Development Of Terahertz Near-field Scanning Microscopy

THz(1 THz=10¹² Hz)radiation locates between the microwave and infrared band in the electromagnetic spectrum,and possesses the unique properties distinguished from other wavelengths,such as strong penetration,good coherence,low ionization energy and high specificity recognition,etc.,it is because of this properties that making THz technique has tremendous application and development potential in various disciplines,and thus known as one of the technologies to change the future.At present,most of the systems used in study of THz spectroscopy techniques are THz time domain spectroscopy system（THz-TDS）,electronic solid-state source frequency multiplied system,differential frequency continuous scanning system,quantum cascade laser（QCL）system and so on,however,these techniques above mainly use the far-field scanning/imaging method to study the samples,and its physical principle follows the traditional Fresnel diffraction law,which indicates that the optimized resolution obtained by these technique cannot be better than half of the incident wavelength because of the Abbe diffraction limit.With the rapid development of the biochemistry and materials science,people found that the conventional far-field technique couldn’t satisfy the emergent need of super-resolution scanning or imaging of samples,meanwhile,the current commercial THz near-field instruments are not mature yet and quite expensive,and thus cannot be popular in the THz research works.Therefore,it is emergent to develop a new kind of near-field system to realize achieving an ultra-high resolution in THz band to satisfy the need of the development of various disciplines.As to our work presented in this dissertation,we intend to develop a kind of THz scattering scanning near-field microscopy based on the reported optics/THz near-field techniques to realize the micro-nano-resolution imaging in lower THz frequency band,and at the same time make a summary of the signal detection techniques and the system construction process,further to promote the development of THz near-field technology and instruments.Considering the difficulties of system development,it is impossible to complete the work at one time.Therefore,on the basis of a thorough investigation of the literatures in this field and the previous works,the research work was divided into three stages to advance at different levels in order to achieve the ultimate goal.The three stages of work are listed as:research and development of near-field system with a resolution of tens/hundreds of microns,and then single-micron systems and subsequently nano-resolution systems based on the experiences of previous two parts.After years of efforts,we have successfully completed three stages of work,and in the first two stages,we have developed a phased micron-resoluted THz-SNOM system prototype with intellectual property rights and can be used independently in the testing,meanwhile providing a substantial foundation for the final third stage work.The main research results of this dissertation are listed as below:（1）In the first stage,a near-field system based on the difference-frequency THz scanning system with a best resolution of 40μm at 500 GHz was developed.This work proves that the frequency-difference THz source is not the most proper to be used in the THz-SNOM,and scattering method and the modulation/demodulation calculations of this work can be used in the development of the THz-SNOM;（2）In the second stage of work,a home-built W-band microscope based on the continuous wave source operating at 110 GHz with a resolution of 1.0μm was developed,which belongs to advanced level in the W band;（3）In the third stage of work,a homebuilt the scattering scanning near-field microscopy coupled with the AFM and the 110 GHz solid-state source with a resolution of³00nm was developed,the resolution is about 1/10000 of the incident wavelength,which belongs to advanced level in the THz-SNOM systems.This stage of work demonstrates the smallest operable and controllable tip-sample separation and the corresponded resolution is 1μm,and verifies the accuracy of simulation results and the signal detection and calculations,further indicating the necessity of the third stage works;（4）Calculate and summary the signal detection techniques in the scattering scanning near-field microscopy,and provide a go-to source for the researchers in this field.The experimental results showed that the system had a good stability and could obtain good experimental results in the ambient conditions,which indicated that the systems had a promising commercial prospect.This significant work may pave the way of the upgrading of THz spectral technology and the popularization and development of THz near-field instruments.