Activation of color centers in bismuth glass by femtosecond laser radiation

The activation of color centers in the bismuth aluminum-boron-phosphate glass by high-intensity femtosecond laser radiation is experimentally studied. The dynamics of the laser-induced emitting centers in the bulk of sample is characterized. The photoactivation of bismuth glasses is possibly related to the recharging of structural precursors, which serve as effective electron traps and whose possible configuration is discusses. The effect of the femtosecond irradiation involves the initiation of the nonlinear ionization of the glass matrix and the generation of plasma with the concentration of carriers that is sufficiently high to provide almost complete recharging of precursors over several laser pulses.     

Effect of atomic displacements in covalent solids

In covalent materials the electronic wave functions are very sensitive even to small displacements, while with displacements of increasing magnitude the stability of the covalent bond gradually decreases and eventually another type of bond becomes more favourable. This has consequences for some of the properties of covalent solids even at low temperature and produces significant changes with increasing temperature and on melting.     

Relaxation of electronic excitations in wide-gap crystals studied by femtosecond interferometry technique

Time-resolved interferometry with a 100-fs temporal resolution was applied for the first time to studying the relaxation of electronic excitations in complex oxides, namely, tungstates CDWO₄ with a crystal lattice of the wolframite-type and CaWO₄ with a scheelite-type lattice. Two stages of charge carrier relaxation, namely, very fast carrier trapping in 200 fs resulting in self-trapped exciton formation and a relatively slow picosecond relaxation process probably due to configurational relaxation within the oxyanion molecule and modification of the surrounding lattice, are revealed in tungstate crystals. Corresponding models of self-trapped exciton creation in tungstate crystals are discussed. The text was submitted by the authors in English.     

Femtosecond reflectivity study of coherent phonons in doubly photoexcited bismuth

Ultrafast optical excitation generates coherent A_1g optical phonons in bismuth via the mechanism of displacive excitation of coherent phonons (DECP). Here, femtosecond pump-probe reflectivity spectroscopy examines the dynamics of coherent phonons in doubly photoexcited bismuth. Two successive pump pulses are employed to generate optic phonons and thus to investigate the phonon dynamics including bond softening and dephasing with variable pump fluence and inter-pump separation times. Combined with thermal analysis, it distinguishes thermal and nonthermal effects of photoexcitation on the phonon dynamics. It also reveals that photocarriers relax via electron-hole recombination and diffusion out of the optical penetration depth on ultrafast timescales of 10–20?ps.     

Space-time resolved reflectivity measurements of picosecond laser-pulse induced phase transitions in (111) silicon surface layers

The changes in reflectivity of a silicon surface, irradiated by a green picosecond pulse, are probed during and following that pulse with a spatial resolution of 10 m. The data indicate the development of a liquid phase, and a resolidification either into a single crystal or an amorphous phase. The latter has a characteristic ring-type pattern, and occurs only at locations where the incident picosecond laser fluence lies between 0.2 and 0.26 J/cm². The reflectivity data appear to be in good quantitative agreement with a simple heating model, in which the electrons and phonons maintain a local thermodynamic equilibrium on a picosecond time scale.On leave from Philips Research Laboratories, D-2000 Hamburg 54, F. R. Germany     

Investigation of coherent phonons in bismuth by femtosecond laser and X-ray pulse probing

Using femtosecond laser pulses, coordinated oscillations (coherent phonons) of Bi single-crystal atoms have been excited and recorded. The comparison with experimental results on the X-ray probing of coherent phonons at the same excitation energy density demonstrates that the observed lifetime and frequency shift of the oscillations are similar in both cases. Moreover, it has been revealed that the relaxation (incoherent) contributions are identical. This coincidence of the photoinduced response parameters indicates that probing in the visible spectrum correctly reflects the coherent dynamics of the Bi crystal lattice.     

Strong atomic displacements around nitrogen in tantalum determined by energy-dispersive x-ray diffraction

The nitrogen induced attenuation of Bragg reflections (static Debye-Waller Factor) of tantalum single crystals containing nitrogen has been measured by energy-dispersive x-ray diffraction. N has been dissolved in Ta up to a concentration ofc=3.3% N/Ta. From a detailed study of the concentration andK dependence (K=scattering vector) of the static Debye-Waller Factor exponent 2L we conclude:i) 2L increases linearly with the nitrogen content forc1.8% N/Ta,ii) The displacements of the atomic shells due to N on the octahedral sites areu ₁=0.45 (5) Å,u ₂=(–)0.11 (2) Å.u ₃=0.25 (2) Å,iii) The thermal vibrations of those atoms close to N are changed, due to the strong static displacements that they experience. The unusual strong and extended displacements around N in Ta are in contradiction to calculations where two-force models have been used. We propose arguments that displacements are responsible for the specific site chosen by hydrogen atoms when they are trapped by nitrogen in Ta.     

Damage induced by femtosecond laser in optical dielectric films

Both the nature of avalanche ionization （AI） and the role of multi-photon ionization （MPI） in the studies of laser-induced damage have remained controversial up to now. According to the model proposed by Stuart et al., we study the role of MPI and AI in laser-induced damage in two dielectric films, fused silica （FS） and barium aluminum borosilicate （BBS）, irradiated by 780-nm laser pulse with the pulse width range of 0.01 - 5 ps. The effects of MPI and initial electron density on seed electron generation are numerically analyzed. For FS, laser-induced damage is dominated by AI for the entire pulse width regime due to the wider band-gap. While for BBS, MPI becomes the leading power in damage for the pulse width T less than about 0.03 ps. MPI may result in a sharp rise of threshold fluence Fth on r, and AI may lead to a mild increase or even a constant value of Fth on r. MPI serves the production of seed electrons for AI when the electron density for AI is approached or exceeded before the end of MPI. This also means that the effect of initial electron can be neglected when MPI dominates the seed electron generation. The threshold fluence Fth decreases with the increasing initial electron density when the latter exceeds a certain critical value.     

Study of quantum effects on atomic displacements in quartz

The crystal structure of quartz (SiO₂) was analyzed by neutron powder diffraction at several temperatures in the range of 10-250 K. The temperature dependence of the structure parameters was consistent with our previous results obtained using single-crystal X-ray diffraction above room temperature. Atomic displacements are order parameters for displacive structural phase transitions. The temperature evolution of Si atomic displacement in quartz was analyzed by studying the quantum expansion of the Landau potential. The expansion was found to accurately describe the evolution of the atomic displacement over the entire temperature interval. To the best of our knowledge, such a verification of atomic displacement is the first of its kind. A proportional relationship between spontaneous strain and the square of the atomic displacement was observed over the entire temperature interval. The validity of the obtained characteristic temperature for the quantum effect is discussed and compared with the results of previous Raman-scattering studies.     

Recombination processes in atomic clusters irradiated by superintense femtosecond laser pulses

Various mechanisms of recombination of electrons with multiply charged atomic ions in atomic clusters irradiated by superintense femtosecond laser pulses are discussed. All of the recombination mechanisms are shown to take a time considerably longer than the laser pulse duration and, hence, they can develop only in a homogeneous, fairly rarefied cluster plasma after pulse termination. All autoionization states of multiply charged ions in a dense cluster plasma have been found to be destroyed by the Holtsmark electric field.     

Population transfer by femtosecond laser pulses in a ladder-type atomic system

The population transfer in a ladder-type atomic system driven by linearly polarized sech-shape femtosecond laser pulses is investigated by numerically solving Schr6dinger equation without including the rotating wave approximation （RWA）. It is shown that population transfer is mainly determined by the Rabi frequency （strength） of the driving laser field and the chirp rate, and that the ratio of the dipole moments and the pulse width also have a prominent effect on the population transfer. By choosing appropriate values of the above parameters, complete population transfer can be realized.     

Isotropy of optical excitations in few-layer graphenes

The geometric and the most band structures of monolayer and AB-stacked bilayer graphenes exhibit strong anisotropy. Nevertheless, the absorption spectra are isotropic for the polarization vector on graphene plane. The velocity matrix elements dominate this property. These results suggest that AA- and AB-stacked few-layer graphenes and graphites manifest this feature.     

Multiplying the repetition rate of passive mode-locked femtosecond lasers by an intracavity flat surface with low reflectivity

By inserting a low-reflectivity flat surface inside the oscillator cavity, we demonstrate a flexible and phase-insensitive method for multiplying the repetition rate of a femtosecond passive mode-locked solid-state laser. Without mode matching and feedback control, we successfully multiplied the repetition rate of a passively mode-locked Cr:forsterite laser from 124 MHz to 1.24 GHz. High-repetition-rate femtosecond optical pulses with average power of >100 mW can be obtained with the demonstrated method.     

Nonlinear nanoscale localization of magnetic excitations in atomic lattices

Reviewed here is the nonlinear intrinsic localization expected for large amplitude spin waves in a variety of magnetically ordered lattices. Both static and dynamic properties of intrinsic localized spin wave gap modes and resonant modes are surveyed in detail. The modulational instability of extended nonlinear spin waves is discussed as a mechanism for dynamical localization of spin waves in homogeneous magnetic lattices. The interest in this particular nonlinear dynamics area stems from the realization that some localized vibrations in perfectly periodic but nonintegrable lattices can be stabilized by lattice discreteness. However, in this rapidly growing area in nonlinear condensed matter research the experimental identification of intrinsic localized modes is yet to be demonstrated. To this end the study of spin lattice models has definite advantages over those previously presented for vibrational models both because of the importance of intrasite and intersite nonlinear interaction terms and because the dissipation of spin waves in magnetic materials is weak compared to that of lattice vibrations in crystals. Thus, both from the theoretical and the experimental points of view, nonlinear magnetic systems may provide more tractable candidates for the investigation of intrinsic localized modes which display nanoscale dimensions as well as for the future exploration of the quantum properties of such excitations.     

Cascades of atomic displacements in solids: The ballistic stage

The evolution of cascades of atomic displacements in solids is analyzed. The charge-exchange, ionization, and elastic scattering cross sections are calculated for the atoms and ions involved in cascade evolution. The effects due to the material density are taken into account. These results are used to perform the first Monte Carlo computations of cascades based on the knowledge of microscopic processes without invoking phenomenological potentials. The proposed approach is unique in that detailed characteristics of atomic processes are obtained by ab initio calculation and applied to analyze cascades of atomic displacements. Subcascade development is described, and a relation between the number of Frenkel pairs and the energy of the primary knock-on atom is found for a wide energy range. This provides a basis for characterizing the dose dependence of the hardening of reactor pressure vessel steel and for comparing the effects of primary radiation damage for fission and fusion reactors.     

Quantum leakage of collective excitations of atomic ensemble induced by spatial motion

We generalize the conception of quantum leakage for the atomic collective excitation states. By making use of the atomic coherence state approach, we study the influence of the atomic spatial motion on the symmetric collective states of 2-level atomic ensemble due to inhomogeneous coupling. In the macroscopic limit, we analyze the quantum decoherence of the collective atomic state by calculating the quantum leakage for a very large ensemble at a finite temperature. Our investigations show that the fidelity of the atomic system will not be good in the case of atom numberN→∞. Therefore, quantum leakage is an inevitable problem in using the atomic ensemble as a quantum information memory. The detailed calculations shed theoretical light on quantum processing using atomic ensemble collective qubit.     

Controllable optical superregular breathers in the femtosecond regime

We investigate optical superregular breathers in the femtosecond regime under dispersion management in an inhomogeneous fiber governed by the nonautonomous higher-order nonlinear Schr o¨dinger equation(NLSE). Exact solutions describing the dynamics of superregular breathers are obtained. Furthermore, we discuss the propagation behaviors of controllable superregular breathers, including stabilization and recurrence in an exponential dispersion fiber and a periodic distributed fiber system. Particularly, the nonlinear dynamics of superregular modes evolved from an identical initial small-amplitude modulation is analyzed in detail.     

Analysis of mesoporous thin films by X-ray reflectivity, optical reflectivity and grazing incidence small angle X-ray scattering

It is well-established that X-ray reflectivity (XR) is an invaluable tool to investigate the structure of thin films. Indeed, this technique provides under correct analysis, the electron density profile of thin films in the direction perpendicular to the substrate. For thin films that exhibit lateral ordering at the nanometer scale, grazing incidence small angle X-ray scattering (GISAXS) ideally complements the XR technique to measure the scattering in off-specular directions. As typical examples, XR and GISAXS data of mesoporous silica thin films and porous materials are presented. The analysis of the XR curve allows to determine the porosity of the film. We also show that the combination of X-ray and visible optical reflection provides information about the index of refraction of thin films. Finally we report how capillary condensation of water can be monitored by XR and GISAXS.