Quantum Dot Decorated Nano-Pyramid Fiber Tip For Scanning Near-Field Optical Microscopy

The optical properties of nano-emitters,such as quantum dots,fluorescent molecules and rare earth elements doped nanocrystals are sensitive to their local environment.They can therefore be used as a nanosensor to probe the local physical properties.Since the size of nano-emitters is extremely small(single quantum dot～10 nm,Nv center～1 nm),a high spatial resolution can be achieved.Motivated by this idea,various nanometer-resolution measurements have been demonstrated by scanning a nano-emitter decorated atomic force microscope(AFM)tip,but this approach can only be used for thin transparent samples because AFM tips are lack of light collection capability.To tackle this issue,in this work we developed a new type of single-quantum dot decorated fiber nano-tip,which not only can perform a stable topography measurement but also can collect optical signal with high efficiency(～10%),providing a robust tool for quantum dot based near-field optical microscopy techniques.Current fiber tips are facing three major issues.They are fragile,their light collection efficiency is low,and the stability is often poor in near-field optical measurements due to the sticky surface of the decorated tips.In this paper,we have studied the design and fabrication of fiber tips in order to solve the above issues.The key parameter related to the collection efficiency and mechanical strength of fiber tips is the apex angle.A small apex angle not only makes the tip fragile,but also leads to low light collection efficiency because of the small collection angle and the leakage of light in the long neck of the fiber tip where propagation modes are not supported.However,the apex angle of tips made by current fabrication methods(namely heating-pulling and etching)is commonly smaller than 40°.To circumvent the limitation,we propose a nano-pyramid fiber tip which can be fabricated by the template based technique(UV imprint technique)efficiently and reproducibly.The nano-pyramid tip has a large apex angle(70.5°),and as a result,the collection efficiency of the nano-pyramid tip is 17 times better than that of the tip fabricated by heating-pulling method.In addition,a thin oxide layer is deposited on the tip in order to solve the sticky surface issue,which prevents us from having stable scanning results.In practice,this additional layer can also serve as a protection layer for the QD on the tip,which is sensitive to the oxygen and moisture in the environment.Experimental results show that significant improvement for both topography and optical mapping can be achieved with the new QD decorated nano-pyramid tips.The optical resolution can reach 20 nm,smaller than k/20.It provides a powerful and reliable tool for near-field optical measurements.This article is organized as follows.In the first chapter,the research background related to SNOM was introduced.In the second chapter,the fabrication process of single quantum dot-modified nano-pyramid probe was described in detail.In the third chapter,light collection efficiencies of different probes were compared experimentally and theoretically.In the fourth chapter,the performance of the new tips was tested in nanostructure measurement experiments.