Time-sequenced optical nuclear magnetic resonance of gallium arsenide

Citation

Buratto, Steven Keith (1993) Time-sequenced optical nuclear magnetic resonance of gallium arsenide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HHY0-JX87. https://resolver.caltech.edu/CaltechETD:etd-11092004-164546

Abstract

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This dissertation describes the development of a new method of optical NMR, time-sequenced optical NMR (TSONMR), of GaAs for which both sensitivity and resolution are optimized. In this method, the three processes of optical nuclear polarization (ONP), nuclear magnetic resonance (NMR), and optical detection (OD) occur in distinct sequential periods achieving order-ofmagnitude improvements insensitivity and resolution relative to the earlier quasi-steady-state methods. The TSONMR experiment is also flexible, allowing both continuous wave (CW) and time-domain TSONMR experiments to be performed. The underlying physics is used to develop a quantitative theory of sensitivity which is in good agreement with experiment.

Applications to epitaxial p-type GaAs samples show that the CW and Fourier transform (FT) versions of TSONMR are sensitive to different nuclear sites. The locus of the method is an unknown optically relevant defect (ORD) present at [...]. The CW experiment is sensitive to the more numerous bulk spins far from the ORD, while the FT experiment is sensitive to the low abundance sites which are quadrupole-perturbed by proximity to the ORD. The difference between the two experiments can be understood by accounting for and manipulating the role of spin diffusion. Nutation experiments and the dependence on sample orientation rule out the alternative interpretation that only strongly perturbed sites are seen. Thus, the range of the optically induced hyperfine coupling at the ORD far exceeds that of valence-band electric field perturbations.

The FT-TSONMR experiment is sample specific as is demonstrated for two electrically-equivalent p-type samples (both containing [...]). One sample exhibits much stronger electric field gradient perturbations than the other indicating differences at the atomic level in the defects responsible for localization.

It is also shown on a bulk sample that the method allows high-resolution measurement of the optically induced Knight shift which is proportional to the electron density at the nucleus. Extensions of these observations to quantum wells are proposed. For a [...] quantum well, sample calculations show that the optical Knight shift will provide spectral resolution of individual atomic layers. Optical-rf multiple-pulse sequences are described to minimize other contributions to the linewidth. It should therefore be possible to map out the electron probability across the GaAs layer of a quantum well and give great insight into the electronic wavefunction near the interface as well as the center of the well.

Thesis Files