Helicity independent optically-pumped nuclear magnetic resonance in gallium arsenide

Abstract

We present new phenomenology for optically-pumped nuclear magnetic resonance ͑OPNMR͒ of gallium arsenide. When pumping at low irradiation intensity, the OPNMR signal becomes independent of light helicity. The results are consistent with a mechanism in which the bulk signal represents competition between nuclear quadrupolar and electron-nuclear hyperfine relaxation. This mechanism is further supported by the scaling behavior of OPNMR for isotopes with varying hyperfine and quadrupolar interactions. These results indicate the magnitude and sign of nuclear polarization in the sample may be controlled as a function of depth by tuning photon energy and laser intensity, portending submicron scale patterning of nuclear magnetization.

Optically rewritable patterns of nuclear magnetization in gallium arsenide

“…2 correspond directly to the sign and magnitude of nuclear magnetization as a function of depth in the sample. At the lowest illumination intensity (4 mW cm − 2 ), the polarization is positive throughout the sample, and is nearly independent of helicity, consistent with the bulk experiments reported previously 12 . At the highest illumination intensity (102 mW cm − 2 ) the sign of the polarization throughout much of the sample may be controlled with light polarization, consistent with many previous bulk studies in this regime 8,16 .…”

“…Both the magnitude and sign of the polarization of nuclei undergoing DNP may be controlled by the polarization of the incident light. More recently, we identifi ed a second mechanism 12,13 for nuclear polarization that arises from the interaction of a nuclear electric quadrupole moment with fl uctuating electric fi eld gradients at a recombination centre. Th ese gradients arise from the spin-exchange process at recombination centres and result in a positive nuclear polarization by facilitating thermal relaxation.…”

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

“…This sample belongs to a lot identical to that used in prior studies: [17][18][19][20][21] It has resistivity greater than 10 7 cm, mobility greater than 6000 cm 2 /(V s), and the surface orientation is [100]. Secondary ion beam spectrometry (SIMS) data reveal the presence of S impurities at a concentration of 7 × 10 14 ; other impurities including H, C, O, Si, Cr, Mn, and Fe are either absent or below the SIMS detection limit.…”

“…As long recognized, this channel of nuclear spin relaxation is key to inducing helicity-dependent nuclear spin order, and thus forms the foundation of OPNMR. 22 More recently a second mechanism was proposed 23 and verified experimentally 20,21 whereby fluctuating electric field gradients in the vicinity of the intermittently photoelectron-occupied defects lead to nuclear spin relaxation via quadrupolar interactions. Such fluctuations are inherently insensitive to light helicity, such that quadrupolar relaxation always drives the nuclear spin ensemble towards equilibrium at the applied magnetic field and temperature.…”

“…Such fluctuations are inherently insensitive to light helicity, such that quadrupolar relaxation always drives the nuclear spin ensemble towards equilibrium at the applied magnetic field and temperature. As shown previously 20,21 the dominating mechanism can be experimentally dictated using, for example, laser light intensity to change the fractional photoelectron occupation of the defect site to a higher value (driving hyperfine-mediated optical pumping) or to a lower value (driving quadrupolar relaxation to thermal equilibrium).…”