Time-resolved Kerr Rotation Research Articles

Electron g-factor and spin decoherence in GaAs quantum nanodisks fabricated by fully top-down lithography

GaAs nanodisks (NDs), with a thickness of 8nm and a diameter of 15nm, were directly fabricated from GaAs quantum wells (QW) by damage-free neutral-beam etching using bio-nanotemplates. We observed the electron g-factor and spin dephasing in NDs with different barrier heights in the lateral direction by means of time-resolved Kerr rotation. The magnitude of the g-factor depends on the degree of lateral confinement originating from enhanced penetration of the electron wavefunction from an ND into the surrounding AlGaAs barriers. The spin-dephasing time is also observed to be altered by ND formation.

Robustness of n-GaAs carrier spin properties to 5 MeV proton irradiation

Modern electronic devices utilize charge to transmit and store information. This leaves the information susceptible to external influences, such as radiation, that can introduce short timescale charge fluctuations and, long term, degrade electronic properties. Encoding information as spin polarizations offers an attractive alternative to electronic logic that should be robust to randomly polarized transient radiation effects. As a preliminary step towards radiation-resistant spintronic devices, we measure the spin properties of n-GaAs as a function of radiation fluence using time-resolved Kerr rotation and photoluminescence spectroscopy. Our results show a modest to negligible change in the long-term electron spin properties up to a fluence of 1 × 1014 (5 MeV protons)/cm2, even as the luminescence decreases by two orders of magnitude.

Optical detection of anisotropic g-factor and nuclear spin polarization in a single CdTe quantum well

Longitudinal and in-plane electron g-factors, and a nuclear spin polarization (NSP) have been evaluated precisely in a CdTe/Cd0.85Mg0.15Te single quantum well by using the time-resolved Kerr rotation and double lock-in detection techniques. Resident electron spin polarization (RESP) was formed via the negative trion formation and recombination, and RESP gave rise to NSP in an oblique magnetic field configuration. We observed the effective nuclear field of a few mT which was weak compared with that in III–V semiconductor nanostructures as expected, but the nuclear field can be converted to the maximal NSP of 12% in Faraday geometry.

Anisotropic spin dephasing of impurity-bound electron spins in ZnO

We investigate the electron spin dynamics of n-type c-axis oriented bulk zinc oxide (ZnO) by using time-resolved Kerr rotation and resonant spin amplification measurements. Calculating resonant spin amplification using an anisotropic spin dephasing model reveals that there are two species involved in the spin dynamics, which we attribute to conduction and impurity-bound electron spins, respectively. We find that the impurity-bound electron spin dephasing mechanism is strongly anisotropic due to anisotropic exchange interactions. The identification of the two spin species and their dephasing mechanisms is further supported by the temperature, power, and wavelength dependence of the spin coherence measurements.

Negative initial phase shift of Kerr rotation generated from the building-up process of resident electron spin polarization in a CdTe single quantum well

Initial phase shift in a precessional motion of resident electron-spin polarization is studied in a ${\mathrm{CdTe}/\mathrm{Cd}}_{0.85}{\mathrm{Mg}}_{0.15}\mathrm{Te}$ single quantum well using a time-resolved Kerr rotation technique. The generation dynamics of resident electron-spin polarization involve the formation and transformation of the associated optically excited states and are complicated particularly in the early time region. A careful analysis of the phase shift gives a deep understanding of the generation processes. In the experiments, the negative phase shift of the resident electron-spin polarization is observed, and the mechanism associated especially with a quick hole spin flip in negative trions is studied through the dependences on excitation power and magnetic field strength.

High temperature electron spin dynamics in bulk cubic GaN: Nanosecond spin lifetimes far above room-temperature

The electron spin dynamics in n-doped bulk cubic GaN is investigated for very high temperatures from 293 K up to 500 K by time-resolved Kerr-rotation spectroscopy. We find extraordinarily long spin lifetimes exceeding 1 ns at 500 K. The temperature dependence of the spin relaxation time is in qualitative agreement with predictions of Dyakonov-Perel theory, while the absolute experimental times are an order of magnitude shorter than predicted. Possible reasons for this discrepancy are discussed, including the role of phase mixtures of hexagonal and cubic GaN as well as the impact of localized carriers.

Hole-spin dynamics and holeg-factor anisotropy in coupled quantum well systems

Due to its $p$-like character, the valence band in GaAs-based heterostructures offers rich and complex spin-dependent phenomena. One manifestation is the large anisotropy of Zeeman spin splitting. Using undoped, coupled quantum wells (QWs), we examine this anisotropy by comparing the hole-spin dynamics for high- and low-symmetry crystallographic orientations of the QWs. We directly measure the hole $g$ factor via time-resolved Kerr rotation, and for the low-symmetry crystallographic orientations (110) and ($113a$), we observe a large in-plane anisotropy of the hole $g$ factor, in good agreement with our theoretical calculations. Using resonant spin amplification, we also observe an anisotropy of the hole-spin dephasing in the (110)-grown structure, indicating that crystal symmetry may be used to control hole-spin dynamics.

Determination ofs-dexchange coupling in GaMnN by time-resolved Kerr rotation spectroscopy

Coherent electron-spin dynamics in ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{N}$ has been investigated by time-resolved Kerr rotation spectroscopy. The effective electron $g$ factor shows a linear increase with the Mn concentration due to the s-d exchange coupling between the conduction electrons and the $d$-shell electrons of ${\mathrm{Mn}}^{3+}$ impurities. The magnitude and sign of the s-d exchange constant are determined precisely to be ${N}_{0}\ensuremath{\alpha}=+0.23\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, indicative of a ferromagnetic s-d exchange coupling in GaMnN. The determined ${N}_{0}\ensuremath{\alpha}$ is consistent with the typical value found in most diluted magnetic semiconductors and reveals that GaMnN is indeed not an exception.

Electron spin dephasing and optical pumping of nuclear spins in GaN

We have measured the donor-bound electron spin dynamics in cubic GaN by time-resolved Kerr rotation experiments. The ensemble electron spin dephasing time in this quantum dot like system characterized by a Bohr radius of 2.5 nm is of the order of 1.5 ns as a result of the interaction with the fluctuating nuclear spins. It increases drastically when an external magnetic field as small as 10 mT is applied. We extract a dispersion of the nuclear hyperfine field {\delta}Bn $\sim$ 4 mT, in agreement with calculations. We also demonstrate for the first time in GaN based systems the optical pumping of nuclear spin yielding the build-up of a significant nuclear polarization.

All-optical NMR in semiconductors provided by resonant cooling of nuclear spins interacting with electrons in the resonant spin amplification regime

Resonant cooling of different nuclear isotopes manifested in optically induced nuclear magnetic resonances (NMR) is observed in $n$-doped CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)Se quantum wells and for donor-bound electrons in ZnSe:F and GaAs epilayers. By time-resolved Kerr rotation used in the regime of resonant spin amplification, we can expand the range of magnetic fields where the effect can be observed up to nuclear Larmor frequencies of 170 kHz. The mechanism of the resonant cooling of the nuclear spin system is analyzed theoretically. The developed approach allows us to model the resonant spin amplification signals with NMR features.

Effective Nuclear Field Measurement in a Single Quantum Well via Time-Resolved Kerr Rotation Technique

Effective magnetic field of nuclear spin polarization (NSP) under circularly polarized pumps excitation and the dephasing time of resident electron spin polarization under pump and control excitation has been detected in a single CdTe quantum well (QW) by a time-resolved Kerr rotation (TRKR) technique. We deduced that the experimental method is verified with the effect, to a certain extent, through confirming the external magnetic field results. In addition, the nuclear field is revealed to be increased with the increasing electron spin component due to the enhanced electron-nuclear hyperfine interaction. Significant nuclear field is observed as 1.85 mT with the applied magnetic field tilted by about 15 degree. What is more, we found the spin dephasing time (SDT) has little relation with the created NSP field, but it decreases largely with the increasing spin polarization magnitude under pump excitation and the spin vector dispersions induced by control pulses.

Magnetic field effect on electron spin dynamics in (110) GaAs quantum wells

We study the electron spin relaxation in both symmetric and asymmetric GaAs/AlGaAs quantum wells (QWs) grown on (110) substrates in an external magnetic field B applied along the QW normal. The spin polarization is induced by circularly polarized light and is detected using the time-resolved Kerr rotation technique. In the asymmetric structure, where a δ-doped layer on one side of the QW produces the Rashba contribution to the conduction-band spin–orbit splitting, the lifetime of electron spins aligned along the growth axis exhibits an anomalous dependence on B in the range 0 < B < 0.5 T; this results from the interplay between the Dresselhaus and Rashba effective fields which are perpendicular to each other. For larger magnetic fields, the spin lifetime increases, which is a consequence of the cyclotron motion of the electrons and is also observed in (001)-grown quantum wells. The experimental results are in agreement with the calculation of the spin lifetimes in (110)-grown asymmetric quantum wells described by the point group Cs, where the growth direction is not the principal axis of the spin-relaxation-rate tensor.

Room temperature spin transport in undoped (110) GaAs/AlGaAs quantum wells

We are reporting on our first observation of a micrometer-order electron spin transport in a (110) GaAs/AlGaAs multiple quantum well (QW) at room temperature using a space- and time-resolved Kerr rotation technique. A 37-μm transport was observed within an electron spin lifetime of 1.2 ns at room temperature when using an in-plane electric field of 1.75 kV/cm. The spatio-temporal profiles of electron spins were well reproduced by the spin drift-diffusion equations coupled with the Poisson equation, supporting the validity of the measurement. The results suggest that (110) QWs are useful as a spin transport layer for semiconductor spintronic devices operating at room temperature.

Tuning of the Landé g-factor in AlxGa1−xAs/AlAs single and double quantum wells

We report on the spin dynamics of a high mobility two-dimensional electron gas in a AlxGa1−xAs/AlAs double quantum well structure. For high electron density samples, the g-factor was measured using time-resolved Kerr rotation technique. The g-factor tuning capability was observed by changing the aluminum content x independently in each well. Experiments demonstrated an unusual spin dephasing time robustness for high excitation power. The effect of the interaction between wells was analyzed in samples with different tunneling barriers. Results were compared with experiments on single well systems demonstrating higher spin polarization generation, longer spin dephasing time, and coupling for the double structures.

Measurement of the magneto-optical response of Fe and CrO2epitaxial films by pump-probe spectroscopy: Evidence for spin-charge separation

We have investigated the magneto-optical response of Fe and CrO${}_{2}$ epitaxial films by pump-probe polarimetry, showing that charge and spin dynamics can be unambiguously disentangled. The time-resolved Kerr ellipticity and rotation in the metallic sample are essentially identical after the initial transient (shorter than a picosecond), but they considerably differ in the oxide film, even tens of picoseconds past the optical excitation. These differences are determined by the combined effects of photoexcited charge carriers and spins on the Kerr signal, but a detailed polarimetric analysis can explicitly unravel these contributions. In addition, the diagonal and off-diagonal terms of the dielectric tensor can be retrieved, providing the complete dynamical characterization of magnetic and optical properties in a ferromagnet, which is of utmost importance to understand spin evolution in magnetically correlated complex oxide materials.

A strong anisotropy of spin dephasing time of quasi-one dimensional electron gas in modulation-doped GaAs/AlGaAs wires

We investigated the wire orientation dependence of ensemble electron spin dynamics by using a time-resolved Kerr rotation technique in wires made from a modulation-doped GaAs/AlGaAs quantum well. We observed a strong anisotropy of the electron spin dephasing time of as large as 40 depending on the wire orientation at 5 K by tuning the spin-orbit interaction. The wire width dependence of the spin dynamics is reproduced in a Monte Carlo simulation. In addition, two characteristic spin decay rates were observed in wires whose widths were close to the spin precession length. The dependence of the magnetic field direction on the spin dynamics of the wires was also studied to determine the difference from that of two dimensional electron gas.

Long electron spin coherence in ion-implanted GaN: The role of localization

The impact of Ga and Au ion implantation on the electron spin dynamics in bulk wurtzite GaN is studied by time-resolved Kerr-rotation spectroscopy. The spin relaxation time increases strongly by up to a factor of 20 for increasing implantation doses. This drastic increase is caused by a transition from delocalized to localized electrons. We find a characteristic change in the magnetic field dependence of spin relaxation that can be used as a sensitive probe for the degree of localization.

Electron spin dynamics study of bulk p-GaAs: The screening effect

In this work, the electron spin dynamics of bulk p-GaAs doped with Be grown by molecular beam epitaxy is investigated by using the time-resolved magneto-optical Kerr rotation technique. The spin relaxation/dephasing times T1 and T2* are systematically investigated as a function of hole doping density and photo-excitation density as well as temperature. The complex hole doping density dependence of spin relaxation times T1 and T2* is observed experimentally, which agrees well with predictions of the kinetic spin Bloch equation theory published previously [J. Jiang and M. Wu, Phys. Rev. B 79, 125206 (2009)]. D'yakonov-Perel's mechanism [M. Dyakonov and V. Perel, Sov. Phys. Solid State 13, 3023 (1972)] is discussed to dominate the electron spin relaxation process in p-GaAs, with the effect of hole screening proven to play an important role.

Long-lived electron spins in a modulation doped (100) GaAs quantum well

We have measured T1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. T1 lifetimes in excess of 1 μs were measured at 1.5 K and 5.5 T, exceeding the typical T2* lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently localized subsets of electrons.

Electron and hole spins in InP/(Ga,In)P self-assembled quantum dots

The properties of electron and hole spins in InP/(Ga,In)P self-assembled quantum dots are studied through their coherent dynamics using time-resolved Kerr rotation. From these studies information about the $g$ factor and dephasing of the spin excitations is extracted. The electron spin shows a behavior similar to that of electron spins in quantum dots of different material: The $g$ factor is isotropic in the dot plane and with increasing applied magnetic field the spin dephasing accelerates due to variations in the dot ensemble. On the other hand, the hole spin demonstrates a behavior different from other dot systems. Namely, the signal decay on a time scale of about 100 ps does not depend on magnetic field, and the $g$ factor is isotropic in the dot plane. These findings underline previous suggestions that only the electrons are well localized in the InP/(Ga,In)P quantum dots, while the holes are weakly confined and carry bulklike character.