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terahertz scanning tunneling microscopy (thz-stm) is a new analysis technology that has both atomic scale spatial resolution and ultrafast time resolution.the principle is to combine a cryogenic scanning tunneling microscope with an ultrafast terahertz pulse laser, and focus the ultrafast terahertz pulse laser to the stm tunnel junction through a lens. the terahertz pulse generates a local transient electric field on the tunnel junction. thereby controlling the tunneling process of current on an ultrafast time scale. ultrafast time resolution is achieved in stm by controlling the carrier envelope phase (cep) of terahertz pulses and using pump-probe technology.because the interaction between matter and terahertz light contains very rich physical and chemical information, terahertz stm technology is of great significance for exploring low-energy excitation and dynamic processes in condensed matter physics and chemistry.however, the construction of thz-stm needs to overcome many technical difficulties. currently, only a few groups in the world have achieved ultra-high vacuum and low-temperature thz-stm with both ultra-fast time resolution and ultra-high spatial resolution.

institute of physics, chinese academy of sciences/state key laboratory of surface physics, beijing national research center for condensed matter physics, group sf09after years of relay research and repeated optimization, a completely independently designed new ultra-high vacuum and low-temperature thz-stm system has been successfully built.this system does not use the traditional low-temperature dewar-type stm design, but innovatively uses a liquid helium-free continuous flow refrigeration method for refrigeration. the simple structure of this low-temperature insert allows the stm core probe and ultra-high vacuum cavity to be designed in a more compact style. placing the focusing lens outside the ultra-high vacuum cavity can still achieve good coupling between thz pulses and low-temperature stm, but but it greatly improves the convenience of operation. in the thz optical path part, they used linbo₃the crystal tilted wavefront method generates thz ultrafast pulses, which are focused to the stm tunnel junction through a parabolic mirror to achieve coupling between stm and thz pulses. by confocaling two beams of thz pulses with adjustable time delays to the stm tunnel junction, a thz pump-probe experiment can be performed, thereby achieving time resolution. because this novel design brings operational convenience and can efficiently achieve better thz focusing,the photocurrent can reach a maximum level of 100pa at a repetition frequency of 1mhz, corresponding to each pulse driving 1000 electrons to be emitted from the tip. this photocurrent intensity has not been reported in the past literature.

figure: (left) the stm scanning head is suspended from the bottom of the cold head and the ultra-high vacuum chamber through springs. (right) atomic resolution image of the double-pulse autocorrelation signal on the ag(111) surface and the photocurrent signal extracted using chopper-phase locking on the fese surface.

at present, this system has observed stable photocurrent signals in a variety of materials, and can achieve sub-picosecond time resolution and atomic level spatial resolution.for example, imaging using thz photocurrent on a fese sample observed a clear lattice of se atoms, and observed that the thz photocurrent exhibits unique behavior near defects, meaning that the photocurrent is able to carry dc tunneling current that cannot the information reflected; the thz photocurrent double-pulse autocorrelation signal obtained on the ag(111) surface has a full width at half maximum of 700fs, indicating that this thz-stm can achieve sub-picosecond time resolution. the successful construction of this system has opened up a new world for the realization of ultrafast time-resolved stm technology, providing a powerful tool for studying dynamic processes in condensed matter physics. at the same time, many innovative designs in this system have also provided information for other optically coupled stms. new ideas.

this work was funded by the national natural science foundation of china and the chinese academy of sciences. researchers wu kehui, researcher chen lan and associate researcher cheng peng of the sf09 group of the institute of physics guided doctoral students zhang huaiyu, tian dacheng, liu zijia and ma chen to complete this work. related work was published under the title "the development of a low-temperature terahertz scanning tunneling microscope based on a cryogen-free scheme"Rev. Sci. Instrum. 95, 093703(2024).