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Optical Excitation (optical + excitation)
Selected AbstractsEmission Colour Tuning in Semiconducting Polymer Nanotubes by Energy Transfer to Organo- Lanthanide Dopants,ADVANCED MATERIALS, Issue 18 2007S. Moynihan Tuning of emission chromaticity in poly(N -vinylcarbazole) nanotubes by efficient energy transfer to luminescent Eu(dbm)3(Phen) organo-lanthanide chelate dopants is demonstrated. Nanotubes are synthesized by solution assisted template wetting. Under optical excitation, undoped tubes luminesce in the blue while doped tubes exhibit red europium ion emission with good color purity at the single nanotube level. [source] Femtosecond electron diffraction: Direct probe of ultrafast structural dynamics in metal filmsMICROSCOPY RESEARCH AND TECHNIQUE, Issue 3 2009Shouhua Nie Abstract Femtosecond electron diffraction is a rapidly advancing technique that holds a great promise for studying ultrafast structural dynamics in phase transitions, chemical reactions, and function of biological molecules at the atomic time and length scales. In this paper, we summarize our development of a tabletop femtosecond electron diffractometer. Using a delicate instrument design and careful experimental configurations, we demonstrate the unprecedented capability of detecting submilli-ångström lattice spacing change on a subpicosecond timescale with this new technique. We have conducted an in-depth investigation of ultrafast coherent phonon dynamics induced by an impulsive optical excitation in thin-film metals. By probing both coherent acoustic and random thermal lattice motions simultaneously in real time, we have provided the first and unambiguous experimental evidence that the pressure of hot electrons contributes significantly to the generation of coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized. Based on these observations, we also propose an innovative approach to measure the electronic Grüneisen parameter in magnetic materials at and above room temperature, which provides a way to gain new insights into electronic thermal expansion in ferromagnetic transition metals. Microsc. Res. Tech. 2009. © 2009 Wiley-Liss, Inc. [source] Fabrication of dendrite-like Au nanostructures and their enhanced photoluminescence emissionPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2007Ying Hu Abstract Special hierarchical dendrite-like Au (DLAu) nanostructures were fabricated facilely between the gaps of Au electrodes by an electrochemical method. The composition, morphology and crystallinity of the DLAu nanostructures were characterized using energy dispersive spectroscopy, field emission scanning electron microscopy and X-ray diffraction, respectively. The formation of these nanostructures is attributed to the distribution of the local electrical field between the Au electrodes and a diffusion-limited aggregation process. Photoluminescence (PL) having an emission peak near 530 nm is observed from these nanostructures, which is attributed to the recombination of the s,p band electrons near the Fermi energy with the d band holes in the DLAu nanostructures generated by optical excitation. We believe that such PL enhancement compared to a smooth Au film is due to the local-field enhancement from the surface plasmon resonance of the DLAu nanostructures. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Exciton relaxation in bulk wurtzite GaN: the role of piezoelectric interactionPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2003G. Kokolakis Abstract A theoretical study on the relaxation of coupled free carriers and excitons after non-resonant optical excitation in bulk wurtzite GaN is presented. In particular the effect of the acoustic piezoelectric scattering is taken under consideration, and the respective rates have been calculated, including screening effects. Results show that the piezo-acoustic rates are bigger in the wurtzite phase of GaN with respect to the cubic phase, and they are really sensitive to the background doping of the sample. Simulations of the full dynamics of the system are performed by using an Ensemble Monte Carlo method under which all the relevant scattering mechanisms are included. The set of semiclassical Boltzmann equations for electron and hole populations is complemented by an additional equation for the exciton distribution and is coupled by reaction terms describing phonon-mediated exciton binding and dissociation. The temporal evolution is studied in the short range time (100 ps) after photo-excitation. It shows that a high background doping prevents the electrons to relax toward low energy states. [source] Polarized electrons, trions, and nuclei in charged quantum dotsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003A. S. Bracker Abstract We have investigated spin polarization in GaAs quantum dots. Excitons and trions are polarized directly by optical excitation and studied through polarization of photoluminescence. Electrons and nuclei are polarized indirectly through subsequent relaxation processes. Polarized electrons are identified by the Hanle effect for exciton and trion photoluminescence, while polarized nuclei are identified through the Overhauser effect in individual charged quantum dots. [source] Single-photon and photon-pair emission from CdSe/Zn(S,Se) quantum dotsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003S. Strauf Abstract We report on the generation of triggered single photons and photon pairs relying on pulsed optical excitation of epitaxially grown self-assembled CdSe quantum dots (QDs). Single-photon emission is studied on individual CdSe/Zn(S,Se) QDs for temperatures up to 200 K. At low temperatures nearly perfect single-photon emission is achieved whereas at higher temperatures an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring QDs is observed. The multi-photon emission probability of a bare QD (background subtracted) is strongly suppressed. Furthermore, the polarization cross correlation of the biexciton,exciton cascade has been investigated in a CdSe/ZnSe QD at low temperatures. A strong polarization correlation of the emitted photon pairs of ,74% is observed in a linear detection basis. [source] Low-temperature MBE-grown GaBiAs layers for terahertz optoelectronic applicationsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2009Vaidas Pa, ebutas Abstract Gallium bismide arsenide epitaxial layers were grown by molecular-beam-epitaxy at low substrate temperatures and investigated for their suitability in terahertz optoelectronic applications. Optical pump-terahertz probe measurements on these layers have shown that carrier dynamics can be described using two characteristic times. The faster decay component has characteristic times shorter than 1 ps, whereas the slower component decays in several tens of picoseconds. Fitting the electron lifetimes dependence on optical excitation level the electron trapping cross-section and trap density were determined. The possible mechanism of carrier recombination was discussed. The photoconductive terahertz emitters and detectors made from GaBiAs layers have been manufactured and used in time-domain spectroscopy system with a signal bandwidth larger than 4.5 THz. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Room temperature intraband Raman emission and ultrafast carrier relaxation in GaN/AlN quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009L. Nevou Abstract We report on the intraband emission at room-temperature from GaN/AlN quantum dots grown by plasma-assisted molecular beam epitaxy as well as on the measurements of GaN quantum dot intraband relaxation using femtosecond spectroscopy. The quantum dots exhibit s-pz intraband absorption peaked at 1.55 ,m. The pz -s intraband luminescence at , = 1.48 ,m is observed under optical excitation at , = 1.34 ,m. The population of the pz state arises from Raman scattering by GaN A1 longitudinal optical phonons. Based on the emission spectral shape, we estimate that the homogeneous linewidth of the s-pz intraband transition is less than 4 meV. The carrier lifetime of the pz state (T1) is measured by femtosecond pump-probe spectroscopy. T1 = 165 fs while the thermalization of electrons in the ground state proceeds in 1.5 ps. The saturation intensity of the s-pz intraband absorption is estimated to be in the 15 to 137 MW/cm2 range taking into account the uncertainty on the pump beam waist. These results demonstrate the promising capabilities of GaN/AlN QDs for application to telecom intraband lasers as well as multi-Tbit/s all-optical switching devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dynamical model for coherent optical manipulation of a single spin state in a charged quantum dotPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2009Gabriela Slavcheva Abstract The optically-induced coherent spin dynamics of a single spin confined in a charged quantum dot (QD) is theoretically studied employing coupled vector Maxwell-pseudospin formalism. Generalized pseudospin master equation is derived for description of the time evolution of spin coherences and spin populations including spin population transfer and dissipation in the system through spin relaxation processes. The equation is solved in the time domain self-consistently with the vector Maxwell equations for the optical wave propagation coupled to it via macroscopic medium polarisation. Using the model the long-lived electron spin coherence left behind a single resonant ultrashort optical excitation of the electron-trion transition in a charged QD is simulated in the low- and high-intensity Rabi oscillations regime. Signatures of the polarised photoluminescence (PL), predicted by the model, such as the appearance of a second echo pulse after the excitation and characteristic PL trace shape, are discussed for realization of high-fidelity schemes for coherent readout of a single spin polarisation state. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Impact of traveling phonon wave packets on the optical response of quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2009J. Huneke Abstract The influence of phonon wave packets created by the optical excitation of a quantum dot (QD) structure on four-wave-mixing (FWM) signals is analyzed theoretically. Two different structures are compared: a single QD in a half-space geometry located close to the surface, where the emitted phonon wave packet after reflection at the surface reenters the QD and a pair of QDs in an infinite medium, where the phonon wave packet created in one QD travels across the other QD. Although the strain fields are very similar pronounced differences in the FWM polarizations are found. In the single QD system we observe clear signatures of the reflected wave packet in the FWM signal at positive delay times, which can also be observed in the time-integrated FWM signal from an inhomogeneously broadened ensemble of such QDs. In the two QD case no signatures of the traveling wave packet are seen in the signal at positive delays and, consequently, in the ensemble signal. However, they are clearly seen in the FWM signal at negative delay times. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Population inversion of photoexcited electrons and holes in graphene and its negative terahertz conductivityPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008Victor Ryzhii Abstract We demonstrate that sufficiently strong optical excitation may result in the population inversion in graphene, so that the real part of the ac conductivity can be negative in the terahertz range of frequencies. We study also how the heating of the electron-hole system influences the effect of negative ac conductivity. The effect of population inversion and negative ac conductivity might be used in graphene-based coherent sources of terahertz electromagnetic radiation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Defect density dependence of luminescence efficiency and lifetimes in AlGaN active regions exhibiting enhanced emission from nanoscale compositional inhomogeneitiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006G. A. Garrett Abstract AlGaN epilayers grown by plasma-assisted molecular beam epitaxy and exhibiting high internal quantum efficiency (up to 30%) are incorporated into double-heterostructure devices grown on base layers of varying defect density. Growth of these AlGaN active layers, having increased emission from localization of carriers in regions of nanoscale compositional inhomogeneities, is found to benefit from base layers of reduced defect density, including thick AlGaN templates grown by hydride vapor phase epitaxy. Nonlinear radiative processes are observed at high optical excitation for layers grown on lower defect base layers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The evolution of the electric field in an optically excited semiconductor superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2005Alvydas Lisauskas Abstract We report on time-resolved photocurrent spectroscopy of an intrinsic GaAs/Al0.3Ga0.7As superlattice subsequent to femtosecond optical excitation. Information on the spatio-temporal evolution of the densities of electrons and holes and on the internal electric field is obtained by tracing Wannier-Stark photocurrent spectra as a function of delay time for various bias fields and pump excitation intensities. The experimental results are supplemented by simulations. We employ the combined information to define the conditions to be met for succesful pump-probe Bloch gain experiments. In particular, we find that field screening sets on upper limit for the carrier density of 1016 cm,3, and that the time window during which gain should be found is defined by the duration of the sweep-out of the optically injected electrons from the superlattice which occurs within about 10 ps after excitation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic excitations and optical spectra of Pt2 and Pt4 on Cu(001) modeled by a clusterPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2010George Pal Abstract The photoabsorption spectra of Pt2 and Pt4 clusters on the Cu(001) surface are computed using two different theoretical methods: the symmetry-adapted cluster expansion configuration interaction from quantum chemistry and a recently developed linear response approach to treat electron,hole correlations in the presence of an external electromagnetic field. Comparing the energetically low-lying optical excitations, we find very good agreement between the two methods. For different orientations of the adsorbed clusters with respect to the surface, we find that the most intense optical peaks occur when the polarization of the applied laser pulse is parallel to the surface. [source] | ||||||||||