Short-laser-pulse-driven emission of energetic ions into a solid target from a surface layer spalled by a laser prepulse

An efficient emission of picosecond bunches of energetic protons and carbon ions from a thin layer spalled from a organic solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (∼20–30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of its small thickness, a layer produced by laser spallation efficiently absorbs ultrashort laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm. Received: 13 March 2001 / Accepted: 20 March 2001 / Published online: 20 June 2001     

Density matrix renormalization group studies on one-dimensional -conjugated organic ferromagnet with side radicals

By using the density matrix renormalization group (DMRG) and the self-consistent numerical method, we obtain a high spin ground state with localized spin density describing spin localization and the soliton describing the distortion of the lattice configurations along the main chain. Different electron-phonon interactions result in different configurations of solitons. When the electron-phonon coupling along the main chain is larger than a critical value , a transition from a single soliton-like distortion to a pair of soliton-like distortions along the main chain takes place. Such critical value depends mainly on the intersite Coulomb interactions. The spin density wave along the main chain is always localized around the center of soliton-like distortions. Received 2 July 2001 and Received in final form 25 September 2001     

Quantum theory of van der Waals interactions between excited atoms and birefringent dielectric surfaces

A theory of van der Waals (vdW) interaction between an atom (in ground or excited state) and a birefringent dielectric surface with an arbitrary orientation of the principal optic axis (C-axis) is presented. Our theoretical approach is based on quantum-mechanical linear response theory, using generalized susceptibilities for both atom and electromagnetic field. Resonant atom-surface coupling is predicted for excited-state atoms interacting with a dispersive dielectric surface, when an atom de-excitation channel gets into resonance with a surface polariton mode. In the non-retarded regime, this resonant coupling can lead to enhanced attractive or repulsive vdW surface forces, as well as to a dissipative coupling increasing the excited-state relaxation. We show that the strongly non-scalar character of the interaction with the birefringent surface produces a C-axis-dependent symmetry-breaking of the atomic wavefunction. Changes of the C-axis orientation may also lead to a frequency shift of the surface polariton mode, allowing for tuning on or off the resonant coupling, resulting in a special type of engineering of surface forces. This is analysed here in the case of cesium 6D _3/2 level interacting with a sapphire interface, where it is shown that an adequate choice of the sapphire C-axis orientation allows one to transform vdW surface attraction into repulsion, and to interpret recent experimental observations based on selective reflection methods [H. Failache etal., Phys. Rev. Lett. 83, 5467 (1999)]. Received 24 January 2001     

Luminescence and damage thresholds of cerium-doped LaF3 for ns-pulsed laser excitation at 248 nm

3 following excitation with 248 nm/14 ns laser pulses at room temperature for the two Ce concentrations 0.03 and [%mol]1. The relative intensities of the 5d-4f bands emitted from Ce³⁺ at regular and at perturbed lattice sites were found to vary linearly with time for the higher concentration and quadratically for the lower one. This can be explained by radiative energy transfer between the two sites and generation of new perturbed sites at a rate that only shows up for the low Ce concentration. Lifetimes of the respective emission bands were determined to be about 18 ns and 41 ns. Despite resonant absorption of the 5 eV photons, surprisingly high ablation thresholds – 16 J/cm² for 0.03% Ce, and 10 J/cm² for 1% Ce – were observed by the probe-beam deflection technique. The reason is the strong energy loss due to intense fluorescence and deposition of the nonradiative energy fraction in the bulk rather than at the surface. The depth of energy deposition was revealed by scanning electron microscopy in the form of distinctly different ablation morphologies for the two Ce concentrations. Received: 30 July 1998     

Ultrafast carrier dynamics in laser-excited materials: subpicosecond optical studies

2 , MgO and Al₂O₃) irradiated by short laser pulses (70 fs to 1.3 ps) at intensities below and above breakdown threshold. This is achieved with the help of time-resolved interferometry in the frequency domain, which was successfully used to study the dynamics of photoexcited carriers in insulators. The results obtained under different experimental conditions, distance from the surface, pump intensities and duration, during or after the pump pulse, are discussed and compared to the models recently developed to explain optical breakdown. Received: 20 September 1998