Versatile polarization-converted non-diffractive Bessel beams based on fully phase-modulated metasurfaces.

In this paper, we propose a device generating Bessel beam that can assist microscope to enhance resolution of microscope to λ/3. As the conventional optical devices of generating Bessel beam have some drawbacks, such as low efficiency, and bulky volume, we design plane-axicons for generating Bessel beam using the principle of phase superposition. The designed plane-axicons can generate Bessel beams with zero-order and higher-order by changing topological charge n. We also demonstrate the FWHMs of plane-axicons about as small as λ/3 and have FWHMs with weak fluctuations independent of wavelength across the visible spectrum, which means the Bessel beam can be used to increase resolution of microscope. These performances make the plane-axicons can also enable other advanced research and applications related to Bessel beams ranging from laser fabrication to optical manipulation.

We report zero-th and high-order acoustic Bessel beams with broad depth-of-field generated using acoustic holograms. While the transverse field distribution of Bessel beams generated using traditional passive methods is correctly described by a Bessel function, these methods present a common drawback: the axial distribution of the field is not constant, as required for ideal Bessel beams. In this work, we experimentally, numerically and theoretically report acoustic truncated Bessel beams of flat-intensity along their axis in the ultrasound regime using phase-only holograms. In particular, the beams present a uniform field distribution showing an elongated focal length of about 40 wavelengths, while the transverse width of the beam remains smaller than 0.7 wavelengths. The proposed acoustic holograms were compared with 3D-printed fraxicons, a blazed version of axicons. The performance of both phase-only holograms and fraxicons is studied and we found that both lenses produce Bessel beams in a wide range of frequencies. In addition, high-order Bessel beam were generated. We report first order Bessel beams that show a clear phase dislocation along their axis and a vortex with single topological charge. The proposed method may have potential applications in ultrasonic imaging, biomedical ultrasound and particle manipulation applications using passive lenses.

This work innovatively designs an all-dielectric metasurface fabricated using 3D printing technology with alumina as the substrate material. This metasurface successfully generates vectorial non-diffracting Bessel beams at a specific frequency of 0.1THz. Initially, through comprehensive simulation analysis of the interaction between the metasurface and the incident beam, it was discovered that the metasurface can precisely manipulate the properties of the transmitted beam based on the polarization state of the incident beam. Specifically, when illuminated by a left-handed circularly polarized (LCP) Gaussian beam, the transmitted beam manifests as a radially polarized Bessel beam. Conversely, under right-handed circularly polarized (RCP) Gaussian beam illumination, the transmitted light transforms into an azimuthally polarized Bessel beam. Building upon the theoretical simulations and considering experimental constraints, further experimental validation was conducted using linearly polarized light, confirming the effectiveness and feasibility of the proposed design. Experimental results demonstrate that the presented all-dielectric alumina metasurface, fabricated via 3D printing, successfully achieves the generation of vector nondiffracting Bessel beams. On one hand, the combination of alumina material and 3D printing technology significantly reduces manufacturing costs, laying a foundation for large scale application. On the other hand, leveraging the high precision of 3D printing ensures the accurate fabrication of the metasurface structure, thereby guaranteeing the high quality and stability of the generated vector beams. This work provides a novel approach for the efficient generation of Bessel beams and is expected to have profound implications in the field of terahertz technology.

Bessel beams have been generated using different methods, such as Axicon lens, digital micromirror device (DMD), etc. Due to the infinite energy requirement of ideal Bessel beams in all space, the generated Bessel beams are more appropriately called Bessel-like beams in practice, which are approximations of the ideal Bessel beams. In this work, we theoretically investigated the generation of Bessel-like beams using annular patterns loaded on a DMD based on the scalar diffraction theory. The model predictions were compared and verified with our previous experimental results. For the first time, the theoretical study shows that the DMD-generated Bessel-like beams have an additional amplitude term depending on the annular radius, ring thickness, and incident angle compared with ideal Bessel beams. Furthermore, we modeled the superposition of two Generated Bessel-like beams using two coaxial annular patterns on a DMD, which revealed the periodic intensity distribution along the z axis as predicted previously based on the superposition of two ideal coaxial Bessel beams. Both the simulations and experiments give a similar periodic length that is close to the theoretical values. The modeling results show that the DMD-based method could not only generate a reasonable approximation of the ideal Bessel beams with good controllability and engineering applications but more importantly, provide explicit formulas to guide the design of the annular patterns on the DMD in order to generate and control Bessel-like beams for practical applications.

Generation of parallel Bessel beams and cosine Bessel beams based on metasurface

In this article, a graphene‐based reflective metasurface which can generate Bessel beams at arbitrary directions is proposed at 1.52 THz. It chooses the classical cross structure as the unit cell and extends the coverage of the reflection phase to 360° by adjusting two key parameters while maintaining a relatively high‐reflection amplitude. Combined with the excellent impedance characteristics of graphene in the terahertz regime and the long energy transmission distance of the nondiffraction beam, the reflective metasurface finally realize the pseudo‐Bessel beam with a maximum propagation distance of about 3358.85 um and a deflection of about 15° in yoz‐plane as expected. Two forms of efficiency are proposed for the metasurface, one is the focusing efficiency, the other is the conventional radiation efficiency. The simulation results show that the former shows a trend of increasing first and then decreasing, which conforms to the beam‐forming property of Bessel beam; the latter can be basically maintained at high value of 70% within the nondiffraction propagation distance, which conforms to the energy transmission characteristic of the nondiffraction beam. All the studies show that the graphene‐based metasurface combined with the nondiffraction theory has the advantages of highly controllable beam deflection, long focusing distance, and high‐energy transfer efficiency in the terahertz regime, so it will have a broad application prospect in terahertz telecommunication, wireless energy transmission, near‐field detecting, imaging and many other fields.

Bessel beams are nondiffracting light beams with large depth-of-focus and self-healing properties, making them suitable as a serial beam writing tool over surfaces with arbitrary topography. This property breaks the inherent resolution vs. depth-of-focus tradeoff of photolithography. One approach for their formation is to use circularly symmetric diffraction gratings. Such a ring grating was designed and fabricated for the extreme ultraviolet (EUV) wavelength of 13.5 nm, a candidate wavelength for future industrial lithography. Exposure of the aerial images showed that a Bessel beam with an approximately 1 mm long z-invariant central core of 223 nm diameter had been achieved, in good agreement with theory. Arbitrary patterns were written using the Bessel spot, demonstrating possible future application of Bessel beams for serial beam writing. Lithographic marks of ~30 nm size were also observed using a high resolution Bessel beam.

Bessel beam, known to have an amplitude that follows a Bessel function of the first kind has attracted considerable attention these last decades, especially due to their propriety of being non-diffractive beam over short distances. In this paper, we propose to generate and control Bessel beams by using a reconfigurable metasurface. This structure allows both to reconfigure the operating frequency and to change the spatial position of the generated Bessel beam. Both numerical simulations and experimental measurements are performed at microwave frequencies to validate the concept.

Based on the temperature-controlled phase transition property of vanadium dioxide (VO2), a tunable broadband polarization converter in the terahertz band has been designed, which achieves a flexible and reversible conversion between transmission and reflection modes. Our results show that the polarization conversion of this converter can be achieved by changing the phase transition of VO2. When VO2 in the insulating state, the transmission polarization conversion (TPC) can be realized to obtain the ideal broadband linear-to-linear (LTL) polarization conversion, and the conversion efficiency is more than 0.99 in the range of 0.99–2.94 THz. When VO2 is in the metallic state, reflection polarization conversion (RPC) can be realized, achieving relatively perfect linear-to-right-hand circular polarization (LTRHCP) and linear-to-left-hand circular polarization (LTLHCP) conversions in different frequency bands. A right-hand circular polarization (RHCP) wave can be obtained from 0.8–1.87 THz, and a left-hand circular polarization (LHCP) wave can be obtained from 2.46–3.15 THz. The points of metasurface polarization converter proposed have the advantages of multi-conversion mode, multi-function, high polarization conversion efficiency, broadband, etc The working principle of the transmission and reflection polarization conversion mode is explained by surface current and electric field. In addition, the structure is insensitive to incident angle and has huge application potential in the fields of communication, imaging, sensing, etc

A 5mm biophotonic catheter was conceived for optical coherence tomography (OCT) with collimation optics, an axicon lens, and custom design imaging optics, yielding a 360 degree scan aimed at imaging within concave structures such as lung lobes. In OCT a large depth of focus is necessary to image a thick sample with a constant high transverse resolution. There are two approaches to achieving constant lateral resolution in OCT: Dynamic focusing or Bessel beam forming. This paper focuses on imaging with Bessel beams. A Bessel beam can be generated in the sample arm of the OCT interferometer when axicon optics is employed instead of a conventional focusing lens. We present a design for a 5mm catheter that combines an axicon lens with imaging optics and the coupling of a MEMS mirror attached to a micromotor that allow 360 degree scanning with a resolution of about 5 microns across a depth of focus of about 1.2mm.

Employing Bessel beams in imaging takes advantage of their self-reconstructing properties to achieve small focal points while maintaining a large depth of focus. Bessel beams are efficiently generated using axicons, and their utility in scanning imaging systems, such as optical coherence tomography (OCT), has been demonstrated. As these systems are miniaturized to allow, for example, endoscopic implementations, micro-axicons are required to assure the maintenance of a large depth of focus. We demonstrate here the design, fabrication, and application of molded micro-axicons for use in silicon-based micro-optical benches. It is shown that arrangements of multiple convex and concave axicons may be implemented to optimize the depth of focus in a miniaturized OCT system, using a telescopic optical arrangement of considerably shorter optical system length than that achievable with classical micro-optics.

We present a theoretical perspective on the notion of a highly sensitive multimode fiber optic evanescent wave absorption-based sensor by exploiting a zero-order Bessel–Gauss beam to determine the concentration of sodium chloride (NaCl) from its aqueous solution. The phenomenon of excited waveguide modes inside the sensor structure has been assessed by using a classical wave-optic model. By harnessing the advantages of the Bessel–Gauss beam, the difference in transmitted output power is evaluated for various concentrations of NaCl, ranging from 0 to 360 g/L. To corroborate our theoretical predictions, the computer-aided simulation in Mode Solutions software has been performed on the proposed sensing configuration. In contrast to the conventional concentration sensor using Gaussian beam, the projected scheme yields a maximum 14.40-fold superior sensitivity of 0.072 dB / gL − 1 with a commendable sensing resolution of ∼0.013 g / L. Also, attention has been paid to the Bessel–Gauss beam shined U-bent fiber-optic absorption-based concentration sensor, where the sensor response has been numerically investigated for different bending radii, and it is concluded that the sensitivity can be enhanced appreciably by gently reducing the bending radius. Due to ultrahigh sensitivity, the present paradigm is very much alluring and evocative, establishing pivotal implication in chemical and biological sensing fields.

A new interpretation of the non-diffracting Bessel beams is given as a superposition of two fields described by Hankel functions. Within our picture, supported by the Sommerfeld radiation condition, we find that Bessel beams must be of finite transverse extension. Our approach also easily explains propagation characteristics of Bessel-Gauss beams and, in general, the propagation of amplitude modulated Bessel beams. Consequences are discussed and contrasted with the classic theory of Bessel beams.

The optical binding force (BF) of stratified chiral particles induced by Bessel beam is computed by using the Generalized Lorenz Mie theory. The axial intensity profile of such non-diffracting beams can suppress the influence of the axial intensity which then depended on the interactions between inter-particles. The arbitrarily polarized high order Bessel beam (HOBB) is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz-Mie theories (GLMT). We discuss the influence of the beam half-conical angles and chirality parameter in detail. Different illuminating Bessel beams with arbitrary polarizations are considered, and the corresponding BFs are analyzed. It is shown that the optical BF of stratified chiral spheres can be significantly depending on the incident beam polarizations and the interacting chiral particles handedness. In binding stratified chiral spheres, the out-layered chirality of the stratified chiral spheres should be chosen in accordance with the incident polarizations. This finding may give a recipe to understand the physical mechanism of interactions between multiple chiral stratified particles by using the analytical approach, which plays an important role in manipulating complex multi-layered chiral structures self-arrangement.

In this paper, a straightforward approach is presented to generate Bessel beam sources in three-dimensional finite-difference time-domain (FDTD) method. Based on the angular spectrum representation (ASR), the incident Bessel beam is described as a superposition of plane waves whose wavevectors covering a conical surface. This decomposition of Bessel beam is then approximated by a finite collection of plane waves, which are injected into FDTD simulation domain using the total-field/scattered-field (TF/ST) method. The present method's correctness and accuracy are verified by comparing the reconstructed field in FDTD with the original field. Far-field scattered diagrams of a dielectric sphere and a spheroid particle illuminated by a zero-order or a higher-order Bessel beam are calculated using FDTD. The results are compared with those calculated using the generalized Lorenz-Mie theory (GLMT) and surface integral equation method (SIEM). Very good agreements have been achieved, which partially indicate the correctness of our method. Internal and near-surface field distributions for a two-layer hemisphere particle, which are illuminated by Bessel beams, are also displayed to show the potentials of this approach in solving scattering problems of complex particles. This approach can also be applied to generate other structured beam sources in FDTD, which provides an access to solve structured beam scattering by complex particles using FDTD.