arXiv subjects

D. R. Yakovlev

Publications and source records attributed to D. R. Yakovlev.

At least 91 records · Page 5Linked to original sources

arXiv2017

Extended pump-probe Faraday rotation spectroscopy of the submicrosecond electron spin dynamics in n-type GaAs

We develop an extended pump-probe Faraday rotation technique to study the submicrosecond electron spin dynamics with picosecond time resolution in a wide range of magnetic fields. The electron spin dephasing time $T_2^*$ and the longitudinal spin relaxation time $T_1$, both approaching $250$ ns in weak fields, are measured thereby in $n$-type bulk GaAs. By tailoring the pump pulse train through increasing the contained number of pulses, the buildup of resonant spin amplification is demonstrated for the electron spin polarization. The spin precession amplitude in high magnetic fields applied in the Voigt geometry shows a non-monotonic dynamics deviating strongly from a mono-exponential decay and revealing slow beatings. The beatings indicate a two spin component behavior with a $g$-factor difference of $Δg \sim 4\times10^{-4}$, much smaller than the $Δg$ expected for free and donor-bound electrons. This $g$-factor variation indicates efficient, but incomplete spin exchange averaging.

cond-mat.mes-hall↗

arXiv2017

Long-range p-d exchange interaction in a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure

The exchange interaction between magnetic ions and charge carriers in semiconductors is considered as prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range $p-d$ exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10~nm thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10~nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift $Δ_S$ on the external magnetic field $B$. We show that in strong magnetic field $Δ_S$ is a linear function of $B$ with an offset of $Δ_{pd} = 50-100~μ$eV at zero field from the FM induced effective exchange field. On the other hand, the $s-d$ exchange interaction between conduction band electrons and FM, as well as the $p-d$ contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure.

cond-mat.mtrl-sci↗

arXiv2017

High resolution two-dimensional optical spectroscopy of electron spins

Multidimensional coherent optical spectroscopy is one of the most powerful tools for investigating complex quantum mechanical systems. While it was conceived decades ago in magnetic resonance spectroscopy using micro- and radio-waves, it has recently been extended into the visible and UV spectral range. However, resolving MHz energy splittings with ultrashort laser pulses has still remained a challenge. Here, we analyze two-dimensional Fourier spectra for resonant optical excitation of resident electrons to localized trions or donor-bound excitons in semiconductor nanostructures subject to a transverse magnetic field. Particular attention is devoted to Raman coherence spectra which allow one to accurately evaluate tiny splittings of the electron ground state and to determine the relaxation times in the electron spin ensemble. A stimulated step-like Raman process induced by a sequence of two laser pulses creates a coherent superposition of the ground state doublet which can be retrieved only optically due to selective excitation of the same sub-ensemble with a third pulse. This provides the unique opportunity to distinguish between different complexes that are closely spaced in energy in an ensemble. The related experimental demonstration is based on photon echo measurements in an n-type CdTe/(Cd,Mg)Te quantum well structure detected by a heterodyne technique. The difference in the sub-$μ$eV range between the Zeeman splittings of donor-bound electrons and electrons localized at potential fluctuations can be resolved even though the homogeneous linewidth of the optical transitions is larger by two orders of magnitude.

cond-mat.mtrl-sci↗

arXiv2017

Damping of Rabi oscillations in intensity-dependent photon echoes from exciton complexes in a CdTe/(Cd,Mg)Te single quantum well

We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20~nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps-pulses. We demonstrate that the transient profile of the photon echo from the localized trion (X$^-$) and the donor-bound exciton (D$^0$X) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than $π$ due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D$^0$X). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are required to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for the X$^-$ and the D$^0$X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as $μ($X$^-$)=73~D and $μ($D$^0$X)=58~D.

cond-mat.mes-hall↗

arXiv2017

The effect of dynamical compressive and shear strain on magnetic anisotropy in low symmetry ferromagnetic film

Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of magnetic state of thin films and nanostructures via inverse magnetostriction on a picosecond time scale. Here we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga) either by injecting picosecond strain pulse into it from a substrate or by generating dynamical strain of complex temporal profile in the film directly. In the former case we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case optically-generated strain emerged abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession anymore. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on mixed, compressive and shear, character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study.

cond-mat.mes-hall↗

arXiv2017

Direct Measurements of Magnetic Polarons in Cd$_{1-x}$Mn$_x$Se Nanocrystals from Resonant Photoluminescence

In semiconductors, quantum confinement can greatly enhance the interaction between band carriers (electrons and holes) and dopant atoms. One manifestation of this enhancement is the increased stability of exciton magnetic polarons in magnetically-doped nanostructures. In the limit of very strong 0D confinement that is realized in colloidal semiconductor nanocrystals, a single exciton can exert an effective exchange field $B_{\rm{ex}}$ on the embedded magnetic dopants that exceeds several tesla. Here we use the very sensitive method of resonant photoluminescence (PL) to directly measure the presence and properties of exciton magnetic polarons in colloidal Cd$_{1-x}$Mn$_x$Se nanocrystals. Despite small Mn$^{2+}$ concentrations ($x$=0.4-1.6\%), large polaron binding energies up to $\sim$26~meV are observed at low temperatures via the substantial Stokes shift between the pump laser and the resonant PL maximum, indicating nearly complete alignment of all Mn$^{2+}$ spins by $B_{\rm{ex}}$. Temperature and magnetic field-dependent studies reveal that $B_{\rm{ex}} \approx$ 10~T in these nanocrystals, in good agreement with theoretical estimates. Further, the emission linewidths provide direct insight into the statistical fluctuations of the Mn$^{2+}$ spins. These resonant PL studies provide detailed insight into collective magnetic phenomena, especially in lightly-doped nanocrystals where conventional techniques such as nonresonant PL or time-resolved PL provide ambiguous results.

cond-mat.mes-hall↗

arXiv2017

Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic-field-induced circular polarization of photoluminescence, $P_c$, is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behaviour of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $τ_{se} = 33~μ$s, and hole, $τ_{sh} = 3~μ$s, bound in the exciton.

cond-mat.mes-hall↗

arXiv2017

Transient photon echoes from donor-bound excitons in ZnO epitaxial layers

The coherent optical response from 140~nm and 65~nm thick ZnO epitaxial layers is studied using transient four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (D$^0$X$_\text{A}$) at temperature of 1.8~K we evaluate optical coherence times $T_2=33-50$~ps corresponding to homogeneous linewidths of $13-19~μ$eV, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time $T_1=30-40$~ps, while pure dephasing is negligible in the studied high quality samples even for strong optical excitation. Temperature increase leads to a significant shortening of $T_2$ due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D$^0$X$_\text{B}$) is significantly faster ($T_2=3.6$~ps) and governed by pure dephasing processes.

cond-mat.mtrl-sci↗

arXiv2017

Excitation of magnon accumulation by laser clocking as a source of long-range spin waves in transparent magnetic films

Optical tools are of great promise for generation of spin waves due to the possibility to manipulate on ultrashort time scales and to provide local excitation. However, a single laser pulse can inject spin waves only with a broad frequency spectrum, resulting in a short propagation distance and low amplitude. Here we excite a magnetic garnet film by a train of fs-laser pulses with 1 GHz repetition rate so that pulse separation is smaller than decay time of the magnetic modes which allows to achieve collective photonic impact on magnetization. It establishes a quasi-stationary source of SWs, namely a coherent magnon accumulation ("magnon cloud"). This approach has several appealing features: (i) the source is tunable; (ii) the SW amplitude can be significantly enhanced; (iii) the spectrum of the generated SWs is quite narrow that provides longer propagation distance; (iv) the periodic pumping results in almost constant in time SW amplitude up to 100 um away from the source; and (v) the SW emission shows a pronounced directionality. These results expand the capabilities of ultrafast coherent optical control of magnetization and pave a way for applications in data processing, including the quantum regime. The quasi-stationary magnon accumulation might be also of interest for the problem of magnon Bose-Einstein condensate.

cond-mat.mes-hall↗

arXiv2017

Spin dynamics of quadrupole nuclei in InGaAs quantum dots

Photoluminescence polarization is experimentally studied for samples with (In,Ga)As/GaAs selfassembled quantum dots in transverse magnetic field (Hanle effect) under slow modulation of the excitation light polarization from fractions of Hz to tens of kHz. The polarization reflects the evolution of strongly coupled electron-nuclear spin system in the quantum dots. Strong modification of the Hanle curves under variation of the modulation period is attributed to the peculiarities of the spin dynamics of quadrupole nuclei, which states are split due to deformation of the crystal lattice in the quantum dots. Analysis of the Hanle curves is fulfilled in the framework of a phenomenological model considering a separate dynamics of a nuclear field BNd determined by the +/- 12 nuclear spin states and of a nuclear field BNq determined by the split-off states +/- 3/2, +/- 5/2, etc. It is found that the characteristic relaxation time for the nuclear field BNd is of order of 0.5 s, while the relaxation of the field BNq is faster by three orders of magnitude.

cond-mat.mes-hall↗

arXiv2017

Photon echoes from (In,Ga)As quantum dots embedded in a Tamm-plasmon microcavity

We report on the coherent optical response from an ensemble of (In,Ga)As quantum dots (QDs) embedded in a planar Tamm-plasmon microcavity with a quality factor of approx. 100. Significant enhancement of the light-matter interaction is demonstrated under selective laser excitation of those quantum dots which are in resonance with the cavity mode. The enhancement is manifested through Rabi oscillations of the photon echo, demonstrating coherent control of excitons with picosecond pulses at intensity levels more than an order of magnitude smaller as compared with bare quantum dots. The decay of the photon echo transients is weakly changed by the resonator indicating a small decrease of the coherence time $T_2$ which we attribute to the interaction with the electron plasma in the metal layer located close (40nm) to the QD layer. Simultaneously we see a reduction of the population lifetime $T_1$, inferred from the stimulated photon echo, due to an enhancement of the spontaneous emission by a factor of 2, which is attributed to the Purcell effect, while non-radiative processes are negligible as confirmed from time-resolved photoluminescence.

cond-mat.mes-hall↗

arXiv2017

Dynamics of exciton magnetic polarons in CdMnSe/CdMgSe quantum wells: the effect of self-localization

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells using time-resolved photoluminescence (PL). The magnetic field and temperature dependencies of this dynamics allow us to separate the non-magnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wavefunction. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200 - 400 ps with increasing magnetic field and temperature.

cond-mat.mtrl-sci↗

arXiv2016

Enhancement of electron hot spot relaxation in photoexcited plasmonic structures by thermal diffusion

We demonstrate that in confined plasmonic metal structures subject to ultra-fast laser excitation electron thermal diffusion can provide relaxation faster than the energy transfer to the lattice. This relaxation occurs due to excitation of nanometer-sized hot spots in the confined structure and the sensitivity of its optical parameters to the perturbation in these regions. Both factors become essential when the plasmonic resonance condition is met for both excitation and detection. A pump-probe experiment on plasmonic gold lattices shows sub-picosecond relaxation with the characteristic times well-described by a two-temperature model. The results suggest that dynamical optical response in plasmonic structures can be tuned by selection of the structural geometry as well as the choice of wavelength and polarization of the excitation and detection light.

cond-mat.mtrl-sci↗

arXiv2016

Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells

The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and Manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy-hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field.

cond-mat.mes-hall↗

arXiv2016

Large anisotropy of electron and hole g factors in infrared-emitting InAs/InAlGaAs self-assembled quantum dots

A detailed study of the $g$-factor anisotropy of electrons and holes in InAs/In$_{0.53}$Al$_{0.24}$Ga$_{0.23}$As self-assembled quantum dots emitting in the telecom spectral range of $1.5-1.6$ $μ$m (around 0.8 eV photon energy) is performed by time-resolved pump-probe ellipticity technique using a superconducting vector magnet. All components of the $g$-factor tensors are measured, including their spread in the quantum dot (QD) ensemble. Surprisingly, the electron $g$ factor shows a large anisotropy changing from $g_{\mathrm{e},x}= -1.63$ to $g_{\mathrm{e},z}= -2.52$ between directions perpendicular and parallel to the dot growth axis, respectively, at an energy of 0.82 eV. The hole $g$-factor anisotropy at this energy is even stronger: $|g_{\text{h},x}|= 0.64$ and $|g_{\text{h},z}|= 2.29$. On the other hand, the in-plane anisotropies of electron and hole $g$ factors are small. The pronounced out-of-plane anisotropy is also observed for the spread of the $g$ factors, determined from the spin dephasing time. The hole longitudinal $g$ factors are described with a theoretical model that allows us to estimate the QD parameters. We find that the QD height-to-diameter ratio increases while the indium composition decreases with increasing QD emission energy.

cond-mat.mes-hall↗

arXiv2016

Photon echo transients from an inhomogeneous ensemble of semiconductor quantum dots

An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot sub-ensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for π/2 pulses, while for 3π/2 the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations.

cond-mat.mes-hall↗

arXiv2016

Dynamics of nuclear spin polarization induced and detected by coherently precessing electron spins in fluorine-doped ZnSe

We study the dynamics of optically-induced nuclear spin polarization in a fluorine-doped ZnSe epilayer via time-resolved Kerr rotation. The nuclear polarization in the vicinity of a fluorine donor is induced by interaction with coherently precessing electron spins in a magnetic field applied in the Voigt geometry. It is detected by nuclei-induced changes in the electron spin coherence signal. This all-optical technique allows us to measure the longitudinal spin relaxation time $T_{1}$ of the $^{77}\text{Se}$ isotope in a magnetic field range from 10 to 130~mT under illumination. We combine the optical technique with radio frequency methods to address the coherent spin dynamics of the nuclei and measure Rabi oscillations, Ramsey fringes and the nuclear spin echo. The inhomogeneous spin dephasing time $T_{2}^{*}$ and the spin coherence time $T_{2}$ of the $^{77}\text{Se}$ isotope are measured. While the $T_{1}$ time is on the order of several milliseconds, the $T_{2}$ time is several hundred microseconds. The experimentally determined condition $T_{1}\gg T_{2}$ verifies the validity of the classical model of nuclear spin cooling for describing the optically-induced nuclear spin polarization.

cond-mat.mes-hall↗

arXiv2015

Förster energy transfer of dark excitons enhanced by a magnetic field in an ensemble of CdTe colloidal nanocrystals

We present a systematic experimental study along with theoretical modeling of the energy transfer in an ensemble of closely-packed CdTe colloidal nanocrystals identified as the Förster resonant energy transfer (FRET). We prove that at low temperature of 4.2 K, mainly the ground dark exciton states in the initially excited small-size (donor) nanocrystals participate in the dipole-dipole FRET leading to additional excitation of the large-size (acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of the bright exciton states to the dark states. The admixture takes place even in zero magnetic field and allows the radiative recombination of the dark excitons. An external magnetic field considerably enhances this admixture, thus increasing the energy transfer rate by a factor of 2-3 in a field of 15T, as well as the radiative rates of the dark excitons in the donor and acceptor nanocrystals. The theoretical modeling allows us to determine the spectral dependence of the probability for the NC to serve as a donor for larger nanocrystals, to evaluate the energy transfer rates as well as to predict their dependencies on the magnetic field, to describe the spectral shift of the photoluminescence maximum due to the energy transfer and to reproduce the experimentally observed spectral dependencies of the photoluminescence recombination dynamics in the magnetic field.

cond-mat.mes-hall↗