1. Introduction

In 1987, BBs were first studied by Durnin^[1] and have been widely employed in applications related to both optics^[2][3] and acoustics^[4][5][6]. In acoustics, BBs are generally used in applications such as ultrasound imaging systems^[7][8][9]. Their extended DOF and slender beam-width provide a precise scanning of the transmitted beam, whereas their self-recovering properties contribute toward extraordinary robustness to tissue scattering. Moreover, its diffraction-free feature provides a perpetual deep imaging resolution. The diffraction-free self-healing features of the BB permits it to penetrate deep into the volume of a sample, resist against refractions in chaotic environments, and provides an axial resolution as compared to that of GBs. The fields, formed from coherent mixtures of BBs, reveal a more than ten-fold rise in their undistorted penetration to diffraction-limited beams. Recently, vortex beams, to be specific, BBs, have gathered a lot of interest because of their distinctive properties for particle trapping^{[10][11][12][13]}, particle handling/rotation applications^[14][15][16] or acoustic radiation force strategies in liquids^[17]. BBs have also been employed in photopolymerization^[18] as well as in material processing^[19]. However, their prospects for high-throughput 3D printing have not been effusively investigated.

2. Optical Trapping with BBs

The techniques of tweezing and optical trapping are established on the forces that emerge as a result of the rule of momentum preservation in the reflection, refraction, and absorption of the laser beam at the particle^[20][21]. For effective optical handling, i.e., consistent microparticles trapping, a high gradient of the optical power density is required. For instance, a sharply focused laser light beam is essential to trap the microparticle. This considerably shrinks the working area owing to diffraction, tighter focusing outcomes in faster radiation divergence, and the issue cannot be solved regarding the Gaussian optics. Nevertheless, Durnin^[1] has demonstrated in his remarkable work that the diffractive divergence can be practically removed with a special class of non-diffracting light fields known as BBs. Practically speaking, BBs display a limited propagation length, which is dependent on the width of the original collimated beam. As the BBs preserve their focus throughout the way, the placement of the manipulated object can be considerably altered, hence allowing practical flexible micromanipulation systems. Additionally, the employment of BBs unveils novel prospects in micro-fabrication^[22], manipulation of micro-machines^[23] and numerous “lab-on-a-chip” applications^[24].

Optical traps based on focused BBs offer highly superior capabilities for manipulation of individual glass beads in the three spatial directions in comparison with standard optical tweezers based on focused GB^[25]. Interesting possibilities for the simultaneous trapping and rotating of various types of dielectric particles (with high and low refractive indexes) provide multi-ringed vortex beams^[26]. The angular momentum density of BBs is similar to that of multi-ringed LG beams^[27][28]. However, high-order BBs are easier to generate experimentally^[29].

The field of optical trapping has grown into an essential phenomenon in the biological examination, cold atom physics, and genetics^[30][31]. It has swiftly bee established from its first verification by Ashkin in the 1970s^[20]. Regardless of its extensive implementation, the optical trapping of airborne particles employing a single-beam gradient-force was not verified until 1997^[32]. Even nowadays, most of the laser tweezer operations are being used to trap fairly translucent specks in a liquid. A particle trapping via a single laser beam in the air is a more challenging task than in a liquid medium, as the optical trap should be robust to conquer gravity and disruption in the air. Moreover, as opposed to a particle in a liquid medium, a particle in the air has a higher relative refractive index, which induces a sturdy scattering force that tends to disrupt the trap^[33]. Nevertheless, for particle delivery strategies, the potentiality to trap airborne particles is essential^[34] as well as developing aerosol classification methods that incorporate optical trapping with examination practices, for instance, Raman spectroscopy^[33][35].

Consequently, significant attention has been given to develop novel methods to capture airborne particles utilizing either the photophoretic force or the radiative pressure force. In recent times, the photophoretic force has been used to permit optical trapping of airborne particles with absorbing properties. Several beam shapes such as vortex beams^[36], bottle beams^[37], and hollow cones produced via a single beam^[38], tapered rings^[39], two counter-propagating beams^[40] and optical lattices ^[41], among others, have been employed. In^{[36][37][38][39][40]}, the particles are trapped in the low-intensity area, which is contrasted to laser tweezers where the gradient force traps the particles near a laser beam’s high-intensity focal point. As several photophoretic traps are only successful for particles with a specific geometry^[42], the morphology of the absorbing particles poses additional technical hitches. Nevertheless, some applications include the ability to capture the particles irrespective of their construction and absorption.

An interesting method is proposed in^[43], where a single formed laser beam is used to form a region of low light intensity for photophoretic trapping of absorbing particles but minimizing the dispersion force near the focal point at the same time. As a result, radiative pressure found that the trapping of translucent particles is facilitated as shown in Figure 1a–d. Besides, this low dispersion force permits airborne particles to be trapped by radiative pressure-based trapping via relatively low numerical aperture optics. A comparable method has lately been demonstrated to allow optical trapping of high relative refractive index non-absorbing particles utilized to trap airborne aerosol droplets^[44].

Figure 1. (a) Illustration of the optical trapping setup. The diameter of an expanded laser beam is managed with a tunable iris before travelling through the two axicons to form a collimated hollow beam. The aspheric lens creates a hollow conical focus within a glass chamber where airborne particles are trapped, adapted with permission from^[43]. (b) Calculated intensity profile near the focal spot, adapted with permission from^[43]. (c) Image of the conical focal region produced inside the chamber, adapted with permission from^[43]. (d) Image of a spore captured in the air close to the focal point, adapted with permission from^[43].

Due to the transfer of momentum from photons to particles, the radiative pressure force, the gradient force and the scattering force produced from a single laser beam emerges. Regardless of the realization of optical tweezers in maneuvering particles in the fluid, optical traps based on radiative pressure utilizing a single laser beam are still not suitable for grabbing airborne particles. Bearing in mind the radiative pressure force as an amalgamation of a gradient force and a scattering force, this problem can be solved. The gradient force drags a particle close to the focus of the laser beam into the high-intensity region at the focus, ensuring the restoring force is mandatory to catch a particle. Instead, the scattering force moves the particle in the direction of the propagation of light and off-limits the reinstating force needed. In general, optical trapping is only probable when the gradient force overpowers the dispersion force^[32]. Airborne particle trapping is a challenging task due to the high refractive index of a particle in the air comparative to the ambient medium results in a sturdy scattering force. To form a robust gradient force to trap airborne particles, high numerical aperture optics (typically NA > 0.9) are obligatory^[32]. In principle, by utilizing a counter-propagating beam configuration, the scattering force can be concealed^[45]. However, an accurate orientation is required, which is practically not possible in many applications. If a laser light falls on absorbing airborne particles, a portion of the light is absorbed and transformed into heat. Consequently, an interplay between a non-uniformly heat-emitting particle and the ambient gas molecules leads to the photophoretic force. As a result, absorbing airborne particles are captured by the photophoretic force. For instance, if an absorbing airborne particle is hit by a light from one side, due to impact with the hot part of the particle, the gas molecules on the elevated temperature side of the particle will have greater velocities, imposing a net force that moves the particle to its cold side. The photophoretic force can be ~5 orders of magnitude more robust than the gradient force for an intensely absorbing particle, usually employed in optical tweezers^[46].

3. Material Processing via Ultrafast BBs

These days, micro- and nano-technologies rely on the growth of detailed and manageable manufacturing tools that are capable of structuring materials with full precision and minimum collateral loss. In a range of applications, from entertainment to medicine, lasers have proven to be flexible instruments over the years. The individual usage of this system is dependent on different parameters of the radiation emitted, such as wavelength, energy, and pulse duration. When considering some applications, the beam structure is essential^[47][48][49]. A typical GB is appropriate for some of them. The ultra-short laser processing technique has grown into a vital technology capable of delivering fundamental processing skills well into the size of the nanoscale^[50][51][52]. This counts on incomparable material processing capabilities through nonlinear excitation and inadequate thermal diffusion, resulting in high-end applications where energy localization in space and time by ultrashort pulses is important. Novel models of laser material processing are developed based on the spatiotemporal design of irradiation rendering to the material response to optimize the structuring process concerning quality and scale^[53][54][55]. To synergistically associate irradiation and material reaction to energy load, an advanced processing strategy involves a detailed understanding of the irradiation and material transformation method.

For material processing applications in ultrafast modes, a new class of ultrafast laser beams has recently emerged with the potential to achieve processing precision beyond the diffraction limit, deep into the nanoscale domain. This relies on non-diffractive concepts, especially the Bessel-Gauss beam^[56], where non-diffractive propagation can be used to design the multi-dimensional interaction segment. In transparent materials, the ability of these Bessel-Gauss beams is entirely utilizable, with an energy gap larger than photon energy. BBs are helpful in Bragg grating inscription^[57], microchannel forming^[58] and photopolymerization^[59] due to an elongated focal area. BBs have a ring-shaped spatial spectrum. It is conceivable to attain Mathieu beams with elliptical intensity distribution by modifying the phase and amplitude of the BB. For instance, the elliptical beam structure has been shown to cause directional glass cracking. Zeroth and higher-order BBs are among the most distinguished non-diffracting beams and can be comprehended utilizing diffractive holograms or by focusing Laguerre GBs via conical prisms (axicons).

In^[60], a novel method is developed for the realization of superimposed complex BB vortices of different orders, single and superimposed Mathieu beams of different topologies, as well as parabolic non-diffracting beams. Furthermore, their amplitudes and phases are controlled in the focal region independently to assist the creation of complex patterns not only in the transverse plane but also along the focal line. The experimental verification using a conical prism together with a geometrical phase element is presented for the transparent materials. In^[61] the higher-order vector BBs are produced for transparent material processing applications.

Three-dimensional integrated circuits are an enticing option to replace standard two-dimensional ones as high-efficiency, low power exhaustion and miniaturized foot-print microelectronic devices^[62]. One of the primary enduring problems, however, is the processing of high-aspect-ratio through-silicon vias (TSVs), which is a critical technology for the assembly of three-dimensional silicon integrated circuits. As a simple and environmentally friendly substitute with less manufacturing stages due to the exclusion of photolithography, TSV manufacturing through direct laser drilling has been suggested. This has been demonstrated with the production of 20 μm diameter holes in a 250 μm thick silicon substrate employing nanosecond UV laser percussion^[63]. However, for the assembly of three-dimensional silicon integrated circuits, the deep drilling of holes smaller than 10 μm in diameter remains a major future challenge. Ultrafast laser treatment has recently been shown to be a desirable method for material processing, as it facilitates sub-diffraction-limit processing with heat-affected zone elimination^[64]. High-quality cutting has been successfully checked for planar silicon elements with 780 nm femtosecond lasers^[65]. Furthermore, in many important research and engineering applications, fast speed and high aspect ratio drilling of through-holes in different materials utilizing ultrafast laser processing have become an attention-grabbing theme. In different silica glasses, femtosecond BBs have been employed to create microstructures with aspect ratios of up to 10^2–10³, without requiring sample transformation due to the elongated field depth.

In^[66], TSVs are fabricated by using femtosecond BBs with wavelength tuning of 400 nm to 2400 nm. The manufacture of fine TSVs utilizing a 1.5 μm femtosecond BB is demonstrated in^[67]. A Bessel femtosecond beam is customized via a specifically fabricated binary phase plate to remove the extreme ablation brought by the sidelobes of a traditional BB. Three separate forms of femtosecond laser beams such as a Gaussian beam (GB), a conventional BB (CBB) and a tailored BB (TBB) were used to determine the theory of the suggested method as shown in Figure 2a–c. It is established that the customized femtosecond BB (without sidelobe destruction) can be employed to form a two-dimensional periodic arrangement of ~10 μm TSVs on a silicon substrate of thickness 100 μm, indicating a possible use in the three-dimensional assembly of three-dimensional silicon integrated circuits. Two-dimensional profiles of CBB and TBB in the x-y plane together with the on-axis intensity distributions (orange curve) are experimentally measured as displayed in Figure 2d,e, respectively. The experimentally obtained transverse intensity distributions of the CBB and TBB at z = z_max are presented in Figure 2f,g, respectively, together with a charged coupled device taken pictures of the particular beams in the insets. Figure 2h–j illustrate the SEM pictures of the TSVs formed in 50 μm thick samples employing the GB, CBB and TBB, respectively.

Figure 2. Diagram of the beams generation setup and intensity profiles for, (a) GB, (b) CBB and (c) TBB. Experimentally measured two-dimensional profiles in the x-y plane in cooperation with the on-axis intensity distribution (Orange curve) of, (d) CBB and (e) TBB. Measured transverse intensity distributions at z = z_max along the CCD captured images of (f) CBB and (g) TBB. SEM images of the cross-sectional view of TSVs manufactured silicon substrate using (h) GB, (i) CBB and (j) TBB. Adapted with permission from^[67].

4. Free-Space long-Distance Self-Healing BBs

In addition to the non-diffractive feature, BBs are self-healing which means that they have a self-reconstruction ability after a hurdle comes across their transmission path^[68]. The intensity arrangement at z and z + δ_z is formed by different sections of the crossing planar beams as shown in Figure 3. This property comes from the selective constructive interference occurring between multiple coherent plane waves propagating at an equal angle concerning the optical axis. The redevelopment distance of the beam relies on the size of the hurdle and the angle defining the conical superposition of the planar beams. Consequently, BBs are usually more prone to dispersion than most other traditional Gaussian beams. Due to the unusual non-diffractive and self-recovery properties, BBs have gathered particular attention in biomedical physics, laser processing and metrology^[69].

Figure 3. Self-healing features of an axicon-produced BB: an obstacle located in the center of the Bessel region blocks the beam for the smallest distance Z_min, the Bessel field reconstructs after travelling for some distance. The beam profiles before and after the obstacle are also shown. Inspired by^[70].

A novel method for creating long-distance self-recovering BBs generated by an annular lens and a 4f-configuration spherical lens is stated in^[71]. This shows the diffraction-free light evolution of a zeroth-order BBs over several meters and addresses the available scaling prospects that exceed present technologies. Additionally, it has been shown that this setup can be modified to generate BB superpositions, realizing the longest optical conveyor beam and helicon beam. Last but not least, the self-healing properties of the beams are verified concerning robust opaque and transparent scatterers, which underscores the pronounced perspective of this novel approach^[71].

Underwater wireless communication is supposed to have high data rates to relay big data over several meters via a proper wavelength. It has already gained more and more recognition with a massive rise in underwater applications, for instance, unmanned underwater vehicles, submarines and sensors in the oceanic environment^[72][73][74], among others. The optical turbulence is primarily caused by variations in temperature and salinity in underwater environments, such as in oceans. The standard of communication of the orbital angular momentum (OAM)-based underwater wireless communication system has been seriously impaired by this turbulence^[75]. A hypothetical analysis of the effect of temperature and salinity variations on the typical strength of the Gaussian Schell-model vortex beams in the turbulent ocean has shown that partially coherent beams have a more effective resistance to turbulence than full coherent beams^[76]. The transmission of partially coherent Laguerre-GBs in the tempestuous ocean is studied in^[77][78]. Furthermore, the influence of the ocean instabilities on the oceanic OAM-based underwater wireless communication system’s channel capacity has also been observed in^[79]. In^[76] [126] the statistical characteristics of the vortex beams such as the power, polarization, and coherence travelling in oceanic turbulence are theoretically studied.

Vortex beams bearing OAM with helical phase fronts have been commonly used as beam sources to mitigate the turbulence effect on any optical vortex^[80]. A Bessel-Gaussian (BG) beam is an imperative part of the pseudo-non diffraction vortex beams family. In the case of blockades, BG beams can heal themselves, which is an important feature for optical communications based on the line-of-sight operations^[81]. Consequently, for the OAM-based underwater communication systems^[82][83], the non-diffraction and self-recovery properties of BG beams make them a valuable commodity. The transmission characteristics of BG beams in the free space environment was explored^[84] and the non-obstruction characteristics of BG beams in a free-space optical communication system were studied in^[85].

An experimental study of the underwater transmission and self-recovering properties of BG beams has recently been provided in^[81]. A GB with a wavelength of 532 nm is transmitted from a laser diode and then a neutral density filter (NDF) is employed to reduce the intensity of the GB. The GB matches its polarization to the optimized working polarization of the selected polarization-sensitive spatial light modulator (SLM) after passing through the polarizer (Pol.) and a half-wave plate (HWP). When the polarized GB is illuminated on an SLM, the desired BG mode is generated, where the special phase hologram grating is revealed on the liquid crystal screen of the SLM. The BG beam carrying the OAM mode is then transferred through a water tank simulating the underwater environment. A rectangular water tank of one meter in length was used to replicate the underwater atmosphere. The temperature variations were regulated by a heater within the container, and salinity variations were accompanied by adding different salt bulks in the water. The experimental setup to analyze the BG beam properties underwater is shown in Figure 4a, whereas the recorded beam profiles at the receiver are shown in Figure 4b. For more detailed information, discuss^[81].

Figure 4. (a) The investigational arrangement for examining BG beam properties in the underwater situation. (b) The influence of underwater optical turbulence on the transmission of BG beam versus free space atmosphere. Adapted with permission from^[81].

5. BB for Optical Coherence Tomography

Optical coherence tomography (OCT) is an imaging process that offers in situ, non-invasive, high resolution, cross-sectional pictures of the biological tissues. Given the depth of penetration of a light beam into biological tissue, endoscopic probes are needed to examine the conditions of wall linings inside the organs such as the liver, stomach, lung, colon and wider arteries within the body. Various types of endoscopic OCT systems have been developed after the first endoscopic applications of OCT in 1996^[86]. Traditional GBs are currently used in endoscopic probes. Some are relocated to the targeted site by moving a wire backwards and forward in and out of the body to steer the catheter. These probe types work at the low numerical aperture and have a lateral resolution of ~20 μm. Some probes use higher numerical aperture optics; nevertheless, they are limited to the smaller DOF. A fine focus modification is needed for these probes which cannot be accomplished by wire adjustment. A gradient index (GRIN) lens rod-based probe was then demonstrated to focus on targeted points in a sample by shifting a stage out of the body^[87]. Nevertheless, some endoscopic applications are constrained by the nonflexible rod. The growth of elongated DOF imaging arrangements is therefore an active research area of OCT^[88]. Several methods have been applied, such as the Swept-source, time and spectral-domain OCT. Of these methods, the spectral-domain is favored over others, since it is simple to implement and does not require a complex structure. Initially, the OCT system was arranged only in free optics, but the advent of optical fiber improves the versatility of the system and paves the way for a new area of operation^[89]. In OCT, the sample beam interferes with the reference beam to produce interference spectra that are further analyzed by the dedicated sample imaging tools. OCT may be achieved by either dual-path or a common-path interferometry techniques. The later methodology can solve the problems of polarization mismatch, group velocity dispersion and ambience vibration. The common path configuration eliminates device and computational complexity since there is no need for extra modules and algorithms to compensate for dispersion.

Imaging based on BB, produced by axicon optics, differs from GB imaging in that it allows the focusing range to be extended without losing resolution^[90]. However, the trade-off is a reduction in the illumination and collection efficiency. OCT employing BB has become a trend to benefit from a long DOF and tighter spot size. Bessel beams for illumination/detection were used to break the limit of focus range and transversal resolution given by the GB profile^[91][92][93]. The use of axicon optics in time-domain OCT was originally considered by Ding et al.^[94].  Lately, a non-dual path imaging scheme for imaging of biological samples has been suggested employing Fourier-domain OCT (FD-OCT)^[95]. A custom-made micro-optic axicon is used to produce a BB to lighten and image in biological samples using spectral-domain OCT (SD-OCT)^[96]. Concerning the GB, BB concurrently produces a high DOF and a tight focus point. It is not possible to produce perfect BB, but the GB can be converted into a BB structure known as a Bessel-Gauss beam and is often referred to as a BB. Several methods are discussed to transform the standard GB to BB. Traditionally, the axicon lens may be the best alternative for the transformation of the beam. A lot of impressive work has been performed on the generation of BB via the axicon lens^[97][98]. However, BB axicon lens construction uses free optics, which are not appropriate for applications where there is a need for a small central spot size.

To eliminate group velocity dispersion and polarization incompatibility between the reference and the sample arm, common-path optical coherence tomography (CP-OCT) is used so that both arms share the same physical path. Current CP-OCT implementations usually involve one to incorporate an additional cover glass into the beam path of the sample arm to provide a reference signal. This step is further reduced by making direct use of the back-reflected signal, provided by the conical lens-tip fiber, as a reference signal as shown in Figure 5a^[99]. The conical lens, which is directly manufactured by a simple selective-chemical etching process on the optical fiber tip (inset of Figure 8a), performs two functions: (1) it can be used as an imaging lens; (2) as the self-aligning reference plane.

Researchers have been involved in producing such axicon structures within the optical fiber such that they can be used for imaging and sensing applications. Considerable work is being conducted on BB construction using optical fibers and deep-sealed negative axicons (DSNAs) ^[100][101]. The DSNA is manufactured by standard chemical etching in hydrofluoric acid (HF) at the tip of the single-mode highly doped photosensitive optical fiber and produces BB^[102]. A tomography methodology for fluorescence microscopy scanning is introduced in^[103], which enables the volume image to be captured in a single frame scan. Volumes are photographed by concurrently recording four independent projections at various angles using temporally multiplexed, tilted BBs.

An FD-OCT method is used to exhibit the effectiveness of this method upon biological tissue. An in-fiber CP-OCT technique can prove to be potentially useful in endoscopic OCT imaging. The usefulness of employing axicon micro-optics has been solely established by imaging a biological sample, precisely an African frog tadpole, at different places attained with an incident power of 25 mW and an irradiation time of 50 μs as shown in Figure5b The detailed study on SD-OCT can be found here^[96].

Figure 5. (a) Illustration of a light field travelling from the fiber to the sample and back. The inset displays the SEM image of the fiber probe where the conical microlens can be seen. Adapted with permission from^[99], (b) 1 mm × 1 mm SD-OCT images of an African from tadpole located at various d values acquired using a 600 μm effective axicon micro-optic lens (top row) and a 0.037 NA conventional lens (bottom row). Adapted with permission from^[96].