Polarized nonequilibrium Bose-Einstein condensates of spinor exciton polaritons in a magnetic field

The effect of a magnetic field on a spinor exciton-polariton condensate has been investigated. A quenching of a polariton Zeeman splitting and an elliptical polarization of the condensate have been observed at low magnetic fields B<2 T. The effects are attributed to a competition between the magnetic field induced circular polarization buildup and the spin-anisotropic polariton-polariton interaction which favors a linear polarization. The sign of the circular polarization of the condensate emission at B<3 T is negative, suggesting that a dynamic condensation in the excited spin state rather than the ground spin state takes place in this magnetic field range. From about 2T on, the Zeeman splitting opens and from then on the slope of the circular polarization degree changes its sign. For magnetic fields larger than the 3 T, the upper spin state occupation is energetically suppressed and circularly polarized condensation takes place in the ground state.     

Steady state and time-resolved magneto-luminescence in ZnSe/(Zn,Mn)Se Heterostructures

We report on optical measurements performed on two 53Å and 106Å wide ZnSe quantum wells separated by a 350Å thick Zn_0.73Mn_0.27Se barrier. The measurements were performed by means of cw photoluminescence up to 20T, cw photoluminescence excitation and time-resolved photoluminescence spectroscopies up to 9T, at low temperature (4.2K). The fundamental optical transition changes its nature from a type-I light hole excition to a type-II heavy hole exciton as a function of applied magnetic field. Calculations taking into account the strain, Zeeman, and excitonic effects support the experimental findings and allows us to specify the value of the relative valence band offset.     

Dynamics of a Spinor Exciton–Polariton System in Laterally Strained GaAs Microcavities under Resonant Photoexcitation

The evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.     

Spin dynamics of a vortical condensate of exciton polaritons in a GaAs microcavity

The temporal dynamics of a spinor exciton-polariton condensate in a high-quality anisotropic GaAs microcavity under pulsed resonant excitation with light possessing a nonzero orbital angular momentum is investigated. The phenomenon of spatial separation of the spin components of the polariton condensate upon pumping with a coherent superposition of two beams with opposite circular polarizations and orbital angular momenta is observed. The key factors for the observation of this effect are the lateral anisotropy of the microcavity that causes a splitting between the linear components of the polariton ground state and the occurrence of opposite orbital angular momenta for the two spin components of the condensate. The experimental results are in qualitative agreement with the theoretical model of the phenomenon developed in JETP Lett. 104, 827 (2016).     

Room-temperature polariton parametric scattering driven by a one-dimensional polariton condensate

We demonstrate a novel way to realize room-temperature polariton parametric scattering in a one-dimensional ZnO microcavity. The polariton parametric scattering is driven by a polariton condensate, with a balanced polariton pair generated at the adjacent polariton mode. This parametric scattering is experimentally investigated by the angle-resolved photoluminescence spectroscopy technique under different pump powers and it is well described by the rate equation of interacting bosons. The direct relation between the intensity of the scattered polariton signal and that of the polariton reservoir is acquired under nonresonant excitation, exhibiting the explicit nonlinear characteristic of this room-temperature polariton parametric process.     

Controlled switching between quantum states in the exciton–polariton condensate

Optically controlled switching between modes of a polariton laser having different symmetries has been demonstrated experimentally. The microscopic shift of the optical excitation spot dramatically changes the shape of the polariton condensate formed in a cylindrical micropillar on the basis of the planar semiconductor microcavity. Switching between the ring and lobed condensate is achieved owing to the violation of the cylindrical symmetry of the effective potential formed by the lateral surface of the pillar and by the cloud of incoherent excitons created by optical pumping.     

Mixing of excitonic states containing light and heavy holes in an isolated GaAs/AlGaAs quantum well in a magnetic field

The Zeeman splitting of the ground states 1s(hh) and 1s(lh) of excitons with heavy and light holes, respectively, in a 15-nm isolated Al_0.3Ga_0.7As/GaAs quantum well in magnetic fields of up to 20 T is investigated according to the photoluminescence excitation spectra in the Faraday geometry (σ⁺− σ⁻ components). The observed anomalous pattern of nonlinear Zeeman splitting and the nonmonotonic behavior of the effective hole g factor are interpreted in terms of the strong mixing of the magnetoexcitonic states containing light and heavy holes. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 52–56 (10 July 1996)     

Optical pumping of the Zeeman components in the rubidium vapor

We investigated the optical pumping of the Zeeman components of rubidium atoms, in the presence of the external magnetic field ranging from the geomagnetic up to 130 Gauss. Using the saturated absorption spectroscopy with linearly polarized pump and probe laser beams, the rubidium Doppler-free spectra at different magnetic field strengths were measured. The dips (negative intensity signals) in the saturated absorption spectra of the ⁸⁷Rb hyperfine transition lines were observed. They come as a result of the alignment process induced by the incoherent population transfer due to the hyperfine optical pumping. By inspection of the dips for different magnetic field strengths we were able to conclude about the dynamics of the alignment process in the external magnetic field. Present work is a part of the investigations concerning the influence of the magnetic field on the velocity selective optical pumping of the rubidium atoms induced by femtosecond frequency comb [D. Aumiler, T. Ban, H. Skenderovi?, G. Pichler, Phys. Rev. Lett. 95 (2005) 233001; T. Ban, D. Aumiler, H. Skenderovi?, G. Pichler, Phys. Rev. A 73 (2006) 043407].     

Nonlinear optical spin Hall effect and long-range spin transport in polariton lasers

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.     

Renormalized dispersion of elementary excitations in spinor polariton condensates

We analyse the polarization of spinor polariton condensates and corresponding dispersions of elementary excitations. We have considered the effects of magnetic field induced splitting in circular polarizations and residual splitting in linear polarizations in the ground state provided by the cavity asymmetry. We show that anisotropic polariton–polariton interactions fully compensate the Zeeman splitting in circular polarizations below the critical magnetic field, thus leading to the spin-Meissner effect for the polariton condensates. We also analyzed the effect of polariton–polariton interactions on the stability of the gap in linear polarizations characteristic for anisotropic microcavities. It was shown that in realistic systems this gap increases with concentration of the particles, thus contributing to the stability of the pinning of linear polarization of photoemission in semiconductor microcavities for pump intensities above the stimulation threshold.     

A Rb-magnetometer for a wide range and high sensitivity

A Rb-magnetometer was constructed for a wide range of application. It uses a very small optically pumped resonance cell with an active volume of less than 0.1 cm³ as the sensor element. The cell is housed in a small probe containing polarizer, rf-coil, heater and photoelement. The pumping light is brought to the resonance cell by use of a 3 m long fibre optics. The magnetometer allows continuous monitoring or stabilizing magnetic fields from about 3·10^?8T to beyond 0.3T. The ultimate sensitivity of the instrument is of the order of 10^?10T. Observing the Hanle signal in the transverse mode of optical pumping the instrument can be used for zero field detection and for the measurement for weak fields by means of field compensation.     

Magnetic field-induced charge transfer to a GaAs/(Ga,Al)As quantum well interface studied by C(e,A0i) photoluminescence

The photoluminescence (PL) from a 300 Å GaAs((Ga,Al)As quantum well (QW) has been studied for a range of excitation powers, in magnetic fields up to 16 T applied both perpendicular to and in the plane of the QW. Particular attention was paid to the intensity of the (e,A₀ⁱ) transition due to Carbon acceptors located at one interface of the QW, in the presence of in-plane fields. The low power in-plane field dependence of the PL is a competition between two effects. At fields up to 12 T charge transfer is observed to and from the interface of the QW, resulting in an increase and subsequent decrease of the acceptor PL intensity. For field values exceeding 12 T the acceptor PL intensity is found to increase again. Whereas the first effect is well described by a composite oscillator model, the latter is suggested to be due to the decreased efficiency of electronic traps, located at the QW interface, for in-plane magnetic fields. These suggestions are confirmed by the excitation power dependence of the PL intensity for in-plane fields.     

Effect of the Excitation Radiation Coherence on Oscillations of the Photon Echo Intensity

The physical reasons for observing the splitting of optical lines several orders of magnitude smaller than the spectral width of a laser pulse are investigated. A theory of coherent and incoherent photon echo (PE) in an external static magnetic field and in the presence of a pulsed magnetic field, which causes oscillations of the PE intensity, is elaborated. It is shown that the periods of oscillations in the echo intensity, the echo duration, and the dimensions of the regions in the inhomogeneous line, where the excited ions are coherent, do not depend on the degree of coherence of the laser pulse and on the external static magnetic field. As follows from the theory, in the case of the coherent excitation of the echo, the amplitude of the intensity oscillations is independent of the external static magnetic field if the inhomogeneous line is symmetric. It is shown that the amplitude of the oscillations at the incoherent excitation of the echo is equal to the autocorrelation function of the distribution function of the transition frequency along the inhomogeneous line with the argument equal to the Zeeman splitting of the optical line in the external magnetic field. In this case, the experimental values of the oscillation amplitude are in good agreement with the calculated values of the autocorrelation function for the total inhomogeneous line in LuLiF₄:Er³⁺ (⁴I_15/2?F_9/2 transition). In the same way, the autocorrelation function has been obtained for YLiF₄:Er³⁺ on the same transition.     

Spontaneous pattern formation in a polariton condensate

Exciton-polariton condensation can be regarded as a self-organization phenomenon, where phase ordering is established among particles in the system. In such condensed systems, further ordering can occur in the particle density distribution, under particular experimental conditions. In this work we report on spontaneous pattern formation in a polariton condensate under nonresonant optical pumping. The slightly elliptical ring-shaped excitation laser that we employ forces condensation to occur into a single-energy state with periodic boundary conditions, giving rise to a multilobe standing-wave patterned state.     

Zeeman effect and stark splitting of the electronic states of the rare-earth ion in the paramagnetic terbium garnets Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Zeeman effect in the ⁷ F ₆ → ⁵ D ₄ absorption band of the Tb³⁺ ion in the paramagnetic garnets Tb₃Ga₅O₁₂ and Tb₃Al₅O₁₂ was studied. The field dependences of the Zeeman splitting of some absorption lines are found to exhibit unusual behavior: as the magnetic field increases, the band splitting decreases rather than increases. Symmetry analysis relates these lines to 4f → 4f electron transitions of the doublet-quasi-doublet or quasi-doublet-doublet type, for which the field dependences of the splitting differ radically from the well-known field dependences of the Zeeman splitting for quasi-doublet-quasi-doublet or quasi-doublet-singlet transitions in a longitudinal magnetic field.     

Photoluminescence of ZnMnTe quantum-well structures in a magnetic field

Photoluminescence spectra of strained structures Zn_{1 ? x} Mn _x Te/Zn_{1 ? y} Mg _y Te with magnetic quantum wells and nonmagnetic barriers are studied. The Zeeman splitting of the heavy exciton is found to follow an unusual behavior: both spin components shift down in energy. The heavy-exciton photoluminescence Zeeman components are observed to be inversely distributed in intensity, with the higher energy component being stronger than the lower energy component. The Zeeman splitting of the exciton in a magnetic field is calculated. The data obtained permit refinement of some parameters of the energy spectrum and magnetic properties of these structures.     

Optical spin orientation of a single manganese atom in a quantum dot

The magnetic state of a single magnetic atom (Mn) embedded in an individual semiconductor quantum dot is optically probed using micro-spectroscopy. A high degree of spin polarization can be achieved for an individual Mn atom localized in a quantum dot using quasi-resonant or fully-resonant optical excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. The Mn spin distribution prepared by optical pumping is fully conserved for a few microseconds. This opens the way to full optical control of the spin state of an individual magnetic atom in a solid state environment.     

Nonlinear Dynamics of Exciton-Polariton Condensates in a One-Dimensional Lattice

We consider the problem of exciton-polariton-condensate formation in a semiconductor microcavity in the strong coupling regime. The condensate is confined in a one-dimensional periodic potential and coupled to an exciton reservoir that is formed by the external cw pumping. The condensate dynamics is studied in the center and in the edges of the Brillouin zone (BZ). Modeling the formation of the condensate from weak initial noise shows that besides steady states the macroscopic oscillations of polariton density can also occur. Within the framework of the mean field approach, we obtain important analytic relations for the condensate eigenstates using the developed simplified model for three coupled spatial harmonics. A numerical analysis verifies the correctness of the analytic results.     

Dark-state imaging for two-dimensional mapping of a magnetic field

By imaging the dark states that are due to coherent population trapping in Na atomic vapor, we have succeeded in depicting a spatially inhomogeneous magnetic field. Highly resolved dark lines represent cross sections of the surfaces of constant magnetic fields, and they agree well with the predicted hyperfine Zeeman splitting and the two-photon selection rules of the Na atom. Mapping was made two dimensionally in real time, but extension to three dimensions is straightforward. Furthermore, unlike the previous techniques based on optical pumping, this method can be used for any magnetic-field directions.