We report a theoretical study of a spaser with an active medium containing three-level molecules or quantum dots, generating one-dimensional axisymmetric plasmon polaritons in a circular cross-section nanowire with a dielectric core and a silver shell. Within the quantum approach to the description of a three-level system and a plasmon polariton, equations of the spaser dynamics are derived and some of their parameters are calculated. The ranges of relaxation rates between excited states of a three-level system are found, at which plasmons of a higher or lower frequency, as well as of both frequencies simultaneously, are generated. It is shown that the stationary number of plasmons of a higher frequency increases linearly with increasing pump rate, and the number of plasmons of a lower frequency either increases nonlinearly or remains constant. Different dependences of the number of plasmons with a higher and lower frequency on the inner radius of the nanowire shell are obtained.

The effect of plasmonic nanoparticles (NPs) on the fluorescence and phosphorescence intensity of organic dye molecules was studied theoretically and experimentally. A theoretical model that takes into account nonradiative transfer of excitation energy from a molecule to an NP and the changes in the rates of spontaneous emission and light absorption by a molecule near an NP was proposed to calculate the luminescence intensity of a molecule in the presence of a plasmonic NP. Numerical estimates for an erythrosine molecule and a silver NP showed that the greatest increase in luminescence was observed at distances of 4–8 nm between the molecule and the NP surface. Experimentally observed changes in luminescence spectra and shortening of the erythrosine triplet state lifetime in poly(vinyl alcohol) films doped with silver NPs were explained using the proposed model.

To explain the experimentally observed effect of silver nanoparticles on the fluorescence of organic dyes and the nonradiative intermolecular transfer of electronic excitation energy in multilayer nanostructures, the previously proposed theoretical model of plasmon resonance in spherical nanoparticles of metals was used. The rates of radiative and nonradiative (FRET) processes in film structures with Ag nanoparticles were calculated for fluorescein and rhodamine B molecules, as well as for two-component systems fluorescein-nile red (NR) and rhodamine B-NR. A version of the model was used that takes into account the effect of NPs on FRET between molecules, the radiative decay of donor and acceptor molecules, and the energy transfer from the dye to plasmonic nanoparticles. The calculation of the UDA rate for pairs with different energy transfer efficiency showed a greater increase in the UDA parameter for the fluorescein-nile red pair than for the rhodamine B-nile red pair. Estimation of the fluorescence enhancement factor of donor and energy acceptor molecules and the rate of energy transfer from the dye to silver NPs showed their insignificant contribution to the formation of the resulting energy transfer efficiency enhancement in the presence of plasmonic NPs. Keywords: energy transfer, silver nanoparticles, plasmon, model

The effects of the influence of the magnetic field on the luminescence intensity of methoxypolyphenylene vinylene (MEH-PPV) films at various air pressures above the samples are investigated. An inversion of the sign of the magnetic field effect was detected with a smooth change in the oxygen concentration in the system. Based on the balance equations, a theoretical model of processes is proposed that takes into account the formation of populations of triplet (T) excitons of MEH-PPV and singlet oxygen during the reactions of triplet-triplet annihilation, T-3Sigmag(O2)-quenching and T-1Deltag(O2)-cross-annihilation of electronic excitations. According to the proposed theoretical model, the magnetic field effects of delayed fluorescence at different concentrations of O2 molecules in MEH-PPV films are calculated, consistent with the experimental results. Keywords: triplet excitons, π-conjugated polymers, magnetically dependent fluorescence, singlet oxygen.

A spectral model is constructed for the luminescence of a two-component exciton-activated quantum dot (QD)–spheroidal plasmon nanoparticle (NP) system in a homogeneous external magnetic field in the approximation of a dipole electric polarizability tensor of the nanoparticle with dissipation of the exciton energy in the NP taken into account. A tensor representation of the dielectric permeability of the magnetized electron plasma of the metal is used, which is responsible for the formation of the characteristics of the electric field in the spheroid. It is found that with a change in the eccentricity of the spheroid, the luminescence spectrum of the system changes, reflecting the effect of the external magnetic field on both the radiative and the dissipative properties of the binary QD–NP complex.

Conformational changes have been studied by molecular dynamics simulation in a complex of two oppositely charged polyelectrolytes and in a polyampholytic block copolymer consisting of these electrolytes on the surface of a spherical metal nanoparticle depending on its electric charge. A mathematical model is presented for the rearrangement of two macromolecular shells of different signs located on a charged spherical nanoparticle, and the rigidity of the polyelectrolyte chain depending on its charge has been assessed. The radial distributions of the average density of atoms of the polyelectrolyte complex and the block copolymer located on the surface of the charged spherical metal nanoparticle have been calculated. The oppositely charged polyelectrolytes in the complex, as well as block copolymer molecules, were found to tightly envelop the neutral spherical nanoparticle; as the absolute value of the nanoparticle charge increased, the macromolecular edge swelled by formation of two layers of oppositely charged polyelectrolytes or block copolymer fragments.

A spectral model of luminescence of the two-component exciton-activated semiconductor quantum dot (QD) layered plasmon composite nanoparticle (CNP) with a dielectric core and a conductive shell in an external magnetic field is constructed, taking into account the inhomogeneity of the quasi-stationary electric field generated by QD in the CNP region, outside the framework of the approximation of the dipole polarizability of the CNP. The tensor formalism of describing the characteristics of the field in each of the layers of the CNP, as well as outside the CNP, is used. It is established that with a change in the structure of the nanocomposite, the parameters of its core or shell layer, the spectral response of the system to external magnetic field action changes. It is shown that the special form of the response is associated with the characteristic magnetic properties of the nanoparticle components acquired (under the action of the field).

The rate of annihilation delayed fluorescence of organic molecules localized on the surface of a layered spherical nanoparticle is calculated based on an original mathematical model on the assumption that one of the molecules remains stationary, while the other diffusively moves over the particle surface. A composite nanoparticle consists of a ferromagnetic (cobalt, nickel, or magnetite) core and a metal (Au or Ag) plasmon shell. Not only is the external uniform magnetic field into which the particle was placed taken into account, but also a nonuniform anisotropic magnetic field, formed by the ferromagnetic core of the composite, in the outer surface region of the particle. The predominant influence of the structural and geometric parameters of the system (the ratio of core radius to shell thickness) on the delayed fluorescence rate of molecules as a result of a plasmon increase/decrease in the radiative transition probability (more than 30

A quantum theory of electronic energy transfer in a layered nanostructure with molecular J-aggregates of polymethine dyes was proposed. An expression for the exciton-plasmon bond energy depending on various parameters of the system was given. The rate of non-radiative Fὄrster resonance energy transfer (FRET) from surface plasmon polaritons (SPPs) of a metal substrate to Frenkel excitons of J-aggregates was determined and dispersion dependences for hybrid states were obtained. It was established that the energy transfer rate can reach values of 1012–1013 s–1, and the value of the Rabi splitting is up to 100 MeV. The kinetics of the process under strong exciton-plasmon interaction was investigated. The time dependence of the energy exchange between the system components had the form of damped oscillations depending on the relaxation parameters, the Rabi frequency, and the response to resonance. In addition, the exciton FRET between two parallel monolayers of J-aggregates of polymethine dyes separated by a nanometer-thick metal film was investigated. It was found that the presence of the metal layer increases the FRET rate. The spin evolution of a pair of two triplet (T) molecules localized in the nano-cell region under the over-barrier jumps regime in a magnetic field was studied. The influence of the parameters of the two-dimensional potential on the frequency of inter-dimensional motions and the population of triplets was considered. The spin dynamics of molecular T-T pairs in the magnetic field of a ferromagnetic globular nanoparticle under free surface diffusion of a spin-carrying molecule was investigated.

Molecular dynamics has been employed to study conformational rearrangements of polyampholytic polypeptides adsorbed on the surface of a metal nanoparticle, with the rearrangements being caused by changes in the direction of particle polarization at a frequency corresponding to microwave electric field. The time dependences of the dipole moments, as well the radial and angular density distributions of adsorbed polypeptide atoms upon the prepolarization of the nanoparticle, have been calculated. The rearrangements occurring in the conformational structure of a polyampholytic polypeptide adsorbed on the nanoparticle upon periodic changes in the particle polarization direction are accompanied by two effects: fluctuations in the conformational structure of the polypeptide and the formation of a ring of macromolecules in the equatorial region of the nanoparticle.

The properties of a specially created analytical model of conformational rearrangements of a Gaussian macromolecular chain adsorbed on the surface of a metal nanoparticle in an external electric field are investigated. The results of calculations based on this model of the structure of polyelectrolyte chains and molecular dynamics (MD) modeling of polypeptide conformations near a gold nanoparticle are presented. It is found that an increase in the strength of the external electric field leads to a displacement of the links of the macromolecular edge to one of the poles of the polarized nanoparticle.

Magneto-sensitive delayed photoluminescence (DPL) of the deaerated MEH-PPV films in the presence of an external magnetic field is investigated. The obtained results have shown that the sign of the magnetic field effect on DPL can be inverted by varying molecular oxygen concentration in the films. For explaining the “sign-inversion” of the magnetic field effect on DPL, the mathematical model based on the superposition of exciton-initiated reactions was proposed. The magnetic field dependencies of DPL based on the theoretical model were calculated. The calculations of the magnetic field effects on DPL confirm the “sign-inversion effect”. Also, the critical molecular oxygen concentration when the magnetic field effect on DPL disappears was determined theoretically. The obtained results can be used for the detection of spin-selective exciton-initiated reactions as well as for the determination of molecular oxygen concentration in MEH-PPV films.

Исследованы свойства специально созданной аналитической модели конформационных перестроек гауссовой макромолекулярной цепи, адсорбированной на поверхности металлической наночастицы во внешнем электрическом поле. Представлены результаты расчетов на основе этой модели структуры цепей полиэлектролитов, а также молекулярно-динамического моделирования конформаций полипептидов вблизи золотой наночастицы. Установлено, что увеличение напряженности внешнего электрического поля приводит к смещению звеньев макромолекулярной опушки на один из полюсов поляризованной наночастицы.

Anisotropic properties of aspherical nanoparticles make it possible to significantly expand their spectral-optical applications associated with local field effects. The plasmon resonance characteristics of such particles have additional features that conductive nanoglobules do not possess. The results of synthesis and morphological analysis of ellipsoidal hematite nanoparticles coated with a gold shell and spherical magnetite nanoparticles by atomic force microscopy are presented. The absorption spectra of aqueous solutions of such nanoparticles, as well as conductive nanoparticles with chain molecules adsorbed on them, are obtained. Based on the methods of dynamic light scattering, histograms of particle size distribution are constructed. The influence of spheroidal magnetic nanoparticles coated with a gold shell on the luminescence intensity of molecules of an organic dye (fluorescein) and poly[2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylenevinylene] is studied. An increase in the luminescence intensity of both types of luminophors at certain concentrations of metallized spheroidal nanoparticles is detected. A mathematical model of radiative and nonradiative processes in the studied colloidal system is presented, as well as the results of calculations of spectral and velocity characteristics based on well-known applied FDTD packages. Characteristic graphs are constructed for the spectral density of the number of photons emitted at a certain frequency of the combined system “molecule–nanoparticle” for various positions of the nanoparticle relative to the molecule and two characteristic directions of the transition dipole moment vector. The relationship between the frequencies of two plasmon resonances and the eccentricity of the spheroid revealed in the model is clearly manifested in the obtained spectral curves. The closer the spheroid eccentricity to 0, the smaller the frequency interval between two spaced plasmon resonance peaks, which merge into a single spectral band at zero eccentricity. The results obtained can be used in the development and modification of sensors with an adjustable conformational structure of macrochains, such as luminescent optical testers for the concentration of molecular oxygen (including singlet oxygen) and chemical sensors based on the effects of surface plasmon resonance and surface-enhanced Raman scattering.

Conformational rearrangements of polyampholytic polypeptides adsorbed on the surface of a charged prolate gold nanospheroid with a periodic change in time of its polarity along the rotation axis have been studied using molecular dynamics simulation. The radial distributions of the density of polypeptide atoms in the equatorial region of the nanospheroid have been calculated, as well as the distributions of the linear density of polypeptide atoms along the major axis of the nanospheroid. At a low simulation temperature, a girdle polyampholytic fringe was formed in the central region of the nanospheroid and its ordering by layers, depending on the type of units, occurred with an increase in the charge of the nanospheroid with a simultaneous increase in the width of the macromolecular fringe along the rotation axis. The thickness of such a fringe along the cross section depends on the distance between the oppositely charged units in the polyampholyte. At high temperatures and high absolute values of the total charge of the spheroidal nanoparticle, there were periodic displacements of the polyampholytic fringe toward the poles of the nanospheroid, being in antiphase for oppositely charged metallic nanospheroids. A mathematical model is presented for describing the conformational structure of a polyampholyte macromolecule on a prolate nanospheroid in an alternating electric field with the approximation of a prolate spheroid by a spherical cylinder.

A mathematical model has been presented for the formation of the conformational structure of chain units in a polyelectrolyte adsorbed on a flattened conducting charged nanospheroid polarized in an external electric field, which harmonically varies at a frequency much lower than the plasma frequency of the nanospheroid metal. Molecular dynamics has been employed to study the rearrangements in the conformational structure of uniformly charged polypeptides adsorbed on the surface of the oppositely charged flattened gold nanospheroid in an external alternating electric field, the strength vector of which varies along the rotation axis of the nanospheroid. One-dimensional density distributions along the rotation axis, as well as radial distributions, have been plotted for atoms of the polypeptides adsorbed on the nanospheroid surface. At a low temperature, a narrow ring-shaped polyelectrolyte fringe is formed in the equatorial region of the flattened metal nanospheroid, and the fringe density increases with the total charge of the nanospheroid and the number of charged units in polyelectrolyte macrochains. At a high temperature, the formed narrow macromolecular ring periodically shifts along the rotation axis of the nanospheroid with redirections of the polarizing electric field vector. The amplitude of the shifts increases with a decrease in the total charge of the nanospheroid and an increase in the fraction of charged units in a polyelectrolyte.

A study is performed of conformational changes of polypeptides with a uniform distribution along a macrochain of units with charges of the same sign on the surface of a gold, oblate nanospheroid polarized along the axis of rotation. A mathematical model of the spheroid–polyelectrolyte system is proposed that considers the effect entropy has on the formation of conformations of an adsorbed Gaussian chain on a curved surface, and the interaction between charged segments of it and the field of a charged or polarized conducting nanospheroid. Characteristic distribution maps of the density of polyelectrolyte chain units are obtained using calculations based on this model. Molecular dynamics is used to calculate distributions of the average linear density of atoms along the axis of rotation and the radial distributions of the average density of atoms of polypeptides. Adsorption of charged amino acid residues of the polypeptide is observed on half the surface of the nanospheroid, charged opposite to the sign of the polyelectrolyte unit. Most units are concentrated near its equator, and their number falls as they approached the pole. Loops of neutral polypeptide units form a corona around the oppositely charged half of the oblate nanospheroid.

Based on the generalized model of a Gaussian chain in a superposition electric field, which takes into account the complex distribution of the surface charge density of a polarized adsorbent, a mathematical model of the bulk structure of a polymer fringe on the surface of a prolate spheroidal nanoparticle in an external alternating electric field was developed. Using molecular dynamics, electrically induced conformational changes in generally neutral polyampholytes, as well as polyelectrolytes with a uniform distribution of units of the same sign along the macrochain, adsorbed on the surface of a prolate gold nanospheroid charged or polarized along the major axis, including those with a periodic change in time of its polarity, were investigated. On the surface of a charged prolate nanospheroid, the macromolecular fringe swelled with an increase in the value of the total charge of the nanospheroid. On the surface of a polarized prolate nanospheroid, units of uniformly charged polyelectrolytes shifted to the oppositely charged polar region, as well as the formation of a dumbbell-shaped macromolecular region during the adsorption of generally neutral polyampholytes. On the surface of a prolate nanospheroid, with a periodic change in time of its polarity along the major axis, the formation of a girdle macromolecular fringe in its equatorial region was observed, the width of which depended on the amplitude of the external polarizing alternating electric field.

The plasmon-exciton interaction between a spherical nanoparticle with a dielectric core and a metal shell and a quantum dot in the mode of strong or weak confinement has been theoretically investigated. The rate of nonradiative transfer of electron excitation energy from the quantum dot to the nanoparticle and the rate of spontaneous emission of the quantum dot in the presence of the nanoparticle are calculated. It is shown that at the radii of the nanoparticle core, for which the frequency of the plasmon oscillation coincides with the frequency of the electronic transition in the quantum dot, the rates of radiative and nonradiative processes increase sharply. The kinetics of the energy exchange between the nanoparticle and the quantum dot has been studied, and values of the parameters of the system under consideration, at which the kinetics has the character of damped oscillations, have been established.

A mathematical model for the volume structure of the polymer fringe on the surface of a prolate spheroidal nanoparticle in the external electric field is developed. Using this model and the molecular dynamics method the conformational changes of uniformly charged polypeptides adsorbed by a charged metal nanospheroid the nature of the polarization of which along the major axis periodically changes with time are studied. An analytical model for the interaction of polyelectrolyte units with the charged nanospheroid is constructed relying on the generalized Gaussian chain model in the superposition electric field taking into account the complex pattern of the surface charge density distribution of the polarized adsorbent. Unidimensional atomic density distributions of polypeptides along the major axis of the nanospheroid, as well as the radial atomic density distributions of polypeptides in the equatorial region of the nanospheroid, are calculated. It is shown that at a low temperature in the central region of the nanospheroid the enveloping polyelectrolyte fringe is formed with its width being dependent on the full charge of the nanospheroid, the amplitude of the external electric field, and the fraction of charged units in the adsorbed macromolecule. At a higher temperature the nature of conformational rearrangements of the adsorbed polyelectrolyte macromolecule changes: either the periodic displacement of polyelectrolyte units on the oppositely charged pole of the nanospheroid or a periodic shift of the formed polyelectrolyte ring relative to the equator along the major axis occurs.