Searcharxiv⌕ Search

arXiv subjects

A. K. Popov

Publications and source records attributed to A. K. Popov.

At least 19 recordsLinked to original sources
arXiv2017

Hyperbolic carbon nanoforest for phase matching of ordinary and backward electromagnetic waves: second harmonic generation

We show that deliberately engineered dispersive metamaterial slab can enable the co-existence and phase matching of contra-propagating ordinary fundamental and extraordinary backward second harmonic surface electromagnetic modes. Energy flux and phase velocity are contra-directed in the backward waves which is the phenomenon that gives rise to unique nonlinear optical propagation processes. We show that frequencies, phase, and group velocities, as well as nanowaveguide losses inherent to the electromagnetic modes supported by such metamaterial, can be tailored to maximize conversion of frequencies and to reverse propagation direction of the generated wave. Such a possibility, which is of paramount importance for nonlinear photonics, is proved with a numerical model of the hyperbolic metamaterial made of carbon nanotubes standing on the metal surface. Extraordinary properties of the backward-wave second harmonic generation in the reflection direction and of the corresponding frequency doubling metareflector in the THz are investigated with a focus on the pulsed regime.

physics.optics↗
arXiv2016

Phase matched backward-wave second harmonic generation in a hyperbolic carbon nanoforest

We show that deliberately engineered spatially dispersive metamaterial slab can enable co-existence and phase matching of contra-propagating ordinary fundamental and backward second harmonic electromagnetic modes. Energy flux and phase velocity are contra-directed in backward waves which determines extraordinary nonlinear-optical propagation processes. Frequencies, phase and group velocities, as well as nanowavequide losses inherent to the electromagnetic modes supported by the metamaterial can be tailored to optimize nonlinear-optical conversion of frequencies and propagation directions of the coupled waves. Such a possibility, which is of paramount importance for nonlinear photonics, is proved with numerical model of the hyperbolic metamaterial made of carbon nanotubes standing on metal surface. Extraordinary properties of backward-wave second harmonic in the THz and IR propagating in the reflection direction are investigated with focus on pulsed regime.

physics.optics↗
arXiv2006

Negative-Index Metamaterials: Second-Harmonic Generation, Manley-Rowe Relations and Parametric Amplification

Second harmonic generation and optical parametric amplification in negative-index metamaterials (NIMs) are studied. The opposite directions of the wave vector and the Poynting vector in NIMs results in a "backward" phase-matching condition, causing significant changes in the Manley-Rowe relations and spatial distributions of the coupled field intensities. It is shown that absorption in NIMs can be compensated by backward optical parametric amplification. The possibility of distributed-feedback parametric oscillation with no cavity has been demonstrated. The feasibility of the generation of entangled pairs of left- and right-handed counter-propagating photons is discussed.

physics.optics↗
arXiv2005

Controlling Silver Nanoparticle Size and Morphology with Photostimulated Synthesis

Photo-induced synthesis and control over the size and shape of colloidal silver nanoparticles is investigated in contrast to photo-stimulated aggregation of small nanoparticles into large fractal-type structures. The feasibility of light-driven nanoengineering which enables manipulation of the sizes and shapes of the isolated nanoparticles is studied by varying the amount and type of the stabilizing agent and the type of optical irradiation.

physics.optics↗
arXiv2005

Laser-stimulated Synthesis of Large Nanostructured Fractal Silver Aggregates

A Laser-stimulated synthesis of large silver nanoaggregates consisting of hundreds to thousands of nanoparticles is investigated. It is shown that the morphology of the synthesized nanostructures can be controlled with light and monitored via the evolution of the colloid absorption spectra. A solution method is demonstrated that enables production of a bulk amount of metal nanoaggregates, which are of paramount importance for subwavelength concentration and dramatic enhancement of the electromagnetically-induced processes at the nanoscale.

physics.chem-ph↗
arXiv2005

Manipulative resonant nonlinear optics: nonlinear interference effects and quantum control of nonlinearity, dispersion, transparency and inversionless amplification in an extended strongly-absorbing inhomogeneously-broadened medium

Specific features of nonlinear interference processes at quantum transitions in near- and fully-resonant optically-dense Doppler-broadened medium are studied. The feasibility of overcoming of the fundamental limitation on a velocity-interval of resonantly coupled molecules imposed by the Doppler effect is shown based on quantum coherence. This increases the efficiency of nonlinear-optical processes in atomic and molecular gases that possess the most narrow and strongest resonances. The possibility of all-optical switching of the medium to opaque or, alternatively, to absolutely transparent, or even to strongly-amplifying states is explored, which is controlled by a small variation of two driving radiations. The required intensities of the control fields are shown to be typical for cw lasers. These effects are associated with four-wave mixing accompanied by Stokes gain and by their interference in fully- and near-resonant optically-dense far-from-degenerate double-Lambda medium. Optimum conditions for inversionless amplification of short-wavelength radiation above the oscillation threshold at the expense of the longer-wavelength control fields, as well as for Raman gain of the generated idle infrared radiation, are investigated. The outcomes are illustrated with numerical simulations applied to sodium dimer vapor. Similar schemes can be realized in doped solids and in fiber optics.

quant-ph↗
arXiv2003

Inversionless light amplification and optical switching controlled by state-dependent alignment of molecules

We propose a method to achieve amplification without population inversion by anisotropic molecules whose orientation by an external electric field is state-dependent. It is based on decoupling of the lower-state molecules from the resonant light while the excited ones remain emitting. The suitable class of molecules is discussed, the equation for the gain factor is derived, and the magnitude of the inversionless amplification is estimated for the typical experimental conditions. Such switching of the sample from absorbing to amplifying via transparent state is shown to be possible both with the aid of dc and ac control electric fields.

quant-ph↗
arXiv2003

Adiabatic passage and dissociation controlled by interference of two laser-induced continuum structures

We have developed a theory of three-pulse coherent control of photochemical processes. It is based on adiabatic passage and quantum coherence and interference attributed to the lower-lying dissociation continuum and excited upper discrete states, which are otherwise not connected to the ground state by one-photon transitions. Novel opportunities offered by the proposed scheme are demonstrated through extensive numerical simulations with the aid of a model relevant to typical experiments. The opportunities for manipulating the distribution of the population among discrete and continuous states with any necessary ratio by the end of the pulses are demonstrated.

quant-ph↗
arXiv2003

Theory of four-wave mixing and accompanying dissociation and population transfer controlled with laser-induced continuum structures

A theory is developed and applied to the study of opportunities and specific features of coherent control of four-wave mixing as well as of the accompanying processes in the continuous-wave regime, which involve transitions between bound and free quantum states. Such opportunities become feasible through constructive and destructive interference of quantum pathways. Two coupling schemes of practical importance are investigated. In the first, a ladder energy level scheme, fully-resonant sum-frequency nonlinear-optical generation of short-wavelength radiation driven by several strong fields is investigated. The relaxation processes as well as absorption of the fundamental and generated radiations, which play an important role, are taken into consideration. It is shown that the generation output can be considerably increased through the appropriate adjustment of several laser-induced continuum structures. In the second, a folded scheme, a possible control of two-photon dissociation (Lambda-scheme) using auxiliary laser radiation applied to the adjacent bound-free transition (V-configuration) is investigated. Besides dissociation, the proposed method enables one to control population transfer between two upper discreet levels via the lower-energy dissociation continuum, while direct transition between these states is not allowed. The opportunities of manipulating these processes as well as of four-wave-mixing-based spectroscopy are explored both analytically and through a numerical simulation for Na_2 dimers.

quant-ph↗
arXiv2003

Optics of Nanostructured Fractal Silver Colloids

Based on the theory of the optical properties of fractal clusters, which is an operator-based modification of the coupled-dipole method, an alternate solution is proposed for the problem of adequately describing the evolution of optical spectra of any polydisperse silver colloid with particles falling within the range of most characteristic sizes (5 - 30 nm). This is the range over which the results of the application of the well-known methods of classical electrodynamics, including the Mie theory, disagree with experimental data. The effect of variation of the parameters of such media on optical spectra is studied by a numerical simulation, which accounts for particle electrodynamic dipole-dipole interactions. Indeed, such interactions are shown to be a key factor in determining the broadening of the sol absorption spectra during the course of fractal aggregation. A quantitative explanation is given for the reasons for the appearance of individual specific features in the contours of the spectral absorption of different types of silver colloids.

physics.optics↗
arXiv2003

Theory of deep ultraviolet generation at maximum coherence assisted by Stark-chirped two-photon resonance

A scheme is analyzed for effcient generation of vacuum ultraviolet radiation through four-wave mixing processes assisted by the technique of Stark-chirped rapid adiabatic passage. These opportunities are associated with pulse excitation of laddertype short-wavelength two-photon atomic or molecular transitions so that relaxation processes can be neglected. In this three-laser technique, a delayed-pulse of strong oR-resonant infrared radiation sweeps the laser-induced Stark-shift of a two-photon transition in a such way that facilitates robust maximum two-photon coherence induced by the first ultraviolet laser. A judiciously delayed third pulse scatters at this coherence and generates short-wavelength radiation. A theoretical analysis of these problems based on the density matrix is performed. A numerical model is developed to carry out simulations of a typical experiment. The results illustrate a behavior of populations, coherence and generated radiation along the medium as well as opportunities of effcient generation of deep (vacuum) ultraviolet radiation.

quant-ph↗
arXiv2000

Ienversionless gain in an optically-dense resonant Doppler-broadened medium

Resonant nonlinear-optical interference processes in four-level Doppler-broadened media are studied. Specific features of amplification and optical switching of short-wavelength radiation in a strongly-absorbing resonant gas under coherent quantum control with two longer wavelength radiations, are investigated. The major outcomes are illustrated with virtual experiments aimed at inversionless short-wavelength amplification, which also address deficiencies in this regard in recent experiments. With numerical simulations related to the proposed experiment in optically-dense sodium dimer vapor, we show optimal condition for optical switching and the expected gain of the probe radiation, which is above the oscillation threshold.

physics.optics↗
arXiv2000

Four-wave mixing at maximum coherence and eliminated Doppler broadening controlled with the driving fields

New feasibity of coherent quantum control of four-wave mixing processes in a resonant Doppler-broadened medium are studied. We propose a technique which enables one to enhance the quantum efficiency of nonlinear optical conversion. At the same time, it allows one to decrease the required intensities of the fundamental beams compared to those necessary in the approach based on coherent population trapping. The major outcomes of the analysis are illustrated with numerical simulation addressed within a practical medium.

physics.optics↗
arXiv2000

Coherence-controlled transparency and far-from-degenerate parametric gain in a strongly-absorbing Doppler-broadened medium

An inversionless gain of anti-Stokes radiation above the oscillation threshold in an optically-dense far-from-degenerate double-Lambda Doppler-broadened medium accompanied by Stokes gain is predicted. The outcomes are illustrated with numerical simulations applied to sodium dimer vapor. Optical switching from absorption to gain via transparency controlled by a small variation of the medium and of the driving radiation parameters which are at a level less than one photon per molecule is shown. Related video/audio clips see in: A.K. Popov, S.A. Myslivets, and T.F. George, Optics Express Vol. 7, No 3, 148 (2000)(http://epubs.osa.org/oearchive/source/22947.htm) or download: http://kirensky.krasn.ru/popov/opa/opa.htm

quant-ph↗
arXiv2000

Inversionless gain in a three-level system driven by a strong field and collisions

Inversionless gain in a three-level system driven by a strong external field and by collisions with a buffer gas is investigated. The mechanism of populating of the upper laser level contributed by the collision transfer as well as by relaxation caused by a buffer gas is discussed in detail. Explicit formulae for analysis of optimal conditions are derived. The mechanism developed here for the incoherent pump could be generalized to other systems.

quant-ph↗
arXiv2000

Sub-Doppler resolution with double coherently driving fields

We propose a four-level model where sub-Doppler resolution as well as enhanced absorption of a weak probe field are realized by using two coherently driving fields. We show that spectral resolution can be improved by modifying the coherent fields intensity and frequencies.

physics.optics↗
arXiv2000

Interference at quantum transitions: lasing without inversion and resonant four-wave mixing in strong fields at Doppler-broadened transitions

An influence of nonlinear interference processes at quantum transitions under strong resonance electromagnetic fields on absorption, amplification and refractive indices as well as on four-wave mixing processes is investigated. Doppler broadening of the coupled transitions, incoherent excitation, relaxation processes, as well as power saturation processes associated with the coupled levels are taken into account. Both closed (ground state is involved) and open (only excited states are involved) energy level configurations are considered. Common expressions are obtained which allow one to analyze the optical characteristics (including gain without inversion and enhanced refractive index at vanishing absorption) for various V, Lambda and H configurations of interfering transitions by a simple substitution of parameters. Similar expressions for resonant four-wave mixing in Raman configurations are derived too. Crucial role of Doppler broadening is shown. The theory is applied to numerical analysis of some recent and potential experiments.

quant-ph↗