1,721,004 research outputs found
Light-induced valley currents and magnetization in graphene rings
No full textWe study the nonequilibrium dynamics in a mesoscopic graphene ring excited by picoseconds shaped electromagnetic pulses. We predict an ultrafast buildup of charge polarization, currents, and orbital magnetization. Applying the light pulses identified here, nonequilibrium valley currents are generated in a graphene ring threaded by a stationary magnetic flux. We predict a finite graphene ring magnetization even for a vanishing charge current; the magnetization emerges due to the light-induced difference of the valley populations.
Excitons in Si nanocrystals
No full textThe states of electron-hole pairs in spherical silicon nanocrystals are theoretically studied using the "multiband" effective-mass approximation in the limit of an infinitely high potential barrier at the boundary. The degeneracy of the states at the top of the valence band is taken into account in the spherical approximation, and the ellipsoidal character of the electronic spectrum in the conduction band is allowed for. Coulomb interaction-induced corrections to the energy of an electron-hole pair are found. (C) 2004 MAIK "Nauka / Interperiodica".
Enhancement of erbium luminescence in c-Si by terahertz radiation
No full textWe suggest a possible mechanism for enhancement of erbium luminescence in crystalline silicon by terahertz (THz) radiation under conditions of constant band-to-band pumping. The suggested mechanism consists of Auger excitation of an Er3+ ion into the second excited state by recombination of an electron-hole pair assisted by THz radiation. After the excitation Er3+ ion undergoes nonradiative relaxation into the first exited state, from which the radiative transition takes place. We estimate the value of the excitation probability for reasonable experimental parameters and show that the excitation process has a strong dependence on the frequency of the THz radiation and may be observed for frequencies in the range of 15-35 THz. (C) 2003 Elsevier Science B.V. All rights reserved.
Polarized light bursts from kicked quantum rings
No full textNonequilibrium quantum rings emit circular-polarized subterahertz radiation with a polarization degree controllable on nano- to picosecond time scales. This we conclude using a theory developed here for the time-dependent detection of the circular polarization of polychromatic radiations, valid for time scales comparable to the reciprocal of characteristic emission frequencies. The theory is applied to driven quantum rings whereby the influence of radiative and nonradiative processes on the properties of the emitted light is incorporated.
Excitation mechanism of erbium photoluminescence in bulk silicon and silicon nanostructures
No full textWe present a short review of the theoretical and experimental results concerning the problem of excitation mechanism of erbium photoluminescence in silicon and silicon nanostructures. The excitation process consists of two stages, the first being absorption of radiation by bulk silicon matrix or nanocrystals while the second is the Auger excitation of erbium ions by recombining electron-hole pairs. The large values of Auger excitation cross-section under optical pumping in semiconductor matrices are due to large values of band-to-band absorption coefficient of bulk silicon or silicon nanocrystals exceeding by several orders of magnitude the absorption coefficient of erbium in dielectric SiO2 matrix. The specific features of Auger process in silicon nanocrystals when excitation of erbium ions is produced by quantum-confined electron-hole pairs are discussed. (C) 2003 Elsevier B.V. All rights reserved.
Photo-induced spin filtering in a double quantum dot
No full textWe investigate the spin-dependent electron dynamics in a double quantum dot driven by sub-picosecond asymmetric electromagnetic pulses. We show analytically that applying the appropriate pulses, specified here, allows a spin separation on a femtosecond time scale in the sense that states with a desired spin projection are localized mainly on one of the dots. It is shown how to maintain in time this photo-induced spin-dependent filtering. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3660227]
Attosecond tracking of light absorption and refraction in fullerenes
No full textThe collective response of matter is ubiquitous and widely exploited, e. g., in plasmonic, optical, and electronic devices. Here we trace on an attosecond time scale the birth of collective excitations in a finite system and find distinct features in this regime. Combining quantum chemical computation with quantum kinetic methods, we calculate the time-dependent light absorption and refraction in fullerene that serve as indicators for the emergence of collective modes. We explain the numerically calculated transient features by an analytical model and point out the relevance for ultrafast photonic and electronic applications. A scheme is proposed to measure the predicted effects via the emergent attosecond metrology.
Microscopic theory for excitation of erbium ions via silicon nanocrystals in silicon dioxide
No full textIn this work, we present the results of microscopic calculation for the probability of excitation of Er ions by electron-hole pairs confined in Si nanocrystals surrounded by silicon dioxide. For simplicity we consider the case of low pumping of nanocrystals when we may take into account only one electron-hole pair inside of a single nanocrystal. We have found the probability of Er ion excitation in three principally different cases for the location of the Er ion relatively to the nanocrystal: (i) Er ion is situated inside of the nanocrystal, (ii) Er ion is situated at the boundary of the nanocrystal or very close to the boundary, and (iii) Er ion is situated at the considerable distance from the nanocrystal so that the tunneling of electrons and holes may be neglected. (c) 2005 Elsevier B.V. All rights reserved.
Photovoltaic effect of light carrying orbital angular momentum on a semiconducting stripe
No full textWe investigate the influence of a light beam carrying an orbital angular momentum on the current density of an electron wave packet in a semiconductor stripe. It is shown that due to the photo-induced torque the electron density can be deflected to one of the stripe sides. The direction of the deflection is controlled by the direction of the light orbital momentum. In addition the net current density can be enhanced. This is a photovoltaic effect that can be registered by measuring the generated voltage drop across the stripe and/or the current increase. (C) 2012 Optical Society of America
Rotational bifurcation of localized dissipative structures
No full textA topic of great interest for pattern-forming systems is the possibility of a spontaneous change in symmetry and dynamics as one slowly varies an external parameter. In this letter, we identify that a stationary localized structure without rotational symmetry, such as a pair of bound dissipative solitons, can spontaneously begin to rotate. The underlying mechanism is similar to the widely studied mechanism of the drift bifurcation in which structures begin to drift at constant velocity. We find a particular example of this new bifurcation for a 3-component reaction-diffusion system in 2 dimensions, and show that it can precede the drift bifurcation.
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