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）的尖端-样品间隙处时，电磁场在尖锐尖端下方被强烈增强。利用这一现象，可以实现超越光学衍射极限的极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，该系统由超高真空低温STM、高灵敏度光子探测系统和可调谐激光系统组成。作为第一个实验，我们测量了在785 nm激发光下沉积在Au（111）表面的单壁碳纳米管（SWNT）的尖端增强拉曼光谱。在本实验中，我们使用了金尖端，预计会显示出较大的电磁增强。在饱和NaCl水溶液中采用电化学刻蚀法制备了金针尖。通过调节电解液的温度和pH值，我们成功地生产出了曲率半径小于50nm的金尖端。使用Au尖端，我们观察样品的STM图像，直到发现单个SWNT束。然后将STM尖端固定在目标SWNT束上，随后测量拉曼光谱作为尖端-样品距离的函数。SWNT的g波段（~ 1590 cm^<-1>）的拉曼强度随着尖端-样品距离的增加而迅速降低，直至~ 10 nm。这一结果清楚地显示了尖端增强的散射效应。此外，为了研究金纳米结构附近的一般局部场，我们观察了金纳米线阵列在线宽为40 ~ 60nm的多面NaCl（110）衬底上产生的光学二次谐波。我们发现，垂直于纳米线轴线极化的入射场的振幅被纳米线中诱导的去极化场大大减小。

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.