Showing papers on "Bessel beam published in 2018"
TL;DR: The proposed method provides an efficient approach to control the radius of vortex beam carrying OAM mode in microwave wireless applications for medium-short range distance.
Abstract: In this paper, ultra-thin metalenses are proposed to generate converging and non-diffractive vortex beam carrying orbital angular momentum (OAM) in microwave region. Phase changes are introduced to the transmission cross-polarized wave by tailoring spatial orientation of Pancharatnam-Berry phase unit cell. Based on the superposition of phase profile of spiral phase plate and that of a converging lens or an axicon, vortex beam carrying OAM mode generated by the metalens can also exhibit characteristics of a focusing beam or a Bessel beam. Measured field intensities and phase distributions at microwave frequencies verify the theoretical design procedure. The proposed method provides an efficient approach to control the radius of vortex beam carrying OAM mode in microwave wireless applications for medium-short range distance.
214 citations
TL;DR: In this paper, a single trapped 40Ca+ ion serves as a localized and positioned probe of the vortex light field and observes the relative strengths of different transitions, depending on the ion's transversal position with respect to the center of the Vortex light field.
Abstract: We analyze the multipole excitation of atoms with twisted light, i.e, by a vortex light field that carries orbital angular momentum. A single trapped 40Ca+ ion serves as a localized and positioned probe of the exciting field. We drive the transition and observe the relative strengths of different transitions, depending on the ion's transversal position with respect to the center of the vortex light field. On the other hand, transition amplitudes are calculated for a twisted light field in form of a Bessel beam, a Bessel–Gauss and a Laguerre–Gauss mode. Analyzing experimental obtained transition amplitudes we find agreement with the theoretical predictions at a level of better than 3%. Finally, we propose measurement schemes with two-ion crystals to enhance the sensing accuracy of vortex modes in future experiments.
82 citations
TL;DR: Using a temporally shaped femtosecond laser Bessel-beam-assisted chemical etching method, the energy deposition efficiency was improved by adjusting the pulse delay to yield a stronger material modification and, thus, a higher etching depth.
Abstract: We proposed combining temporally shaped (double-pulse train) laser pulses with spatially shaped (Bessel beam) laser pulses. By using a temporally shaped femtosecond laser Bessel-beam-assisted chemical etching method, the energy deposition efficiency was improved by adjusting the pulse delay to yield a stronger material modification and, thus, a higher etching depth. The etching depth was enhanced by a factor of 13 using the temporally shaped Bessel beam. The mechanism of etching depth enhancement was elucidated by localized transient-free electrons dynamics-induced structural and morphological changes. Micro-Raman spectroscopy was conducted to verify the structural changes inside the material. This method enables high-throughput, high-aspect-ratio microchannel fabrication in fused silica for potential applications in microfluidics.
68 citations
TL;DR: In this paper, the authors demonstrate several efficient, polarization-independent, all-silicon dielectric metasurfaces in the terahertz regime, which can be easily fabricated using etching technology for semiconductors.
Abstract: Dielectric metasurfaces have achieved great success in realizing high-efficiency wavefront control in the optical and infrared ranges. Here, we experimentally demonstrate several efficient, polarization-independent, all-silicon dielectric metasurfaces in the terahertz regime. The metasurfaces are composed of cylindrical silicon pillars on a silicon substrate, which can be easily fabricated using etching technology for semiconductors. By locally tailoring the diameter of the pillars, full control over abrupt phase changes can be achieved. To show the controlling ability of the metasurfaces, an anomalous deflector, three Bessel beam generators, and three vortex beam generators are fabricated and characterized. We also show that the proposed metasurfaces can be easily combined to form composite devices with extended functionalities. The proposed controlling method has promising applications in developing low-loss, ultra-compact spatial terahertz modulation devices.
67 citations
TL;DR: Three-photon fluorescence imaging with an axially extended Bessel focus is demonstrated, using an axicon-based module that allowed for the generation of Bessel foci of varying numerical apertures and axial lengths to image mouse brain slices and for in vivo imaging of the mouse brain.
Abstract: Volumetric imaging tools that are simple to adopt, flexible, and robust are in high demand in the field of neuroscience, where the ability to image neurons and their networks with high spatiotemporal resolution is essential. Using an axially elongated focus approximating a Bessel beam, in combination with two-photon fluorescence microscopy, has proven successful at such an endeavor. Here, we demonstrate three-photon fluorescence imaging with an axially extended Bessel focus. We use an axicon-based module that allowed for the generation of Bessel foci of varying numerical apertures and axial lengths, and apply this volumetric imaging tool to image mouse brain slices and for in vivo imaging of the mouse brain.
51 citations
TL;DR: In this article, a radially periodic 2-D leaky-wave (LW) antenna is proposed for the generation of zeroth-order Bessel beams within a limited spatial region and over a wide-frequency range.
Abstract: A radially periodic 2-D leaky-wave (LW) antenna is studied for the generation of zeroth-order Bessel beams within a limited spatial region and over a wide-frequency range. The antenna design is wideband and based on an annular metal strip grating placed on the top of a grounded dielectric slab, supporting a cylindrical leaky wave (CLW) with a fast backward spatial harmonic. The focusing capabilities of the relevant LW aperture fields are investigated over the considered frequency range (15–21 GHz), in conjunction with the dispersion analysis of the optimized structure, which is developed by means of an efficient in-house method of moments code. Full-wave simulations using a commercial tool including a simple coaxial feeder are presented and discussed, demonstrating the desired wideband operation. The antenna design is validated by means of measurements performed on a manufactured prototype, considering different frequencies and components of the electric field within the nondiffracting range of the system. The proposed design represents an attractive simple and low-cost solution potentially able to generate arbitrary-order Bessel beams at microwaves as well as in the millimeter-wave and terahertz frequency regions.
49 citations
TL;DR: Simulation results verify the ability of the MS lens to achieve OAM beam focusing, which is advantageous for enhancing the propagation directivity and increasing the gain in the main lobes of vortex waves, of particular importance in microwave wireless communication applications.
Abstract: Vortex electromagnetic (EM) waves hold promise for their ability to significantly increase the transmission capacity of wireless communication systems via the torsion resistance defined by different topological charges associated with the orbital angular momentum (OAM). However, the application of vortex waves in remote distance transmission is limited by its characteristic of divergence. In this paper, a lens based on a phase-modulation metasurface (MS) is proposed that enables vortex EM waves to converge, thereby improving their propagation performance at microwave frequencies. A phase-shift distribution on the plane of the MS is obtained based on the concept of the optical converging axicon, which can convert a Laguerre-Gaussian (LG) beam to a Bessel beam based on changing the propagation direction. Simulation results verify the ability of the MS lens to achieve OAM beam focusing, which is advantageous for enhancing the propagation directivity and increasing the gain in the main lobes of vortex waves. This is of particular importance in microwave wireless communication applications.
48 citations
TL;DR: In this paper, a metasurface-based reflective-type approach was proposed to generate a first-order Bessel beam carrying OAM. And the beam was integrated with a planar feeding source in the same single-layer printed circuit board.
Abstract: Metasurfaces, orbital angular momenta (OAM), and non-diffractive Bessel beams have been attracting worldwide research. Combining the benefits of these three promising techniques, this paper proposes a metasurface-based reflective-type approach to generate a first-order Bessel beam carrying OAM. To validate this approach, a millimeter-wave metasurface is analyzed, designed, fabricated, and measured. Experimental results agree well with simulation. Moreover, this reflective-type metasurface, generating a Bessel beam with OAM, is inherently integrated with a planar feeding source in the same single-layer printed circuit board. Therefore, the proposed design features low profile, low cost, easy integration with front-end active circuits, and no alignment error between the feeding source and the metasurface.
46 citations
TL;DR: A large-focal-depth welding method to bond materials by using non-diffractive femtosecond laser Bessel beams, which increases the focal-position tolerant zone for effective welding many-fold compared to traditional Gaussian beam welding.
Abstract: It is known that ultrashort laser welding of materials requires an accurate laser beam focusing and positioning onto the samples interface. This puts forward severe challenges for controlling the focus position particularly considering that the tightly focused Gaussian beam has a short, micron-sized Rayleigh range. Here we propose a large-focal-depth welding method to bond materials by using non-diffractive femtosecond laser Bessel beams. A zero-order Bessel beam is produced by an axicon and directly imaged on the interface between silicon and borosilicate glass to write welding lines, ensuring a non-diffractive length in the 500 μm range and micron-sized FWHM diameter. The focal-position tolerant zone for effective welding increases thus many-fold compared to traditional Gaussian beam welding. The shear joining strength of the sample welded by this method could be as high as 16.5 MPa. The Raman spectrum and element distribution analyses within the cross section of welding line reveal that substance mixing has occurred during laser irradiation, which is considered as the main reason for femtosecond laser induced bonding.
44 citations
TL;DR: This work has characterized two-dimensional spatio-temporal beam behavior and demonstrated all stages of pulse reshaping during the propagation, including X-shape pulse forming, as well as estimating phase velocity via propagation distance and comparing existing experimantal results.
Abstract: Terahertz pulse time-domain holography is the ultimate technique allowing the evaluating a propagation of pulse broadband terahertz wavefronts and analyze their spatial, temporal and spectral evolution. We have numerically analyzed pulsed broadband terahertz Gauss-Bessel beam’s both spatio-temporal and spatio-spectral evolution in the non-paraxial approach. We have characterized two-dimensional spatio-temporal beam behavior and demonstrated all stages of pulse reshaping during the propagation, including X-shape pulse forming. The reshaping is also illustrated by the energy transfer dynamics, where the pulse energy flows from leading edge to trailing edge. This behavior illustrates strong spatio-temporal coupling effect when spatio-temporal distribution of Bessel beam’s wavefront depends on propagation distance. The spatio-temporal and spatio-spectral profiles for different spectral components clearly illustrate the model where the Bessel beam’s wavefront at the exit from the axicon can be divided into radial segments for which the wave vectors intersect. Phase velocity via propagation distance is estimated and compared with existing experimantal results. Results of the phase velocity calculation depend strongly on distance increment value, thus demonstrating superluminal or subluminal behavior.
43 citations
TL;DR: A simple method for non-ideal axicon-generated Bessel beam reconstruction by tilting the axicon perpendicular to its optical axis is introduced and an optimum axicon tilt angle is found where beam distortions can be compensated by inducing additional astigmatic aberrations.
Abstract: It is known that Bessel beam generation with a non-ideal axicon induces beam pattern distortions. In this paper, we introduce a simple method for non-ideal axicon-generated Bessel beam reconstruction by tilting the axicon perpendicular to its optical axis. We found an optimum axicon tilt angle where beam distortions can be compensated by inducing additional astigmatic aberrations. At optimal tilt angle, the central spot symmetry and focal depth was increased. By this method we could control crack formation symmetry in the bulk of glass, which is essential for many transparent material processing applications.
TL;DR: An axicon-based Bessel beam module with continuously adjustable depth of focus (CADoF), that turns frame rate into volume rate by extending the excitation focus in the axial direction while maintaining high lateral resolutions is reported.
Abstract: Understanding how neural circuits control behavior requires monitoring a large population of neurons with high spatial resolution and volume rate Here we report an axicon-based Bessel beam module with continuously adjustable depth of focus (CADoF), that turns frame rate into volume rate by extending the excitation focus in the axial direction while maintaining high lateral resolutions Cost-effective and compact, this CADoF Bessel module can be easily integrated into existing two-photon fluorescence microscopes Simply translating one of the relay lenses along its optical axis enabled continuous adjustment of the axial length of the Bessel focus We used this module to simultaneously monitor activity of spinal projection neurons extending over 60 µm depth in larval zebrafish at 50 Hz volume rate with adjustable axial extent of the imaged volume
TL;DR: The propagation of Bessel beams through aberrated obstacles is considered and it is shown that the self-healing is not guaranteed, but rather a function of the severity of the aberration.
Abstract: It is understood from the conical wave picture that Bessel beams may self-heal after certain opaque obstructions, but the extrapolation to transparent phase screens is not self-evident. Here we consider the propagation of Bessel beams through aberrated obstacles and show that the self-healing is not guaranteed, but rather a function of the severity of the aberration. Paradoxically, we explain why strong aberrations may show self-healing while weak aberrations will not, and highlight the parameters that influence this. Finally, we combine aberrations to pass the Bessel beam through turbulence, and debunk the myth that Bessel beams are resilient to such perturbations.
TL;DR: Comparison between the calculated Bessel beam for an ideal axicon and the quasi-Bessel beam generated and measured by the fabricated axicon reveals excellent agreement, verifying the precise manufacturing method.
Abstract: We report on the fabrication of an axicon by applying a two-step manufacturing process including a 1030 nm femtosecond and a 10.6 µm CO2 laser. First, the pre-defined axicon geometry is generated by high-precision femtosecond layer-by-layer ablation. In order to meet high surface quality requirements, inevitable stipulated for optical use, the surface of the thus structured axicon is smoothened by a subsequent CO2 laser polishing process. The finalized axicon fulfills optical quality as the surface roughness Ra is significantly reduced from 0.56 µm to 34 nm. For the evaluation of the optical quality, the axicon is placed in a measurement setup including the femtosecond laser. Comparison between the calculated Bessel beam for an ideal axicon and the quasi-Bessel beam generated and measured by the fabricated axicon reveals excellent agreement, verifying our precise manufacturing method.
TL;DR: This work reports how Bessel beams reduce streaking artifacts and produce high-fidelity quantitative data demonstrating a fivefold increase in sensitivity to calcium transients and a 20- fold increase in accuracy in the detection of activity correlations in functional imaging.
Abstract: Light-sheet microscopy (LSM), in combination with intrinsically transparent zebrafish larvae, is a choice method to observe brain function with high frame rates at cellular resolution. Inherently to LSM, however, residual opaque objects cause stripe artifacts, which obscure features of interest and, during functional imaging, modulate fluorescence variations related to neuronal activity. Here, we report how Bessel beams reduce streaking artifacts and produce high-fidelity quantitative data demonstrating a fivefold increase in sensitivity to calcium transients and a 20 fold increase in accuracy in the detection of activity correlations in functional imaging. Furthermore, using principal component analysis, we show that measurements obtained with Bessel beams are clean enough to reveal in one-shot experiments correlations that can not be averaged over trials after stimuli as is the case when studying spontaneous activity. Our results not only demonstrate the contamination of data by systematic and random errors through conventional Gaussian illumination and but,furthermore, quantify the increase in fidelity of such data when using Bessel beams.
TL;DR: The use of a light droplet illumination whose side lobes are suppressed by interfering Bessel beams of specific k-vectors is demonstrated, providing a more efficient energy localization without loss in transverse resolution.
Abstract: An ideal illumination for light sheet fluorescence microscopy entails both a localized and a propagation invariant optical field. Bessel beams and Airy beams satisfy these conditions, but their non-diffracting feature comes at the cost of the presence of high-energy side lobes that notably degrade the imaging contrast and induce photobleaching. Here, we demonstrate the use of a light droplet illumination whose side lobes are suppressed by interfering Bessel beams of specific k-vectors. Our droplet illumination readily achieves more than 50% extinction of the light distributed across the Bessel side lobes, providing a more efficient energy localization without loss in transverse resolution. In a standard light sheet fluorescence microscope, we demonstrate a two-fold contrast enhancement imaging micron-scale fluorescent beads. Results pave the way to new opportunities for rapid and deep in vivo observations of large-scale biological systems.
TL;DR: The theoretical results are validated with the experimental realization of partially coherent Bessel beam arrays, anticipated to be useful for multi-particle trapping and micromanipulation, optical metrology and microscopy, as well as for 3D imaging.
Abstract: We propose a protocol for generating high-quality, partially coherent (quasi-)Bessel beam arrays with controllable beam order and spatial distributions. Our protocol involves, apart from beam intensity shaping, coherence engineering of recently introduced optical coherence lattices. Our theoretical results are validated with the experimental realization of partially coherent Bessel beam arrays. The novel beam arrays are anticipated to be useful for multi-particle trapping and micromanipulation, optical metrology and microscopy, as well as for 3D imaging.
TL;DR: This work compute the wavefronts and the caustics associated with the solutions to the scalar wave equation introduced by Durnin in elliptical cylindrical coordinates generated by the function A(ϕ)=ceν( ϕ,q)+iseν(η,q) and concludes that this type of Mathieu beam is more stable than plane waves, Bessel beams, parabolic beams, and those generated by A(ξ, η).
Abstract: In this work we compute the wavefronts and the caustics associated with the solutions to the scalar wave equation introduced by Durnin in elliptical cylindrical coordinates generated by the function A(ϕ)=ceν(ϕ,q)+iseν(ϕ,q), with ν being an integral or nonintegral number. We show that the wavefronts and the caustic are invariant under translations along the direction of evolution of the beam. We remark that the wavefronts of the separable Mathieu beams generated by A(ϕ)=ceν(ϕ,q) and A(ϕ)=seν(ϕ,q) are cones and their caustic is the z axis; thus, they are not structurally stable. However, in general, the Mathieu beam generated by A(ϕ)=ceν(ϕ,q)+iseν(ϕ,q) is stable because locally its caustic has singularities of the fold and cusp types. To show this property, we present the wavefronts and the caustics for the Mathieu beams with characteristic value aν=0 and q=0,0.2,0.3,0.5. For q=0, we obtain the Bessel beam of order zero; in this case, the wavefronts are cones and the caustic coincides with the z axis. For q≠0, the wavefronts are deformations of conical ones, and the caustic surface, for some values of q, has singularities of the cusp ridge type. Furthermore, we remark that the set of Mathieu beams with characteristic value aν=0 and 0≤q<1 has associated a caustic with singularities of the swallowtail type, which is structurally stable. Therefore, we conclude that this type of Mathieu beam is more stable than plane waves, Bessel beams, parabolic beams, and those generated by A(ϕ)=ceν(ϕ,q) and A(ϕ)=seν(ϕ,q). To support this conclusion, we present experimental results showing the pattern obtained after obstructing a plane wave, the Bessel beam of order m=5, and the Mathieu beam of order m=5 and q=50 with complex transversal amplitude given by Ce5(ξ,50)ce5(η,50)+iSe5(ξ,50)se5(η,50), where (ξ, η) are the elliptical coordinates on the plane.
TL;DR: This work provides the precise experimental and theoretical bases for the comprehension and application of a terahertz vortex Bessel beam.
Abstract: A vortex Bessel beam combines the merits of an optical vortex and a Bessel beam, including a spiral wave front and a non-diffractive feature, which has immense application potentials in optical trapping, optical fabrication, optical communications, and so on. Here, linearly and circularly polarized vortex Bessel beams in the terahertz (THz) frequency range are generated by utilizing a THz quarter wave plate, a spiral phase plate, and Teflon axicons with different opening angles. Taking advantage of a THz focal-plane imaging system, vectorial diffraction properties of the THz vortex Bessel beams are comprehensively characterized and discussed, including the transverse (Ex, Ey) and longitudinal (Ez) polarization components. The experimental phenomena are accurately simulated by adopting the vectorial Rayleigh diffraction integral. By varying the opening angle of the axicon, the characteristic parameters of these THz vortex Bessel beams are exhibited and compared, including the light spot size, the diffraction-free range, and the phase evolution process. This work provides the precise experimental and theoretical bases for the comprehension and application of a THz vortex Bessel beam.
TL;DR: The all-fiber and seamlessly integrated structure of the proposed scheme can find ample potential as a micro-optical probe in in situ characterization and manipulation of multiple bio-cells with refractive indices lower than that of the liquid bath.
Abstract: We proposed and experimentally demonstrated 3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam. The Bessel beam was produced by concatenating single-mode fiber and a step index multimode fiber, which was then focused by a high refractive index glass microsphere integrated on the fiber end facet. The focused Bessel beam provided two dark fields along the axial direction, where stable trapping of low refractive index bio-cells was realized in a high refractive index liquid bath. The all-fiber and seamlessly integrated structure of the proposed scheme can find ample potential as a micro-optical probe in in situ characterization and manipulation of multiple bio-cells with refractive indices lower than that of the liquid bath.
TL;DR: In this paper, the authors presented a design of a transmitted acoustic metasurface based on a single row of Helmholtz resonators with varying geometric parameters, which not only steer an acoustic beam as expected from the generalized Snell's law of refraction, but also exhibits various interesting properties and potential applications such as insulation of two quasi-intersecting transmitted sound waves, ultrasonic Bessel beam generator, frequency broadening effect of anomalous refraction and focusing.
University of Insubria1, Vilnius University2, Jean Monnet University3, Saint Petersburg State University of Information Technologies, Mechanics and Optics4, Centre national de la recherche scientifique5, Franche Comté Électronique Mécanique Thermique et Optique Sciences et Technologies6, Université Paris-Saclay7
TL;DR: Bhuyan et al. as mentioned in this paper investigated the nonlinear absorption of laser energy in the bulk of transparent dielectrics for femtosecond and picosecond laser pulses focused by a conical lens.
Abstract: We investigate the nonlinear absorption of laser energy in the bulk of transparent dielectrics for femtosecond and picosecond laser pulses focused by a conical lens. We highlight the influence of the pulse duration, laser pulse energy, and cone angle on laser energy absorption in transparent dielectrics. We provide a semi-analytical model allowing the calculation of maps for the density of nonlinear absorption of energy in BK7 and in SiO2 as a function of the pulse duration and peak fluence in the focal region. The comparison of the density of nonlinear absorption of energy with the available energy density determines optimal pulse durations and Bessel beam cone angles compatible with uniform laser energy deposition in the Bessel zone. The results reproduce quantitatively the transmission measurements for experiments in BK7 with picosecond pulses and suggest that the loss of propagation invariance and uniform laser energy deposition is responsible for a previously reported transition between different types of damage morphology in SiO2 [M. K. Bhuyan et al., Appl. Phys. Lett. 104, 021107 (2014)].
TL;DR: In this article, the elastic Rayleigh scattering of twisted light and the polarization of the scattered photons have been analyzed within the framework of second-order perturbation theory and Dirac's relativistic equation.
Abstract: The elastic Rayleigh scattering of twisted light and, in particular, the polarization (transfer) of the scattered photons have been analyzed within the framework of second-order perturbation theory and Dirac's relativistic equation. Special attention was paid hereby to the scattering on three different atomic targets: single atoms, a mesoscopic (small) target, and a macroscopic (large) target, which are all centered with regard to the beam axis. Detailed calculations of the polarization Stokes parameters were performed for ${\mathrm{C}}^{5+}$ ions and for twisted Bessel beams. It is shown that the polarization of scattered photons is sensitive to the size of an atomic target and to the helicity, the opening angle, and the projection of the total angular momentum of the incident Bessel beam. These computations indicate more that the Stokes parameters of the (Rayleigh) scattered twisted light may significantly differ from their behavior for an incident plane-wave radiation.
TL;DR: Experimental and simulation results of using a segmented deformable mirror to generate zero- and higher-order Bessel beams that have a controllable transverse and longitudinal shape are reported.
Abstract: Bessel beams with tunable spot size are desirable for many applications such as laser material processing, optical trapping, and imaging. In this paper, we report experimental and simulation results of using a segmented deformable mirror to generate zero- and higher-order Bessel beams that have a controllable transverse and longitudinal shape. The tilt angle and piston position of the mirror segments are optimized to recreate the phase structure of a reflective axicon. Zero-order Bessel beams are generated at various beam converging angles, and their core diameter, peak intensity, and depth-of-focus are found to agree with the calculated results. By applying a phase ramp along the azimuthal direction, the first-order Bessel beam is generated with the characteristic annular shape. Because deformable mirrors have low absorption and dispersion and operate at a fast frame rate, they are a promising candidate for spatial beam shaping of high-power ultrafast lasers, which are used in material processing applications.
TL;DR: In this article, axicon-generated asymmetrical Bessel beams were used for glass dicing. But the axicon tilt operation was introduced to add additional astigmatic aberrations for Bessel beam asymmetry control.
TL;DR: An analytical theory is presented that formulates interactions of an arbitrary-order Bessel beam with an arbitrarily located sphere as a superposition of interactions with a series of Bessel beams of different orders whose axis is through the object's center.
Abstract: This paper presents an analytical theory that formulates interactions of an arbitrary-order Bessel beam with an arbitrarily located sphere as a superposition of interactions with a series of Bessel beams of different orders whose axis is through the object's center. The analysis is via a parallel-axis relation that is derived to represent the incident Bessel beam as a superposition of a series of Bessel beams of different orders along a parallel, shifted axis. By the superposition, summing on-axial formulas gives off-axial formulas, including multipole expansion of the incident beam, scattered fields, powers of scattering and absorption, and axial radiation forces. Features of the off-axial scattering and interactions are hence accessed from prior studies on the axially centered illuminations. Dependence on the object location is contained in a weighting function in the superposition that also depends on the beam orders. Transverse and azimuthal forces are interpreted as a result of interactions between the beams of different orders in the superposition.
TL;DR: In this article, a general bianaisotropic metasurface represented by a generalized sheet transition condition is modeled by the finite-difference time-domain (FDTD) method.
Abstract: A general bianaisotropic metasurface represented by a generalized sheet transition condition is modeled by the finite-difference time-domain (FDTD) method. The dispersive susceptibilities of the metasurface are approximated by FDTD-compatible rational functions which are incorporated in the update equations by the piecewise linear recursive convolution technique. Compared to the existing methods, the proposed technique requires less arithmetic operations. The method is verified by comparing the results with those of the finite-difference frequency-domain method through simulation of wave propagation in five test cases: generalized refraction metasurface, polarization rotator, Bessel beam generator, orbital angular momentum multiplexer, and reflectionless omega-type metasurface.
TL;DR: Variable meta-axicons composed of rectangular nano-apertures arranged in several concentric rings that can focus left circularly polarized (LCP) light into a real Bessel beam and defocus right circular polarized light to form a virtual beam are experimentally demonstrated.
Abstract: Metasurfaces are two-dimensional metamaterials composed of a carefully designed series of subwavelength meta-atom (antenna or aperture) arrays. These surfaces can manipulate the phase, amplitude and polarization of output light by changing the shapes and orientations of the meta-atoms on a subwavelength scale. Using these properties, we experimentally demonstrate variable meta-axicons composed of rectangular nano-apertures arranged in several concentric rings that can focus left circularly polarized (LCP) light into a real Bessel beam and defocus right circular polarized (RCP) light to form a virtual beam. A desired phase discontinuity in cross-polarized transmitted light is introduced along the interface by controlling the orientations of the nano-apertures. In addition, the meta-axicons can generate Bessel beams of arbitrary orders by suitable design of the phase profile along the surface. The meta-axicons demonstrate broadband optical properties that can switch the wavelength of the incident light from 690 nm to 1050 nm. These variable meta-axicons open a path towards the development of new applications using integrated beam shaping devices.
TL;DR: In this paper, a method was proposed for detecting birefringence of mechanically free parabolic gradient-index lenses based on the astigmatic transformation of the zero-order Bessel beam.
TL;DR: In this paper, an iterative optimization based approach was proposed to engineer the on-axis intensity of Bessel beams through design and fine-tuning processes, which is suitable for higher-order Bessel beam by adding appropriate vortex phases into the designed phase mask.
Abstract: The Bessel beam belongs to a typical class of non-diffractive optical fields that are characterized by their invariant transverse profiles with the beam propagation. The extended and uniformed intensity profile in the axial direction is of great interest in many applications. However, ideal Bessel beams only rigorously exist in theory; the Bessel beams generated in the experiment are always quasi-Bessel beams with finite focal extensions and varying intensity profiles along the propagation axis. The ability to shape the on-axis intensity profile to meet specific needs is essential for many applications. Here, we demonstrate an iterative optimization based approach to engineer the on-axis intensity of Bessel beams through design and fine-tune processes. Starting with a standard axicon phase mask, the design process uses the computed on-axis beam profile as a feedback in the iterative optimization process, which searches for the optimal radial phase distribution that can generate a so-called generalized Bessel beam with the desired on-axis intensity profile. The fine-tune process repeats the optimization processing by using the adjusted target on-axis profile according to the measured one. Our proposed method has been demonstrated in engineering several quasi-Bessel beams with customized on-axis profiles. The high accuracy and high energy throughput merit its use in many applications. This method is also suitable to engineer higher-order Bessel beams by adding appropriate vortex phases into the designed phase mask.