The Stability Paradox in Central Asia

Baev responds to Cooley's essay on external powers and political stability in Central Asia

Ab initio studies of two-photon absorption of some stilbenoid chromophores

Two-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to second-order response function. Several important issues in modeling are highlighted which must be addressed for a reliable reproduction of the experimental results. It is evident that the correct order of magnitude of calculated two-photon absorption cross sections can only be obtained if proper account is taken of vibrational broadening of the absorption profiles. A comparison of the theoretical results with the experimental ones indicates that the computed two-photon absorption cross sections are in rough agreement with our previous report, although the observed systematic increase of the cross sections with the electron acceptor strength is not well reproduced. It is suggested that this disagreement may be due not only to the deficiencies of the computations but also to a variety of factors contributing to the experimental value of the effective two-photon absorption cross section, which are not taken into account in the ab initio calculations

Free-space Excitation of Resonant Cavities formed from Cloaking Metamaterial

We propose a new class of resonant electromagnetic structures, and study their response to free-space illumination. The structures consist of partial cylindrical shells that have cloaking material properties proposed by Pendry et al. These metamaterial shells have apertures that allow the propagation of incident irradiation into an interior resonant cavity. We use full wave time-harmonic analysis to study the field distribution inside the cavity, and show that an analogue of Whispering Gallery Modes (WGMs) can be efficiently excited via free-space illumination.Comment: to appear J. Modern Optic

Negative refractivity assisted optical power limiting

We propose an approach for achieving optical power limiting (OPL) using planar multilayered media. The approach involves the use of multiple bilayer structures that consist of a negative index (NIM) layer followed by a nonlinear two-photon absorbing (TPA) layer. The NIM layers refocus light in the TPA layers to increase intensity and nonlinear absorption. We perform parametric two-dimensional finite difference time domain simulations and compute the power transmittance of the media as a function of the incident light intensity, the number of layers, the layer thickness, and the material properties. Our analysis demonstrates proof-of-concept and indicates that the TPA-based OPL can be enhanced and optimized using NIM focusing.This work was in part supported by a grant from the office of Vice-President for Research at the University at Buffalo and in part by the Chemistry and Life Sciences Directorate of the Air Force Office of Scientific Research

Asymmetric properties between the forward and backward stimulated emission generated by ultrafast three- and four-photon excitation

This paper presents the observation of asymmetric behavior between the forward and backward stimulated emission, generated in multiphoton active dye solutions, through three- or four-photon excitation of subpicosecond laser pulses. At a pump energy level considerably higher than the lasing threshold value, the peak wavelengths of the forward stimulated emission are 20–30-nm shorter than those of the backward stimulated emission for the two investigated stilbazolium dye solutions (PRL-L3 and PRL-L10). This obvious spectral asymmetry can be explained by the following three considerations: (i) the difference of spatial/temporal sequences between the forward and backward stimulated emission pulses; (ii) blueshift of the peak wavelength of transient gain experienced by the forward stimulated emission pulse; and (iii) saturation of reabsorption at the forward lasing wavelength range. These proposed explanations are verified by a specially designed pump-probe experiment, utilizing a white-light continuum as the probe beam and the ∼1300-nm laser radiation as the pump beam for three-photon excitation. The experimental results have clearly shown the existence of the saturation effect of reabsorption and the gain-peak blueshift effect as well as their transient features

Zwischen alter und neuer Weltordnung: die Logik der Russischen Außen- und Sicherheitspolitik

Politische Entscheidungen im System von Präsident Wladimir Putin folgen einer militärisch-technischen Logik und militärische Stärke ist ein Hauptinstrument russischer Außenpolitik. Die Modernisierung des nuklearen Potentials wird als Faustpfand in der Abschreckung vor allem gegenüber den USA gesehen, das Russland zugleich auf Augenhöhe mit dem Gegner bringt. Ebenso legitim ist für den Kreml die hybride Kriegsführung mit Desinformation, Cyber-Angriffen, Fake-News und von Russland gesteuerten Auslandsmedien

X-Ray absorption spectroscopy of H3O+

We report the X-ray absorption of isolated H3O+ cations at the O 1s edge. The molecular ions were prepared in a flowing afterglow ion source which was designed for the production of small water clusters, protonated water clusters, and hydrated ions. Isolated H2O+ cations have been analyzed for comparison. The spectra show significant differences in resonance energies and widths compared to neutral H2O with resonances shifting to higher energies by as much as 10 eV and resonance widths increasing by as much as a factor of 5. The experimental results are supported by time-dependent density functional theory calculations performed for both molecular cations, showing a good agreement with the experimental data. The spectra reported here could enable the identification of the individual molecules in charged small water clusters or liquid water using X-ray absorption spectroscopy

Multi-Photon Interactions with a Time Structure

The present thesis concerns aspects of the interaction ofmatter in gas, liquid and solid phases, with electromagneticradiation, ranging from the optical to the X-ray region. Overthe last decade the availability of ultrashort strong laserpulses as well as of high power synchrotron sources of tunableX-ray radiation has stimulated a rapid development of newexperimental techniques which makes it possible to analysedifferent physical, chemical and biological processes inunprecedented detail. All of this urges a concomitantdevelopment of adequate theoretical language and methodscombined with simulation techniques. The first part of the thesis addresses nonlinear propagationof strong optical pulses. This study is motivated by thebreakthrough in synthesis of novel organic materials possessingprespecified nonlinear optical properties and which has led toa multitude of potential applications such as, for example, 3Dimaging and data storage, optical limiting and photodynamiccancer therapy. In order to clarify the underlying physics, astrict solution has been derived of the density matrixequations of a material aiming at an explicit treatment of itsnonlinear polarization without addressing a conventional Taylorexpansion over field amplitudes. Such a formalism is developedfor many-level molecules, allowing to solve the coupledMaxwell's and density matrix equations for the propagation of afew interacting laser pulses through a nonlinear molecularmedium. The theory presented is capable to account formulti-photon processes of an arbitrary order and for differentsaturation effects. The theory is applied to simulations oftwo- and three-photon absorption as well as to upconvertedstimulated emission of organic molecules in solvents. The second part of the thesis is devoted to resonant X-rayRaman scattering from free molecules, solutions and polymerfilms. The temporal analysis of the spectral profiles isperformed using the technique of scattering duration whichallows to select physical processes with different time scales.The slowing-down/speeding-up of the scattering by frequencydetuning provides insight in the formation of the differentparts of the scattering profile like atomic and molecularbands, resonant and vertical scattering channels, anomalousenhancement of the Stokes doubling effect. The lifetimevibrational interference (LVI), playing a crucial role inresonant scattering, is found to strongly influence thedispersion of the Auger resonances of polymers in agreementwith experiment. An almost complete quenching of the scatteringcross section by LVI is observed for the N2molecule. It is found that the interferenceelimination of the scattering amplitude gives valuableinformation on molecular geometry. The electron Doppler effectis minutely studied making use of a wave packet technique. Thesimulations show an "interference burning" of a narrow hole onthe top of the Doppler broadened profile of the Auger spectraof molecular oxygen. For the SF6molecule the Auger Doppler effect is found to besensitive to the detuning due to the scattering anisotropy. Inall of these studies the temporal language was foundconstructive and enormously helpful for understanding theunderlying physical processes. Most theoretical predictionsmade have been verified by experiments.NR 2014080

Radicals from the gas-phase pyrolysis of a lignin model compound: -coumaryl alcohol

The intermediate radicals produced in the gas-phase pyrolysis of one of the main building blocks of lignin - -coumaryl alcohol (-CMA) - were investigated using the low temperature matrix isolation technique interfaced with electron paramagnetic resonance spectroscopy (LTMI-EPR). An anisotropic EPR spectrum characterized by a high -value (\u3e2.0080) and a relatively low saturation coefficient (∼1.40) throughout the high pyrolytic temperature region (700 to 1000 °C) was observed. Theoretical calculations revealed plausible decomposition pathways for -CMA comprising highly delocalized aromatic radicals. The results provide evidence for a dominant role of oxygen-centered radicals during the pyrolysis of -CMA

Armenia’s Velvet Revolution: Challenges to Regional Foreign Policy

ISSN:1867-932