Two photon photoemission.

• Main Author: Goh, Teck Wee.
• Other Authors: Huan Cheng Hon, Alfred
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Science::Physics::Atomic physics::Solid state physics
• Online Access: http://hdl.handle.net/10356/53657
• Institution: Nanyang Technological University

2PPE Setup In this work, setting up a two photon photoemission (2PPE) system was attempted. The 2PPE setup consists of the third harmonic generation (THG) setup and optical components leading the THG output to the UHV chamber. Power conversion was at 30 mW maximum, which translates to maximum conversion e ciency to three percent from the fundamental light. Challenges and Experiments Performed Over the course of this work, key challenges lie in optimizing the THG setup. Several problems were encountered in the THG setup, mainly low e ciency of THG conversion, long duration required to vary photon wavelength and stabilizing its output, and the instability in modelocking of Mira due to degradation of BBO used for SHG. Preliminary 2PPE experiments, namely angle-resolved 2PPE (AR-2PPE) and photon energy dependent 2PPE measurements could still be performed. The sample which was chosen was a graphene nanoribbon (GNR) sample on gold substrate. This sample was chosen since there were not much 2PPE works on it with only one published work [3] at current time of writing.Results and Concluding Remarks AR-2PPE and photon energy dependent 2PPE were successfully performed on GNR/Au and the electronic structure of the unoccupied and occupied states of GNR were observed. Three unoccupied states shows energy dispersion, indicating charge mobilities in the material. Photon energy dependent 2PPE measurements showed the state positions of these energy levels. This gives a picture of the electronic structure of the GNR/Au, showing a glimpse of the potential 2PPE technique has in complementing optical spectroscopic techniques to perform full characterization of the system. Ongoing Work in Setting Up TR-2PPE Ongoing work is in progress in setting up TR-2PPE in our lab. Optics directing the third harmonic (pump beam) and second harmonic (probe beam) light to the UHV chamber were set up. Existing electronics for the electron analyzer were modi ed by setting up a summing ampli er to combine the analog outputs of individual channeltrons to a single output for data acquisition. Existing pump probe software was modi ed to make it compatible with the electronics built for TR-2PPE. Details about the work in setting up the individual components of the TR-2PPE setup can be found in Section 4.5. Time-resolved measurements could not be performed since time-zero was not found due to the low e ciency of the THG output and the problem of spatial overlap of the pump and probe on the sample. Future work entails optimizing the THG setup for TR-2PPE experiments and address the problems faced in setting up TR-2PPE.