Research on Passive Near-field Optical Scanning Imaging Based on Semiconductor Nanowire/Tapered Microfiber Probe *

: In this paper, we propose a passive near-field scanning imaging system by using the structure of cadmium sulfide (CdS) nanowire/tapered microfiber probe, which combines the near-field scanning structure and the nanowire/microfiber coupling technology. The passive near-field scanning imaging system adopts a passive nanoprobe to detect the intensity change of the reflected light field on the sample surface, which not only retains the advantage of the nanoprobe for the strong restriction of the reflected light on the sample surface, but also reduces the interference of strong excitation light during detection. Through the high efficiently evanescent field coupling between the CdS nanowire and the tapered microfiber, the collected light signal is transmitted to the photodetector at the far field, and finally the imaging of the target sample morphology can be realized. At first, the light field model of the nanowire/tapered microfiber probe structure is verified by the finite element analysis method. The calculated collection efficiency from the sample to the probe is about 4.65‰ and the transmission efficiency from the nanowire to the tapered microfiber is about 74.47%. The collection efficiency is improved by an order of


实验所用光源可被看做一组经物镜聚焦后具有不同振动方向和相同光强度
的偏振光， 物镜聚焦点处主要为纵向光场分布 [26][27][28] 被动式近场探针对纵向偏振分量的响应更加敏感，因此参数 样品宽度的归一化测量结果.Abstract: In this paper, we propose a passive near-field scanning imaging system by using the structure of cadmium sulfide (CdS) nanowire/tapered microfiber probe, which combines the near-field scanning structure and the nanowire/microfiber coupling technology.The passive near-field scanning imaging system adopts a passive nanoprobe to detect the intensity change of the reflected light field on the sample surface, which not only retains the advantage of the nanoprobe for the strong restriction of the reflected light on the sample surface, but also reduces the interference of strong excitation light during detection.Through the high efficiently evanescent field coupling between the CdS nanowire and the tapered microfiber, the collected light signal is transmitted to the photodetector at the far field, and finally the imaging of the target sample morphology can be realized.
At first, the light field model of the nanowire/tapered microfiber probe structure is verified by the finite element analysis method.The calculated collection efficiency from the sample to the probe is about 4.65‰ and the transmission efficiency from the nanowire to the tapered microfiber is about 74.47%.The collection efficiency is improved by an order of magnitude compared with traditional metal-coated near-field probe.In the experiments, a scanning step of 20 nm and a probe-sample distance of 230 nm were selected.The nanowire/tapered microfiber probe and traditional tapered fiber probe are both used to measure the width of different CdSe nanoribbons samples and the atomic force microscopy measurement are used as the benchmark to calculate their measurement error, which is increased by about 3 times.By changing the angle θ between the probe and the sample on the imaging quality, it is found that the resolution obtained using the designed nanowire/microfiber probe is always higher than that only using the tapered microfiber probe.Compared with the tapered microfiber probe scheme, the measurement error is reduced to be less than 7.2%.
In addition, compared with the active luminescence probe scheme, this passive near-field scanning scheme reduces the preparation complexity of the optical probe and the detection structure complexity of the optical system.The large microscopic illumination area can avoid the influence of the small laser spot size on imaging, and the imaging range is only determined by the travel distance of the linear stage.Therefore, our work may provide an attractive approach for developing new near-field scanning microscopy systems in the future.

Fig. 5
The width measurement results of S1-S3 samples with the nanowire/tapered microfiber probe and traditional tapered microfiber probe at θ = 23°: (a)(d)(g) Imaging results of the samples S1-S3 scanned by the nanowire/tapered microfiber probe; (b)(e)(h) Measurement results of the width of the samples S1-S3 by the nanowire/tapered microfiber probe; (c)(f)(i) Measurement results of the width of the samples S1-S3 by tapered microfiber probe.

Table 1 .
Comparison of measurement results between CdS nanowire/tapered microfiber probe and Sun L, Zhou B K 2016 Scanning near-field optical microscope and nano-optical measurement .(Beijing:science press) p167 and p247(in Chinese) Near-field optical scanning, passive microscopy imaging technology, nanowire/microfiber optical probe, cadmium sulfide nanowire, evanescent field coupling Keywords:* Project supported by the Excellent Youth Science Fund Project of National Natural Science Foundation of China (Grant Nos.6212200740).