This Dissertation presents the magneto-optical properties of monolayer (ML) transition metal dichalcogenide (TMDC) materials using our several magneto-optical setups that were developed at UB. In this Dissertation, we discuss a magneto-photoluminescence (PL) setup, a broadband magneto-FTIR setup, and a two-color spectroscopy setup in detail. We also discuss the double modulation technique, which we use in two-color spectroscopy.
The primary results of this work include magneto-PL measurements of ML WSe2 on YIG. We pump these materials with circularly polarized light and analyze with a circular polarizer. We reported a 30% polarization and 10 nm peak shift in a localized state with an applied magnetic field. We see a polarization up to T = 80 K. By changing the magnetic field from –7 Tesla to +7 Tesla, localized impurity-bound exciton states show strong polarization under optical excitation of opposite helicity. Right circularly polarized PL peaks are shifted to lower energies and their PL become stronger than left circularly polarized PL peaks. This is opposite for left circularly polarized peaks. They shift to higher energies (shorter wavelengths) and become weaker than right circularly polarized peaks. We also found that localized states show more polarization than free exciton and trion peaks on YIG substrate.
We also investigated Kerr rotation and Kerr ellipticity properties of ML MoS2 and ML WSe2 on YIG with our new broadband magneto—FTIR optical setup. Samples and substrate do not show any Kerr ellipticity features when exposed to a changing magnetic field. All samples show strong magnetic field dependent Kerr rotation signal but we found that ML MoS2 by itself does not show any magnetic field dependent Kerr rotation signal. We found that there are two broad peaks in the YIG and ML WSe2 on YIG Kerr rotation spectrum. YIG’s two broad peak centers are located at around 1800 cm–1 and 2300 cm–1 and ML WSe2 on YIG peak centers are located at around 1900 cm –1 and 2500 cm–1. For both samples, these peak intensities are linear with the magnetic field and they are symmetric with respect to B = 0 T. ML WSe2 on YIG peaks are shifted to higher energies with respect to YIG peak. We also report that the center of the peaks has no shift with a magnetic field.
With our two-color spectroscopy setup, we have tested Imamoglu’s theory that predicts a splitting of dark 2p states at B = 0 Tesla. A circularly polarized laser and a linearly polarized IR laser were used together to excite electrons to dark states. We used red or green laser and CO or CO2 IR laser together in our experimental setup. Samples are ML MoS2 on sapphire and ML WS2 on Si/SiO2. Within a sensitivity of 10 µrad, we did not see any splitting at B = 0 Tesla on any samples.
|Commitee:||Zeng, Hao, Zutic, Igor|
|School:||State University of New York at Buffalo|
|School Location:||United States -- New York|
|Source:||DAI-B 80/07(E), Dissertation Abstracts International|
|Subjects:||Physics, Condensed matter physics|
|Keywords:||Broadband infrared, Lock-in technique, Magneto optical setups, Magneto polarimetry, Monolayer transition metal dichalcogenides, Two-color spectroscopy|
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