Stokes diagnostics of simulated solar magneto-convection

We present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow.Comment: Astronomy & Astrophysics, in pres

THEORY OF THE STRUCTURE OF THE SELF-TRAPPED EXCITON IN QUARTZ

Quartz is an insulator with an extremely wide band gap in the vacuum ultra-violet. However, under irradiation from high-energy electrons or X-rays, samples of high purity emit a luminescence band in the blue, corresponding to a Stokes shift of approximately 7 eV. This large Stokes shift has been ascribed to the self-trapping of an exciton in an otherwise perfect lattice owing to the distortion it induces; the authors review the evidence for this assignment, and describe electronic-structure calculations which reveal the structure of the distorted configuration and also explain various experimentally determined properties of the centre. The self-trapping process they postulate is a novel one as it is driven primarily by the electron component of the exciton

A Large Blue Shift of the Biexciton State in Tellurium Doped CdSe Colloidal Quantum Dots

The exciton-exciton interaction energy of Tellurium doped CdSe colloidal quantum dots is experimentally investigated. The dots exhibit a strong Coulomb repulsion between the two excitons, which results in a huge measured biexciton blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very narrow hole wave function localized around the defect, which is manifested by a large Stokes shift. Moreover, we show that the biexciton blue shift increases linearly with the Stokes shift. This result is highly relevant for the use of colloidal QDs as optical gain media, where a large biexciton blue shift is required to obtain gain in the single exciton regime.Comment: 9 pages, 4 figure

Radiation reaction force induced nonlinear mixing of Raman sidebands of an ultra-intense laser pulse in a plasma

Stimulated Raman scattering of an ultra-intense laser pulse in plasmas is studied by perturbatively including the leading order term of the Landau-Lifshitz radiation reaction force in the equation of motion for plasma electrons. In this approximation, radiation reaction force causes phase shift in nonlinear current densities that drive the two Raman sidebands (anti-Stokes and Stokes waves), manifesting itself into the nonlinear mixing of two sidebands. This mixing results in a strong enhancement in the growth of the forward Raman scattering instability

Phase-locking in Multi-Frequency Brillouin Oscillator via Four Wave Mixing

Stimulated Brillouin scattering (SBS) and Kerr-nonlinear four wave-mixing (FWM) are among the most important and widely studied nonlinear effects in optical fibres. At high powers SBS can be cascaded producing multiple Stokes waves spaced by the Brillouin frequency shift. Here, we investigate the complex nonlinear interaction of the cascade of Stokes waves, generated in a Fabry-Perot chalcogenide fibre resonator through the combined action of SBS and FWM. We demonstrate the existence of parameter regimes, in which pump and Stokes waves attain a phase-locked steady state. Real-time measurements of 40ps pulses with 8GHz repetition rate are presented, confirming short-and long-term stability. Numerical simulations qualitatively agree with experiments and show the significance of FWM in phase-locking of pump and Stokes waves. Our findings can be applied for the design of novel picosecond pulse sources with GHz repetition rate for optical communication systems

Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals

The photoluminescence spectra of spherical CdTe nanocrystals with zincblende structure are studied by size-selective spectroscopic techniques. We observe a resonant Stokes shift of 15 meV when the excitation laser energy is tuned to the red side of the absorption band at 2.236 eV. The experimental data are analyzed within a symmetry-based tight-binding theory of the exciton spectrum, which is first shown to account for the size dependence of the fundamental gap reported previously in the literature. The theoretical Stokes shift presented as a function of the gap shows a good agreement with the experimental data, indicating that the measured Stokes shift indeed arises from the electron-hole exchange interaction.Comment: 8 pages, 4 figures, LaTe

Raman G and D band in strongly photoexcited carbon nanotubes

We observe clear differences in the spectral shift of the Raman D and G bands when heating double wall carbon nanotubes through intense photon irradiation and by varying the temperature in a thermostat. These spectral differences are attributed to modifications of the defect induced double-resonance Raman process, and are consistent with Stokes–anti-Stokes anomalies observed for single and double wall carbon nanotubes, not present in graphite. We find that the Raman intensity for double wall carbon nanotubes increases superlinearly in the red spectral region and sublinearly in the UV spectral region