Optical control of the spin of a magnetic atom in a semiconductor quantum dot

International audienceThe control of single spins in solids is a key but challenging step for any spin-based solid-state quantum-computing device. Thanks to their expected long coherence time, localized spins on magnetic atoms in a semiconductor host could be an interesting media to store quantum information in the solid state. Optical probing and control of the spin of individual or pairs of Mn atoms (S=5/2) have been obtained in II-VI and III-V semiconductor quantum dots during the last years. In this paper , we review recently developed optical control experiments of the spin of an individual Mn atoms in II-VI semiconductor self-assembled or strain free quantum dots. We first show that the fine structure of the Mn atom and especially a strained induced magnetic anisotropy is the main parameter controlling the spin memory of the magnetic atom at zero magnetic field. We then demonstrate that the energy of any spin state of a Mn atom or pairs of Mn atom can be independently tuned by using the optical Stark effect induced by a resonant laser field. The strong coupling with the resonant laser field modifies the Mn fine structure and consequently its dynamics. We then describe the spin dynamics of a Mn atom under this strong resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that the Mn spin population can be trapped in the state which is resonantly excited. This effect is modeled considering the coherent spin dynamics of the coupled electronic and nuclear spin of the Mn atom optically dressed by a resonant laser field. Finally, we discuss the spin dynamics of a Mn atom in strain free quantum dots and show that these structures should permit a fast optical coherent control of an individual Mn spin

Polariton spin beats and polariton fine structure in semiconductor microcavities

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Exciton and polariton spin beats in a CdMnTe based microcavity

International audienceExciton and polariton spin beats are observed in a CdMnTe quantum well embedded in a microcavity using time-resolved Kerr rotation experiments under magnetic field. Photoinduced linear birefiringence phenomenon allows to comprehend the polariton spin beats using linear polarized pumping in Faraday geometry. Exciton spin beats can be detected in the standard configuration with circularly polarized pump pulse and under in-plane magnetic field

Spin quantum beats in CdMnTe microcavity

International audienceCdMnTe quantum well embedded in CdMgTe/CdMnTe microcavity is studied by time-resolved Kerr rotation experiments under in-plane magnetic field and quantum beatings corresponding to at least three distinct frequencies are observed. Whereas the lowest gigahertz frequency is clearly identified with Mn spin precession in the quantum well, terahertz beatings can be due to both polariton formation and valence band states mixing in the magnetic field. However, the experiments indicate unambiguously that there is no beatings between coherently excited polariton states in the absence of magnetic field, in contrast with the proposed model, which predicts the beatings at Rabi frequency in zero field. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Optically enhanced nuclear spin polarization in InP

International audienceOptically induced nuclear spin polarization is studied via time-resolved magnetooptical Kerr effect in Voigt configuration in n-doped InP. The hyperfine field acting on electrons is detected via a phase shift of the electron spin precession. The dependence of the hyperfine field on external field and the polarization time of the nuclei are discussed within a simple model neglecting nuclear spin diffusion

Effect of holes on the dynamic polarization of nuclei in semiconductors

International audienceIn semiconductors optically enhanced polarization of nuclei is known to be primarily due to photoexcited electrons. We show that holes play a role in this process via the spin-dependent recombination of the carriers. Our results are obtained in n-type InP where spin-dependent recombination leads to the inversion of the nuclear field direction due to the donor spins cooling under optical excitation

Do thermal treatments influence the ultrafast opto-thermal processes of eumelanin?

After light absorption, melanin converts very rapidly the energy gained into heat. The time scale of this process ranges from tens of femtoseconds to a few nanoseconds. Femtosecond transient absorption allows for exploration of such photo-induced carrier dynamics to observe the de-excitation pathways of the biological complex. Here, we report on the ultrafast relaxation of suspensions of Sepia melanin in DMSO at room temperature using a femtosecond broadband pump and probe technique by photoexciting in the UV and probing in the entire visible range. In particular, we focus on the possible role that different heat treatments, performed in the temperature range 30-80 degrees C might have on the relaxation of charge carriers photogenerated by UV radiation in such suspensions. Experimental data indicate that in all the investigated suspensions, photoexcited carriers always follow a tri-exponential route to relaxation. Moreover, we find that the relaxation time constants are essentially the same in all cases, within the experimental error. We take this as evidence that all the investigated suspensions essentially exhibit the same relaxation dynamics, regardless of the temperature at which the heat treatment has been performed and of the heat-induced denaturation of the proteinaceous compounds bound to the photoactive pigment. Our experiments represent a significant step towards the understanding of the stability of melanin with respect to temperature changes

Photolumines,cence of "dark" excitons in CdMnTe quantum well, embedded in a microcavity

International audienceA diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic, resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole-light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination. (c) 2007 Elsevier Ltd. All rights reserved