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

Scanning Near-field Optical Microscopy(SNOM)is a scanning-probe-based optical microscopy technique.The central idea is to use a sub-wavelength probe to convert the high spatial frequency near-field components of light into detectable farfield signal and to achieve super spatial resolution.Therefore,the optical probe is the key for near-field optical microscopy,and its properties determines the capability of the entire system.In this work,we focus on an important branch of the near-field optical probes,namely the fluorescent nanoparticle-based probes.Here,fluorescent nanoparticles refer to quantum dots,fluorescent molecules,upconverted nano fluorescent particles and color centers,to name a few.Under the excitation of pump light,fluorescent nanoparticles emit fluorescent signals,and the signals are related to the local environment,including light field strength,temperature,pressure,electric field,magnetic field and local density of states.This unique property makes fluorescent nanoparticles a powerful tool for multi-physics detection.In addition,the size of the fluorescent particles is generally small(?10 nm for quantum dot,? 1 nm for color center and fluorescent molecules),and therefore a spatial resolution better than 10 nm can be achieve.Moreover,thanks to the environment-dependent emission properties of the fluorescence nanoparticles,the nano-fluorescent probe potentially allows us to measure the multiple physical fields simultaneously with a nanometer spatial resolution.In this work,we focus on the two major challenges of fluorescent nanoparticlebased probes,namely the efficiency and robustness.Optical fiber probes fabricated by traditional methods,such as thermal pulling and chemical etching,often suffer from a small apex angle(< 40°),which make probe very fragile and the transmission efficiency low.To address this issue,we propose a new type of pyramidal fiber probe,which can be fabricated on the end face of the micro-fiber by template-based replication technique.The pyramid angle can reach 70.2 °,and the neck length is short.It can therefore subsequentially improve the light transmission efficiency.Meanwhile,tip diameter less than 50 nm is achieved,enabling high lithography resolution and great robustness.Another important properties which determines the performance of fluorescent nanoparticle probes is their stability during the scanning process,i.e.,whether the tipsample distance can be kept constant and whether the particle is stable enough for long term scanning.Here,we use a quartz tuning fork as the force sensor,and realize the stable control of the probe-sample distance through the detection of its resonance signal by an external circuit.Moreover,we use PECVD method to protect the probes with an additional oxide coating,and this leads to a significant improvement of the scanning stability.The Au nanoarray was scanned and imaged with the new probe,and 20 nm optical resolution was achieved.This thesis is organized as follows.In the first chapter,development of SNOM and key breakthroughs are reviewed.In the second chapter,fabrication process of the single quantum dot modified nano-pyramid probe is introduced in detail.In the third chapter,near-field scanning imaging using nano-pyramid probe is verified experimentally.The fourth chapter is summary and prospect of this work.