Showing papers on "Bessel beam published in 2019"

Efficient manipulations of circularly polarized terahertz waves with transmissive metasurfaces.

Abstract: The unrestricted control of circularly polarized (CP) terahertz (THz) waves is important in science and applications, but conventional THz devices suffer from issues of bulky size and low efficiency. Although Pancharatnam–Berry (PB) metasurfaces have shown strong capabilities to control CP waves, transmission-mode PB devices realized in the THz regime are less efficient, limiting their applications in practice. Here, based on Jones matrix analysis, we design a tri-layer structure (thickness of ~λ/5) and experimentally demonstrate that the structure can serve as a highly efficient transmissive meta-atom (relative efficiency of ~90%) to build PB metadevices for manipulating CP THz waves. Two ultrathin THz metadevices are fabricated and experimentally characterized with a z-scan THz imaging system. The first device can realize a photonic spin Hall effect with an experimentally demonstrated relative efficiency of ~90%, whereas the second device can generate a high-quality background-free CP Bessel beam with measured longitudinal and transverse field patterns that exhibit the nondiffracting characteristics of a Bessel beam. All the experimental results are in excellent agreement with full-wave simulations. Our results pave the way to freely manipulate CP THz beams, laying a solid basis for future applications such as biomolecular control and THz signal transportation. Ultrathin metasurfaces that can efficiently manipulate circularly polarized terahertz waves in transmission rather reflection have been demonstrated by scientists in China. The devices were fabricated by Min Jia and colleagues from Fudan University and Capital Normal University. The tri-layer structures operate at frequencies of around 0.6 THz and suit future on-chip applications for terahertz photonics. Various designs were tested resulting in a circularly polarized terahertz Bessel beam generator and a device exhibiting the photonic spin Hall effect. The metasurfaces consist of a periodic array of meta-atoms, each composed of three thin layers of metal in a U-shape that are separated and surrounded by polyimide. Changing the angular orientation of the meta-atoms allows different phase gradients to be programmed into the metasurfaces bringing the desired functionality.

Generating Bessel beams with broad depth-of-field by using phase-only acoustic holograms.

Abstract: 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.

Photonic tractor beams: a review

Abstract: Usually, an unfocused light beam, such as a paraxial Gaussian beam, can exert a force on an object along the direction of light propagation, which is known as light pressure. Recently, however, it was found that an unfocused light beam can also exert an optical pulling force (OPF) on an object toward the source direction; the beam is accordingly named an optical tractor beam. In recent years, this intriguing force has attracted much attention and a huge amount of progress has been made both in theory and experiment. We briefly review recent progress achieved on this topic. We classify the mechanisms to achieve an OPF into four different kinds according to the dominant factors. The first one is tailoring the incident beam. The second one is engineering the object’s optical parameters. The third one is designing the structured material background, in which the light–matter interaction occurs, and the fourth one is utilizing the indirect photophoretic force, which is related to the thermal effect of light absorption. For all the methods, we analyze the basic principles and review the recent achievements. Finally, we also give a brief conclusion and an outlook on the future development of this field.

High-efficiency Bessel beam array generation by Huygens metasurfaces

Abstract: Abstract Bessel beams have attracted considerable interest because of their unique non-diffractive, self-healing characteristics. Different approaches have been proposed to generate Bessel beams, such as using axicons, diffractive optical elements, composite holograms, or spatial light modulators. However, these approaches have suffered from limited numerical aperture, low efficiency, polarization-dependent properties, etc. Here, by utilizing dielectric Huygens metasurfaces as ultrathin, compact platforms by integrating the functionalities of Dammann gratings and axicons, we successfully demonstrate multiple Bessel beam generation with polarization-independent property. The number of two-dimensional arrays can be controlled flexibly, which can enhance information capacity with a total efficiency that can reach 66.36%. This method can have various applications, such as parallel laser fabrication, efficient optical tweezers, and optical communication.

Generation of a Nondiffracting Superchiral Optical Needle for Circular Dichroism Imaging of Sparse Subdiffraction Objects.

Abstract: Chirality describes not only the structural property of three-dimensional objects, but also an intrinsic feature of electromagnetic fields. Here we report a strategy to realize a Bessel beam superchiral ``needle'' by focusing a twisted radially polarized beam on a planar dielectric interface. By tailoring the light spatial distribution in the pupil plane of a high numerical aperture lens, the chirality of the local field at the focus can be enhanced by 11.9-fold than that of a circular polarized beam. Through a combined interaction of chiral and achiral transitions, the dimension of the region with enhanced chiral sensitivity can be shrunk down to $\ensuremath{\lambda}/25$. This theoretical work paves the way towards a completely new label-free imaging technique using the enhanced circular dichroism for sparse subdiffraction chiral objects (e.g., individual molecules).

Generation of a double-ring perfect optical vortex by the Fourier transform of azimuthally polarized Bessel beams.

Abstract: The perfect optical vortex (POV), the ring size being independent of its topological charge, has found potential applications in optical tweezers and optical communications. In this Letter, we report a new kind of POV, termed as double-ring POV (DR-POV), whose diameters of the two rings are independent of topological charge. We theoretically demonstrate that such a vortex is the Fourier transform of an azimuthally polarized Bessel beam. Experimental results agree well with theoretical prediction. We further investigate the vortex nature of the DR-POV through an interferometric method, showing that the two rings of the vortex have the same topological charge value (magnitude and sign). The specular properties of the DR-POV may find application in optical tweezers, such as trapping and rotating of low-refractive-index particles in the dark region between the two rings.

Extremely high-aspect-ratio ultrafast Bessel beam generation and stealth dicing of multi-millimeter thick glass

Abstract: We report on the development of an ultrafast beam shaper capable of generating Bessel beams of high cone angle that maintain a high-intensity hot spot with subwavelength diameter over a propagation distance in excess of 8~mm. This generates a high-intensity focal region with extremely high aspect ratio exceeding 10~000:1. The absence of intermediate focusing in the shaper allows for shaping very high energies, up to Joule levels. We demonstrate proof of principle application of the Bessel beam shaper for stealth dicing of thick glass, up to 1~cm. We expect this high energy Bessel beam shaper will have applications in several areas of high intensity laser physics.

Extremely high-aspect-ratio ultrafast Bessel beam generation and stealth dicing of multi-millimeter thick glass

Abstract: We report on the development of an ultrafast beam shaper capable of generating Bessel beams of high cone angle that maintain a high intensity hot spot with subwavelength diameter over a propagation distance in excess of 8 mm. This generates a high intensity focal region with extremely high aspect ratio exceeding 10 000:1. The absence of intermediate focusing in the shaper allows for shaping very high energies, up to Joule levels. We demonstrate a proof of principle application of the Bessel beam shaper for stealth dicing of thick glass, up to 1 cm. We expect that this high energy Bessel beam shaper will have applications in several areas of high intensity laser physics.

Optical pulling at macroscopic distances

Abstract: Optical tractor beams, proposed in 2011 and experimentally demonstrated soon after, offer the ability to pull particles against light propagation. It has attracted much research and public interest. Yet, its limited microscopic-scale range severely restricts its applicability. The dilemma is that a long-range Bessel beam, the most accessible beam for optical traction, has a small half-cone angle, θ0, making pulling difficult. Here, by simultaneously using several novel and compatible mechanisms, including transverse isotropy, Snell's law, antireflection coatings (or impedance-matched metamaterials), and light interference, we overcome this dilemma and achieve long-range optical pulling at θ0 ≈ 1°. The range is estimated to be 14 cm when using ~1 W of laser power. Thus, macroscopic optical pulling can be realized in a medium or in a vacuum, with good tolerance of the half-cone angle and the frequency of the light.

Glass dicing with elliptical Bessel beam

Abstract: In this paper the possibility to optimize the glass dicing process by controlling the axicon-generated Bessel beam ellipticity is presented. Single-shot intra-volume modifications in soda-lime glass followed by dicing experiments of 1 mm-thick samples are performed. The Bessel beam ellipticity is essential for glass dicing process. Such beam generates intra-volume modifications with transverse crack propagation in dominant direction. Orientation of these modifications parallel to the dicing direction gives significant advantages in terms of processing speed, glass breaking force and cutting quality.

Electron Bessel beam diffraction for precise and accurate nanoscale strain mapping

Abstract: Strain has a strong effect on the properties of materials and the performance of electronic devices. Their ever shrinking size translates into a constant demand for accurate and precise measurement methods with a very high spatial resolution. In this regard, transmission electron microscopes are key instruments thanks to their ability to map strain with a subnanometer resolution. Here, we present a method to measure strain at the nanometer scale based on the diffraction of electron Bessel beams. We demonstrate that our method offers a strain sensitivity better than 2.5 × 10−4 and an accuracy of 1.5 × 10−3, competing with, or outperforming, the best existing methods with a simple and easy to use experimental setup.

Axial resolution enhancement of light-sheet microscopy by double scanning of Bessel beam and its complementary beam.

Abstract: The side lobes of Bessel beam will create significant out-of-focus background when scanned in light-sheet fluorescence microscopy (LSFM), limiting the axial resolution of the imaging system. Here, we propose to overcome this issue by scanning the sample twice with zeroth-order Bessel beam and another type of propagation-invariant beam, complementary to the zeroth-order Bessel beam, which greatly reduces the out-of-focus background created in the first scan. The axial resolution can be improved from 1.68 μm of the Bessel light-sheet to 1.07 μm by subtraction of the two scanned images across a whole field-of-view of up to 300 μm × 200 μm × 200 μm. The optimization procedure to create the complementary beam is described in detail and it is experimentally generated with a spatial light modulator. The imaging performance is validated experimentally with fluorescent beads as well as eGFP-labeled mouse brain neurons.

Reflection-mode switchable subwavelength Bessel-beam and Gaussian-beam photoacoustic microscopy in vivo.

Abstract: We have developed a reflection-mode switchable subwavelength Bessel-beam (BB) and Gaussian-beam (GB) photoacoustic microscopy (PAM) system. To achieve both reflection-mode and high resolution, we tightly attached a very small ultrasound transducer to an optical objective lens with numerical aperture of 1.0 and working distance of 2.5 mm. We used axicon and an achromatic doublet in our system to obtain the extended depth of field (DOF) of the BB. To compare the DOF performance achieved with our BB-PAM system against GB-PAM system, we designed our system so that the GB can be easily generated by simply removing the lenses. Using a 532 nm pulse laser, we achieved the lateral resolutions of 300 and 270 nm for BB-PAM and GB-PAM, respectively. The measured DOF of BB-PAM was approximately 229 μm, which was about 7× better than that of GB-PAM. We imaged the vasculature of a mouse ear using BB-PAM and GB-PAM and confirmed that the DOF of BB-PAM is much better than the DOF of GB-PAM. Thus, we believe that the high resolution achieved at the extended DOF by our system is very practical for wide range of biomedical research including red blood cell (RBC) migration in blood vessels at various depths and observation of cell migration or cell culture.

Reversals of Acoustic Radiation Torque in Bessel Beams Using Theoretical and Numerical Implementations in Three Dimensions

Abstract: Acoustic radiation torque (ART) plays an important role in the subject of acoustophoresis, which could induce the spinning rotation of particles on their own axes. The partial wave series method (PWSM) and T matrix method (TMM) are employed to investigate the three-dimensional radiation torques of both spherical and nonspherical objects in a single Bessel beam over broader frequencies (from Rayleigh scattering to geometric-optics regimes), with emphasis on the parametric conditions for torque reversal and the corresponding physical mechanisms. For elastic objects, the dipole, quadrupole, and even several higherscattering modes are dominant to induce the axial ARTs beyond the Rayleigh limit with a relatively large offset from the particle centroid to the beam axis in Bessel beams. The reversals appear with parametric conditions (including wave number, cone angle, and offset) for the extrema or null values of the corresponding cylindrical Bessel functions. The reversal of transverse ART from a rigid nonspherical particle is also investigated in an ordinary Bessel beam and the geometrical surface roughness is briefly studied for the effect on the radiation torques. This suggests the possibility of acoustic tweezers controlling the spinning motion of particles within and beyond the Rayleigh limit.

Common-Path Optical Coherence Tomography Using the Bessel Beam From Negative Axicon Optical Fiber Tip

Abstract: In this work, a deep-seated negative axicon fiber tip (DSNA-FT) generating the Bessel–Gauss beam is proposed as an optical probe for a common-path optical coherence tomography (CPOCT) imaging. The axicon at the optical fiber tip is fabricated by etching the tip of a selective optical fiber in hydrofluoric acid under the influence of capillary action. The nondiffracting type quasi-Bessel beam from the DSNA-FT having a long depth of field, ∼0.7 mm, is used for the CPOCT imaging of different type of samples, i.e., sticky tape, fish scale, and rice grain. The CPOCT with unique and simple optical fiber probe has achieved lateral resolution at least ∼3.3 μm maintained up to 3 mm working distance in air. This optical fiber tip is an excellent candidate for an endoscopic probe design.

Controllable rotation of multiplexing elliptic optical vortices

Abstract: We propose a highly efficient phase modulation method for generating an elliptic optical vortex with an arbitrary controllable rotation angle, that can be recognized as a controllable degree of freedom. A transformation matrix and rotation matrix were used to realize the modulation of the light field. We numerically and experimentally demonstrate the control capability of Bessel beam and higher-order Laguerre–Gauss vortex modes. The rotation angle β, elliptic degree e and multiplexing elliptic optical vortices could be freely modulated by using the proposed method. This work is important for the progress of programmable optical tweezers, as well as for automated optical transport operations which are of interest in colloidal physics and biophysics. It can also be applied in high-dimensional and multiphoton quantum experiments.

Two-photon Bessel beam tomography for fast volume imaging.

Abstract: Light microscopy on dynamic samples, for example neural activity in the brain, often requires imaging volumes that extend over several 100 µm in axial direction at a rate of at least several tens of Hertz. Here, we develop a tomography approach for scanning fluorescence microscopy which allows recording a volume image in a single frame scan. Volumes are imaged by simultaneously recording four independent projections at different angles using temporally multiplexed, tilted Bessel beams. From the resulting projections, three-dimensional images are reconstructed using inverse Radon transforms combined with convolutional neural networks (U-net).

Diffractive Elements for Zero-Order Bessel Beam Generation With Application in the Terahertz Reflection Imaging

Abstract: Novel diffractive elements are presented to produce zero-order Bessel beams in the terahertz (THz) band. First, diffractive elements are designed as phase plates to change the direction of the incident Gaussian beam. Then, they are fabricated by the emerging three-dimensional printing technology. Last, zero-order Bessel beams are generated by the diffractive elements at 0.3 THz. To verify the feasibility of this approach, the generated zero-order Bessel beams are monitored and compared with the ones generated by the axicons. The comparison confirms that the diffractive elements can produce Bessel beam effectively and efficiently. In addition, the diffractive elements are applied to introduce zero-order Bessel beam to a THz reflection imaging system. A resolution test phantom is imaged by the THz reflection imaging system with zero-order Bessel beam and a conventional THz reflection imaging system. The results indicate that the THz reflection imaging system with Bessel beam offers a significantly extended depth of field and high anti-interference capability.

Solving Fresnel equation for refractive index using reflected optical power obtained from Bessel beam interferometry

Abstract: This work demonstrates an interferometric technique to estimate the reflected powers from dielectric interfaces and the reflection coefficient using the Fresnel equation for measurement of the refractive index (RI) of liquid samples. It uses low-coherence common-path optical interferometry that is commonly used for optical imaging. A uniquely designed optical fiber tip generating a high-quality non-diffractive Bessel beam probes liquid samples in a glass container non-invasively. The light reflected from different interfaces of the container is recollected by the same optical fiber tip. The reflected beams interfere with the reference beam generated at the fiber tip itself. This interference spectrum is further processed using fast-Fourier transform to measure reflected powers from the respective interfaces. The acquired powers are used to solve the Fresnel equation to find RI of liquid samples. As a proof of concept, experiments have been performed on several liquid samples including turbid media such as blood. This non-invasive interferometric technique could also be an ideal example confirming the Fresnel equation for reflection of light. Unlike other optical fiber-based RI sensors, this technique does not require temperature compensation. The method can be employed for inspection of the production process in terms of RI in pharmaceutical and chemical process plants, etc.

Light-sheet microscopy with length-adaptive Bessel beams

Abstract: In light-sheet microscopy, a confined layer in the focal plane of the detection objective is illuminated from the side. The illumination light-sheet usually has a constant beam length independent of the shape of the biological object. Since the thickness and the length of the illumination light-sheet are coupled, a tradeoff between resolution, contrast and field of view has to be accepted. Here we show that scanned Bessel beams enable object adapted tailoring of the light-sheet defined by its beam length and position. The individual beam parameters are obtained from automatic object shape estimation by low-power laser light scattered at the object. Using Arabidopsis root tips, cell clusters and zebrafish tails, we demonstrate that Bessel beam light-sheet tailoring leads to a 50% increase in image contrast and a 50% reduction in photobleaching. Light-sheet tailoring requires only binary amplitude modulation, therefore allowing a real time illumination adaptation with little technical effort in the future.

Ultrashort Bessel beam photoinscription of Bragg grating waveguides and their application as temperature sensors

Abstract: Ultrashort pulsed Bessel beams with intrinsic nondiffractive character and potential strong excitation confinement down to 100 nm can show a series of advantages over Gaussian beams in fabricating efficient Bragg grating waveguides (BGWs). In this work, we focus on parameter management for the inscription of efficient BGWs using the point-by-point method employing Bessel beams. Due to their high aspect ratio, the resulting one-dimensional void-like structures can section the waveguides and interact efficiently with the optical modes. Effective first-order BGWs with low birefringence can then be fabricated in bulk fused silica. By controlling the size and the relative location of grating voids via the Bessel pulse energy and scan velocities, the resonant behaviors of BGWs can be well regulated. A high value of 34 dB for 8 mm length is achieved. A simple predictive model for BGWs is proposed for analyzing the influences of processing parameters on the performance of BGWs. The technique permits multiplexing several gratings in the same waveguide. Up to eight grating traces were straightforwardly inscribed into the waveguide in a parallel-serial combined mode, forming the multiplex BGWs. As an application, the multiplex BGW sensor with two resonant peaks is proposed and fabricated for improving the reliability of temperature detection.

Fabrication of high-aspect-ratio structures using Bessel-beam-activated photopolymerization

Abstract: Microfabrication based on photopolymerization is typically achieved by scanning a focal spot within the material point by point, which significantly limits fabrication speed. In this paper, we explore a method for rapid fabrication of high-aspect-ratio microstructures based on photopolymerization using a femtosecond laser beam that is converted into a Bessel beam by an axicon. With stationary exposure, a polymer fiber measured at 200 μm in length and 400 nm in width (500∶1 aspect ratio) was fabricated within 50 ms of exposure time. The exposure conditions can be adjusted to produce fibers with variable widths. A phenomenological polymerization-threshold model is adapted for Bessel-beam exposure. The revised model is applied to analyze the structure width and estimate the order of multi-photon absorption. Examination of the cross section of the fibers shows that they are nearly monolithic, suggesting that active species diffuse during photopolymerization. By scanning the Bessel beam in the plane transverse to the direction of beam propagation, mesh structures are fabricated with a single-pass scan, showing the potential of this method for rapid fabrication of large-scale high-aspect-ratio microstructures for applications in photonics, micro-machines, and tissue engineering.

Approach to fully decomposing an optical force into conservative and nonconservative components

Abstract: One of the theoretical challenges in studying optical trapping is the decomposition of the optical force into the gradient force (conservative component) and scattering force (nonconservative component), which can be achieved either for Raleigh particles or for very large particles in the regime of ray optics. However, for the moderate particles in between these two limits, the scenario is still a mystery. In this paper we present a theoretical approach to bridge this gap and fully split the optical force acting on a spherical particle immersed in a generic monochromatic free-space optical field into two such essentially different components, which is efficient even for large particles with the exact consideration of light polarization, thus offering a benchmark for examining the effective range for application of ray optics. Our approach models general optical fields by a series of homogeneous plane waves. The analytical expressions for the gradient and scattering parts of the optical force exerted on a spherical particle of arbitrary size illuminated by multiple interferential plane waves are then derived. As examples of applications, we investigate the gradient and scattering forces acting on a dielectric particle immersed in the Bessel beam. Our results are in excellent agreement with those obtained based on ray optics methods when the illuminated particle is large enough, while exhibiting effects of Mie resonance that are totally missing in the ray optics for moderate particle sizes. Finally, we study the effect of particle size on the gradient force acting on a spherical particle sitting in multiple interferential plane waves. Our extensively numerical results, up to a size as large as 2000 illuminating wavelengths, suggest an overall decreasing tendency in the ratio of the magnitude of the gradient force to that of the total force as the particle size increases.

Extended depth-of-field all-optical photoacoustic microscopy with a dual non-diffracting Bessel beam.

Abstract: All-optical photoacoustic microscopy (AOPAM) facilitates high-sensitivity, wide-bandwidth, volumetric imaging without coupling media. However, the rapid divergence of the Gaussian beam restricts the stability and depth-of-field in typical Gaussian AOPAM (G-AOPAM). Here we report an extended depth-of-field AOPAM using a dual non-diffracting Bessel beam (B-AOPAM). Benefiting from the designing, the B-AOPAM has the unique advantages of increasing depth resolving ability and improving photoacoustic detection sensitivity. The proposed scheme shows optimal lateral resolution of 2.4 μm and a long depth-of-focus of 635 μm, which is 10-fold larger than that of the G-AOPAM. The scattering phantoms and in vivo animal experiments demonstrated the imaging feasibility and capability of the B-AOPAM, which can provide noncontact, high spatial resolution imaging of non-flat tissue and contribute to future clinical applications.

Imaging Enhancement of Light-Sheet Fluorescence Microscopy via Deep Learning

Abstract: The complementary beam subtraction (CBS) method can reduce the out-of-focus background and improve the axial resolution in light-sheet fluorescence microscopy (LSFM) via double scanning a Bessel and the complementary beams. With the assistance of a compressed blind deconvolution and denoising (CBDD) algorithm, the noise and blurring incurred during CBS imaging can be further removed. However, this approach requires double scanning and large computational cost. Here, we propose a deep learning-based method for LSFM, which can reconstruct high-quality images directly from the conventional Bessel beam (BB) light-sheet via a single scan. The image quality achievable with this CBS-Deep method is competitive with or better than the CBS-CBDD method, while the speed of image reconstruction is about 100 times faster. Accordingly, the proposed method can significantly improve the practicality of the CBS-CBDD system by reducing both scanning behavior and reconstruction time. The results show that this cost-effective and convenient method enables high-quality LSFM techniques to be developed and applied.

Modulation of acoustic waves by a broadband metagrating

Abstract: Metasurface has recently attracted a lot of attentions for controlling wave fields. Based on the diffraction effects of phase gratings, we demonstrate a broadband acoustic metagrating which can concentrate the diffracted waves in the first (±1) orders and achieve multifunctional wave steering such as broadband anomalous diffraction. In the acoustic metagrating, the subwavelength rectangular waveguides (SRWs) function as the periodic elements to replace the fences in ordinary gratings. Thus, we can achieve a group of phase delay from 0 to 2π independently with frequency just by reconfiguring the relative locations of the effective apertures. With the iterative algorithm, the acoustic metagrating can be used to record the phase profile and then control the output waveform. We further demonstrate that the broadband metagrating can be used to achieve the acoustic Gaussian beam. By rotating the periodic elements into a two-dimensional structure, the Bessel beam is further obtained.