Scanning Near-field Optical Microscopy:Instrumentation And Applications

Scanning near-field optical microscopy(SNOM)is a scanning probe microscope based optical technique developed in 1980s.Compared with traditional optical microscopy,it can break the diffraction limit.Combined with spectroscopy-resolved and time-resolved technology,SNOM provide a versatile and powerful tool for nanoscale analysis in various fields from physics,chemistry to material science and life sciences.In this work,we focus on the instrumentation part of SNOM,including its feedback system,synchronization of multiple data channels and software,as well as its application in quantum dot(QD)samples.In the introduction part,we give a brief retrospect of the development and current situation of scanning near-field optical microscopy.The fundamentals of near-field optics are introduced.In Chapter 2,we discuss the laser confocal scanning microscopy(LCSM)which is an important part for a modern SNOM system.A homebuilt confocal system was presented.It includes all the basic functions and modules of a LCSM,including scanning module,signal detection module and software which control and synchronize all the hardware modules.In Chapter 3,we discuss the details of the instrumentation of SNOM.First,we study the high quality factor quartz tuning fork resonators which are used an ultra-sensitive force sensor for monitoring tip-sample interactions.In practice,the quartz crystal was excited electrically,and its current signal was amplified subsequently by a pre-amplifier and a lock-in amplifier,which can retrieve both the amplitude and phase of the resonator.A nanometer precision piezo-stage and a proportional-integral-derivative(PID)feedback controller were used to control maintain the strength of tip-sample interaction and consequently their distance.Together with raster scanning system,we are able to measure samples’ local height information point-by-point and finally their whole topography.In addition to the hardware,we designed and wrote a control software system using Lab VIEW.It can find the resonance peak of the quartz resonator,approach the tip to the sample,and scan the sample automatically.Finally,with the homebuilt system,we scanned a standard sample,a 1 μm grating made of polymer,and clean and stable topography images can be obtained routinely.In Chapter 4,we integrated spectroscopic and fluorescence lifetime detection in our system.For the spectral mapping,we synchronize the spectrograph with scanner using a 2 channel trigger system,acquire the spectra at each point of the sample,and finally reconstruct the full spectral image using the homemade software.For the lifetime measurement,a commercial TCSPC module was used.Similar to the spectral mapping function,we synchronize the scanning system with TCSPC module to obtain 2D images.To demonstrate capability of the system,we measured both the spectra and lifetime of patterned CsPbBr3 and CdSe QDs.With the detailed spectral information,CsPbBr3 and CdSe QDs can be distinguished easily.In summary,we built a multifunction scanning near-field optical microscopic system,which can collect topographic information,spectra and lifetime information,providing a powerful nanoscale characterization tool for various fields.Thanks to its modular design,this system is flexible and open,and other channels,such as optical force,thermal and magnetic can also be added in the future.