Honors Theses and Capstones

Date of Award

Summer 2022

Project Type

Senior Thesis

College or School

CEPS

Department

Physics

Program or Major

Physics

Degree Name

Bachelor of Science

First Advisor

Shawna Hollen

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.

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