Existence and stability of modulating pulse solutions in Maxwell’s equations describing nonlinear optics

Modulating pulse solutions play a big rôle in modern long distance high speed communication. Such solutions consist of a traveling pulse-like envelope modulating an underlying electromagnetic wave. In this paper we show that under certain assumptions such solutions exist and are dynamically stable for the associated nonlinear partial differential equations, namely Maxwells integro-differential equations describing nonlinear optics. The analysis is worked out in detail for bulk media, and we discuss how the results extend to optical fibers and to parametrically forced systems.     

Interferometrische Messung der Mikrostruktur von synthetischen Fasern

Zusammenfassung Bei den optisch anisotropen Fasern stellen der Brechungsindex bzw. die Doppelbrechung kennzeichnende Größen für die Struktur des Materials dar. Anhand von Beispielen wird die interferenzmikroskopische Messung der Brechungszahlen erläutert und insbesondere eine bisher noch nicht praktizierte Methode zur detaillierten Erfassung der Inhomogenitäten über den Faserquerschnitt vorgestellt. Einige Meßergebnisse für Polyamidfasern werden mitgeteilt. Es wird ein quantitativer Zusammenhang zwischen den Brechungsindizes und der Kristallinität der Faser angegeben. Aufgrund der vorliegenden Brechzahlmessungen ist z. B. ein Unterschied im Kristallinitätsgrad zwischen Mitte und Rand der Faser von 1–2% noch deutlich nachweisbar.

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Summary For optically anisotropic fibres the refractive index and the double refraction are parameters characterizing the structure of the material. Examples are used to explain the measurement of refractive indices by interference microscopy and especially to introduce a method, not yet applied so far, for the detailed determination of the inhomogeneities over the cross-section of the fibre. Some test readings for polyamide fibres are communicated. A quantitative relationship is given between the refractive indices and the crystallinity of the fibre. In accordance with the refractive index measurements available, it is still possible to distinctly prove, e.g., a difference in the degree of crystallinity between the centre and the rim of the fibre of 1–2%.

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Herrn Prof. Dr.Otto Bayer zu seinem 70. Geburtstag gewidmet.

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Für die umsichtige und sorgfältige Ausführung der interferenzmikroskopischen Messungen und der Auswertungen sei an dieser Stelle FrauAntje Mazzucco herzlich gedankt.     

Improving the mechanical properties of CNF films by NMMO partial dissolution with hot calender activation

Reinforcing of cellulose nanofibril (CNF) films by partial dissolution with N-methylmorpholine-N-oxide (NMMO) was investigated. The method investigated is composed of impregnation of CNF film with liquid solution of NMMO followed by dry heat activation. The heat activation of the impregnated film was carried out using a heated calendering nip, which enabled simultaneous heating and compression. The partial dissolution of cellulose by NMMO caused a significant increase in the transparency of CNF film due to the decrease of film porosity and increased surface smoothness. The dry strength of the reinforced film was increased from 122 up to 195 MPa. Furthermore, the wet strength of the reinforced film was up to 70% greater than the dry strength of pure CNF film. The changes in the fibrillar structure were investigated with topographical imaging (SEM and AFM) and spectroscopically using NMR and FTIR. No significant changes in the fibril structure or cellulose morphology were observed. Moreover, the treated film resisted significant water pressure, highlighting CNF film’s permanent water resistance. The partial dissolution process with NMMO was also capable of reinforcing a CNF composite film with macro scale structural elements (lyocell short-cut fibres). The strategy investigated is a robust and fast method to improve the mechanical properties of fibrillary cellulose films, allowing them utilization in applications where improved water resistance and fully cellulosic character are required properties.     

Photodissociation mass spectra and mass-selected resonant (1+1)-photodissociation spectroscopy of some alkyl iodide radical cations

Photodissociation (PD) mass spectra and mass selected (1+1)-photodissociation spectra of C(2)H(5)I(+?), C(2)D(5)I(+?),1- C(3)H(7)I(+?), 2-C(3)H(7)I(+?), 1-C(4)H(9)I(+?) and 2- C(4)H(9)I(+?) radical cations were studied within the ? ← X~ absorption band. The photodissociation mass spectra within the range 13,600-15,900 cm(-1) (1.68-1.97 eV) evidence only a simple cleavage of the C-I bond and formation of the corresponding alkyl ions. The resonant (1+1)-photodissociation spectra of C(2)H(5)I(+?) and C(2)D(5)I(+?) show intense vibrational structure in the excited ? state. The thresholds for formation of the states of C(2)H(5)I(+?) and C(2)D(5)I(+?) were estimated to be (13,278 ± 12) cm(-1) (1.6462 ± 0.0014 eV)and (13,363 ± 12) cm(-1) (1.6586 ± 0.0014 eV), respectively. Whereas a few resonant vibronic excitations could be identified with 1-C(3)H(7)I(+?) and 1- C(4)H(7)I(+), no vibrational features were observable with 2- C(3)H(7)I(+?) and 2-C(4)H(9)I(+?). It is concluded that 1- and 2-iodoalkane radical cations do not rearrange, even under the conditions of electron ionisation used to generate the molecular ions.     

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light‐responsive systems in materials and life sciences. ortho‐Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ‐electron‐withdrawing F atoms ortho to the N?N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para‐electron‐withdrawing groups (EWGs) work in concert with ortho‐F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho‐fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.     

Observation of bi-polarons in blends of conjugated copolymers and fullerene derivatives

From a fundamental and application point of view it is of importance to understand how charge carrier generation and transport in a conjugated polymer (CP):fullerene blend are affected by the blend morphology. In this work light-induced electron spin resonance (LESR) spectra and transient ESR response signals are recorded on non-annealed and annealed blend layers consisting of alkyl substituted thieno[3,2-b]thiophene copolymers (pATBT) and the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at temperatures ranging from 10 to 180 K. Annealing of the blend sample leads to a reduction of the steady state concentration of light-induced PCBM anions within the blend at low temperatures (T = 10 K) and continuous illumination. This is explained on the basis of the reducing interfacial area of the blend composite on annealing, and the high activation energy for electron diffusion in PCBM blends leading to trapped electrons near the interface with the CP. As a consequence, these trapped electrons block consecutive electron transfer from an exciton on a CP to the PCBM domain, resulting in a relatively low concentration charge carriers in the annealed blend. Analysis of the transient ESR data allows us to conclude that in annealed samples diamagnetic bi-polaronic states on the CPs are generated at low temperature. The formation of these states is related to the generation and interaction of multiple positive polarons in the large crystalline polymer domains present in the annealed sample.