Honors Theses and Capstones

Date Completed

Summer 2022

Abstract

Scanning tunneling microscopes allow for atomic spatial resolution but the resulting images are necessarily time-averaged and fast dynamics are lost. Pump-probe spectroscopy is a common optical technique used to measure ultrafast electronic dynamics but the integration of optical pump-probe spectroscopy into an STM requires specialized knowledge and equipment. Alternatively, an all-electronic pump-probe spectroscopy technique has recently been developed for use with an STM that replaces the laser pulses of optical pump-probe with voltage pulses. In this paper, I implemented an all-electronic pump-probe scheme into an existing scanning tunneling microscope using an arbitrary waveform generator and a lock-in amplifier. I developed a python package and associated graphical user interface to control the pump-probe instrumentation and preform all-electronic pump-probe spectroscopy on a sample of the transition-metal dichalcogenide 1T-TaS2. A charge density wave transition occurs in 1T-TaS2 with hysteresis in a temperature range around 180~K to 220~K. I test the possibility to force this transition to occur with a voltage pump pulse. Average resulting dynamics can be measured by use of a voltage probe pulse. Pump-probe spectroscopy was taken at the surface of 1T-TaS2 at 11~K, 180~K, and 220~K, and is also taken on HOPG at 11~K and 220~K as a control. TaS2 at 11K and 180K and HOPG both showed clear autocorrelation functions but with large ringing-like artifacts. The autocorrelation functions measured here are a clear indication that our system is working as intended and it remains only to either reduce the artifacts or understand them. A major source of these artifacts was identified as the lack of high-frequency wiring used to transfer the pump-probe pulse train to the sample.Scanning tunneling microscopes allow for atomic spatial resolution but the resulting images are necessarily time-averaged and fast dynamics are lost. Pump-probe spectroscopy is a common optical technique used to measure ultrafast electronic dynamics but the integration of optical pump-probe spectroscopy into an STM requires specialized knowledge and equipment. Alternatively, an all-electronic pump-probe spectroscopy technique has recently been developed for use with an STM that replaces the laser pulses of optical pump-probe with voltage pulses. In this paper, I implemented an all-electronic pump-probe scheme into an existing scanning tunneling microscope using an arbitrary waveform generator and a lock-in amplifier. I developed a python package and associated graphical user interface to control the pump-probe instrumentation and preform all-electronic pump-probe spectroscopy on a sample of the transition-metal dichalcogenide 1T-TaS2. A charge density wave transition occurs in 1T-TaS2 with hysteresis in a temperature range around 180~K to 220~K. I test the possibility to force this transition to occur with a voltage pump pulse. Average resulting dynamics can be measured by use of a voltage probe pulse. Pump-probe spectroscopy was taken at the surface of 1T-TaS2 at 11~K, 180~K, and 220~K, and is also taken on HOPG at 11~K and 220~K as a control. TaS2 at 11K and 180K and HOPG both showed clear autocorrelation functions but with large ringing-like artifacts. The autocorrelation functions measured here are a clear indication that our system is working as intended and it remains only to either reduce the artifacts or understand them. A major source of these artifacts was identified as the lack of high-frequency wiring used to transfer the pump-probe pulse train to the sample.

First Advisor

Shawna Hollen

College or School

CEPS

Department or Program

Physics

Degree Name

Bachelor of Science

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