Nondiffracting Beams Research Articles

Nondiffractive beam shaping for enhanced optothermal control in metal additive manufacturing.

High thermal gradients and complex melt pool instabilities involved in powder bed fusion–based metal additive manufacturing using focused Gaussian-shaped beams often lead to high porosity, poor morphological quality, and degraded mechanical performance. We show here that Bessel beams offer unprecedented control over the spatiotemporal evolution of the melt pool in stainless steel (SS 316L) in comparison to Gaussian beams. Notably, the nondiffractive nature of Bessel beams enables greater tolerance for focal plane positioning during 3D printing. We also demonstrate that Bessel beams significantly reduce the propensity for keyhole formation across a broad scan parameter space. High-speed imaging of the melt pool evolution and solidification dynamics reveals a unique mechanism where Bessel beams stabilize the melt pool turbulence and increase the time for melt pool solidification, owing to reduced thermal gradients. Consequently, we observe a distinctively improved combination of high density, reduced surface roughness, and robust tensile properties in 3D-printed test structures.

Non-Diffracting Light Wave: Fundamentals and Biomedical Applications

The light propagation in the medium normally experiences diffraction, dispersion, and scattering. Studying the light propagation is a century-old problem as the photons may attenuate and wander. We start from the fundamental concepts of the non-diffracting beams, and examples of the non-diffracting beams include but are not limited to the Bessel beam, Airy beam, and Mathieu beam. Then, we discuss the biomedical applications of the non-diffracting beams, focusing on linear and nonlinear imaging, e.g., light-sheet fluorescence microscopy and two-photon fluorescence microscopy. The non-diffracting photons may provide scattering resilient imaging and fast speed in the volumetric two-photon fluorescence microscopy. The non-diffracting Bessel beam and the Airy beam have been successfully used in volumetric imaging applications with faster speed since a single 2D scan provides information in the whole volume that adopted 3D scan in traditional scanning microscopy. This is a significant advancement in imaging applications with sparse sample structures, especially in neuron imaging. Moreover, the fine axial resolution is enabled by the self-accelerating Airy beams combined with deep learning algorithms. These additional features to the existing microscopy directly realize a great advantage over the field, especially for recording the ultrafast neuronal activities, including the calcium voltage signal recording. Nonetheless, with the illumination of dual Bessel beams at non-identical orders, the transverse resolution can also be improved by the concept of image subtraction, which would provide clearer images in neuronal imaging.

Efficient and wavelength-dependent directional launching of a nondiffracting surface plasmon polariton beam device

The high-efficiency excitation and dynamic manipulation of the nondiffracting surface plasmon polariton (SPP) beam are important prerequisites for practical applications including the next-generation on-chip devices, near field optical trapping, and micromanipulation. Here we proposed two kinds of high-efficiency coupling and wavelength-dependent nondiffracting SPP beam unidirectional devices, which can generate and manipulate Bessel-like SPP beam or SPP Bottle beam, respectively. Different from the conventional groove or ridge structure that equally split SPP power to propagate from the boundary to both sides, the compact coupling element directs all of the SPP power of the matched wavelength to one side, resulting in higher collecting efficiency. Besides, as the wavelength of the incident light is changed, the generated Bessel-like SPP beam or SPP Bottle beam can be directionally excited on one side of the device. The design of the proposed devices provides a new means for constructing plasmonic devices with wavelength-dependent dynamic manipulation of nondiffracting SPP beams and has potential applications in on-chip interconnect circuits and near-field optical trapping.

Propagation-invariant space-time caustics of light.

Caustics are responsible for a wide range of natural phenomena, from rainbows and mirages to sparkling seas. Here, we present caustics in space-time wavepackets, a class of pulsed beams featuring strong coupling between spatial and temporal frequencies. Space-time wavepackets have attracted much attention with their propagation-invariant intensity profiles that travel at tunable superluminal and subluminal group velocities. These intensity profiles, however, have been largely restricted to an X-shape or similar pattern. We show that space-time caustics combine the propagation invariance of space-time wavepackets with the flexible design of caustics, allowing for customizable intensity patterns in space-time wavepackets. Our method directly provides the phase distribution needed to realize user-designed caustic patterns in space-time wavepackets. We show that space-time caustics can feature in a broad range of intriguing optical phenomena, including backward traveling caustics formed from purely forward propagating waves, and nondiffracting beams that evolve with time. Our findings should open the doors to an even wider range of structured light with spatiotemporal coupling.

Thermal and mechanical limitations to processing resolution in volume non-diffractive ultrafast laser structuring

The minimum feature size and spatial resolution are key factors for ultrafast laser structuring, defining the resulting function of the structured material. With the aim to improve the processing resolution achievable for non-diffractive beams, we analyze in volume the hydrodynamic and thermomechanical material responses during laser structuring defining the affected zone. The extent of laser induced cavitation on a 100 nm scale, accompanied by local annealing, and internal fracture under stress on µm scale are revealed using a combination ion beam milling, chemical etching and electron microscopy. Melting and mechanical stress acting at different planes are proved to be primary factors for restricting the structuring resolution to a critical distance where cooperative effects appear. Their extent is controllable via the local intensity and, hence, the pulse duration. A parametric control on the accumulated energy density limiting the thermomechanical action range and maximizing resolution is then reported.

Acoustic manipulation on microbubbles along arbitrary trajectories and adjustable destination

Non-diffracting Bessel acoustic beam that propagates in three dimensions has the properties of self-healing, self-bending, and self-acceleration, giving them many potential applications. Here, we report a zero-order Bessel-like acoustic beam that propagates along a curved trajectory with a phase-modulating acoustic holographic lens. Its pressure distribution on the transverse section approximately conforms to the zero-order Bessel function, and the self-healing property is found. In addition, the maximum point of acoustic intensity on the beam mainlobe can be adjusted by the change in the acoustic frequency. Microbubbles can be forced toward the maximum point along the predefined curve. Therefore, such a beam can be used for the directional transportation of microbubbles with a predefined curve and an adjustable destination. We have numerically calculated the primary Bjerknes force, including the oscillation of the microbubbles with the shell and experimentally proved the curved trajectory and the adjustable destination of the transportation of microbubbles. The proposed method uses the conservative part of the radiation force, which controls the trajectory of microbubbles by the amplitude gradient. The relatively free design of the transport trajectory and destination makes the approach suitable in vivo.

Ultradeep microaxicons in lithium niobate by focused Xe ion beam milling

Refractive axicons are conically shaped optical devices that are capable of generating nondiffracting Bessel-like beams over extended depths-of-focus (DOFs). In addition to the substantially longer DOF compared to those produced by parabolic focusing lenses, the axicons can generate beams with better resolution for the same form-factor of the optical element, e.g., its diameter and sag height. These properties make the axicons useful in numerous applications in imaging, particle trapping, and many others. Miniaturized refractive axicons or microaxicons are challenging to realize in hard substrates due to the lack of sufficiently precise and rapid fabrication technologies. Here, we report on the rapid fabrication of ultradeep microaxicons in lithium niobate using high-current focused Xe ion beam milling. Microaxicons with 230-μm diameter with ultradeep sag heights between 21 and 48 μm were milled using 200 nA of beam current. Furthermore, the microaxicons were milled in single-crystal lithium niobate—a material with a high refractive index of >2.2 but which inertness makes it a challenging material in microfabrication. The performance of the lenses was characterized by mapping the transmitted intensity at different positions. The measured spot sizes of the produced beams are in excellent agreement with the theoretical expectations and range from 750 down to 250 nm (∼λ/2) beam spot size for the shallowest and the deepest microaxicons in this study, respectively. The corresponding DOFs are from 500 down to ∼50 μm for the ultradeep microaxicon. The results verify the applicability of high-current milling with a focused Xe ion beam for the fabrication of high-performance optical elements.

High-efficiency unidirectional wavefront manipulation for broadband airborne sound with a planar device

In the past decade, one-way manipulation of sound has attracted rapidly growing attention with application potentials in a plethora of scenarios ranging from ultrasound imaging to noise control. Here we propose a design of a planar device capable of unidirectionally harnessing the transmitted wavefront for broadband airborne sound. Our mechanism is to use the broken spatial symmetry to give rise to different critical angles for plane waves incident along opposite directions. Along the positive direction, the incoming sound is allowed to pass with high efficiency and be arbitrarily molded into the desired shape while any reversed wave undergoes a total reflection. We analytically derive the working bandwidth and incident angle range, and present a practical implementation of our strategy. The performance of our proposed device is demonstrated both theoretically and numerically via distinct examples of production of broadband anomalous refraction, acoustic focusing and non-diffractive beams for forward transmitted wave while virtually blocking the reversed waves. Bearing advantages of simple design, planar profile, broad bandwidth and high efficiency, our design opens the possibility for novel one-way acoustic device and may have important impact on diverse applications in need of special control of airborne sound.

Tricomi beams and nondiffracting sheet beams

We introduce a non-diffracting beam family as a new beam solution of the scalar Helmholtz equation in Cartesian coordinates whose complex amplitude is mainly described by the Tricomi functions and characterized by four parameters. Based on the proposed beam model, the on-axial, off-axial, and asymmetrical Bessel beams can be obtained as special cases through appropriate selection of the complex parameters. We show that both the intensity and phase distributions can be tailored in a controlled manner through modulation of the angular spectra in the frequency space, enabling the transverse intensity pattern to be varied from a concentric ring structure, through a concentric arc configuration, to a single sheet pattern with different orientations and central positions.

Simplified non-diffracting beam propagation calculation for axial distribution using ray tracing and energy conservation

We propose a method to compute the axial distribution of non-diffracting beams. The approach stands on simple ray tracing and energy conservation principles. The proposed method is applied for four different Bessel beam generators, such as the refractive axicon, to compute the output axial intensity profile for a given input beam. We show great agreement between the results and the CODE V diffractional simulations. The method can also be used to calculate analytically the incident illumination pattern needed for a target output profile or to design an optical element surface to reach the same goal.

Comparative numerical analysis between the multipole expansion of optical force up to quadrupole terms and the generalized Lorenz–Mie theory

We explore graphical comparisons of the radiation forces calculated via the multipole expansion of optical force up to electric and magnetic quadrupole terms with the rigorous and exact generalized Lorenz–Mie theory (GLMT) on a homogeneous spherical particle. Examples are presented for a Gaussian beam in its fundamental mode and for a higher-order ideal Bessel beam. We also compute forces for a nondiffracting structured beam composed of a discrete superposition of copropagating Bessel beams with the same order and frequency, known in the literature as frozen waves. The results show a great agreement for moderately sized Mie particles, up to radius of about one-fifth the wavelength, thus suggesting an intrinsic connection between both methods and the possibility to extend recent theoretical works related to the identification between the Rayleigh limit of the GLMT and the dipole theory of forces.

Spatiotemporal Helicon Wavepackets

Propagation-invariant or non-diffracting optical beams have received considerable attention during the last two decades. However, the pulsed nature of light waves and the structured property of optical media like waveguides are often overlooked. We here present a four-dimensional spatiotemporal approach that extends and unifies both concepts of conical waves and helicon beams, mainly studied in bulk media. By taking advantage of tight correlations between the spatial modes, the topological charges, and the frequencies embedded in an optical field, we reveal propagation-invariant (dispersion- and diffraction-free) space-time wavepackets carrying orbital angular momentum (OAM) that evolve on spiraling trajectories in both time and space in bulk media or multimode fibers. Besides their intrinsic linear nature, we show that such wave structures can spontaneously emerge when a rather intense ultrashort pulse propagates nonlinearly in OAM modes. With emerging technologies of pulse/beam shaping, multimode fibers and modal multiplexing, our proposed scheme to create OAM-carrying helicon wavepackets could find a plethora of applications.

1 Bit Non-Diffractive Vortex Beam Generator Based on FSS in Microwave Region

In this article, a low-profile planar 1 bit non-diffractive vortex beam generator is proposed based on frequency selective surface (FSS) in microwave region. The characteristic of Bessel beam is introduced to control the divergence of the vortex beam. The proposed unit cell is analyzed through equivalent circuit model. The main resonance is provided by the coupling between the patches and the circular slot grid. A cross patch is embedded in the slot to form another resonance. Two elements with 0° and 180° transmission phase are designed to discretize the phase distribution of the metalens. The simulation results show that the metalens can generate fulfill non-diffracting transmission upon a distance of the $32\lambda _{\mathbf {0}}$ , verifying the effectiveness of the proposed method.

Glass tube cutting with aberration-corrected non-diffracting ultrashort laser pulses

The separation of complex inner and outer contours of glass articles with curved surfaces using ultrashort pulsed lasers is reported. Single-pass, full-thickness modifications along the entire substrate are achieved using a processing optics that allows for beam shaping of non-diffracting beams and, additionally, for aberration compensation of phase distortions occurring at the curved interface. The glass articles finally separated by thermal stress or via selective etching meet the demands of the medical industry in terms of micro-debris, surface quality and processing speed.

Non-diffractive acoustic beams produce negative radiation force in certain regions

A method of particle manipulation, one based on the force of acoustic radiation, has drawn wide attention. However, the real concept behind “acoustic tweezers”—negative acoustic radiation force (ARF)—has not been realized in experiments. In this paper, a prediction of a negative ARF generated by the non-diffractive acoustic beam is proposed. Its underlying physical mechanism is also analyzed in detail. On the basis of an analysis of energy flux density, the analytical region of negative radiation produced by the non-diffractive beam is solved completely. Forecast methods based on this solution are proposed that lay the foundation for realizing acoustic tweezers and offer the possibility of designing devices that produce negative ARFs. In addition, the negative propagation of acoustic beams in normal materials is realized, raising a possible alternative means to accomplish acoustic beam control.

Coherent interaction of atoms with a beam of light confined in a light cage

Controlling coherent interaction between optical fields and quantum systems in scalable, integrated platforms is essential for quantum technologies. Miniaturised, warm alkali-vapour cells integrated with on-chip photonic devices represent an attractive system, in particular for delay or storage of a single-photon quantum state. Hollow-core fibres or planar waveguides are widely used to confine light over long distances enhancing light-matter interaction in atomic-vapour cells. However, they suffer from inefficient filling times, enhanced dephasing for atoms near the surfaces, and limited light-matter overlap. We report here on the observation of modified electromagnetically induced transparency for a non-diffractive beam of light in an on-chip, laterally-accessible hollow-core light cage. Atomic layer deposition of an alumina nanofilm onto the light-cage structure was utilised to precisely tune the high-transmission spectral region of the light-cage mode to the operation wavelength of the atomic transition, while additionally protecting the polymer against the corrosive alkali vapour. The experiments show strong, coherent light-matter coupling over lengths substantially exceeding the Rayleigh range. Additionally, the stable non-degrading performance and extreme versatility of the light cage provide an excellent basis for a manifold of quantum-storage and quantum-nonlinear applications, highlighting it as a compelling candidate for all-on-chip, integrable, low-cost, vapour-based photon delay.

Deep Penetration Microscopic Imaging with Non-Diffracting Airy Beams.

We report a deep penetration microscopic imaging method with a non-diffracting Airy beam. The direct mapping of volume imaging in free space shows that the axial imaging range of the Airy beam is approximately 4 times that of the traditional Gaussian beam along the axial direction while maintaining a narrow lateral width. Benefiting from its non-diffracting property, the microscopic imaging with Airy beam illumination can acquire image structures through turbid medium and capture a volumetric image in a single frame. We demonstrate the penetration ability of the Airy microscopic imaging through a strongly scattering environment with 633 nm and 780 nm lasers. The performances of the volumetric imaging method were evaluated using HeLa cells and isolated mouse kidney tissue. The thick sample was scanned layer by layer in the Gaussian mode, however, in the Airy mode, the three-dimensional (3D) structure information was projected onto a two-dimensional (2D) image, which vastly increased the volume imaging speed. To show the characteristics of the Airy microscope, we performed dynamic volumetric imaging on the isolated mouse kidney tissue with two-photon.

Spiral antennas for the generation of bessel beams with tunable nondiffractive range

Backward cylindrical leaky waves radiating in the direction of their central axis represent an efficient solution for generating nondiffracting Bessel-like beams. The structures proposed so far to support such waves, constituted by concentric metal rings printed on a grounded dielectric slab, ensure the formation of nondiffractive field patterns with preset unchangeable configurations at a given frequency. Here, we propose the adoption of subwavelength spiral filaments with controlled patterning of the envelope figure by dynamic mechanical deformations, in order to achieve spatial tuning of the nondiffractive region in its size and shape. Precise guidelines and examples are provided to describe the proposed design technique and underline the inherent advantages of this alternative technological solution.

Controllable manipulation of composite multi-singularity vortex array

This study proposes a composite optical vortex array with multiple singularities and controllable radius, phase, sign, and intensity based on the coaxial interference of Bessel–Gaussian​ (BG) beams. Different from the general petal-like beam, we destroy the symmetry of the composite vortex by changing the order of the Bessel function, resulting in the occurrence of positive and negative alternating unit vortex chains in the interference region. The rule of the influence of the topological charges (TCs) and the order of Bessel function on the locations and number of singularities is systematically investigated. Additionally, due to the effect of Gouy phase shift, the coaxial interference characteristics of non-diffractive BG beams with spatial mismatch are examined numerically using a split-step Fourier transform algorithm. The multi-singularity technology demonstrated in this study facilitates the control of complex particles and large-capacity optical communication; furthermore, it is of substantial significance for optical tweezers and optical imaging applications.

Analytical method for the description of important obstructed optical beams and the Poisson-Arago spot.

In this work, we describe analytically the diffraction of some important beams due to a circular obstacle. In order to obtain the desired results, we deal with the wave equation in paraxial approximation together with the diffraction Fresnel integral and apply the analytical method proposed by Zamboni-Rached et al. [Appl. Opt.51, 3370-3379 (2012)APOPAI0003-693510.1364/AO.51.003370]. As a byproduct of our method, we notice the formation of the Poisson-Arago spot for ordinary beams (plane wave and Gaussian beam) and a reconstruction of the beam for nondiffracting beams (Bessel beam). Then, we pass to a vectorial analysis for better describing the electromagnetic beams.