Publications (7)11.71 Total impact
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Article: Biexciton, single carrier, and trion generation dynamics in single-walled carbon nanotubes
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ABSTRACT: We present a study of free carrier photo-generation and multi-carrier bound states, such as biexcitons and trions (ionized excitons), in semiconducting single-walled carbon nanotubes. Pump-and-probe measurements performed with fs pulses reveal the effects of strong Coulomb interactions between carriers on their dynamics. Biexciton formation by optical transition from exciton population results in an induced absorption line (binding energy 130 meV). Exciton-exciton annihilation process is shown to evolve at high densities towards an Auger process that can expel carriers from nanotubes. The remaining carriers give rise to an induced absorption due to trion formation (binding energy 190 meV). These features show the dynamics of exciton and free carriers populations.02/2013; -
Article: All-optical trion generation in single-walled carbon nanotubes.
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ABSTRACT: We present evidence of all-optical trion generation and emission in pristine single-walled carbon nanotubes (SWCNTs). Luminescence spectra, recorded on individual SWCNTs over a large cw excitation intensity range, show trion emission peaks redshifted with respect to the bright exciton peak. Clear chirality dependence is observed for 22 separate SWCNT species, allowing for determination of electron-hole exchange interaction and trion binding energy contributions. Luminescence data together with ultrafast pump-probe experiments on chirality-sorted bulk samples suggest that exciton-exciton annihilation processes generate dissociated carriers that allow for trion creation upon a subsequent photon absorption event.Physical Review Letters 10/2011; 107(18):187401. · 7.37 Impact Factor -
Article: All-optical trion generation in single walled carbon nanotubes
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ABSTRACT: We present evidence of all optical trion generation and emission in undoped single walled carbon nanotubes (SWCNTs). Luminescence spectra, recorded on individual SWCNTs over a large CW excitation intensity range, show trion emission peaks red-shifted with respect to the bright exciton peak. Clear chirality dependence is observed for 22 separate SWCNT species, allowing for determination of electron-hole exchange interaction and trion binding energy contributions. Luminescence data together with ultrafast pump probe experiments on chirality sorted bulk samples suggest that exciton-exciton annihilation processes generate dissociated carriers that allow for trion creation upon a subsequent photon absorption event.06/2011; -
Article: Ultrafast Optical Dynamics of Metal-Free and Cobalt Phthalocyanine Thin Films II: Study of Excited-State Dynamics
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ABSTRACT: We report in this paper in detail on the excited-state dynamics of both metal-free phthalocyanine (H2Pc) and cobalt phthalocyanine (CoPc) thin films measured by pump-and-probe experiments. Samples are excited by femtosecond laser pulses at two different photon energies, which correspond to the two maxima of the Q-band absorption. Using a test beam, we measure the differential transmission ΔT(t)/T dynamics that is spectrally resolved in the range from 1.570 to 2.470 eV (790−502 nm). When compared to a H2Pc sample, the CoPc films show a faster dynamics, which has one very fast contribution on a subpicosecond time scale, a slower part with a time constant of several picoseconds, and a subnanosecond contribution. Since the observed dynamics is independent of excitation intensity, fast exciton−exciton annihilation can be excluded for our measurements. Therefore, we attribute the observed ultrafast dynamics in CoPc to the presence of the Co atoms. In the manifold of long-living triplet states, Co atoms are responsible for a long-lasting induced absorption and, when exciting at 2.032 eV (610 nm), we also observe around the photon energy of the pump beam a Fano-type resonance, whose properties evolve on a time scale of several tens of picoseconds. The latter is tentatively attributed to the presence of a delocalized exciton band originating from the Co atoms, which couple the planar molecules in different film planes and which give rise to an efficient excitation transfer. It is degenerated with the selectively excited Pc molecular states to which it is strongly coupled.09/2010; -
Article: Correlation of structural properties with energy transfer of Eu-doped ZnO thin films prepared by sol-gel process and magnetron reactive sputtering.
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ABSTRACT: We investigated the structural and optical properties of Eu-doped ZnO thin films made by sol-gel technique and magnetron reactive sputtering on Si (100) substrate. The films elaborated by sol-gel process are polycrystalline while the films made by sputtering show a strongly textured growth along the c-axis. X-ray diffraction patterns and transmission electron microscopy analysis show that all samples are free of spurious phases. The presence of Eu(2+) and Eu(3+) into the ZnO matrix has been confirmed by x-ray photoemission spectroscopy. This means that a small fraction of Europium substitutes Zn(2+) as Eu(2+) into the ZnO matrix; the rest of Eu being in the trivalent state. This is probably due to the formation of Eu(2)O(3) oxide at the surface of ZnO particles. This is at the origin of the strong photoluminescence band observed at 2 eV, which is characteristic of the (5)D(0)-->(7)F(2) Eu(3+) transition. In addition the photoluminescence excitonic spectra showed efficient energy transfer from the ZnO matrix to the Eu(3+) ion, which is qualitatively similar for both films although the sputtered films have a better structural quality compared to the sol-gel process grown films.Journal of Applied Physics 06/2010; 107(12):123522. · 2.17 Impact Factor -
Article: Dislocation density and band structure effects on spin dynamics in GaN
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ABSTRACT: We present experimental results obtained on wurtzite epitaxial GaN layers grown on sapphire and SiC substrates. Thanks to a set of samples with different values of the residual strain, we demonstrate that the high dislocation density enhances the spin relaxation rate through the Elliott–Yafet mechanism. This fact is validated by the T<sup>-1</sup> temperature dependence of the spin-relaxation times. The influence of the valence-band structure on the hole-spin relaxation is also highlighted. In particular, a decrease in the hole-spin relaxation rate, accompanied by a strong polarization rate (∼50%) of the differential reflectivity signal (ΔR/R) , is observed when the splitting ΔE<sub>AB</sub> between the heavy-hole and the light-hole bands is larger than the broadening Γ<sub>A</sub> of the A excitonic transition. On the contrary, the overlap of the A and B resonances for Γ<sub>A</sub>≫ΔE<sub>AB</sub> is responsible for a decrease in the ΔR/R polarization rate (∼10%) and an enhancement of the spin relaxation rate.Journal of Applied Physics 02/2009; · 2.17 Impact Factor -
Article: Experimental investigation of excitonic spin relaxation dynamics in GaN
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ABSTRACT: By performing nondegenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflectivity spectra (ΔR∕R) of the two circularly polarized probe components, we are able to identify each spin relaxation channel (electron, light- and heavy-hole spin flips) separately and to extract characteristic times of the different spin relaxation processes. In addition, spectral oscillatory features are observed for negative delays. They show a rapid rise determined by the fast dephasing time T2 of the excitonic transitions. We show that the high density of dislocations increases the spin relaxation of electrons and holes through the Elliot–Yafet mechanism and makes the exciton dephasing time very short. The measured heavy-hole relaxation time, which is not extremely short compared to the electron relaxation time, can be related to the band structure, in which the degeneracy between different spin valence bands is partially lifted.Phys. Rev. B. 77(12).
