Development Of Terahertz Near-field Scanning Microscope

Electromagnetic waves in the terahertz frequency band have important application prospects in high-speed communications,non-destructive detection,biomedical testing,and satellite remote sensing due to their unique properties of strong penetration,good coherence,photon energy much lower than ionization energy,and abundant interactions with matter.The vast majority of current research efforts in terahertz spectroscopy and imaging are based on far-field systems,which are limited by diffraction limit,and the limiting resolution of these systems is about one-half wavelength.Terahertz detection technology plays an irreplaceable role in non-destructive detection,identification of biochemical samples,and detection of carrier distribution in semiconductor devices.The development of terahertz near-field scanning imaging systems to break through the diffraction limit,and realize super-resolution detection in the terahertz band can meet the growing demand for detection in various disciplines such as electronic information,materials,and biology,and is of great significance for further improving the ultrastructural research in various disciplines.In this paper,the research of terahertz near-field scanning imaging system is carried out based on the adequate literature research and preliminary laboratory work.A terahertz near-field scanning probe microscope based on a quartz tuning fork probe was developed starting from the theoretical model and simulation analysis of the electric field enhancement of the terahertz probe tip,using the combining metal probes driven by quartz tuning forks with scanning probe microscopes and further exploring signal extraction techniques in terahertz near-field systems.A radio frequency single-electron transistor scanning probe microscopes was developed starting from the extremely high charge detection sensitivity characteristics of single-electron transistors and the use of silicon on insulator(SOI),providing key technical support for core devices and system integration for the further application in terahertz near-field scanning imaging.The main research content and innovation points of this paper are as follows:(1)The simulation of the probe-sample system was carried out by using FDTD Solutions software.The effects of various parameters such as incident light angle,probe tip curvature radius,tip length,tip-sample distance and different samples on the local electric field enhancement effect were analyzed and the physical mechanisms of the near-field enhancement effects of the probe tip,such as antenna resonance,lightning rod effect,mirror dipole and plasmon resonance effect were discussed.The modulation and demodulation technology of scattering signal,the near-field signal amplification and background noise suppression technology,and the influence of various elements in the system on the enhancement effect of the local electric field at the needle tip in the terahertz scattering-type scanning near-field optical microscope(THz s-SNOM)system are comprehensively and systematically discussed,which provides a theoretical basis for the development of terahertz near-field microscopes.(2)The solution of using a quartz tuning fork assembled with the probe replaces the cantilevered atomic force microscope(AFM)probe commonly used in terahertz near-field systems,and is applied in the self-developed sub-THz s-SNOM and the radio frequency single-electron transistor scanning probe system.The tuning fork probe can be flexibly combined with various probes,and can simultaneously realize the scanning imaging of other multi-physical quantities while realizing the scanning of the topography of the sample surface.Additionally,compared with the cantilever-type AFM probe,the tuning-fork-type probe can be more conveniently applied in special environments such as vacuum and low temperature because it does not require an additional optical path to realize motion feedback.In this paper,a sub-THz band s-SNOM system was realized by using a long metal probe driven by the quartz tuning fork and a charge scanning detection system was realized in a 7 K ultra-low temperature vacuum environment by using a radio frequency single-electron transistor probe also driven by the quartz tuning fork,under the realization of capacitance compensation of tuning fork type probes,Q value control and integration technology with various probes.In the two systems,the imaging of the terahertz near-field scattering signal and the charge distribution on the sample surface were completed respectively with the topography of the sample scanned simultaneously.The introduction and use of the tuning fork probes is the key to the realization of the two systems.The continue exploration of the application of tuning fork probes in terahertz near-field spectroscopy and other multi-physical quantity scanning detection systems can carry out in the future.(3)Completed the development of a scattering-type terahertz near-field scanning optical microscope based on a quartz tuning fork probe.The commonly used cantilever type AFM probe is replaced by a metal probe driven by the quartz tuning fork,which solves the problem of serious mismatch between tip length and incident light wavelength in the terahertz/mm-wave band,and the extraction of near-field scattering signals is realized by homodyne detection technology.The terahertz near-field scanning imaging of a 2 μm period metal grating is completed at 94 GHz with an imaging resolution better than 1 μm(～λ/3000).The relationship between the probe length and the field enhancement factor is also discussed by the finite difference time domain method(FDTD)simulation considering the convergent spot size.The approach curve of the scattered signal demodulated at the first-order of tip vibration frequency is fitted by a point dipole model,and the signal is proved to be a pure near-field signal and discussed.(4)Completed the development of the radio frequency single electron transistor scanning probe system.After realizing the development of the core subsystems such as radio frequency single-electron transistor probe,radio frequency readout circuit,lowtemperature scanning probe microscope and low-temperature&low-noise system,the joint commissioning of the three operating points of quartz tuning fork resonance,radio frequency LC resonance and single-electron transistor Coulomb oscillation was realized.Simultaneous scanning imaging of the surface charge distribution and topography of a 2 μm period SOI grating sample with applied voltage is also achieved using an RF single-electron transistor scanning probe system at 7 K.The charge spatial resolution of the system can reach 60 nm,the operating bandwidth can reach 25 MHz,the time resolution is 0.05 μs and the charge sensitivity of the RF single-electron transistor δq is 1.6 x 10-5e/(?)The development of a scanning probe rmcroscope with the ability to scan and image the dynamic distribution of charge on the sample surface was realized.The idea of using a single-electron transistor probe to achieve terahertz near-field scanning imaging through the measurement of charge/potential changes caused by coupling the incident terahertz waves in a tip-sample system is proposed,based on the ultrahigh sensitivity of the single-electron transistor as an electrostatic meter to neighboring charges.The development of the RF single-electron transistor scanning probe system has basically solved most of the technical problems of the system to achieve terahertz near-field scanning detection,providing the basis for the subsequent final realization of terahertz near-field scanning imaging.