Diffraction-free Beams Research Articles

Phase gradient metasurface for arbitrary three-dimensional shaped near-field

This paper proposes a methodology for generating arbitrary three-dimensional (3D) shaped near-field using phase gradient metasurface (PGMS) based on phase conjugation (PC) backtracking and shaped diffraction-free Bessel beam array techniques. The PGMS is designed by combining MATLAB and electromagnetic (EM) simulation software (CST). To the knowledge of the authors, for the first time, a 3D-shaped near-field is generated without increasing cost, structural and calculating complexity, and a methodology for generating arbitrarily shaped diffraction-free beam array is developed by MATLAB. To verify the technique, a planar PGMS is designed for dual x-polarized 3D-shaped near-fields: qq image “” in direction (θ1 = 5°, φ1 = 0°) and a flower image “” in direction (θ2 = 10°, φ2 = 180°). The advantages of the 3D-shaped near-fields are as follows: high conversion efficiency 30.5% (measured), wideband 60.5%, and low sidelobe -15 dB. The proposed technique is confirmed by the calculated, simulated, and measured results.

Driver circuit design based on NI-6363 data acquisition card for the magnetic fluid deformable mirror

Abstract Commonly used methods for generating diffraction-free beams are prone to signal loss or coding errors. Therefore, a method of generating a diffraction-free beam based on a magnetic fluid deformation mirror (MFDM) with dual-layer actuators is proposed. It can produce the desired wavefront in real-time and has the advantages of a large magnitude of mirror deformation, low manufacturing cost, continuous and smooth surface, and easy expansion of the actuator. In this paper, a driver circuit is designed based on Simulink and NI-6363 data acquisition card for the MFDM with its multi-channel control ability. The experiment result shows that the driver circuit can accurately produce the signal for each channel, and has the advantages of stable output, simple operation, and strong expandability.

Performance analysis of an underwater wireless optical communication link with Lommel beam

In order to mitigate the stochastic interference of underwater channels and improve the quality of underwater communication systems, it is essential to study the performance of the underwater wireless optical communication (UWOC) links utilizing vortex beams with unique attributes. In this paper, the analytical formulae for the bit error rate (BER) and the average capacity of the UWOC link with diffraction-free Lommel beam are derived under the Rytov theory. Simulation results demonstrate that the system with a long wavelength, a high system signal-to-noise ratio(SNR), a small asymmetric parameter and receiving aperture diameter achieves a high average capacity and a low BER. Furthermore, in the underwater channel with a larger kinetic energy dissipation rate per unit mass of fluid and inner scale, a smaller mean-squared temperature dissipation rate, temperature salinity contribution ratio and transmission distance, the performance of the communication link can be improved. Meanwhile, it is found that the performance of the link with carrier Lommel beam are less sensitive to the topological charge, the scaling factor of the beam and the turbulent outer scale. These findings provide theoretical support for the design and implementation of an UWOC link utilizing the Lommel beam.

Terahertz fan-beam computed tomography.

A terahertz (THz) fan-beam computed tomography (CT) system using a 0.3 THz continuous-wave sheet beam is proposed. The diffraction-free sheet beam expands in a fan shape in only one direction and provides propagation-invariant focal lines and extended the depth-of-field. The fan-beam CT based on this beam is the second-generation THz CT. It breaks the conventional 4-f symmetric structure of THz CT using the parallel beam. The fan-beam THz CT allows for use with a linear array detector, which reduces the time required to collect data. To demonstrate its feasibility for three-dimensional (3D) imaging, the 3D structure of a metal rod packed in a carton is reconstructed with the support of the system. The results show that the object's internal structure can be obtained by this new THz CT system while retaining the geometrically magnified features of the cross-sectional structure. The results of our research provide a template for the second-generation THz CT system, which provides an additional method for nondestructive testing.

Generation of diffraction-free beam with winding trajectory based on metasurface holography

The diffraction-free beams with curved trajectories and shaped wavefronts have wide application prospects in many fields. This paper proposes the generation of diffraction-free beam with winding trajectory and spiral wavefront based on holographic metasurface. The holographic metasurface consists of rotated rectangular nanoholes and the winding trajectory for the generated diffraction-free beam may be in two or three dimensional space under the control of the rotated nanoholes. The multiple diffraction-free beams are exemplified and the performance of holographic metasurfaces are testified by the simulation and experiment results. The utilization of compact metasurface enables the flexible generation of the diffraction-free beams with complex trajectories and tailored wavefronts. It may bring more new applications of diffraction-free beams with on-demand trajectories and customized wavefronts.

Generation of Mathieu beam using a single layer transmissive metasurface

Mathieu beams is an important diffraction-free beam. In this letter, we proposed a transmissive metasurface to generate Mathieu beams in microfields. An ultrathin transmissive element with single layer is designed. It consists of two metallic I-shaped structures on both sides of the substrate. By rotating the top layered I-shaped resonator, amplitude adjustment from 0 to 1 with 1-bit phase shift can be achieved when y-polarized incident wave is converted into x-polarized one. A metasurface prototype is designed and fabricated to generate the required Mathieu beam. Both simulation and measurement show that Mathieu beam can be effectively generated and diffraction free feature can be observed in the distance range of 0 36λ (where λ is the wavelength at operating frequency). Compared with traditional devices to generate Mathieu beams, using metasurface reduces the complexity and cost. Our proposed method and metasurface can provide reference for complex field control in 3-dimensional space and may facilitate the uses of Mathieu beams for practical applications.

Acoustic Bessel-like beam generation using phononic crystals

Diffraction-free beams with large depth-of-field have a lot of potential in the field of acoustics, such as imaging, sensing, and particle manipulation. In this study, an acoustic Bessel-like beam is produced using an axicon-sonic crystal lens. The sonic crystal is created using cylindrical glass rods arranged in a triangular shape with a centered square lattice configuration. The numerical simulation between 4 and 8 kHz indicates that the axicon-sonic crystal converts the plane acoustic wave into a Bessel-like beam. The analysis of the beam indicates that the depth of field of this beam depends on the size and periodicity (lattice parameter) of the sonic crystal. The axicon lens also displays variable focal lengths at different frequencies. A graded index layer was implemented to mitigate the reflection caused by the significant impedance mismatch. Experimental validation of acoustic Bessel-like beam formation is also reported for the working frequencies. At 8 kHz, the measured range to the 50% on-axis intensity was 34λ, while the focus width at the same frequency was measured to be 2λ. The integration of three distinct design strategies—axicon shape, sonic crystal, and graded index—expands the possibilities for sound focusing applications.

On-chip 3.2 Tb/s wireless optical interconnects using diffraction-free beam and microcomb

On-chip 3.2 Tb/s wireless optical interconnects using diffraction-free beam and microcomb

Terahertz diffraction-free beam generation technology based on single element

Terahertz diffraction-free beam generation technology based on single element

Terahertz ellipsometry based on the long-distance diffraction-free beam

We introduce a long-distance terahertz (THz) diffraction-free beam (DFB) to the quasi-optical THz ellipsometry. Due to the distinctive characteristics of DFBs, the reflected arm of the ellipsometer does not need a focusing lens to enhance the signal detected. Since the radiation emitted from the generator is linearly polarized, there is no need to place a polarizer on the incident arm. The THz ellipsometry with the lensless reflected arm completed the thickness measurements of non-absorbent films with thicknesses in the wavelength order. The measured values are consistent with the results measured by the micrometer. In addition, we quantitatively analyzed errors that commonly occur in this ellipsometry. The DFB generating system, which consists of two lens-axicon doublets, is fairly simple to implement. We firmly believe that the proposed approach can be easily transplanted to other types of THz ellipsometry.

Non-Diffraction Self-Acceleration Beams With Customized Transverse Intensity Profiles Based on 3-D-Printed Meta-Structure

In this article, two 3-D-printed all-dielectric meta-structures that can produce diffraction-free self-acceleration beams with different transverse intensity profiles are proposed in the microwave frequency band. The first single-layer meta-structure, combining the Fourier lens phase and cubic phase distributions, is engineered to generate Airy beams with highly asymmetric transverse intensity profiles. In order to flexibly customize the propagation trajectory and transverse spot shape, Bessel-like beams with symmetric transverse profiles along arbitrary trajectories are realized by the second compact meta-structure based on phase synthesis. Calculating the phase mask of the metasurface is the most critical part. In this work, phase-modulated meta-structures can map the phase information to height distribution of the cube elements that can be precisely controlled. To verify the validity, two prototypes are printed and measured. From the beam intensities at different cross sections along the propagation trajectory, the oscillating multilobe intensity pattern of the 2-D Airy beam and the symmetric non-spreading central main lobe of the Bessel-like beam can be observed in simulation and measurement. Besides that, two kinds of beams have transverse acceleration behavior and self-reconstruction property. Therefore, the proposed meta-structures have great potential in numerous challenging fields, like microwave power transmission, secure communication, imaging, and detection.

Generation of diffraction-free Bessel beams based on combined axicons

Generation of diffraction-free Bessel beams based on combined axicons

Generation of Bloch surface beams with arbitrarily designed phases.

We proposed a new manipulation method for Bloch surface waves that can almost arbitrarily modulate the lateral phase through in-plane wave-vector matching. The Bloch surface beam is generated by a laser beam from a glass substrate incident on a carefully designed nanoarray structure, which can provide the missing momentum between the two beams and set the required initial phase of the Bloch surface beam. An internal mode was used as a channel between the incident and surface beams to improve the excitation efficiency. Using this method, we successfully realized and demonstrated the properties of various Bloch surface beams, including subwavelength-focused, self-accelerating Airy, and diffraction-free collimated beams. This manipulation method, along with the generated Bloch surface beams, will facilitate the development of two-dimensional optical systems and benefit potential applications of lab-on-chip photonic integrations.

Composite Diffraction-Free Beam Formation Based on Iteratively Calculated Primitives.

To form a diffraction-free beam with a complex structure, we propose to use a set of primitives calculated iteratively for the ring spatial spectrum. We also optimized the complex transmission function of the diffractive optical elements (DOEs), which form some primitive diffraction-free distributions (for example, a square or/and a triangle). The superposition of such DOEs supplemented with deflecting phases (a multi-order optical element) provides to generate a diffraction-free beam with a more complex transverse intensity distribution corresponding to the composition of these primitives. The proposed approach has two advantages. The first is the rapid (for the first few iterations) achievements of an acceptable error in the calculation of an optical element that forms a primitive distribution compared to a complex one. The second advantage is the convenience of reconfiguration. Since a complex distribution is assembled from primitive parts, it can be reconfigured quickly or dynamically by using a spatial light modulator (SLM) by moving and rotating these components. Numerical results were confirmed experimentally.

Multilevel Spiral Axicon for High-Order Bessel-Gauss Beams Generation.

This paper presents an efficient method to generate high-order Bessel-Gauss beams carrying orbital angular momentum (OAM) by using a thin and compact optical element such as a multilevel spiral axicon. This approach represents an excellent alternative for diffraction-free OAM beam generation instead of complex methods based on a doublet formed by a physical spiral phase plate and zero-order axicon, phase holograms loaded on spatial light modulators (SLMs), or the interferometric method. Here, we present the fabrication process for axicons with 16 and 32 levels, characterized by high mode conversion efficiency and good transmission for visible light (λ = 633 nm wavelength). The Bessel vortex states generated with the proposed diffractive optical elements (DOEs) can be exploited as a very useful resource for optical and quantum communication in free-space channels or in optical fibers.

Laser Scribing of Graphene Oxide Using Bessel Beam for Humidity Sensing

Laser-scribed graphene oxide (GO) shows great promise for high-performance, cost-effective humidity sensors. However, when using the commonly employed Gaussian beam, the Rayleigh length is relatively short, leading to potential stability issues during large-area processing, especially when defocusing occurs. In this paper, we utilize a diffraction-free Bessel beam to one-step fabricate reduced graphene oxide (rGO) electrodes specifically designed for humidity sensing applications. The effects of defocusing and laser power on the line width and resistance of the fabricated electrodes are investigated, giving the optimal processing parameters for Bessel laser writing of GO. The line width, resistance, and sheet resistance of the rGO electrode are stable at a defocusing distance within ±1.00 mm. Defocusing also proves to be effective in reducing the ablation region during the fabrication process. The temperature and humidity responses of the electrodes are examined, focusing on those fabricated with typical defocusing settings, and the related mechanisms are discussed. Proof-of-principle rGO/GO/rGO humidity sensors are demonstrated, and were one-step fabricated using a Bessel beam with both focusing and defocusing settings. The corresponding humidity response results evidence that rGO humidity sensors can be fabricated using a Bessel beam, even in the defocusing cases. The investigation into the Bessel-beam-based laser fabrication technique offers promising prospects for rapid, flexible, and cost-effective production of graphene-based humidity sensors. Meanwhile, the study of defocusing may enhance the fabrication stability to withstand defocusing conditions effectively.

Diffraction-free beam propagation at the exceptional point of non-Hermitian Glauber Fock lattices

We construct localized beams in a non-Hermitian Glauber Fock (NGF) lattice of coupled waveguides and show that they can propagate over a long distance withalmost no diffraction. We specifically obtain the diffraction-free beams in a finite NGF lattice at the exceptional point (EP) by using the exact eigenstates of the semi-infinite unidirectional NGF lattice. We provide a numerical approach to finding other lattices that are capable of supporting non-diffracting beams at EPs.

Single optical element to generate a meter-scale THz diffraction-free beam.

Diffraction-free electromagnetic beam propagates in free space without change in its two-dimensional transverse profile. Elongating diffraction-free length can benefit the practical application of this beam. Here, we demonstrate that a THz diffraction-free beam with meter-scale length can be achieved by using only one optical element. By circumscribing the line-shape of spherical harmonic function on a traditional axicon, such optical element is designed, and then can be fabricated by 3D-printing technique. Simulated, experimental, and theoretical results all show that the diffraction-free length of generated beam is over 1000 mm. Further analysis based on Fourier optics theory indicates that the spatial frequency of this beam has a comb distribution, which plays a key role during the beam generation process. Moreover, such distribution also demonstrates the beam generated by our invented optical element is not the Bessel beam, but a new diffraction-free beam. It is believed that this meter-scale THz diffraction-free beam can be useful in a non-contact and non-destructive THz imaging system for large objects.

Excitation of surface plasmon polaritons by diffraction-free and vector beams.

Surface plasmon polaritons (SPPs) are traditionally excited by plane waves within the Rayleigh range of a focused transverse-magnetic (TM) Gaussian beam. Here we investigate and confirm the coupling between SPPs and two-dimensional Gaussian and Bessel-Gauss wave packets, as well as one-dimensional light sheets and space-time wave packets. We encode the incoming wavefronts with spatially varying states of polarization; then we couple the respective TM components of radial and azimuthal vector beam profiles to confirm polarization-correlation and spatial-mode selectivity. Our results do not require material optimization or multi-dimensional confinement via periodically corrugated metal surfaces to achieve coupling at a greater extent, hereby outlining a pivotal, yet commonly overlooked, path towards the development of long-range biosensors and all-optical integrated plasmonic circuits.

Generation of diffraction-free petallike beams based on stationary phase principle

Lately, a kind of so-called “combined half-integer Bessel-like beams,” in which their subfamilies include radial carpet, petallike, and ringlike vortex beams, has been generated using a radial periodic grating. Different from classical petallike beams, we introduce and generate a kind of new petallike beams, that is, diffraction-free petallike beams (DFPBs). Their diffraction pattern is independent of the propagation distance. Firstly, the generating mechanism of diffraction-free petallike beams is theoretically investigated based on the stationary phase principle. Then, we theoretically construct a family of patterns of DFPBs. Subsequently, the DFPBs is generated by using an amplitude radial grating with a sinusoidal transmission function and two phase modulation elements — phase plate and axicon. Experimental results well confirm the theoretical predictions, thereby showing that the stationary phase principle is an effective approach to generating various diffraction-free combined half-integer Bessel-like beams based on radial grating, including diffraction-free radial carpet, petallike, and ringlike vortex beams.