arXiv subjects

Almas F. Sadreev

Publications and source records attributed to Almas F. Sadreev.

At least 19 recordsLinked to original sources

arXiv2020

Interference traps waves in open system: Bound states in the continuum

I review the four mechanisms of bound states in the continuum (BICs) in application to microwave and acoustic cavities open to directional waveguides. The most simple are the symmetry protected BICs which are localized inside the cavity because of the orthogonality of the eigenmodes to the propagating modes of waveguides. However, the most general and interesting is the Friedrich-Wintgen mechanism when the BICs are result of full destructive interference of outgoing resonant modes. The third type of the BICs, the Fabry-Perot BICs, occur in a double resonator system when each resonator can serve as an ideal mirror. At last, the accidental BICs can be realized in the open cavities with no symmetry like the open Sinai billiard in which the eigenmode of the resonator can become orthogonal to the continuum of the waveguide accidentally by a smooth deformation of the eigenmode. We also review the one-dimensional systems in which the BICs occur owing to full destructive interference of two waves separated by spin or polarization or by paths in the Aharonov-Bohm rings. We widely use the method of effective non-Hermitian Hamiltonian equivalent to the coupled mode theory which detects bound states in the continuum (BICs) by finding zero widths resonances.

quant-ph↗

arXiv2020

Giant resonant enhancement of optical binding of dielectric particles

Optical coupling of two identical dielectric particles gives rise to bonding and anti-bonding resonances. The latter is featured by significant narrowing of the resonant width and strong enhancement of the $Q$ factor for the high index micron size particles in subwavelength range. We consider particles shaped as spheres and disks under coaxial illumination of dual incoherent counter propagating Bessel beams. In the case of spheres we derive analytical expressions for the optical binding (OB) force which decreases as $1/L^2$ for large distance $L$ between the spheres and displays two periods of oscillations. For close distances the OB force enormously increases in the resonant regime. The case of two coaxial disks owing to variation of the distance between disks and aspect ratio of each disk is featured by extremal enhancement of the $Q$ factor compared to the case of two spheres. In that case we demonstrate unprecedent enhancement of the OB force up to several decades of nano Newtons. We show that the magnitude and sign of the OB force strongly depend on the longitudinal wave vector of the Bessel beams.

physics.optics↗

arXiv2018

Fibers based on propagating bound states in the continuum

We show that a circular periodic array of $N$ dielectric cylinders supports nearly bound states in the continuum (BICs) propagating along the cylinders. These propagating nearly BICs with extremely large $Q$ factors of order $exp(λN)$ are surrounded by resonant modes weakly leaking into the radiation continuum. We present leaky zones in the vicinity of different types of BICs: symmetry protected nearly BICs with the resonant width proportional to the squared propagation constant $Γ\sim k_z^2$, non-symmetry protected nearly BICs with finite propagation constant $k_c$ with $Γ\sim (k_z-k_c)^2$ and non-symmetry protected nearly BICs with $Γ\sim k_z^4$. The latter propagating nearly BICs can serve for transmission of electromagnetic signal paving a way to novel type of optical fibers. We also demonstrate weakly leaking resonant modes which carry orbital angular momentum.

physics.optics↗

arXiv2018

Interference of Laguerre--Gaussian beams for reflection by dielectric slab

We study reflection of TE Laguerre-Gaussian light beam by dielectric slab and show that the Goos-Hänchen and the Imbert-Federov shifts show resonant behavior following to the behavior of the reflection. Moreover the Imbert-Federov linear and the Goos-Hänchen angular shifts strongly depend on the orbital angular momentum m. Due to destructive interference of two beams reflected from upper and down interfaces of the slab profile of the reflected light beam acquires structure which distinctively displays an amount of m.

physics.optics↗

arXiv2017

Near-bound states in the radiation continuum in circular array of dielectric rods

We consider E polarized bound states in the radiation continuum (BICs) in circular periodical arrays of $N$ infinitely long dielectric rods. We find that each true BIC which occurs in an infinite linear array has its counterpart in the circular array as a near-BIC with extremely large quality factor. We argue analytically as well as numerically that the quality factor of the symmetry protected near-BICs diverges as $e^{λN}$ where $λ$ is a material parameter dependent on the radius and the refraction index of the rods. By tuning of the radius of rods we also find numerically non-symmetry protected near-BICs. These near-BICs are localized with exponential accuracy outside the circular array but fill the whole inner space of the array carrying orbital angular momentum.

physics.optics↗

arXiv2017

Polarized bound state in the continuum and resonances with tunable Q-factor in an anisotropic photonic crystal

We consider a one-dimensional photonic crystal composed of alternating layers of isotropic and anisotropic dielectric materials. Such a system has different band structures for different polarizations of light. We demonstrate that if an anisotropic defect layer is inserted into the structure, the crystal can support an optical bound state in the continuum. By tilting the principle dielectric axes of the defect layer relative to those of the photonic crystal we observe a long-lived resonance in the transmission spectrum. We derive an analytical expression for the decay rate of the resonance that agrees well with the numerical data by the Berreman anisotropic transfer matrix approach. An experimental set-up with a liquid crystal defect layer is proposed to tune the Q-factor of the resonance through applying an external electric field. We speculate that the set-up provides a simple and robust platform for observing optical bound states in the continuum in the form of resonances with tunable Q-factor.

physics.optics↗

arXiv2017

Tuning of bound states in the continuum by waveguide rotation

We consider acoustic wave transmission in non axisymmetric waveguide which consists of cylindrical resonator and two semi-infinite cylindrical waveguides whose axes are shifted relative to the resonator axis and each other by azimuthal angle $Δϕ$. We show that for rotation of one of attached waveguides the coupling matrix elements of the eigenmodes of resonator classified by the integer $m$ and propagating mode of the waveguide acquire phase factor $e^{imΔϕ}$. That crucially effect Fano resonances and creates an analog of faucet opening and closing wave flux under rotation of the waveguide. We show that under the rotation of the waveguide and variation of the length of resonator numerous bound states in the continuum occur complimenting by the Fano resonance collapse.

physics.app-ph↗

arXiv2017

Bound states in the continuum with high orbital angular momentum in a dielectric rod with periodically modulated permittivity

We report bound states in the radiation continuum (BSCs) in a single infinitely long dielectric rod with periodically stepwise modulated permittivity alternating from $ε_1$ to $ε_2$. For $ε_2=1$ in air the rod is equivalent to a stack of dielectric discs with permittivity $ε_1$. Because of rotational and translational symmetries the BSCs are classified by orbital angular momentum $m$ and the Bloch wave vector $β$ directed along the rod. For $m=0$ and $β=0$ the symmetry protected BSCs have definite polarization and occur in a wide range of the radius of the rod and the dielectric permittivities. More involved BSCs with $m\neq 0, β=0$ exist only for a selected radius of the rod at a fixed dielectric constant. The existence of robust Bloch BSCs with $β\neq 0, m=0$ is demonstrated. Asymptotic limits to a homogeneous rod and to very thin discs are also considered.

physics.optics↗

arXiv2017

Temporal oscillations of light transmission through dielectric microparticles subjected to optically induced motion

We consider light-induced binding and motion of dielectric microparticles in an optical waveguide that gives rise to a back-action effect such as light transmission oscillating with time. Modeling the particles by dielectric slabs allows us to solve the problem analytically and obtain a rich variety of dynamical regimes both for Newtonian and damped motion. This variety is clearly reflected in temporal oscillations of the light transmission. The characteristic frequencies of the oscillations are within the ultrasound range of the order of $10^{5}$ Hz for micron size particles and injected power of the order of 100 mW. In addition, we consider driven by propagating light dynamics of a dielectric particle inside a Fabry-Perot resonator. These phenomena pave a way for optical driving and monitoring of motion of particles in waveguides and resonators.

physics.optics↗

arXiv2015

Light trapping above the light cone in one-dimensional array of dielectric spheres

We demonstrate bound states in the first TE and TM diffraction continua (BSC) in a linear periodic array of dielectric spheres in air above the light cone. We classify the BSCs according to the symmetry specified by the azimuthal number $m$, the Bloch wave vector $β$ directed along the array, and polarization. The most simple symmetry protected TE and TM polarized BSCs have $m=0$ and $β=0$ and occur in a wide range of the radius of the spheres and dielectric constant. More complicated BSCs with $m\neq 0$ and $β=0$ exist only for a selected radius of spheres at a fixed dielectric constant. We also find robust Bloch BSCs with $β\neq 0, m=0$. We present also the BSCs embedded into two and three diffraction continua. We show that the BSCs can be easily detected by the collapse of Fano resonance for scattering of electromagnetic plane waves by the array.

physics.optics↗

arXiv2015

Coupled mode theory for acoustic resonators

We develop the effective non-Hermitian Hamiltonian approach for open systems with Neumann boundary conditions. The approach can be used for calculating the scattering matrix and the scattering function in open resonator-waveguide systems. In higher than one dimensions the method represents acoustic coupled mode theory in which the scattering solution within an open resonator is found in the form of expansion over the eigenmodes of the closed resonator decoupled from the waveguides. The problem of finding the transmission spectra is reduced to solving a set of linear equations with a non-Hermitian matrix whose anti-Hermitian term accounts for coupling between the resonator eigenmodes and the scattering channels of the waveguides. Numerical applications to acoustic two-, and three-dimensional resonator-waveguide problems are considered.

quant-ph↗

arXiv2015

Frequency comb generation for wave transmission through the nonlinear dimer

We study dynamical response of a nonlinear dimer to a symmetrically injected monochromatic wave. We find a domain in the space of frequency and amplitude of the injected wave where all stationary solutions are unstable. In this domain scattered waves carry multiple harmonics with equidistantly spaced frequencies (frequency comb effect). The instability is related to a symmetry protected bound state in the continuum whose response is singular as the amplitude of the injected wave tends to zero.

nlin.PS↗

arXiv2015

Gate controlled resonant widths in double-bend waveguides: Bound states in the continuum

We consider quantum transmission through double-bend $Π$- and $Z$-shaped waveguides controlled by the finger gate potential. Using the effective non-Hermitian Hamiltonian approach we explain the resonances in transmission. We show a difference in transmission in the short waveguides that is the result of different chirality in $Z$ and $Π$ waveguides. We show that the potential selectively affects the resonant widths resulting in the occurrence of bound states in the continuum.

cond-mat.mes-hall↗

arXiv2014

Bloch bound states in the radiation continuum in a periodic array of dielectric rods

We consider an infinite periodic array of dielectric rods in vacuum with the aim to demonstrate three types of a Bloch bound states in the continuum (BSC), symmetry protected with a zero Bloch vector, embedded into one diffraction channel with nonzero Bloch vector, and embedded into two and three diffraction channels. The first and second types of the BSC exist in a wide range of material parameters of the rods, while the third occurs only at a specific value of the radius of the rods. We show that the second type supports the power flux along the array. In order to find BSC we put forward an approach based on the expansion over the Hankel functions. We show how the BSC reveals itself in the scattering function when the singular BSC point is approached along a specific path in the parametric space.

physics.optics↗

arXiv2014

Robust bound states in the continuum in Kerr microcavity embedded in photonic crystal waveguide

We present a two-dimensional photonic crystal design with a microcavity of four defect dielectric rods with eigenfrequencies residing in the propagating band of directional waveguide. In the linear case for tuning of material parameters of defect rods the nonrobust bound state in the continuum (BSC) might occur. The BSC is a result of full destructive interference of resonant monopole and quadrupole modes with the same parity. % to trap light interior of the microcavity. A robust BSC arises in a self-adaptive way without necessity to tune the parameters of the microcavity with the Kerr effect. Lack of the superposition principle in nonlinear systems gives rise to coupling of the BSC with injecting light. That forms a peculiar shape of isolated transmittance resonance around BSC frequency. We show if injecting light is switched off the BSC storages light that opens a way for light accumulation.

physics.optics↗

arXiv2013

Effect of gate-driven spin resonance on the conductance of a one-dimensional quantum wire

We consider quasiballistic electron transmission in a one-dimensional quantum wire subject to both time-independent and periodic potentials of a finger gate that results in a coordinate- and time-dependent Rashba-type spin-orbit coupling. A spin-dependent conductance is calculated as a function of external constant magnetic field, the electric field frequency, and the potential strength. The results demonstrate the effect of the gate-driven electric dipole spin resonance in a transport phenomenon such as spin-flip electron transmission.

cond-mat.mes-hall↗

arXiv2013

Symmetry breaking in binary chain with nonlinear sites

We consider a system of two or four nonlinear sites coupled with binary chain waveguides. When a monochromatic wave is injected into the first (symmetric) propagation channel the presence of cubic nonlinearity can lead to symmetry breaking giving rise to emission of antisymmetric wave into the second (antisymmetric) propagation channel of the waveguides. We found that in the case of nonlinear plaquette there is a domain in the parameter space where neither symmetry preserving nor symmetry breaking stable stationary solutions exit. As a result injection of a monochromatic symmetric wave gives rise to emission of nonsymmetric satellite waves with energies different from the energy of the incident wave. Thus, the response exhibits nonmonochromatic behavior.

nlin.CD↗

arXiv2011

Light-induced breaking of symmetry in photonic crystal waveguides with nonlinear defects as a key for all-optical switching circuits

We consider light transmission in 2D photonic crystal waveguide coupled with two identical nonlinear defects positioned symmetrically aside the waveguide. We show that with growth of injected light power there is a breaking of symmetry by two ways. In the first way the symmetry is broken because of different light intensities at the defects. In the second way the intensities at the defects are equaled but phases of complex amplitudes are different. That results in a vortical power flow between the defects similar to the DC Josephson effect if the input power over the waveguide is applied and the defects are coupled. As application of these phenomena we consider the symmetry breaking for the light transmission in a T-shaped photonic waveguide with two nonlinear defects. We demonstrate as this phenomenon can be explored for all-optical switching of light transmission from the left output waveguide to the right one by application of input pulses. Finally we consider the symmetry breaking in the waveguide coupled with single defect presented however by two dipole modes.

nlin.PS↗