Time-resolved Kerr Rotation Spectroscopy Research Articles

Spin relaxation time enhancement induced by polarization field screening in an InGaN/GaN quantum well

A correlation between the spin-polarized carrier transfer and spin relaxation processes of a two-dimensional electron gas (2DEG) in an InGaN/GaN quantum well (QW) is investigated by time-resolved Kerr rotation spectroscopy at low temperature. Upon resonant excitation with the GaN barrier band edge energy, the spin polarization of the 2DEG in the QW is acquired from the transfer of spin-polarized photoexcited carriers. Significantly, the spin relaxation time of the 2DEG is enhanced to be as long as 1 ns along with the carrier transfer. It is demonstrated that by tailoring the Rashba and Dresselhaus spin–orbit couplings to approach a spin-degenerate surface, the screening effect of the polarization field leads to a longer spin relaxation time and effective manipulation of the spin relaxation. The polarization field screening induced enhancement of the spin relaxation time is significant in the way for the development of GaN-based spintronic devices.

Layer-dependent optically induced spin polarization in InSe

Optical control of spin in semiconductors has been pioneered using nanostructures of III-V and II-VI semiconductors, but the emergence of two-dimensional van der Waals materials offers an alternative low-dimensional platform for spintronic phenomena. Indium selenide (InSe), a group-III monochalcogenide van der Waals material, has shown promise for opto-electronics due to its high electron mobility, tunable direct bandgap, and quantum transport. There are predictions of spin-dependent optical selection rules suggesting potential for all-optical excitation and control of spin in a two-dimensional layered material. Despite these predictions, layer-dependent optical spin phenomena in InSe have yet to be explored. Here, we present measurements of layer-dependent optical spin dynamics in few-layer and bulk InSe. Polarized photoluminescence reveals layer-dependent optical orientation of spin, thereby demonstrating the optical selection rules in few-layer InSe. Spin dynamics are also studied in many-layer InSe using time-resolved Kerr rotation spectroscopy. By applying out-of-plane and in-plane static magnetic fields for polarized emission measurements and Kerr measurements, respectively, the $g$-factor for InSe was extracted. Further investigations are done by calculating precession values using a $\textbf{k} \cdot \textbf{p}$ model, which is supported by \textit{ab-initio} density functional theory. Comparison of predicted precession rates with experimental measurements highlights the importance of excitonic effects in InSe for understanding spin dynamics. Optical orientation of spin is an important prerequisite for opto-spintronic phenomena and devices, and these first demonstrations of layer-dependent optical excitation of spins in InSe lay the foundation for combining layer-dependent spin properties with advantageous electronic properties found in this material.

Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy

Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule, and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration. Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy, we investigate the spin/valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy. With fine tuning of the photon energy of both pump and probe beams, the valley relaxation process for the neutral excitons and trions is found to be remarkably different—their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds, respectively. The observed long trion spin lifetime of > 2.0 ns is discussed to be associated with the dark trion states, which is evidenced by the photon-energy dependent valley polarization relaxation. Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D'yakonov-Perel' spin relaxation mechanism in wurtzite GaN.

Effective Manipulation of Spin Dynamics by Polarization Electric Field in InGaN/GaN Quantum Wells at Room Temperature.

III‐nitride wide bandgap semiconductors are favorable materials for developing room temperature spintronic devices. The effective manipulation of spin dynamics is a critical request to realize spin field‐effect transistor (FET). In this work, the dependence of the spin relaxation time on external strain‐induced polarization electric field is investigated in InGaN/GaN multiple quantum wells (MQWs) by time‐resolved Kerr rotation spectroscopy. Owing to the almost canceled two different spin–orbit coupling (SOC), the spin relaxation time as long as 311 ps in the MQWs is obtained at room temperature, being much longer than that in bulk GaN. Furthermore, upon applying an external uniaxial strain, the spin relaxation time decreases sensitively, which originates from the breaking of the SU(2) symmetry. The extracted ratio of the SOC coefficients shows a linear dependence on the external strain, confirming the essential role of the polarization electric field. This effective manipulation of the spin relaxation time sheds light on GaN‐based nonballistic spin FET working at room temperature.

Accuracy and form of off-resonance time-resolved Kerr rotation spectroscopy

We show that off resonance time-resolved Kerr rotation (TRKR) is a quantitatively accurate tool to measure spin dynamics by demonstrating that TRKR scans yield essentially identical temporal profiles when the probe energy is tuned far below, far above, and on resonance with the bandgap. We further show that the observed symmetry of the Kerr rotation with respect to probe energy detuning is inconsistent with time-dependent perturbation theory in the simplest case and resolve the discrepancy by recognizing that the optical lifetime decreases with excess energy above the band edge transition.We show that off resonance time-resolved Kerr rotation (TRKR) is a quantitatively accurate tool to measure spin dynamics by demonstrating that TRKR scans yield essentially identical temporal profiles when the probe energy is tuned far below, far above, and on resonance with the bandgap. We further show that the observed symmetry of the Kerr rotation with respect to probe energy detuning is inconsistent with time-dependent perturbation theory in the simplest case and resolve the discrepancy by recognizing that the optical lifetime decreases with excess energy above the band edge transition.

Long valley relaxation time of free carriers in monolayer WSe2

Monolayer transition metal dichalcogenides (TMDs) feature a valley degree of freedom, giant spin-orbit coupling, and spin-valley locking. These exotic natures have stimulated efforts of exploring potential applications in conceptual spintronics, valleytronics, and quantum computing. Among all the exotic directions, a long relaxation time of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominate the optical response due to an enhanced Coulomb interaction in two dimensions. The valley relaxation time of free carriers remains ambiguous. In this Rapid Communication, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselenide. The valley relaxation time of free carriers is found around 2 ns at 70 K, about three orders of magnitude longer than the excitons of about 2 ps, and 15 times larger than that of trions (130 ps). The extended valley relaxation time of free carriers evidences that an exchange interaction dominates the valley relaxation in optical excitations. The pump-probe spectroscopy also reveals an exciton binding energy of 0.60 eV in monolayer ${\mathrm{WSe}}_{2}$.

Temperature dependence of the electron Landé g-factor in cubic GaN

The temperature dependence of the electron Landé g-factor in bulk cubic GaN is investigated over an extremely broad temperature range from 15 K up to 500 K by time-resolved Kerr-rotation spectroscopy. The g-factor is found to be approximately constant over the full investigated temperature range. Calculations by k·p-theory predict a negligible temperature dependence g(T) in complete agreement with the experiment as a consequence of the large band-gap and small spin orbit splitting in cubic GaN.

Optically initialized robust valley-polarized holes in monolayer WSe2.

A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Although monolayer transition metal dichalcogenides with inherent spin–valley coupling offer a unique platform to develop such valleytronic devices, the anticipated long-lived valley pseudospin has not been observed yet. Here we demonstrate that robust valley-polarized holes in monolayer WSe2 can be initialized by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a long-lived valley polarization for positive trion with a lifetime approaching 1 ns at low temperatures, which is much longer than the trion recombination lifetime (∼10–20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley. The optically initialized valley pseudospin of holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on transition metal dichalcogenides.

Spin Coherence and Dephasing of Localized Electrons in Monolayer MoS₂.

We report a systematic study of coherent spin precession and spin dephasing in electron-doped monolayer MoS2. Using time-resolved Kerr rotation spectroscopy and applied in-plane magnetic fields, a nanosecond time scale Larmor spin precession signal commensurate with g-factor |g0| ≃ 1.86 is observed in several different MoS2 samples grown by chemical vapor deposition. The dephasing rate of this oscillatory signal increases linearly with magnetic field, suggesting that the coherence arises from a subensemble of localized electron spins having an inhomogeneously broadened distribution of g-factors, g0 + Δg. In contrast to g0, Δg is sample-dependent and ranges from 0.042 to 0.115.

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.

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.

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.

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.

Dyakonov-Perel electron spin relaxation in a wurtzite semiconductor: From the nondegenerate to the highly degenerate regime

Electron spin lifetimes are investigated in bulk wurtzite $n$-GaN up to very high doping densities by time-resolved Kerr-rotation spectroscopy. The doping densities range from $5\ifmmode\times\else\texttimes\fi{}{10}^{15}$ to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{19}$ cm${}^{\ensuremath{-}3}$, corresponding to Fermi temperatures as high as 1200 K. The spin lifetime shows a maximum at the onset of degeneracy. The additional determination of momentum scattering times allows for a direct comparison with an analytical expression of the spin relaxation tensor in wurtzite semiconductors for a degenerate electron gas following Dyakonov-Perel theory. We find good agreement with the experiment up to the highest densities without any fitting parameter. The only known theoretical value ${\ensuremath{\alpha}}_{e}=$ 9.0 meV \AA{} of the $k$-linear contribution to spin-orbit coupling in the conduction band is shown to be valid up to the highest doping densities.

Determining the sign of g factor via time-resolved Kerr rotation spectroscopy with a rotatable magnetic field

Time-resolved Kerr rotation spectroscopy is used to determine the sign of the g factor of carriers in a semiconductor material, with the help of a rotatable magnetic field in the plane of the sample. The spin precession signal of carriers at a fixed time delay is measured as a function of the orientation of the magnetic field with a fixed strength B. The signal has a sine-like form and its phase determines the sign of the g factor of carriers. As a natural extension of previous methods to measure the (time-resolved) photoluminescence or time-resolved Kerr rotation signal as a function of the magnetic field strength with a fixed orientation, such a method gives the correct sign of the g factor of electrons in GaAs. Furthermore, the sign of carriers in a (Ga, Mn)As magnetic semiconductor is also found to be negative.

Spin relaxation in submonolayer and monolayer InAs structures grown in a GaAs matrix

Electron-spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3--1 monolayer) embedded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin-relaxation time of the submonolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. The electron-spin-relaxation time and the effective $g$ factor in submonolayer samples were found to be strongly dependent on the photogenerated carrier density. The contribution from both the D'yakonov-Perel' mechanism and Bir-Aronov-Pikus mechanism are discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

OPTICALLY INDUCED DYNAMIC NUCLEAR SPIN POLARIZATION IN QUANTUM HALL REGIME OBSERVED BY A TIME-RESOLVED KERR ROTATION

The dynamics between electron and nuclear spins in quantum Hall regime is investigated by a time-resolved Kerr rotation (TRKR) spectroscopy carried down in tilted-field geometry. The spin-flip energy varies almost linearly with the applied magnetic field under the nuclear spin depolarizing radio frequency field and the helicity modulation of circularly polarized pump, while the spin-flip energy obtained under a circularly polarized pump deviates from linearly behavior. This deviation is ascribed to the Overhauser shift due to dynamic nuclear spin polarization (DNP). The DNP depends markedly on the filling factor and present peaks at even filling factors. These results can be well explained by a simple model based on the detailed balance between electron and nuclear spins.

Ultrafast optical manipulation of single electron spins in quantum dots

Most schemes for quantum information processing require fast single qubit operations as well as initialization and read-out steps. Here we describe recent experimental realizations of schemes for time-resolved initialization, coherent control, and read-out of a single electron spin state in an individual quantum dot using all-optical techniques. The spin state is manipulated via the optical Stark effect using off-resonant, picosecond-scale optical pulses. The coherent rotation of a single electron spin state through arbitrary angles up to π radians is demonstrated, constituting a single qubit gate. This spin manipulation is directly monitored using time-resolved Kerr rotation spectroscopy allowing for the observation of the coherent evolution of a single electron spin state in time as well as revealing the spin lifetime and g -factor.

Spin dynamics of two-dimensional electrons in a quantum Hall system probed by time-resolved Kerr rotation spectroscopy

Time-resolved Kerr rotation spectroscopy under the radio frequency field to depolarize dynamic nuclear polarization reveals the intrinsic spin-relaxation time $({T}_{2}^{\ensuremath{\ast}})$ and $g$ factor of two-dimensional electrons in a quantum Hall system. Out-of-plane magnetic field increases the spin coherence drastically through the Landau level quantization. ${T}_{2}^{\ensuremath{\ast}}$ is enhanced strongly around odd filling factors where a quantum Hall ferromagnet is formed. Collapse of spin coherence and appearance of an anomalous Kerr signal observed around $\ensuremath{\nu}=1$ are discussed in the relation to the formation of Skyrmions.