arXiv subjects

D. R. Yakovlev

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

At least 55 records · Page 3Linked to original sources

arXiv2020

Magneto-toroidal nonreciprocity of second harmonic generation

The Lorentz reciprocity principle is a fundamental concept that governs light propagation in any optically linear medium in zero magnetic field. Here, we demonstrate experimentally a novel mechanism of reciprocity breaking in nonlinear optics driven by the toroidal moment. Using high-resolution femtosecond spectroscopy at optical electronic resonances in the magnetoelectric antiferromagnet CuB$_2$O$_4$, we show that by controlling the nonlinear interference of coherent sources of second harmonic generation originating from the toroidal spin order, applied magnetic field, and noncentrosymmetric crystal structure, we induce a huge nonreciprocity approaching 100% for opposite magnetic fields. The experimental results are corroborated by a convincing theoretical analysis based on the magnetic and crystal symmetry. These findings open new degrees of freedom in the nonlinear physics of electronic and magnetic structures and pave the way for future nonreciprocal spin-optronic devices operating on the femtosecond time scale.

cond-mat.mtrl-sci↗

arXiv2020

Optical detection of electron spin dynamics driven by fast variations of a magnetic field: a simple method to measure $T_1$, $T_2$, and $T_2^*$ in semiconductors

We develop a simple method for measuring the electron spin relaxation times $T_1$, $T_2$ and $T_2^*$ in semiconductors and demonstrate its exemplary application to $n$-type GaAs. Using an abrupt variation of the magnetic field acting on electron spins, we detect the spin evolution by measuring the Faraday rotation of a short laser pulse. Depending on the magnetic field orientation, this allows us to measure either the longitudinal spin relaxation time $T_1$ or the inhomogeneous transverse spin dephasing time $T_2^*$. In order to determine the homogeneous spin coherence time $T_2$, we apply a pulse of an oscillating radiofrequency (rf) field resonant with the Larmor frequency and detect the subsequent decay of the spin precession. The amplitude of the rf-driven spin precession is significantly enhanced upon additional optical pumping along the magnetic field.

cond-mat.mes-hall↗

arXiv2020

Effect of electric current on optical orientation of electrons in AlGaAs/GaAs heterostructure

The effect of a lateral electric current on the photoluminescence H-band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H-band are studied by means of continuous wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H-band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H-band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of the lateral electric field of ~ 100-200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ~ 100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (> 40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons.

cond-mat.mes-hall↗

arXiv2020

Magneto-Stark and Zeeman effect as origin of second harmonic generation of excitons in Cu$_2$O

We report on the experimental and theoretical investigation of magnetic-field-induced second harmonic generation (SHG) and two-photon absorption (TPA) of excited exciton states ($n \geq 3$) of the yellow series in Cu$_2$O. In this centrosymmetric material, SHG can occur due to constructive interplay of electric dipole and electric quadrupole/magnetic dipole transitions for light propagating along the low-symmetry directions [111] or [112]. By application of a magnetic field in Voigt configuration, SHG gets also allowed for excitation along the [110]-axis and even the high-symmetry cubic direction [001]. Combining a symmetry analysis and a microscopic theory, we uncover the two key contributions to the magnetic-field-induced SHG: the Zeeman effect and the magneto-Stark effect. We demonstrate systematic dependencies of the SHG intensity on the linear polarization angles of the ingoing fundamental laser and the outgoing SHG beam. In general, the resulting contour plots in combination with a symmetry analysis allow one to determine uniquely the character of involved transitions. Moreover, we can separate in magnetic field the Zeeman and the magneto-Stark effect through appropriate choice of the experimental geometry and polarization configuration. We present a microscopic theory of the second harmonic generation of excitons in a centrosymmetric cubic semiconductor taking into account the symmetry and the band structure of cuprous oxide. Based on the developed microscopic theory we identify the main contributions to the second-order nonlinear susceptibility of $S$-, $P$- and $D$-excitons. We analyze the redistribution of SHG intensities between the excitonic states both in the absence and presence of the magnetic field and show good agreement with the experimental data. With increasing exciton principal quantum number the magneto-Stark effect overpowers the influence of the Zeeman effect.

cond-mat.mes-hall↗

arXiv2020

The in-plane anisotropy of the hole $g$ factor in CdTe/(Cd,Mg)Te quantum wells studied by spin-dependent photon echoes

We use the two-pulse spin-dependent photon echo technique to study the in-plane hole spin anisotropy in a 20~nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te single quantum well by exciting the donor-bound exciton resonance. We take advantage of the photon echo sensitivity to the relative phase of the electron and hole spin precession and study various interactions contributing to the hole in-plane spin properties. The main contribution is found to arise from the crystal cubic symmetry described by the Luttinger parameter $q=0.095$, which is substantially larger than the one theoretically expected for CdTe or found in other quantum well structures. Another contribution is induced by the strain within the quantum well. These two contributions manifest as different harmonics of the spin precession frequencies in the photon echo experiment, when strength and orientation of the Voigt magnetic field are varied. The magnitude of the effective in-plane hole $g$ factor is found to vary in the range $|\tilde{g_h}|$=0.125--0.160 in the well plane.

cond-mat.mes-hall↗

arXiv2020

Quantum beats in the polarization of the spin-dependent photon echo from donor-bound excitons in CdTe/(Cd,Mg)Te quantum wells

We study the quantum beats in the polarization of the photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the $σ^+$ and $σ^-$ circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole $g$ factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te quantum well. From these quantum beats we obtain the in-plane heavy hole $g$ factor $g_h=-0.143\pm0.005$.

cond-mat.mes-hall↗

arXiv2019

Step-like spectral distribution of photoelectrons at the percolation threshold in heavily $p$-doped GaAs

We study the origin of the step-like shoulder on the high energy side of the low temperature photoluminescence spectrum of heavily $p$-doped GaAs. We show experimentally that it is controlled by the Fermi-Dirac distribution of the holes and by the energy distribution of the photoexcited electrons showing a sharp step-like dependence. This step is attributed to the percolation threshold in the conduction band separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation are used for these studies. We identify two different electron ensembles with substantially different spin lifetimes of 20 and 280~ps, limited by the lifetime of the electrons. Their spin relaxation times are longer than 2~ns. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the high-energy photoluminescence step-like tail. For energies above the percolation threshold the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populate these states.

cond-mat.mes-hall↗

arXiv2019

Spin beats in the photoluminescence polarization dynamics of charged excitons in InP/(In,Ga)P quantum dots in presence of nuclear quadrupole interaction

The spin dynamics of positively (X$^{+}$) and negatively (X$^{-}$) charged excitons in InP/In$_{0.48}$Ga$_{0.52}$P quantum dots subject to a magnetic field is studied. We find that a characteristic feature of the system under study is the presence of nuclear quadrupole interaction, which leads to stabilization of the nuclear and electron spins in a quantum dot in zero external magnetic field. In detail, the nuclear quadrupole interaction leads to pinning of the Overhauser field along the quadrupole axis, which is close to the growth axis of the heterostructure. The nuclear effects are observed only when resident electrons are confined in the quantum dots, i.e. for X$^{-}$ trion photoexcitation. The presence of X$^{-}$ and X$^{+}$ trion contributions to the photoluminescence together with the quadrupole interaction significantly affects the dynamics of optical orientation in Voigt magnetic field. In absence of dynamic nuclear spin polarization the time evolution of the photoluminescence polarization was fitted by a form which describes the electron spin relaxation in "frozen" nuclear field fluctuations. In relatively large external magnetic fields exceeding 60 mT good agreement between theory and experiment is achieved.

cond-mat.mes-hall↗

arXiv2019

Microscopic dynamics of electron hopping in a semiconductor quantum well probed by spin-dependent photon echoes

Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that independent of the particular spin relaxation mechanism, hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei.

cond-mat.mes-hall↗

arXiv2019

Radiofrequency driving of coherent electron spin dynamics in $n$-GaAs detected by Faraday rotation

We suggest a new pump-probe method for studying semiconductor spin dynamics based on pumping of carrier spins by a pulse of oscillating radiofrequency (rf) magnetic field and probing by measuring the Faraday rotation of a short laser pulse. We demonstrate this technique on $n$-GaAs and observe the onset and decay of coherent spin precession during and after the course of rf pulse excitation. We show that the rf field resonantly addresses the electron spins with Larmor frequencies close to that of the rf field. This opens the opportunity to determine the homogeneous spin coherence time $T_2$, that is inaccessible directly in standard all-optical pump-probe experiments.

cond-mat.mes-hall↗

arXiv2019

Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $Γ$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

cond-mat.mes-hall↗

arXiv2019

Nuclear spin dynamics influenced and detected by electron spin polarization in CdTe/CdMgTe quantum wells

Nuclear spin coherence and relaxation dynamics of all constituent isotopes of an n-doped CdTe/(Cd,Mg)Te quantum well structure are studied employing optically detected nuclear magnetic resonance. Using time-resolved pump-probe Faraday ellipticity, we generate and detect the coherent spin dynamics of the resident electrons. The photogenerated electron spin polarization is transferred into the nuclear spin system, which becomes polarized and acts back on the electron spins as the Overhauser field. Under the influence of resonant radio frequency pulses, we trace the coherent spin dynamics of the nuclear isotopes $^{111}$Cd, $^{113}$Cd, and $^{125}$Te. We measure nuclear Rabi oscillations, the inhomogeneous dephasing time $T_2^*$, the spin coherence time $T_2$, and the longitudinal relaxation time $T_1$. Furthermore, we investigate the influence of the laser excitation and the corresponding electron spin polarization on the nuclear spin relaxation time and find a weak extension of this time induced by interaction with the electron spins.

cond-mat.mes-hall↗

arXiv2019

Subsecond nuclear spin dynamics in $n$-GaAs

We use time-resolved detection of the Hanle effect and polarized photoluminescence with dark intervals to investigate the buildup and decay of the spin polarization of nuclei interacting with donor-bound electrons in $n$-doped GaAs. Strong hyperfine coupling defines the millisecond timescale of the spin dynamics of these nuclei, as distinct from the nuclei far from impurity centers, characterized by a thousand times longer spin-relaxation time. The dynamics of spin polarization and relaxation attributed to the nuclei inside the donor orbit is observed on the time scale from 200 to 425 ms.

cond-mat.mes-hall↗

arXiv2018

Electron and hole spin relaxation in InP-based self-assembled quantum dots emitting at telecom wavelengths

We investigate the electron and hole spin relaxation in an ensemble of self-assembled InAs/In$_{0.53}$Al$_{0.24}$Ga$_{0.23}$As/InP quantum dots with emission wavelengths around $1.5$~$μ$m by pump-probe Faraday rotation spectroscopy. Electron spin dephasing due to the randomly oriented nuclear Overhauser fields is observed. At low temperatures we find a sub-microsecond longitudinal electron spin relaxation time $T_1$ which unexpectedly strongly depends on temperature. At high temperatures the electron spin relaxation time is limited by optical phonon scattering through spin-orbit interaction decreasing down to $0.1$~ns at 260~K. We show that the hole spin relaxation is activated much more effectively by a temperature increase compared to the electrons.

cond-mat.mes-hall↗

arXiv2018

Voltage control of the long-range p-d exchange coupling in a ferromagnet-semiconductor quantum well hybrid structure

Voltage control of ferromagnetism on the nanometer scale is highly appealing for the development of novel electronic devices. Here a key challenge is to implement and combine low power consumption, high operation speed, reliable reversibility and compatibility with semiconductor technology. Hybrid structures based on the assembly of ferromagnetic and semiconducting building blocks are attractive candidates in that respect as such systems bring together the properties of the isolated constituents: They are expected to show magnetic order as a ferromagnet and to be electrically tunable as a semiconductor. Here we demonstrate the electrical control of the exchange coupling in a hybrid consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well, separated by a thin non-magnetic (Cd,Mg)Te barrier. The effective magnetic field of the exchange interaction reaches up to 2.5 Tesla and can be turned on and off by application of 1 V bias across the heterostructure. The mechanism of this electric field control is essentially different from the conventional concept, in which wavefunctions are spatially redistributed to vary the exchange interaction, requiring high field strengths. Here we address instead control of the novel exchange mechanism that is mediated by elliptically polarized phonons emitted from the ferromagnet, i.e. the phononic ac Stark effect. An essential parameter of this coupling is the splitting between heavy and light hole states in the quantum well which can be varied by the electric field induced band bending. Thereby the splitting can be tuned with respect to the magnon-phonon resonance energy in the ferromagnet, leading to maximum coupling for flat band conditions. Our results demonstrate the feasibility of electrically controlled exchange coupling in hybrid semiconductor nanostructures at quite moderate electric field strengths.

cond-mat.mes-hall↗

arXiv2018

Polarimetry of photon echo on charged and neutral excitons in CdTe/(Cd,Mg)Te quantum wells

Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver detailed information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle $φ$ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a 20-nm-thick CdTe/(Cd,Mg)Te quantum well at temperature of 1.5~K. Applying narrow-band optical excitation we selectively excite different exciton complexes including the exciton, the trion, and the donor-bound exciton D$^0$X. We find that polarimetry of the photon echo on the trion and D$^0$X is substantially different from the exciton: The echoes of the trion and D$^0$X are linearly polarized at the angle $2φ$ with respect to the first pulse polarization and their amplitudes are weakly dependent on $φ$. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as $\cosφ$.

cond-mat.mes-hall↗

arXiv2018

Optical excitation of single- and multi-mode magnetization precession in Galfenol nanolayers

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of Galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes in a 120-nm layer, and a single mode with effective damping $α_{eff}$ = 0.005 and frequency up to 100 GHz in a 4-nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetization precession approaching THz frequencies should be possible with Galfenol nanolayers.

cond-mat.mes-hall↗

arXiv2018

Transverse Magneto-Optical Kerr Effect at Narrow Optical Resonances

Magneto-optical spectroscopy based on the transverse magneto-optical Kerr effect (TMOKE) is a sensitive method for investigation of magnetically-ordered media. However, in magnetic materials the optical transitions are usually characterized by spectrally broad resonances with widths considerably exceeding the Zeeman splitting in the magnetic field. Here we investigate experimentally and theoretically the TMOKE in the vicinity of relatively narrow optical resonances provided by confined quantum systems. For experimental demonstration we use the exciton resonance in a (Cd,Mn)Te diluted magnetic semiconductor quantum well, where the strong exchange interaction with magnetic ions enables the giant Zeeman splitting of exciton spin states $Δ$ in magnetic fields of a few Tesla. In the weak coupling regime, when the splitting $Δ$ is smaller than the spectral broadening of the optical transitions $Γ$, the TMOKE magnitude grows linearly with the increase of the Zeeman splitting and its spectrum has an S-shape, which remains virtually unchanged in this range. In the strong coupling regime ($Δ>Γ$) the TMOKE magnitude saturates, while its spectrum is strongly modified resulting in the appearance of two separate peaks. The TMOKE is sensitive not only to the sample surface but can be used to probe the confined electronic states in depth if the upper layer is sufficiently transparent. Our results demonstrate that TMOKE of spectrally narrow resonances serves as a versatile tool for probing the charge and spin structure of electronic states in various confined quantum systems and can be used for spin tomography in combination with the conventional polar Kerr effect.

cond-mat.mtrl-sci↗