I. V. Soboleva

Fluorescence emission enhanced by surface electromagnetic waves on one-dimensional photonic crystals

An appreciable increase in the fluorescence emission of an organic chromofore is obtained by exploiting the local field enhancement at the surface of one-dimensional photonic crystals after excitation of surface electromagnetic waves (SEW). Using a properly designed photonic crystal consisting of alternating a-Si1−xNx:H layers with different nitrogen content, efficient emission of R6G dye spun on the surface of the photonic crystal is detected and the intensity spatial distribution of the SEW is visualized by means of far-field fluorescence microscopy. Our results demonstrate potential applications in enhanced fluorescence microscopy with an increased sensitivity and spectral selectivity.

Second- and third-harmonic generation in birefringent photonic crystals and microcavities based on anisotropic porous silicon

One-dimensional anisotropic photonic crystals and microcavities based on birefringent porous silicon are fabricated. The reflectance spectra demonstrate the presence of photonic band gap and microcavity modes with spectral positions tunable upon the sample azimuthal rotation around its normal and/or rotation of polarization plane of incident light. Simultaneous enhancement of secondand third-harmonic generation at the photonic band-gap edge due to the phase matching is observed. The angular positions of the second- and third-harmonic peaks are controllable via the anisotropy of the refractive indices of porous silicon layers.

Evidence for diffusion-controlled electron transfer in exciplex formation reactions. Medium reorganisation stimulated by strong electronic coupling

Diffusion-controlled rates of formation were found from the temperature dependence of apparent quenching rate constants for exciplexes, when the driving force of excited-state electron transfer -0.1 < deltaG(ET)* < +0.1 eV. This is inconsistent with the conventional mechanism of electron-transfer reactions, involving preliminary reorganisation of the medium and reactants, and provides strong support for the mechanism of medium reorganisation stimulated by strong electronic coupling of locally excited and charge-transfer states.

The donor-acceptor properties of exciplexes. Comparison of fluorescence quenching of excited aromatic molecules and their exciplexes

Abstract Exciplex reactivity in electron transfer reactions is investigated. Methods are proposed for the calculation of exciplex ionization potentials and electron affinities. Exciplex quenching rate constants have been measured. It is shown that exciplex quenching follows the same laws as that of excitcd molecules. Conditions of specific exciplex quenching are considered.

Direct measurements of forces induced by Bloch surface waves in a one-dimensional photonic crystal

An experimental study of the interaction between a single dielectric microparticle and the evanescent field of the Bloch surface wave in a one-dimensional (1D) photonic crystal is reported. The Bloch surface wave-induced forces on a 1 μm polystyrene sphere were measured by photonic force microscopy. The results demonstrate the potential of 1D photonic crystals for the optical manipulation of microparticles and suggest a novel approach for utilizing light in lab-on-a-chip devices.

Surface wave-induced enhancement of the Goos-Hänchen effect in one-dimensional photonic crystals

The excitation conditions of surface electromagnetic waves in one-dimensional photonic crystals (Bragg reflectors) are studied. Surface electromagnetic waves are visualized by the far-field optical microscopy of the surface of the photonic crystal. The enhancement of the Goos-Hänchen effect by surface electromagnetic waves excited in one-dimensional photonic crystals has been experimentally observed. The Goos-Hänchen shift reaches 30λ for a wavelength of λ = 532 nm.

Anisotropic Photonic Crystals and Microcavities Based on Mesoporous Silicon

A technique to prepare one-dimensional anisotropic photonic crystals and microcavities based on anisotropic porous silicon exhibiting optical birefringence has been developed. Reflectance spectra demonstrate the existence of a photonic band gap and of an allowed microcavity mode at the photonic band gap center. The spectral position of these bands changes under rotation of the sample about its normal and/or under rotation of the plane of polarization of the incident radiation. The dependence of the shift of the spectral position of the photonic band gap edges and of the microcavity mode on the orientation of the polarization vector of incident electromagnetic wave with respect to the optical axis of the photonic crystals and microcavities was studied.

Exciplex quenching as a method of investigating photochemical reaction mechanisms

Abstract The application of exciplex quenching to the investigation of photochemical reaction mechanisms is investigated. Kinetic analyses of exciplex fluorescence quenching and photochemical reaction quenching are presented. The product of a photochemical reaction is generally formed by one of the following three routes: (a) the product is formed in parallel with the exciplex; (b) the product and the exciplex are both formed from a common non-relaxed charge transfer state; (c) the product is formed from the fluorescent (relaxed) state of the exciplex or the reaction proceeds via the exciplex as an intermediate. Expressions enabling the partial quantum yields of routes (a), (b) and (c) to be determined are obtained. The quenching method is used to analyse the mechanism of halogen photoelimination in 9,10-dichloroanthracene in the presence of diethylaniline. The reaction is shown to proceed via both the non-relaxed and relaxed states of the exciplex.

Effect of solubilization in micelles on the kinetics of electron transfer photoreactions and redox properties of reactants. Quenching of RuL62+ luminescence in SDS micelles

Abstract The oxidative luminescence quenching of tris(2,2′-bipyridyl)ruthenium(II) (RuL62+) by various quenchers (quinones and nitroaromatic compounds) was investigated in homogeneous (organic and aqueous) solutions and in sodium dodecylsulphate (SDS) micelles to reveal the effects of the local microenvironment on the energy of electron transfer and on the reorganization energy of the reaction. Analysis of the reaction kinetics in micellar solutions showed that, in systems in which the redox potential of the quencher E 1 2 ( Q Q ·− ) V , “internal” quenching was observed, i.e. reaction of ∗RuL62+ with a quencher molecule solubilized inside a micelle. In these systems, true intramicellar quenching rate constants were obtained, and the electron transfer reactions in micelles were compared with those in homogeneous solutions. In SDS micelles, the Gibbs energy of electron transfer ΔGet was close to that observed in water; however, the spectral properties of RuL62+ and the value of the limiting diffusion rate constant were typical of media with medium polarity and high viscosity. This suggests a considerable reorganization of the local microenvironment of the RuL62+ complex and quencher during the electron transfer reaction and a specific effect of surrounding water molecules on the reaction kinetics.

Competition and interplay of various intermolecular interactions in ultrafast excited-state proton and electron transfer reactions.

The main features of the photoinduced kinetics of both ultrafast excited-state proton and electron transfer reactions that occur in the picosecond (ps) and femtosecond (fs) time domains are compared. Proton transfer (PT) reaction kinetics can be described in terms of several discrete values of rate coefficients in the form of polyexponential functions where each value of the rate coefficient can be attributed to a definite physical behavior of the reaction mechanism. In contrast, electron transfer (ET) reaction kinetics requires a consideration of a continuous distribution of rate coefficients. This difference can be related to structure of the ground-state reactant pairs for each reaction. Excited-state ET can occur at various configurations of reactant molecules and its rate reflects the fluctuations of the distances and orientations of these molecules. In contrast, excited-state PT requires preliminary formation of a ground-state H-bonded complex with definite structure where the reaction occurs after photoexcitation.