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arXiv:1709.01997 · Physical review. B./Physical review. B

Electron spin dynamics of Ce3+ ions in YAG crystals studied by pulse-EPR and pump-probe Faraday rotation

Д. В. Азамат, V. V. Belykh, Dmitri R. Yakovlev, Felix Fobbe, Donghai Feng, Eiko Evers, Lubomir Jastrabik, Alexandr Dejneka, M. Bayer

Published Aug 30, 2017• 11 citations
arXivHTMLPDF

Abstract

The spin relaxation dynamics of ${\mathrm{Ce}}^{3+}$ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the $4f$ ground state, while pump-probe Faraday rotation also provides access to the excited $5d$ state. We measure a millisecond spin-lattice relaxation time ${T}_{1}$, a microsecond spin coherence time ${T}_{2}$, and a $\ensuremath{\sim}10$ ns inhomogeneous spin dephasing time ${T}_{2}^{*}$ for the ${\mathrm{Ce}}^{3+}$ ground state at low temperatures. The spin-lattice relaxation of ${\mathrm{Ce}}^{3+}$ ions is due to modified Raman processes involving the optical phonon mode at $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}125\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The relaxation at higher temperature goes through a first excited level of the $^{2}\mathrm{F}_{\frac{5}{2}}$ term at about $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}228\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Effects provided by the hyperfine interaction of the ${\mathrm{Ce}}^{3+}$ with the $^{27}\mathrm{Al}$ nuclei are observed.

DOI
10.1103/physrevb.96.075160

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The spin relaxation dynamics of Ce3+{\mathrm{Ce}}^{3+}Ce3+ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the 4f4f4f ground state, while pump-probe Faraday rotation also provides access to the excited 5d5d5d state. We measure a millisecond spin-lattice relaxation time T1{T}_{1}T1​, a microsecond spin coherence time T2{T}_{2}T2​, and a \ensuremath∼10\ensuremath{\sim}10\ensuremath∼10 ns inhomogeneous spin dephasing time T2∗{T}_{2}^{*}T2∗​ for the Ce3+{\mathrm{Ce}}^{3+}Ce3+ ground state at low temperatures. The spin-lattice relaxation of Ce3+{\mathrm{Ce}}^{3+}Ce3+ ions is due to modified Raman processes involving the optical phonon mode at \ensuremath∼125cm\ensuremath−1\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}125\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}\ensuremath∼125cm\ensuremath−1. The relaxation at higher temperature goes through a first excited level of the 2F52^{2}\mathrm{F}_{\frac{5}{2}}2F25​​ term at about \ensuremathℏ\ensuremathω\ensuremath≈228cm\ensuremath−1\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}228\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}\ensuremathℏ\ensuremathω\ensuremath≈228cm\ensuremath−1. Effects provided by the hyperfine interaction of the Ce3+{\mathrm{Ce}}^{3+}Ce3+ with the 27Al^{27}\mathrm{Al}27Al nuclei are observed.
Circularly polarized optical excitation generates electron spin polarization in the lowest 5d state of rare-earth Ce3+ ions in a YAG crystal. The 5d electron spin dynamics is investigated in transverse and longitudinal magnetic fields by time-resolved pump-probe Faraday rotation. Long lived electron spin coherence with a dephasing time of 2.5 ns is found at room temperature. In a transverse magnetic field of 1 T, the electron spin coherence shows a distinct beating-like amplitude modulation due to several slightly different Larmor frequencies corresponding to different electron g factors of magnetically inequivalent positions of the Ce3+ ions in the crystal lattice. Hyperfine coupling between the 5d electron of Ce3+ ions and environmental nuclear spins dominates the spin relaxation, which can be efficiently suppressed by a longitudinal magnetic field as small as 10 mT. The dependence of electron spin relaxation on both the transverse and longitudinal magnetic fields agrees well with the one predicted theoretically for the hyperfine coupling mechanism.
The inhomogeneity of an electron spin ensemble as well as fluctuating environment acting upon individual spins drastically shorten the spin coherence time T_{2} and hinder coherent spin manipulation. We show that this problem can be solved by the simultaneous application of a radio frequency (rf) field, which stimulates coherent spin precession decoupled from an inhomogeneous environment, and periodic optical pulses, which amplify this precession. The resulting resonance, taking place when the rf field frequency approaches the laser pulse repetition frequency, has a width determined by the spin coherence time T_{2} that is free from the effects of inhomogeneity and slow nuclear spin fluctuations. We measure a 50-Hz-narrow electron spin resonance and milliseconds-long T_{2} for electrons in the ground state of Ce^{3+} ions in the yttrium aluminum garnet (YAG) lattice at low temperatures, while the inhomogeneous spin dephasing time T_{2}^{*} is only 25 ns. This study paves the way to coherent optical manipulation in spin systems decoupled from their inhomogeneous environment.
Eu-doped yttrium orthosilicate (Eu(3+) : Y2SiO5) had been a subject being investigated for coherent time-domain optical memory and information processing applications since its ultraslow optical dephasing was discovered several years ago. In this crystal the weakly allowed (7)F0 - (5)D0 transition of europium ions exhibits a sufficient long dephasing time and no spectral difli.usion on a time scale of several hours at low temperature, thus an information pattern or data can be stored as a population grating in the ground state hyperfine levels. On the other hand, the study on photon-echo relaxation shows that the dephasing time T2 of Eu (3+) and other rare-earth ions doped YAG, YAlO3 strongly depends on the intensity of the excitation pulses. In Eu (3+) :YAlO3, an exponential decay of photon-echo with T2 = 53 microseconds if the excitation pulses are weak (5 vJ/pulse) was observed. However, when the excitation pulses are strong (80 pJ/pulse) they observed a much shortened T2 with a highly nonexponential decay pattern. The conclusion they derived is that the intensity-dependent dephasing rate effects are quite general, and it depends on how much the excitation intensity varies. In this paper we use transient grating formation technique showing that a temporal lattice distortion may only occur along crystal c axis, caused by EU (3+) excitation. At high excitation level the produced exciton in conduction band may also couple to the dynamical lattice relaxation process, giving rise to an apparently much shortened dephasing time.
Abstract 167Er3+ doped solids are a promising platform for quantum technology due to erbium’s telecom C-band optical transition and its long hyperfine coherence times. We experimentally study the spin Hamiltonian and dynamics of 167Er3+ spins in Y2O3 using electron paramagnetic resonance (EPR) spectroscopy. The anisotropic electron Zeeman, hyperfine and nuclear quadrupole matrices are fitted using data obtained by X-band (9.5 GHz) EPR spectroscopy. We perform pulsed EPR spectroscopy to measure spin relaxation time T 1 and coherence time T 2 for the 3 principal axes of an anisotropic g tensor. Long electronic spin coherence time up to 24.4 μs is measured for lowest g transition at 4 K, exceeding previously reported values at much lower temperatures. Measurements of decoherence mechanism indicates T 2 limited by spectral diffusion and instantaneous diffusion. Long spin coherence times, along with a strong anisotropic hyperfine interaction makes 167Er3+:Y2O3 a rich system and an excellent candidate for spin-based quantum technologies.
Systematic investigation of the spin dephasing times in GaAs/(Al,Ga)As quantum wells and its depending on parameters like temperature, magnetic fields and doping will be shown in Chapter 3.In Chapter 4 are presented results of investigation of the nuclear spin systems in quantum well heterostructures.The chapter 5 presents the results of studying the carrier spin dynamics in a n-doped (In,Ga)As/GaAs quantum wells by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 mil-liKelvin.In Chapter 6 is devoted to experimental and theoretically studying of optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization.For that purpose the coherent electron spin dynamics in a n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state.on the eigenstates |s z ; ↑ and |s z ; ↓ yields the states themselves independent of time.Along the field axis (longitudinal direction) these quantum-mechanical spin states are thus stationary.Any loss of coherence is induced by coupling of those states with the environment.Applying the time-evolution operator on the spin states perpendicular
Long spin coherence times of carriers are essential for implementing quantum technologies using semiconductor devices for which, however, a possible obstacle is spin relaxation. For the spin dynamics, decisive features are the band structure, crystal symmetry, and quantum confinement. Perovskite semiconductors recently have come into focus of studies of their spin states, notivated by efficient optical access and potentially long-living coherence. Here, we report an electron spin coherence time T2T_2T2​ of the order of 1 ms, measured for a bulk FA0.95_{0.95}0.95​Cs0.05_{0.05}0.05​PbI3_33​ lead halide perovskite crystal. Using periodic laser pulses, we synchronize the electron spin Larmor precession about an external magnetic field in an inhomogeneous ensemble, the effect known as spin mode locking. It appears as a decay of the optically created ensemble spin polarization within the dephasing time T2∗T_2^*T2∗​ of up to 20 ns and its revival during the spin coherence time T2T_2T2​ reaching the millisecond range. This exceptionally long spin coherence time in a bulk crystal is complemented by millisecond-long longitudinal spin relaxation times T1T_1T1​ for electrons and holes, measured by optically-detected magnetic resonance. These long-lasting spin dynamics highlight perovskites as promising platform for the quantum devices with all-optical control.