Research On Terahertz Near-Field Characterization Of Novel Materials And Micro-Nano Structures

Terahertz scattering-type scanning near-field optical microscopy(s-SNOM)is an emerging nano-resolution imaging device that can break through the diffraction limit of conventional optics,opening the door to terahertz nano-imaging and nano-spectroscopy research.It is of great scientific research value that many materials and microstructures exhibit exotic terahertz properties at the nanoscale.In this dissertation,the work related to terahertz s-SNOM is carried out from four aspects.The specific research contents are as follows:1.As a key component of S-SNOM,the atomic force microscope probe plays the role of excitation,detection and enhancement,mainly realized by four physical effects,namely,dipole effect,antenna resonance effect,plasma enhancement effect and lightning rod effect.The interaction between probe and sample caused by these effects will inevitably affect the terahertz s-SNOM measurement results.In this dissertation,the interaction between probe and sample in terahertz s-SNOM is systematically studied from three aspects: near-field excitation,near-field detection and terahertz near-field spectroscopy.We propose feasible solutions to those essential problems in terahertz sSNOM,such as wave vector mismatching and probe interference to the near-field spectrum,providing theoretical guidance in selecting suitable probes in terahertz nearfield measurements.Great significance can be expected for the development of terahertz near-field optics.2.Terahertz s-SNOM,as a novel technical tool,shows a wide range of applications in the study of novel materials.In this dissertation,the near-field properties of various novel materials are investigated using terahertz s-SNOM,and the dielectric response of the materials in the terahertz near-field is analyzed.In this dissertation,the terahertz nearfield properties of monolayer graphene materials are investigated,and the near-field imaging of graphene/gold/silica samples using terahertz s-SNOM is performed to analyze the terahertz near-field properties of materials with different carrier concentrations.The strong terahertz antenna resonance caused by the long tip of the metal probe used experimentally will cause severe interference to the terahertz near-field spectrum.Therefore,we aim to eliminate this interference by using a long cantilever probe and ultimately obtain a purer terahertz near-field spectrum,which can be seen as a breakthrough for terahertz s-SNOM to extract the sample spectrum.In this dissertation,the terahertz near-field nano-spectroscopy of topological insulator phonons is studied based on the long cantilever probe,and significant phonon resonance responses are extracted.3.In addition to characterizing the carrier distribution and mobility of semiconductor materials,s-SNOM can be used for real-space imaging studies of surface plasmon polaritons(SPP)of graphene,which has been extensively reported in the infrared band related literature.However,relevant work in the terahertz band is still scarce.In this dissertation,we investigate the problem of real-space imaging of graphene SPPs in the terahertz band and find two main reasons for the difficulty in observing the graphene SPP interference fringes in the terahertz band: one is the wave vector mismatching problem in the terahertz s-SNOM,and the other is the insufficient number of graphene SPP propagation periods in the terahertz band.For these two problems,corresponding solutions are proposed in this dissertation.The graphene acoustic SPP not only has a great wave vector,but also effectively increases the number of propagation periods.Real-space imaging of graphene acoustic SPP in the terahertz band is expected to be achieved if the graphene relaxation time is appropriately improved.4.In this dissertation,we investigate the near-field properties of various micro-nano structures using terahertz s-SNOM,such as metal gap structures,graphene disk,and bowtie antennas.These three types of micro-nano structures correspond to three different physical problems.The near-field imaging of the metal gap structure contains a complex tip-gap interaction,as the radius of the tip and the width of the gap bring about either enhancement or weakening of the near-field signal amplitude.The near-field imaging of the graphene disk reflects the interference problem of metal probe on the surface nearfield distribution.In the near-field imaging and spectroscopy experiments of the bowtie antenna,different types of probes are used for the measurements.Finally,the resonant spectrum of a single antenna can be successfully observed using silicon probes,expanding the application prospects of s-SNOM in the field of terahertz resonant antenna.