Diffraction Divergence Research Articles

Spin‐Selective Trifunctional Metasurfaces for Deforming Versatile Nondiffractive Beams along the Optical Trajectory

AbstractExploring and taming the diffraction phenomena and divergence of light are foundational to enhancing comprehension of nature and developing photonic technologies. Despite the numerous types of nondiffraction beam generation technologies, the 3D deformation and intricate wavefront shaping of structures during propagation have yet to be studied through the lens of nanophotonic devices. Herein, the dynamic conversion of a circular Airy beam (CAB) to a Bessel beam with a single‐layer spin‐selective metasurface is demonstrated. This spatial transformation arises from the interplay of 1D local and 2D global phases, facilitating the 3D control of non‐diffractive light fields. An additional overall phase gradient and orbital angular momentum are introduced, which effectively altering the propagation direction and transverse fields of complex amplitude beams along the optical path. The manifested samples exhibit superior defect resistance, laying a crucial application in micro/nanolithography technologies. This approach expands the in‐plane spin‐selective mechanism and leverages the out‐of‐plane propagation dimension, allowing for integrated high‐resolution imaging, on‐chip optical micromanipulation, and micro/nanofabrication within a versatile nanophotonic platform.

Self-focusing, as the collapse of the wave vector

This article proposes a GRT approach to the phenomenon of self-focusing. The refractive index is considered as a component of the metric tensor, and the addition to the refractive index due to nonlinear effects in the self-focusing area is considered as an additional curvature that compensates diffraction divergence by pulling photon trajectories inward so that photons in the self-focusing area propagating at certain angles experience collapse and do not go beyond the waveguide region.

Self-focusing and defocusing phenomena of super-Gaussian laser beams in plasmas carrying density gradient

Considering nonlinearities due to relativistic mass variation and ponderomotive force driven depression in the electron density, a model is developed for the self-focusing of a super-Gaussian laser beam in inhomogeneous plasmas. Since paraxial ray approximation is not appropriate for the beams of super-Gaussian profile, the formulation is developed based on the moment theory. To have an in-depth understanding of self-focusing and defocusing phenomena and to develop generalized treatment, three different types of plasma density profiles namely linear, tangent and exponential density profiles, are considered and the results are compared. Due to the saturating nature of nonlinear permittivity with laser intensity, the beam is found to undergo periodic focusing because of competition between the diffraction divergence and nonlinear self-focusing effects. The ponderomotive nonlinearity enhances the self-focusing. As the super-Gaussian beam moves into higher density plasma region up to several Rayleigh lengths, stronger self-focusing is achieved and the value of minimum spot size decreases.

Laguerre-Gaussian beam and plasma interaction under relativistic and ponderomotive nonlinearities

Self-focusing of the laser plays a decisive role in achieving higher intensity. This article investigates the beam width evolution of the Laguerre-Gaussian laser beam in an underdense, cold, and collisionless plasma in the presence of weakly relativistic and ponderomotive nonlinearities. We consider pure radial mode of the LG beam (LG10) as it plays a key role in phenomenon like higher harmonic generation and can dominate diffraction effect in the self-focusing. Wentzel–Kramers–Brillouin (WKB) approach under paraxial approximation is used to reduce the nonlinear wave equation to a differential equation for the beam width parameter to study the self-focusing and defocusing phenomena. The optimum values of the initial intensity and beam waist of the laser beam are then calculated and plotted, for which the diffraction divergence is cancelled out by the self-focusing.

THE METHOD OF SEQUENTIAL TRANSFORMATION OF THE SOUND FIELDS

A rule for describing the sequential transformation of the sound fields when properties of the surfaces or structural elements change due to such basic processes as sound absorption, reflection, diffraction, or sound divergence is proposed. The main assumption is that sound fields are non-coherent, i.e., resonant phenomena and sound interference are not considered. The examples show solutions to such problems: - sound propagation in space if there are artificial structures; - sound propagation in the rooms; - efficiency calculation of the noise protection structures; - calculation of the expected noise level of the machinery and separation of the contribution of noise and vibration sources to sound fields (for example, an external sound field, a sound field in the office, etc.)

Investigation of the filament spatial structure by aberration focusing in air of a femtosecond radiation pulse

The appearance conditions of two filaments propagating at an angle to each other by aberration focusing in air of femtosecond radiation pulse with a 940 nm wavelength are studied. Interest to these beams is due to the formation of highly directed radiation of the white supercontinuum in them with diffraction divergence. It is shown that the pump beam diffraction on plasma and Kerr nonlinearity in filamentation area are responsible for the formation of two additional filaments. The diffraction maxima are beginnings of high-intensity pump beams due to Kerr self-focusing of surrounding energy reservoir of pump radiation. The dynamics of these beams from foci zone till far zone is demonstrated.

Self-defocusing of super-Gaussian laser beam in tunnel ionized plasmas

Based on moment theory, an analytical formalism of high intensity super-Gaussian laser beam propagation through a tunnel ionized gas is developed by including the effects of diffraction divergence and nonlinear refraction. During the interaction of the laser with the plasma, the plasma density is found to have a broad maximum on the axis in the direction of propagation of the laser, causing self defocusing of the laser beam. It is seen that the super-Gaussian beam suffers stronger divergence than a Gaussian beam. The laser beam is found to get more defocused for the cases of higher intensity and smaller spot size of the laser. For in depth study, the calculations are carried out for argon, neon and xenon gases and it is seen that different gases lead to different diffraction divergence.

Effective production of gammas, positrons, and photonuclear particles from optimized electron acceleration by short laser pulses in low-density targets

Electron acceleration has been optimized based on 3D particle-in-cell simulations of a short laser pulse interacting with low-density plasma targets to find the pulse propagation regime that maximizes the charge of high-energy electron bunches. This regime corresponds to laser pulse propagation in a self-trapping mode where the diffraction divergence is balanced by the relativistic nonlinearity such that relativistic self-focusing on the axis does not happen and the laser beam radius stays unchanged during pulse propagation in a plasma over many Rayleigh lengths. Such a regime occurs for a near-critical density if the pulse length considerably exceeds both the plasma wavelength and the pulse width. Electron acceleration occurs in a traveling cavity filled with a high-frequency laser field and a longitudinal electrostatic single-cycle field (“self-trapping regime”). Monte Carlo simulations demonstrated that a high electron yield allows an efficient production of gamma radiation, electron–positron pairs, neutrons, and even pions from a catcher-target.

Generation of high-charge electron beam in a subcritical-density plasma through laser pulse self-trapping

To maximize the charge of a high-energy electron beam accelerated by an ultra-intense laser pulse propagating in a subcritical plasma, the pulse length should be longer than both the plasma wavelength and the laser pulse width, which is quite different from the standard bubble regime. In addition, the laser-plasma parameters should be chosen to produce the self-trapping regime of relativistic channeling, where the diffraction divergence is balanced by the relativistic nonlinearity such that the laser beam radius is unchanged during pulse propagation in a plasma over many Rayleigh lengths. The condition for such a self-trapping regime is the same as what was empirically found in several previous simulation studies in the form of the pulse width matching condition. Here, we prove these findings for a subcritical plasma, where the total charge of high-energy electrons reaches the multi-nC level, by optimization in a 3D PIC simulation study and compare the results with an analytic theory of relativistic self-focusing. A very efficient explicitly demonstrated generation of high-charge electron beams opens a way to a high-yield production of gammas, positrons, and photonuclear particles.

Flame Propagation and Acoustics

An intense effect of acoustic waves on the structure and shape of the burner flame was discovered by Rayleigh. The present paper deals with acoustic waves in the case of flame propagation in a tube. The flame emits acoustic waves generating the flow in the tube. Being actually a wave beam, the flow bounded by the walls experiences friction and also diffraction divergence, resulting in the emergence of secondary waves, i.e., waves of flow disturbances. They induce the formation of a cellular structure of the flame; at high velocities, the flame becomes turbulent. All these processes play important roles in the deflagration-to-detonation transition in tubes. Emission of acoustic waves by the flame is also responsible for the formation of spherical detonation. Spin and pulsed detonation can be interpreted on the same basis.

A wideband transmission metasurface for generating Bessel beam carrying orbital angular momentum

Orbital angular momentum (OAM) microwave waves have received increasing research interest because of their potential to improve communication capacity and radar resolution. However, the traditional orbital angular momentum wave has the problem of diffraction divergence. The ideal Bessel beam with orbital angular momentum has the property of nondiffraction. Hence, the Bessel beam carrying orbital angular momentum has great research potential. In this work, a wideband transmission metasurface for generating Bessel beam carrying OAM is designed, fabricated, and measured. And it is the first experimental to show that the Bessel beam carrying OAM has higher gain and lower crosstalk than the traditional OAM wave in far field transmission.

Change nonlinear-optical response of electro-optic crystal in conditions of contributions incoherent background illumination

Influence of incoherent background on diffraction divergence of narrow light beams of He-Ne laser with wavelength of 633 nm due to refractive index changes of Fe-doped lithium niobate sample is experimentally studied. Incoherent background is produced by light-emitting diodes (LED´s) of different average wavelengths and laser diode emitting in blue region of visible. It has been experimentally demonstrated that nonlinear diffraction of coherent red beams of He-Ne laser with diameters on full width of half maximum (FWHM) near to 15–20 μm may be totally compensated by means of assistance of incoherent background with shorter average wavelengths 455–465 nm and even much lower intensity compared with that for coherent beam.

Simple Wave Transformations in the Channel

The Riemann wave describes the plane flow behind an infinite wavefront, though it is used to describe its propagation in channels. The influence of the walls on the process of wave propagation is not taken into account. However, friction against the walls slows the flow, and the limited diameter of the elementary plane waves, which compensate for the friction, undergo diffraction divergence. As a result, the entropy constancy and laminar flow structure are violated. This means that a simple wave as such may not exist in the channel. In this case, analysis of the friction and diffraction divergence make it possible to explain the turbulence occurrence in the flow and to find a way to form a simple wave of finite aperture with a large Rayleigh length.

Initial Excited State and Divergence of Wave Beams as Factors Responsible for the Realization of the Wave Process that Controls the Growth of Transformation Twins

In the dynamic theory of martensitic transformations the wave mechanism of growth of martensite crystals is controlled by the overlap of wave beams of quasi-longitudinal (or longitudinal) waves that carry deformation of the “tension–compression” type in orthogonal directions. The appearance of wave beams is considered a consequence of the appearance of the initial excited (vibrational) states. The presence of the transformation twins is interpreted as the result of the matched propagation of relatively long-wave (𝓁 waves) and short-wave (s waves) displacements. With the aid of the Fourier transforms, an analysis was performed of the influence of the diffraction divergence of a pair of orthogonal wave beams on the formation of a region of their superposition, including the limiting case of narrow beams with one of the transverse dimensions of the front less than λ/2, where λ is the wavelength. The connection of the distribution of the amplitudes of spatial harmonics with the configuration of the initial excited state is discussed.

Finite-Aperture Riemann’s Wave

The Riemann compression wave describes a 2D flow behind an infinite wave front, but is used for describing its propagation in channels. The effect of walls on the process of its propagation is disregarded in this case. However, friction against the walls and diffraction divergence of elementary plane waves compensating the friction change the conditions of wave propagation. These effects, which are inevitable in a channel, violate the constancy of entropy and the jet nature of the flow, which contradicts the definition of a “simple wave.” Analysis of diffraction divergence makes it possible to solve the problem of formation of a finite-aperture simple wave (a wave beam with a large Rayleigh length). The evolution of friction processes explains the emergence of a turbulent flow and makes it possible to describe the mechanism of a spontaneous transition of deflagration into detonation more accurately.

Propagation of a stable gaseous detonation in a circular arc configuration

We examine numerically the effect that detonation wavefront curvature has on gaseous detonation cellular structure in a circular arc geometry with rigid walls. For the model considered, a planar detonation is weakly unstable. The detonation curvature is induced by diffraction and flow divergence as the wave sweeps around the arc. We examine the effect of arc thickness, inner arc radius and activation energy on the cellular pattern. A combination of pseudo-Schlieren, pseudo-soot foil records and detonation shock and sonic flow loci are used to explore the observed detonation structures. We find that for sufficiently wide arcs, the detonation wavefront curvature renders the detonation hydrodynamically stable, i.e. laminar flow with no cells. The stable configuration consists of a broadly curved wave originating from the inner arc surface, which is typically connected to the outer arc boundary by a stable Mach stem. The stable detonation is driven by a small region of subsonic flow attached to the inner arc surface, outside of which the flow is supersonic. The initiation dynamics of a stable propagating wave case are explored. The evolution dynamics between an unstable (cellular) and stable arc thickness are also examined. Smaller inner arc radii and lower activation energies increase the range of stable solutions.

Spatial Divergence of Multimode Acoustic Beams Formed by a Vertical Array in a Refracting Oceanic Waveguide

We consider the patterns manifested during the formation of the spatial intensity distribution of an acoustic field excited by a vertical array in a deep-water oceanic waveguide with an underwater sound channel open to the surface and an acoustically transparent bottom. The study focuses on the dependence of the spatial divergence of multimode beams on the angle of compensation of an array emitting a tone signal. It is established that if the size of the array aperture significantly exceeds the optimal size, then at a certain angle of compensation appreciably differing from zero, a multimode beam forms with minimal diffraction divergence even in the absence of bending points and smooth extrema of the dependence of the length of the ray cycle on the ray parameter.

Metasurface-assisted orbital angular momentum carrying Bessel-Gaussian Laser: proposal and simulation

Bessel-Gaussian beams have distinct properties of suppressed diffraction divergence and self-reconstruction. In this paper, we propose and simulate metasurface-assisted orbital angular momentum (OAM) carrying Bessel-Gaussian laser. The laser can be regarded as a Fabry-Perot cavity formed by one partially transparent output plane mirror and the other metasurface-based reflector mirror. The gain medium of Nd:YVO4 enables the lasing wavelength at 1064 nm with a 808 nm laser serving as the pump. The sub-wavelength structure of metasurface facilitates flexible spatial light manipulation. The compact metasurface-based reflector provides combined phase functions of an axicon and a spherical mirror. By appropriately selecting the size of output mirror and inserting mode-selection element in the laser cavity, different orders of OAM-carrying Bessel-Gaussian lasing modes are achievable. The lasing Bessel-Gaussian0, Bessel-Gaussian01+, Bessel-Gaussian02+ and Bessel-Gaussian03+ modes have high fidelities of ~0.889, ~0.889, ~0.881 and ~0.879, respectively. The metasurface fabrication tolerance and the dependence of threshold power and output lasing power on the length of gain medium, beam radius of pump and transmittance of output mirror are also discussed. The obtained results show successful implementation of metasurface-assisted OAM-carrying Bessel-Gaussian laser with favorable performance. The metasurface-assisted OAM-carrying Bessel-Gaussian laser may find wide OAM-enabled communication and non-communication applications.

Laser Requirements for High-Order Harmonic Generation by Relativistic Plasma Singularities

We discuss requirements on relativistic-irradiance (I0 > 1018 W/cm2) high-power (multi-terawatt) ultrashort (femtosecond) lasers for efficient generation of high-order harmonics in gas jet targets in a new regime discovered recently (Pirozhkov et al., 2012). Here, we present the results of several experimental campaigns performed with different irradiances, analyse the obtained results and derive the required laser parameters. In particular, we found that the root mean square (RMS) wavefront error should be smaller than ~100 nm (~λ/8). Further, the angular dispersion should be kept considerably smaller than the diffraction divergence, i.e., μrad level for 100–300-mm beam diameters. The corresponding angular chirp should not exceed 10−2 μrad/nm for a 40-nm bandwidth. We show the status of the J-KAREN-P laser (Kiriyama et al., 2015; Pirozhkov et al., 2017) and report on the progress towards satisfying these requirements.

Conical refraction in a magneto-optical biaxial crystal

The paper investigates the influence of an external magnetic field on light propagation through a magneto-optical biaxial crystal. It has been demonstrated that the application of a magnetic field results in the splitting and reconnection of an isofrequency near the self-intersection point and thus it leads to the disappearance of conical refraction in a crystal. This effect makes it possible to control light propagation by means of a magnetic field. In addition, it has been demonstrated that the diffraction divergence of a beam is suppressed in a homogeneous magneto-optical biaxial crystal.