Describing angular momentum conventions in circularly polarized optically pumped NMR in GaAs and CdTe

Michael E West; Erika L Sesti; Matthew M Willmering; Dustin D Wheeler; Zayd L Ma; Sophia E Hayes

doi:10.1016/j.jmr.2021.106980

Describing angular momentum conventions in circularly polarized optically pumped NMR in GaAs and CdTe

J Magn Reson. 2021 Jun:327:106980. doi: 10.1016/j.jmr.2021.106980. Epub 2021 Apr 20.

Authors

Michael E West¹, Erika L Sesti¹, Matthew M Willmering¹, Dustin D Wheeler¹, Zayd L Ma¹, Sophia E Hayes²

Affiliations

¹ Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130, United States.
² Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130, United States. Electronic address: hayes@wustl.edu.

PMID: 33940541
DOI: 10.1016/j.jmr.2021.106980

Abstract

The physical phenomena governing hyperpolarization through optical pumping of conduction electrons continue to be explored in multiple semiconductor systems. One early finding has been the asymmetry between the optically pumped nuclear magnetic resonance (OPNMR) signals when generated by different circular polarizations (i.e., light helicities). Because these resonances are asymmetric, the midpoint between the signals prepared with each of the two circular polarizations is either a positive or negative value, termed an "offset" that is representative of an optical Overhauser enhancement. Both negative offsets (in GaAs) and positive offsets (in CdTe) have been observed. The origins of these offsets in semiconductors are believed to arise from thermalized electrons; however, to the best of our knowledge, no study has systematically tested this hypothesis. To that end, we have adopted two configurations for OPNMR experiments-one in which the Poynting vector of the laser light and magnetic field are parallel, and one in which they are antiparallel, while other experimental conditions are kept the same. We find that the OPNMR signal response to a fixed helicity of light depends on the experimental configuration, and this configuration needs to be accounted for in order to properly describe the OPNMR results. Further, studying the offsets as a function of field strength shows that the optical Overhauser enhancement (the offset) increases in magnitude with field strength. Finally, by describing all angular momentum and phasing conventions unambiguously, we are able to determine that the absorptively-phased appearance of ¹¹³Cd (and ¹²⁵Te) OPNMR in CdTe is a consequence of the sign of the nuclear gyromagnetic ratios for these isotopes.

Keywords: CdTe; GaAs; OPNMR.