Nondiffracting Beams Research Articles

Synthesis of Bessel Beam Using Time-Reversal Method Incorporating Metasurface

In this paper, a synthesis and implementation method for generating Bessel beams based on time-reversal theory incorporating electromagnetic (EM) meta-lens is proposed. As is known, time-reversal EM waves has the unique characteristic of adaptive backtracking. Therefore, with this characteristic, the EM characteristics of the radiation aperture can be obtained and further utilized to generate Bessel waves with any departure angle. Based on this concept, two meta-lenses for generating Bessel beams tilted in different directions were designed. Both meta-lenses were designed at the center frequency of 15 GHz, and the simulation results were consistent with the target expectation. A representative meta-lens was fabricated and measured. The final size of the meta-lens was 350 mm × 350 mm, and a Bessel beam with a 30° emergence angle was generated by this structure. The experimental results were in good agreement with the simulation results and the theoretical derivation. This synthetic method of Bessel beam generation using the time-reversal operation may be of great use for the application of Bessel beams in microwave communications, and it can broaden the application scope of non-diffracting beams.

An Efficient Algorithm Based on Spectrum Migration for High Frame Rate Ultrasound Imaging

The high frame rate (HFR) imaging technique requires only one emission event for imaging. Therefore, it can achieve ultrafast imaging with frame rates up to the kHz regime, which satisfies the frame rate requirements for imaging moving tissues in scientific research and clinics. Lu’s Fourier migration method is based on a non-diffraction beam to obtain HFR images and can improve computational speed and efficiency. However, in order to obtain high-quality images, Fourier migration needs to make full use of the spectrum of echo signals for imaging, which requires a large number of Fast Fourier Transform (FFT) points and increases the complexity of the hardware when the echo frequency is high. Here, an efficient algorithm using the spectrum migration technique based on the spectrum’s distribution characteristics is proposed to improve the imaging efficiency in HFR imaging. Since the actual echo signal spectrum is of limited bandwidth, low-frequency and high-frequency parts with low-energy have little contribution to the imaging spectrum. We transform the effective part that provides the main energy in the signal spectrum to the imaging spectrum while the ineffective spectrum components are not utilized for imaging. This can significantly reduce the number of Fourier transform points, improve Fourier imaging efficiency, and ensure the imaging quality. The proposed method is evaluated on simulated and experimental datasets. Results demonstrated that the proposed method could achieve equivalent image quality with a reduced point number for FFT compared to the complete spectrum migration. In this paper, it only requires a quarter of the FFT points used in the complete spectrum migration, which can improve the computational efficiency; thus, it is more suitable for real-time data processing. The proposed spectrum migration method has a specific significance for the study and clinical application of HFR imaging.

Dual non-diffractive terahertz beam generators based on all-dielectric metasurface.

The applications of terahertz (THz) technology can be greatly extended using non-diffractive beams with unique field distributions and non-diffractive transmission characteristics. Here, we design and experimentally demonstrate a set of dual non-diffractive THz beam generators based on an all-dielectric metasurface. Two kinds of non-diffractive beams with dramatically opposite focusing properties, Bessel beam and abruptly autofocusing (AAF) beam, are considered. A Bessel beam with long-distance non-diffractive characteristics and an AAF beam with low energy during transmission and abruptly increased energy near the focus are generated for x- and y-polarized incident waves, respectively. These two kinds of beams are characterized and the results agree well with simulations. In addition, we show numerically that these two kinds of beams can also carry orbital angular momentum by further imposing proper angular phases in the design. We believe that these metasurface-based beam generators have great potential use in THz imaging, communications, non-destructive evaluation, and many other fields.

Simulations of optical forces by a microstructured continuous superposition of first-order nonparaxial Bessel beams on Rayleigh particles

This paper investigates optical forces on absorptive Rayleigh particles produced by a nondiffracting microstructured vectorial beam (micrometer Frozen Wave) designed from a continuous superposition of first-order Bessel beams, very adaptive to highly nonparaxial conditions. Carrying a non null topological charge and possessing a vector nature, these beams have both orbit and spin angular momentum. Thus, this paper aims to investigate the possibility of setting a Rayleigh particle to follow a circular or elliptical trajectory by these beams. Since energy considerations are also taken into account, optical forces can be conveniently expressed in femtonewtons. Numerical results demonstrate that the micrometer Frozen Wave is able to produce full three-dimensional optical trapping points, thus allowing the manipulation of several particles simultaneously. Additionally, those beams with circular (elliptical) polarization states are capable of setting the particle to follow a circular (elliptical) orbit around the optical axis.

Single-pixel imaging for dynamic targets via a non-diffracting beam

We develop a novel kind of single-pixel imaging (SPI) scheme to reconstruct high-quality images of the axially fast-moving target by utilizing the non-diffracting beam illumination. In this paper, a kind of cosine shaped source is elaborately designed and introduced for the first time in the SPI scheme to realize the reconstruction of the dynamic target. The imaging formula for the SPI system with the cosine shaped source is theoretically derived and numerical examples are given to see clearly the influences of system parameters and motion models of the target on the image quality. Our results show that the image quality and imaging efficiency of the SPI system for the static or dynamic target can be significantly improved by utilizing the cosine shaped source, due to its non-diffracting and orthogonal features. Even there exists strong scattering media, our SPI proposal can still work well for the static or dynamic target. Therefore, our work offers great promise for the SPI real application in forward-looking lidar detection.

Generation of nondiffraction beams by using graphene‐based metasurface in terahertz regime

AbstractIn 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 Beam: Significance and Applications-A Progressive Review.

Diffraction is a phenomenon related to the wave nature of light and arises when a propagating wave comes across an obstacle. Consequently, the wave can be transformed in amplitude or phase and diffraction occurs. Those parts of the wavefront avoiding an obstacle form a diffraction pattern after interfering with each other. In this review paper, we have discussed the topic of non-diffractive beams, explicitly Bessel beams. Such beams provide some resistance to diffraction and hence are hypothetically a phenomenal alternate to Gaussian beams in several circumstances. Several outstanding applications are coined to Bessel beams and have been employed in commercial applications. We have discussed several hot applications based on these magnificent beams such as optical trapping, material processing, free-space long-distance self-healing beams, optical coherence tomography, superresolution, sharp focusing, polarization transformation, increased depth of focus, birefringence detection based on astigmatic transformed BB and encryption in optical communication. According to our knowledge, each topic presented in this review is justifiably explained.

Stealth dicing of sapphire sheets with low surface roughness, zero kerf width, debris/crack-free and zero taper using a femtosecond Bessel beam

Previous approaches for laser beam cutting of sapphire often lead to chipping, debris, large kerf widths, tapering and high surface roughness or nonuniform surfaces. Laser beam stealth dicing can remove kerf width and tapering, reduce defects. However, sidewall uniformity is poor as part of the material is laser cut and part is broken by an external mechanical force. In previous approaches of Bessel beam full depth cutting of sapphire, uniformity can be improved. However, the sidewall surface roughness is poor. There has been lack of an ideal solution to achieving minimum defects and low surface roughness. Here we show a much improved sapphire cutting method with a femtosecond Bessel beam achieving a 200 nm Ra cut surface roughness, which is almost an order of magnitude improvement over the previous Bessel beam cutting approaches without losing the uniformity. By using a diameter reduced Gaussian beam passing through a 20° physical angle axicon lens, a highly uniform non-diffraction Bessel beam is generated. Under circular polarization, the effects of Bessel beam scanning speed on flexural strength and the sidewall surface roughness are analyzed for cutting sapphire sheets of 0.38 mm, 1 mm, and 1.5 mm in thickness. Zero taper, zero kerf width, free of debris/chipping sapphire cutting with both straight and curved lines are demonstrated. The fundamental mechanisms involved are discussed. The uniformity of Bessel beam and appropriate separation of pulses have been identified as the key factors for achieving the low surface roughness.

Flexible tuning of nonlinear non-diffracting array beams using wavelengths and angles

We present a simple method to enable flexible tuning of non-diffracting beams in a two-dimensional nonlinear photonic crystal, based on the interference of two or more non-collinear second-harmonic beams. By manipulating the wavelengths of the beams and the angle of incidence of the fundamental wave, the arbitrary period and propagation length, as well as the wavelength of the generated nonlinear non-diffracting array beams, can be tuned flexibly. These light beams can trap and manipulate multiple particles, create new forms of optical imaging systems, and act within nonlinear devices to bring novel functionalities to integrated optics.

Non-Diffractive Bessel Beams for Ultrafast Laser Scanning Platform and Proof-Of-Concept Side-Wall Polishing of Additively Manufactured Parts.

We report the potential use of non-diffractive Bessel beam for ultrafast laser processing in additive manufacturing environments, its integration into a fast scanning platform, and proof-of-concept side-wall polishing of stainless steel-based additively fabricated parts. We demonstrate two key advantages of the zeroth-order Bessel beam: the significantly long non-diffractive length for large tolerance of sample positioning and the unique self-reconstruction property for un-disrupted beam access, despite the obstruction of metallic powders in the additive manufacturing environment. The integration of Bessel beam scanning platform is constructed by finely adapting the Bessel beam into a Galvano scanner. The beam sustained its good profile within the scan field of 35 × 35 mm. As a proof of concept, the platform showcases its advanced capacity by largely reducing the side-wall surface roughness of an additively as-fabricated workpiece from Ra 10 m down to 1 m. Therefore, the demonstrated Bessel–Scanner configuration possesses great potential for integrating in a hybrid additive manufacturing apparatus.

3-D Printed Terahertz Lens to Generate Higher Order Bessel Beams Carrying OAM

Conventional vortex beams carrying orbital angular momentum (OAM) suffer from the limitation of beam divergence in wireless communications applications. This article proposes novel 3-dimensional (3-D) printed discrete dielectric lenses (DDLs) for generation of nondiffractive OAM beams operating at 300 GHz. By virtue of its arbitrary aperture phase control capability, a terahertz (THz) DDL combining the functionalities of three conventional bulky refractive hyperbolic, spiral phase plane, and aixcon lenses is proposed to generate nondiffractive Bessel vortex beam carrying OAM. An aperture field analysis method is also developed to evaluate the radiation performance of the DDL antennas. Furthermore, to cover the OAM signal in an intended longitudinal region, two DDL synthesis methods are explored to generate extended higher order Bessel beams carrying OAM. The first approach is based on geometric optics, while the second uses the alternating projection method (APM) to optimize the aperture phase distribution of the DDL. Two THz DDLs are conveniently fabricated by 3-D printing technology. Measured results demonstrate that THz nondiffractive OAM beams can be successfully generated by the designed DDLs. The generated THz OAM beam with attractive nondiffractive characteristic may open new opportunities for next-generation ultra-high-speed wireless communications.

Influence of pump beam shaping and noise on performance of a direct diode-pumped ultrafast Ti:sapphire laser.

One of the major factors that limits the widespread use of ultrafast titanium sapphire (Ti:S) lasers in life science is its expensive and complex pump source. Broad area diode laser (BA-DL) based pump sources have high potential to solve this problem, since they are compact, inexpensive and very efficient. However, their non-diffraction limited beam profile makes it challenging to achieve high powers in Kerr-lens mode-locked (KLM) operation of Ti:S lasers. In this work, we show that the ideal way to beam shape two spectrally combined BA-DLs with different beam qualities is to aim for a compromise between matching to the cavity mode diameter and the cavity mode Rayleigh range. We furthermore conclude that the relative intensity noise (RIN) of the BA-DL pumped Ti:S laser, another important parameter for imaging applications, is sufficiently low for a wide range of life science applications. However, for applications that are highly sensitive to noise, new laser diode designs are likely necessary to reduce inherent noise originating within the laser diode.

Extraordinary trapping by Gorkov potential of ordinary and vortex limited-diffraction beams

Trapping behaviors by using standing waves or focused beams have been investigated and widely used for acoustic levitation and tweezers. It is commonly understood that dense and stiff particles are repelled from pressure anti-nodes (pressure maximum) where the Gor'kov potential has a maximum. Here, the trapping behaviors of a small particle by using Gor'kov potential of ordinary and vortex non-diffracting beams are examined. Unlike the standing waves or strongly focused beams, the ordinary non-diffracting beam with proper parameters is found to have a Gor'kov potential minimum at its pressure maximum to trap a relatively dense and stiff particle therein [Fan and Zhang, Phys. Rev. Appl. 11(1), 014055 (2019)]. For a vortex beam to trap a dense and stiff particle at the pressure null, the paraxiality of the beam has to be larger than some values. These seemly to be extraordinary behaviors are interpreted by the contribution of axial velocity to the Gor'kov potential. The results are applicable to beams with weak diffraction or weak focusing. Following from the results, ways to simplify methods for particle manipulation and development of acoustic tweezers are suggested.

Subwavelength generation of nondiffracting structured light beams

Nondiffracting light beams have been attracting considerable attention for their various applications in both classical and quantum optics. Whereas substantial investigations on generation of the nondiffracting beams were made, their lateral dimension is much larger than optical wavelength. Here we present both theoretically and experimentally a study of the generation of nondiffracting light beams at deep-subwavelength scale. The highly localized light field is a result of in-phase interference of high-spatial-frequency waves generated by optical sharp-edge diffraction with a circular thin film. It is shown that the generated beam can maintain its spot size below the optical diffraction limit for a distance of up to considerable Rayleigh range. Moreover, the topological structure of both the phase and polarization of a light beam is found to be preserved when it passes through the diffractive configuration, which enables generating nondiffracting vortex beams as well as transversely polarized vector beams at deep-subwavelength scale. This work opens a new avenue to manipulate higher-order vector vortex beams at subwavelength scale and may find intriguing applications in subwavelength optics, e.g., in superresolution imaging and nanoparticle manipulation.

Multiscale electronic and thermomechanical dynamics in ultrafast nanoscale laser structuring of bulk fused silica

We describe the evolution of ultrafast-laser-excited bulk fused silica over the entire relaxation range in one-dimensional geometries fixed by non-diffractive beams. Irradiation drives local embedded modifications of the refractive index in the form of index increase in densified glass or in the form of nanoscale voids. A dual spectroscopic and imaging investigation procedure is proposed, coupling electronic excitation and thermodynamic relaxation. Specific sub-ps and ns plasma decay times are respectively correlated to these index-related electronic and thermomechanical transformations. For the void formation stages, based on time-resolved spectral imaging, we first observe a dense transient plasma phase that departs from the case of a rarefied gas, and we indicate achievable temperatures in the excited matter in the 4,000–5,500 K range, extending for tens of ns. High-resolution speckle-free microscopy is then used to image optical signatures associated to structural transformations until the evolution stops. Multiscale imaging indicates characteristic timescales for plasma decay, heat diffusion, and void cavitation, pointing out key mechanisms of material transformation on the nanoscale in a range of processing conditions. If glass densification is driven by sub-ps electronic decay, for nanoscale structuring we advocate the passage through a long-living dense ionized phase that decomposes on tens of ns, triggering cavitation.

Generation of nondiffraction beam with arbitrary focusing direction using metasurface

ABSTRACT A planar metasurface is designed to replace the traditional three-dimensional axicon lens to generate a nondiffraction in normal direction at first. Then, to generate Bessel beams that can be tilted in any direction, we add the phase distribution information derived from the generalized Snell’s reflection law to the traditional axicon phase distribution formula. Finally, a metasurface with a nondiffracting beam inclined to φ of 30° and θ of 30° is designed by employing the final modified phase information for demonstration. The simulated results indicate the effectiveness and robustness of the proposed theory and design.

Modulation of Orbital-Angular-Momentum Symmetry of Nondiffractive Acoustic Vortex Beams and Realization Using a Metasurface

Orbital angular momentum (OAM) carried by acoustic vortex attracts extensive research interests due to the possible application such as acoustic tweezer and communications. The OAM in the conventional symmetric acoustic vortex is generally determined by topological charge l. Here, we provide the beam symmetry as an alternative factor to manipulate the OAM beyond the topological charge. We propose a nondiffractive asymmetric acoustic vortex beam, which can be generated by an acoustic metasurface and modulate the OAM to arbitrary values. Rather than the radially symmetric distribution, the intensity of the asymmetric acoustic vortex is characterized with a crescent shape controlled by the asymmetry parameter $\ensuremath{\alpha}$. The ratio between axial OAM and sound energy on the transverse plane depends on the asymmetry parameter and thus is not quantized by the ratio of topological charge l to wave angular frequency $\ensuremath{\omega}$ as in conventional vortex beam. Moreover, we present the realization of the proposed vortex beam by designed metasurface, and reveal the nondiffractive nature of the asymmetric acoustic vortex beam. Our work expands the family of acoustic vortex and demonstrates an alternative route to control acoustic OAM.

Realization of Terahertz Wavefront Manipulation Using Transmission-Type Dielectric Metasurfaces

Metasurfaces, composed of an array of subwavelength artificial structures, have attracted great interest, owing to their high ability in locally manipulating the wavefront of electromagnetic waves. Here, we propose a dielectric metasurface based on a fused silica resonator, consisting of a rectangular-shaped bar placed in the center of a cross net-shaped structure, to manipulate the wavefront of terahertz waves. As proof of concept, several transmission-type devices for spatial modulation are designed at the target frequency of 0.14 THz, including on-axis and off-axis focusing, generation of a non-diffracting Bessel beam, and multi-focus lens. The simulated efficiencies range from 45% to 62%. This novel approach for manipulating THz wavefronts can be also used for information storage and other phase-related techniques in the rapid development of THz applications.

Multifunctional light beam control device by stimuli-responsive liquid crystal micro-grating structures

There is an increasing need to control light phase with tailored precision via simple means in both fundamental science and industry. One of the best candidates to achieve this goal are electro-optical materials. In this work, a novel technique to modulate the spatial phase profile of a propagating light beam by means of liquid crystals (LC), electro-optically addressed by indium-tin oxide (ITO) grating microstructures, is proposed and experimentally demonstrated. A planar LC cell is assembled between two perpendicularly placed ITO gratings based on microstructured electrodes. By properly selecting only four voltage sources, we modulate the LC-induced phase profile such that non-diffractive Bessel beams, laser stretching, beam steering, and 2D tunable diffraction gratings are generated. In such a way, the proposed LC-tunable component performs as an all-in-one device with unprecedented characteristics and multiple functionalities. The operation voltages are very low and the aperture is large. Moreover, the device operates with a very simple voltage control scheme and it is lightweight and compact. Apart from the demonstrated functionalities, the proposed technique could open further venues of research in optical phase spatial modulation formats based on electro-optical materials.

A 2-D Beam-Scanning Bessel Launcher for Terahertz Applications

This article proposes a novel nondiffractive Bessel beam launcher based on discrete dielectric lens (DDL) antenna with 2-D beam-scanning capability operating at 300 GHz. The terahertz (THz) Bessel beam launcher consists of two identical in-plane rotatable DDLs fed by a commercially available pyramid horn. First, the two thin and lightweight DDLs can transform the quasi-Gaussian beam with spherical phase front from the feed horn into a well-collimated and confined pseudo-Bessel beam. Second, we demonstrate that simultaneous in-plane rotation of the two DLLs in the same and opposite directions can steer the generated Bessel beam in the azimuth and elevation, respectively. Hexagonal dielectric post is employed as the element of the DDL, whose height is tuned from pixel to pixel to realize the required aperture phase distribution. 3-D printing technology was utilized to fabricate the two DDLs aiming at simplifying the manufacturing process and reducing the cost. The theoretically calculated, simulated, and measured results reveal that the designed THz DDL can generate 2-D scanning nondiffractive Bessel beam with a large field of view (FoV) of 86.2° and full azimuth coverage. The proposed 2-D scanning THz Bessel beam launcher can find widespread applications in THz fast noncontact detection, tracking, and imaging systems.