Theses Doctoral

Cryogenic Near-field Nanoscopy at Terahertz Frequency

Jing, Ran

This dissertation reports on data acquisition method and the application of world’s first cryogenic apertureless near-field microscope designed for terahertz frequencies. The dissertation briefly summarizes the commonly used data acquisition methods and the existing challenges in applying near-field technology using broadband terahertz sources. We devised, implemented, and validated a novel measurement technique to resolve the challenges. The novel method improves the traditional method by providing the information of the carrier-envelop-phase of the terahertz pulse. The physical properties of WTe₂ microcrystals depend sensitively on the layer number. By applying both the traditional and the novel techniques, we systematically explored the layer-dependent electromagnetic response of mono-layer and few-layer tungsten ditelluride (WTe₂ microcrystals. On tri-layer WTe₂, we discovered the plasmonic response and imaged the real-space pattern of the terahertz plasmon using the novel measurement technique. On bi-layer WTe₂, our measurements support that the band alignment is semi-metallic instead of semi-conducting.

Near-field technology at terahertz frequency is sensitive to the Drude behavior of condensed matters. We imaged the electromagnetic response of the transition of cadmium osmate (Cd₂Os₂O₇) crystals from a high temperature metal to a low temperature magnetic insulator. The result is consistent with the temperature dependence in the direct-current conductivity.

In the end, the dissertation discusses the theory and simulation of imaging hydrodynamic flow of materials with viscous electron systems via nano-photocurrent technique. In anisotropic material, nano-photocurrent measures the geometrical properties of the Shockley-Ramo auxiliary field or flux. As a result, the nano-photocurrent is a good candidate to detect the boundary layer and vortex flow pattern of a viscous electron system.


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More About This Work

Academic Units
Thesis Advisors
Basov, Dmitri N.
Ph.D., Columbia University
Published Here
November 30, 2022