixed over the target SWNT bundle, and subsequently the Raman spectra were measured as a function of the tip-sample distance. The Raman intensity of the G-band (〜1590 cm^<-1>) of SWNT decreases rapidly as the tip-sample distance increases up to 〜10 nm. This result clearly shows the tip-enhanced scattering effect. Furthermore, we have observed optical second harmonic generation from Au nanowire arrays on faceted NaCl(110) substrates with wire widths from 40 to 60nm, in order to investigate the general local field near Au nanostructures. We have found that the amplitude of the incident field polarized perpendicular to the nanowire axes is considerably reduced by the de polarization field induced in the nanowires.