Optically pumped InP: Nuclear polarization from NMR frequency shifts

Abstract

There are a number of mechanisms that can produce frequency shifts in the NMR spectra of optically pumped semiconductors, including the hyperfine interaction, nuclear dipolar fields, and indirect-or J-couplings. Using optically pumped Fe-doped InP, we explore how to experimentally distinguish the shift mechanisms from one another, and then exploit the shifts to measure the absolute nuclear polarization. Furthermore, we optically pump, using circularly polarized light, at a much lower field (2.35 T) than previous work, permitting us to explore the field-dependence of the nuclear polarization rate, the spin-lattice relaxation time, and the NMR photon energy spectrum. We measure similar polarizations as obtained at higher fields, but with a significantly faster nuclear polarization rate, making operation at lower fields attractive for optically pumped InP as a source of nuclear spin polarization.

Temporal and spatial evolution of nuclear polarization in optically pumped InP

“…We have previously shown 7 in semi-insulating InP that this contact mechanism, rather than a direct electron-nucleus dipolar mechanism, 8,10 or coupling of nuclei to conduction electrons, is consistent with the experimental results. In this paper we will present additional evidence of this mechanism.…”

“…These two competing effects of local polarization and penetration depth were sorted out by using stray-field NMR imaging (STRAFI) to measure the 31 P OPNMR signal in InP with micron spatial resolution as a function of depth from the surface. 5 These results, as well as our own results, 7 showed that super-bandgap photons are more efficient at generating high local nuclear polarizations than sub-bandgap photons.…”

“…13 Evidence for the existence of localized electrons at ORD sites comes from the small electron-nucleus hyperfine shifts observed with the light on in OPNMR of both GaAs and InP. 7,[14][15][16] The ORD sites with trapped electrons in the presence of light have generally been assumed to act as shallow donors, with the unpaired electron in a hydrogenic s-orbital having a Bohr radius, a 0 , of many nanometers. The fractional number of ORD sites having trapped electrons depends on the intensity of irradiation.…”

Near-band-gap photoinduced nuclear spin dynamics in semi-insulating GaAs: Hyperfine- and quadrupolar-driven relaxation

“…In particular, one can imagine creating onionlike patterns in which the polarization successively changes sign or amplitude over concentric layers of predefined thickness. The formation of such mesoscale polarization structures could be monitored via the NMR line shifts created by the concomitant long-range fields 31 or could be imaged directly using MRFM. 29 Because the pitch in these structures is ultimately influenced by the nuclear spin relaxation mechanism, such experiments could serve as a platform for significant applications, including the detection of trapped charges or the spatial characterization of hyperfine interactions.…”

Electrical detection of spin hyperpolarization in InP

“…3 A spin polarization far beyond thermal equilibrium is known as "hyperpolarization." [4][5][6] Indium phosphide (InP) features easy electronic spin hyperpolarization using circular-polarized photons, due to its direct band gap of 1.34 eV at room temperature (RT). 7,8 Thus, InP may be a perfect candidate for opto-spinelectronics.…”