Optical Pumping NMR Research Articles

Enhanced NMR with Optical Pumping Yields 75As Signals Selectively from a Buried GaAs Interface.

We have measured the 75As signals arising from the interface region of single-crystal semi-insulating GaAs that has been coated and passivated with an aluminum oxide film deposited by atomic layer deposition (ALD) with optically pumped NMR (OPNMR). Using wavelength-selective optical pumping, the laser restricts the volume from which OPNMR signals are collected. Here, OPNMR signals were obtained from the interface region and distinguished from signals arising from the bulk. The interface region is highlighted by interactions that disrupt the cubic symmetry of the GaAs lattice, resulting in quadrupolar satellites for nuclear [Formula: see text] isotopes, whereas NMR of the "bulk" lattice is nominally unsplit. Quadrupolar splitting at the interface arises from strain based on lattice mismatch between the GaAs and ALD-deposited aluminum oxide due to their different coefficients of thermal expansion. Such spectroscopic evidence of strain can be useful for measuring lattice distortions at heterojunction boundaries and interfaces.

Modelling of OPNMR phenomena using photon energy-dependent 〈Sz〉 in GaAs and InP

We have modified the model for optically-pumped NMR (OPNMR) to incorporate a revised expression for the expectation value of the z-projection of the electron spin, 〈Sz〉 and apply this model to both bulk GaAs and a new material, InP. This expression includes the photon energy dependence of the electron polarization when optically pumping direct-gap semiconductors in excess of the bandgap energy, Eg. Rather than using a fixed value arising from coefficients (the matrix elements) for the optical transitions at the k=0 bandedge, we define a new parameter, Sopt(Eph). Incorporating this revised element into the expression for 〈Sz〉, we have simulated the photon energy dependence of the OPNMR signals from bulk semi-insulating GaAs and semi-insulating InP. In earlier work, we matched calculations of electron spin polarization (alone) to features in a plot of OPNMR signal intensity versus photon energy for optical pumping (Ramaswamy et al., 2010). By incorporating an electron spin polarization which varies with pump wavelength into the penetration depth model of OPNMR signal, we are able to model features in both III-V semiconductors. The agreement between the OPNMR data and the corresponding model demonstrates that fluctuations in the OPNMR intensity have particular sensitivity to light hole-to-conduction band transitions in bulk systems. We provide detailed plots of the theoretical predictions for optical pumping transition probabilities with circularly-polarized light for both helicities of light, broken down into illustrative plots of optical magnetoabsorption and spin polarization, shown separately for heavy-hole and light-hole transitions. These plots serve as an effective roadmap of transitions, which are helpful to other researchers investigating optical pumping effects.

Assignments of transitions in optically-pumped NMR of GaAs/AlGaAs quantum wells on a bulk GaAs substrate

We investigate optically-pumped nuclear magnetic resonance (OPNMR) in GaAs/AlGaAs multiple quantum wells. The photon energy dependence of the ${}^{69}\text{Ga}$ OPNMR shows a sign change in the signal at the light-hole transition for both ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ circular polarized irradiation. The energy for this transition and those of other Landau levels can be predicted by an eight-band $\stackrel{P\vec}{k}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{P\vec}{p}$ theory used to calculate the conduction band spin polarization. Comparison of the spin polarization with the OPNMR measurements shows the origin of many features. In addition, we find features associated with the bulk GaAs substrate. In an external magnetic field, there are regions of low quantum well absorptivity between the Landau levels and in this region, OPNMR signals arising from the bulk GaAs substrate can be observed. The discrete nature of transitions between Landau levels in two-dimensional electron systems compared to the Landau levels in three-dimensional systems leads to the change in the phase of the OPNMR signal at the light-hole transitions.

Evidence for the light hole in GaAs/AlGaAs quantum wells from optically-pumped NMR and Hanle curve measurements

Optically-pumped 69Ga NMR (OPNMR) and optically-detected measurements of polarized photoluminescence (Hanle curves) show a characteristic feature at the light hole-to-conduction band transition in a GaAs/AlxGa1−xAs multiple quantum well sample. OPNMR data are often depicted as a “profile” of the OPNMR integrated signal intensity plotted versus optical pumping photon energy. What is notable is the inversion of the sign of the measured 69Ga OPNMR signals when optically pumping this light hole-to-conduction band energy in OPNMR profiles at multiple external magnetic fields (B0=4.7T and 3T) for both σ+ and σ− irradiation. Measurements of Hanle curves at B0=0.5T of the same sample exhibit similar phase inversion behavior of the Hanle curves at the photon energy for light hole excitation. The zero-field value of the light-hole state in the quantum well can be predicted for the quantum well structure using the positions of each of these signal-inversion features, and the spin splitting term in the equation for the transition energy yields consistent values at 3 magnetic fields for the excitonic g-factor (gex). This study demonstrates the application of OPNMR and optical measurements of the photoluminescence to detect the light hole transition in semiconductors.

Optically pumped NMR: Revealing spin-dependent Landau level transitions in GaAs

We show that high-resolution optically pumped NMR (OPNMR) studies can reveal spin-dependent optical transitions between valence- and conduction-band Landau levels in bulk semiconductors such as GaAs. The OPNMR signal intensity exhibits oscillations as a function of pump photon energy that evolve with magnetic field. In contrast to standard polarized magnetoabsorption measurements, OPNMR is sensitive to the polarization of the photoexcited electron spins (i.e., the difference between spin-up and spin-down electron populations rather than the sum). This allows one to clearly resolve the spin dependence of optical transitions that might normally be obscured in conventional magnetoabsorption studies. The data are in good agreement with theoretical calculations of the transitions from the spin-split light-hole Landau levels in the valence band to the conduction-band Landau levels of GaAs.

Manifestation of Landau level effects in optically-pumped NMR of semi-insulating GaAs

Oscillations in the magnitude of optically generated nuclear spin polarization as a function of photon energy have been observed in bulk semiconductors using optically-pumped NMR (OPNMR). We compare experimental measurements of (69)Ga OPNMR with experimental measurements of magneto-absorption in bulk GaAs at low temperatures (6 K) and multiple magnetic fields. These data show that the photon energy dependence of both the OPNMR signal intensities and the magneto-absorption coefficients are well correlated over the studied energy regime approximately 1.515-1.568 eV, i.e., at and above the bandgap energy. Using the magneto-absorption coefficients as an input to a previously reported model of OPNMR, we are able to capture many of the oscillatory features in the (69)Ga signal, demonstrating that the origin of these oscillations are magnetic in nature, namely the formation of Landau levels. The nuclear spins as probed by OPNMR provide a measure of the circularly-polarized optical interband transitions between Landau levels. This correlation shows the importance of magneto-absorption measurements for understanding the complex photon energy dependence profile of OPNMR.

Helicity asymmetry of optically pumped NMR spectra in GaAs

Department of Chemical Engineering, University of California, Berkeley; Berkeley, CA 94720(Dated: May 9, 2008)The origin of ˙ asymmetries in the optically-pumped NMR signal and hyper ne shift in GaAs isderived analytically and tested experimentally. The ratio of the optically-pumped to the equilibriumelectron polarizations is a key parameter in determining both asymmetries. Variations in asymmetrywith photon energy and laser power reect variations in the local temperature and the electronspin polarization, and these two quantities are extracted from the asymmetry through a simplemethodology. Other contributions to the asymmetry are considered.

Optically pumped nuclear magnetic resonance of semiconductors

Optically pumped NMR (OPNMR) of direct gap and indirect gap semiconductors has been an area of active research interest, motivated by both basic science and technological perspectives. Proposals to enhance and to spatially localize nuclear polarization have stimulated interest in this area. Recent progress in OPNMR has focused on exploring the experimental parameter space in order to elucidate details of the underlying photophysics of optical pumping phenomena. The focus of this review is on recent studies of bulk samples of GaAs and InP, namely, the photon energy dependence, the magnetic field dependence, and the phase dependence of OPNMR resonances. Models for the development of nuclear polarization are discussed.

Physical insights from a penetration depth model of optically pumped NMR

A model of optically pumped NMR (OPNMR) behavior in GaAs that connects the photon energy dependence of the OPNMR signal intensity for (69)Ga with different polarizations of light has been developed. Inputs to this model include experimental conditions--external magnetic field (B(0)), temperature (T), and optical pumping parameters (tau(L), laser helicity)--as well as parameters that arise from sample-specific characteristics--electron spin lifetime (T(1e)), electron lifetime (tau(e)), electron-nuclear correlation time (tau(c)), and sample thickness (z). These various inputs affect the profile of the OPNMR signal intensity as a function of photon energy (E) in a predictable manner. Therefore, the profile can serve as a composite fingerprint by which individual parameters can be inferred when not known. Characteristics of the profile include the photon energy for maximum OPNMR signal intensity and the intensity ratio between sigma(+) and sigma(-) light.

Development of a dynamic nuclear polarization system based on the optical pumping method

We have developed a dynamic nuclear polarization system based on the optical pumping NMR method. It consists of an NMR spectrometer, laser and optical system, optical-pumping double-resonance probe, cryostat and superconducting magnet. The double-resonance probe newly developed for this system is of a top-loading type, characterized by a single-coil double-tuning tank circuit with traps. The system allows simultaneous control of two radio-frequency fields and a light beam irradiated to samples, enabling us to perform cross-polarization experiments under the optical pumping condition. As a test of the system, a cross-polarization experiment has been performed with indium phosphide doped with iron.

Optical pumping NMR in the compensated semiconductor InP:Fe

The optical pumping NMR effect in the compensated semiconductor InP:Fe has been investigated in terms of the dependences of photon energy (E_p), helicity (sigma+-), and exposure time (tau_L) of infrared lights. The {31}P and {115}In signal enhancements show large sigma+- asymmetries and anomalous oscillations as a function of E_p. We find that (i) the oscillation period as a function of E_p is similar for {31}P and {115}In and almost field independent in spite of significant reduction of the enhancement in higher fields. (ii) A characteristic time for buildup of the {31}P polarization under the light exposure shows strong E_p-dependence, but is almost independent of sigma+-. (iii) The buildup times for {31}P and {115}In are of the same order (10^3 s), although the spin-lattice relaxation times (T_1) are different by more than three orders of magnitude between them. The results are discussed in terms of (1) discrete energy spectra due to donor-acceptor pairs (DAPs) in compensated semiconductors, and (2) interplay between {31}P and dipolar ordered indium nuclei, which are optically induced.

Physics of the insulating phase in dilute two-dimensional electron gas

We propose to use the radio-frequency single-electron transistor as an extremely sensitive probe to detect the time-periodic ac signal generated by sliding electron lattice in the insulating state of the dilute two-dimensional electron gas. We also propose to use the optically-pumped NMR technique to probe the electron spin structure of the insulating state. We show that the electron effective mass and spin susceptibility are strongly enhanced by critical fluctuations of electron lattice in the vicinity of the metal-insulator transition, as observed in experiment.

Optical Pumping System for a Qubit Initializer in a Solid-state NMR Quantum Computer

We present an optical pumping NMR system working at low temperatures developed for the qubit initialization in a solid-state NMR quantum computer. Demonstrations of the initialization experiments of 31P nuclear spins in InP:Fe at 4.2 K are also presented, where the signal enhancements of 31P by two orders of magnitude compared to that at thermal equilibrium are achieved.

Development of a nuclear spin polarizer with the optical pumping method

We have developed an optical pumping NMR system for semiconductors as an effective nuclear spin polarizer for a solid-state NMR quantum computer. The system was successfully applied to enhance 31P nuclear spin polarization in InP doped with Fe, and the enhancement of the 31P NMR signal by more than two orders of magnitude was achieved. We also observed the strong dependences of the enhancement on the helicity and the photon energy of incident light. The most effective enhancement was achieved with the helicity σ + and photon energy smaller than the bandgap by 10∼15 meV.

Continuous-flow optical pumping NMR in a closed circuit system.

In a typical continuous-flow optical pumping setup, the chemical shift of xenon in the adsorbed phase depends on the gas flow rate due to warming of the sample surface by the gas stream. Calibration of the system using the (207)Pb resonance of solid lead nitrate is necessary to determine the actual sample temperature. Optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate alters signal intensity ratios and may lead to the complete suppression of signals.

Measurement of minority carrier lifetime and diffusion length in silicon epitaxial layers by means of the photo-current technique : J. Muller, H. Bernt and H. Reichl. Solid-St. Electron.21, 999 (1978)

We have modified the model for optically-pumped NMR (OPNMR) to incorporate a revised expression for the expectation value of the z-projection of the electron spin, 〈Sz〉 and apply this model to both bulk GaAs and a new material, InP. This expression includes the photon energy dependence of the electron polarization when optically pumping direct-gap semiconductors in excess of the bandgap energy, Eg. Rather than using a fixed value arising from coefficients (the matrix elements) for the optical transitions at the k=0 bandedge, we define a new parameter, Sopt(Eph). Incorporating this revised element into the expression for 〈Sz〉, we have simulated the photon energy dependence of the OPNMR signals from bulk semi-insulating GaAs and semi-insulating InP. In earlier work, we matched calculations of electron spin polarization (alone) to features in a plot of OPNMR signal intensity versus photon energy for optical pumping (Ramaswamy et al., 2010). By incorporating an electron spin polarization which varies with pump wavelength into the penetration depth model of OPNMR signal, we are able to model features in both III-V semiconductors. The agreement between the OPNMR data and the corresponding model demonstrates that fluctuations in the OPNMR intensity have particular sensitivity to light hole-to-conduction band transitions in bulk systems. We provide detailed plots of the theoretical predictions for optical pumping transition probabilities with circularly-polarized light for both helicities of light, broken down into illustrative plots of optical magnetoabsorption and spin polarization, shown separately for heavy-hole and light-hole transitions. These plots serve as an effective roadmap of transitions, which are helpful to other researchers investigating optical pumping effects.