A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As quantum wells grown on (110) oriented substrate. The spin lifetime of spins perpendicular to the growth direction is about one order of magnitude shorter compared to spins along (110). The spin lifetimes of both spin orientations decrease monotonically above a temperature of 80 and 120 K, respectively. The decrease is very surprising for spins along (110) direction and cannot be explained by the usual Dyakonov Perel dephasing mechanism. A novel spin dephasing mechanism is put forward that is based on scattering of electrons between different quantum well subbands.The electron spin in semiconductors has recently become a focus of intense research in the context of spinelectronics or spintronics. This new kind of electronics aims to utilize spin for devices with unprecedented properties [1,2,3]. A prime condition for the development of potential applications is the understanding of spin decoherence, i.e. the loss of spin memory, in semiconductor structures [4]. The main reason for spin decoherence at room temperature is the intrinsic spin splitting of the conduction band, which occurs in all binary semiconductors. The spin splitting, which acts as an effective magnetic field, depends on the electron's momentum and is the basis for the Dyakonov-Perel (DP) spin relaxation mechanism [5,6]. Semiconductor heterostructures are in this context of particular interest since spin splitting in conduction and valence band can be controlled via dimensionality and orientation of crystal axes [7]. Ohno et al. observed very long electron spin decoherence times at room temperature in GaAs quantum wells (QWs) grown on (110) oriented substrates that exceeded the coherence times in usual (100) grown QWs by more than one order of magnitude [8,9]. However, slow spin dephasing in (110) QWs had been demonstrated only for electron spins oriented along the crystal growth direction. The dynamics of in-plane spin was left unexplored.Starting point for the theoretical description of the spin dynamics in (110) QWs is the Dresselhaus-Hamilton for binary semiconductorswhere i = x, y, z are the principal crystal axes with i + 3 → i, Γ is the spin-orbit coefficient for the bulk semiconductor, and σ i are the Pauli spin matrices [10]. Comparing eq.(1) with the spin Hamilton for a free electron in a magnetic field (H = 1 2 i µ B σ i B i ) one easily recognizes that random scattering of electrons leads to an effective k dependent random magnetic field with components in x, y, and z direction. This random magnetic field destroys the average spin orientation of an ensemble of electrons by rotating individual spins in different directions. The DP effect increases in bulk semiconductors with temperature due to occupation of higher k-states with larger spin splittings despite a motional narrowing effect at higher temperatures (spin lifetime τ s is inversely proportional to momentum scattering time τ * p ). In (110) QWs, however, the spin splitting (effective magnetic field)

Time-resolved fluorescence spectroscopy is used to investigate relaxation of electronic excitations in films of π-conjugated polymer 1 in the ps time domain. The position of the fluorescence band and its width are measured as a function of time and excitation energy. Both low (15 K) and room-temperature behavior are investigated. For high energy excitation, the fluorescence band shows a continuous red shift with time. The energy associated with the maximum of the fluorescence band E is proportional to log(t), with t being the time after excitation. For excitation in the tail of the lowest absorption band, the fluorescence remains stationary and selective excitation of a subset of chromophoric chain segments is possible. At intermediate excitation energy the time required for the excitations to make their first jump depends on the excitation energy and is longer at lower energy. At low temperature and high energy excitation the fluorescence bands are found to narrow with time, while for low energy excitation a broadening with time is observed. The experimental data are consistent with dispersive relaxation dynamics for the photoexcitations by incoherent hopping between localized states. Monte Carlo simulations are performed to obtain the average energy and the width of the energy distribution for an ensemble of photoexcitations in an energetically disordered molecular solid assuming Förster type energy transfer. A Förster radius R 0 ∼ 30 Å is found to give good agreement between experiment and simulations. In addition, the measurements indicate that for excitation energies >2.94 eV additional relaxation processes, ascribed to ultrafast intrachain vibrational relaxation, are operative.

We observe the noise spectrum of electron spins in bulk GaAs by Faraday-rotation noise spectroscopy. The experimental technique enables the undisturbed measurement of the electron-spin dynamics in semiconductors. We measure exemplarily the electron-spin relaxation time and the electron Landé g factor in -doped GaAs at low temperatures and find good agreement of the measured noise spectrum with a theory based on Poisson distribution probability.

In this Letter, the first spin noise spectroscopy measurements in semiconductor systems of reduced effective dimensionality are reported. The nondemolition measurement technique gives access to the otherwise concealed intrinsic, low temperature electron spin relaxation time of n-doped GaAs (110) quantum wells and to the corresponding low temperature anisotropic spin relaxation. The Brownian motion of the electrons within the spin noise probe laser spot becomes manifest in a modification of the spin noise line width. Thereby, the spatially resolved observation of the stochastic spin polarization uniquely allows to study electron dynamics at equilibrium conditions with a vanishing total momentum of the electron system.

We demonstrate a reduction of the threshold of a semiconductor laser by optically pumping spin-polarized electrons in the gain medium. Polarized electrons couple selectively to one of two possible lasing light modes which effectively reduces the threshold by up to 50% compared to conventional pumping with unpolarized electrons. We theoretically show that our concept can be generalized to an electrically pumped laser.