Flat-top Beam Research Articles

Improving periodic uniformity of area-type LIPSS on Si wafer using a flat-top beam femtosecond NIR laser

Fabrication of line- and area-type laser-induced periodic surface structures (LIPSS) induced by a flat-top beam provides improved periodic uniformity in structure compared to fabrication with a Gaussian beam. Improved periodic uniformity induces high reproducibility and high performance through signal uniformity. Especially, this characteristic makes LIPSS induced by flat-top beam can be applied in structural color, hydrophobicity control and surface-enhanced Raman scattering. To demonstrate the improved periodic uniformity, the periods of the LIPSSs induced by Gaussian and flat-top beams are analyzed. Period uniformity is evaluated by applying the fast Fourier transformation to atomic force microscopy data. Improved periodic uniformity is observed in the area-type LIPSS induced by a flat-top beam. This is because of the peak of line width induced by the Gaussian beam profile characteristic. Consequently, area-type LIPSSs induced by a flat-top beam exhibit improved periodic uniformity.

Fabrication of Single Crystals by Laser Powder Bed Fusion with a Flat-Top Laser Beam

Fabrication of Single Crystals by Laser Powder Bed Fusion with a Flat-Top Laser Beam

Effect of scan strategy on the formation of a pure nickel single-crystal structure using a flat-top laser beam via laser powder bed fusion

ABSTRACT In this study, the role of a scan strategy in the fabrication of a single crystal (SX) structure of face-centered cubic (fcc) pure Ni using a flat-top laser profile by a laser powder bed fusion (LPBF) process was investigated. A flat-top laser with a uniform heat intensity across the beam profile was employed to fabricate the SX structure with a <001> texture parallel to the build direction (║BD). A meander scan strategy with an XY–90° scan rotation was required to achieve the flat-top-derived SX structure. The meander scanning strategy promoted the epitaxial <001> fcc growth along the direction ║BD, leading to the <001>║BD texture formation. Meanwhile, the XY–90° scan rotation induced the epitaxial growth of <001> fcc phase on the hatch direction (HD)–scan direction (SD) plane in the direction deviated by approximately 45° relative to the SD. This 45° deviation relative to the SD occurred to accommodate the beam movement and the circular beam geometry on the HD – SD plane, resulting in the preferred configurations of <011>║SD and HD. Thus, this paper reports the importance of meander scanning with a 90° scan-rotation strategy to yield a flat-top-LPBF-derived SX structure with <001>║BD and <011>║SD and HD textures.

Terahertz metasurface for near-field beam conversion.

A uniform illumination over a screen is crucial for terahertz imaging. As such, conversion from a Gaussian beam to a flattop beam becomes necessary. Most of the current beam conversion techniques rely on bulky multi-lens systems for collimated input and operate in the far-field. We present a single metasurface lens to efficiently convert a quasi-Gaussian beam from the near-field region of a WR-3.4 horn antenna to a flattop beam. The design process is divided into three sections to minimize simulation time, and the conventional Gerchberg-Saxton (GS) algorithm is supplemented with the Kirchhoff-Fresnel diffraction equation. Experimental validation confirms that a flattop beam with an efficiency of 80% has been achieved at 275 GHz. Such high-efficiency conversion is desirable for practical terahertz systems and the design approach can be generally used for beam shaping in the near-field.

"Analytical continuation" of flattened Gaussian beams.

A purely analytical extension of flattened Gaussian beams [Opt. Commun.107, 335 (1994)OPCOB80030-401810.1016/0030-4018(94)90342-5] to any values of beam order is here proposed. Due to it, the paraxial propagation problem of axially symmetric, coherent flat-top beams through arbitrary ABCD optical systems can definitely be solved in closed form via a particular bivariate confluent hypergeometric function.

Design and 3D printing of the Powell lens for sub-terahertz imaging.

The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

Synthesis of Reconfigurable Multiple Shaped Beams of a Concentric Circular Ring Array Antenna Using Evolutionary Algorithms

The approach described in this paper uses evolutionary algorithms to create multiple-beam patterns for a concentric circular ring array (CCRA) of isotropic antennas using a common set of array excitation amplitudes. The flat top, cosec2, and pencil beam patterns are examples of multiple-beam patterns. All of these designs have an upward angle of θ = 0◦. All the patterns are further created in three azimuth planes (φ = 0◦, 5◦, and 10◦). To create the necessary patterns, non-uniform excitations are used in combination with evenly spaced isotropic components. For the flat top and cosecant-squared patterns, the best combination of common components, amplitude and various phases is applied, whereas the pencil beam pattern is produced using the common amplitude only. Differential evolutionary algorithm (DE), genetic algorithm (GA), and firefly algorithm (FA) are used to generate the best 4-bit discrete magnitudes and 5-bit discrete phases. These discrete excitations aid in lowering the feed network design complexity and the dynamic range ratio (DRR). A variety of randomly selected azimuth planes are used to verify the excitations as well. With small modifications in the desired parameters, the patterns are formed using the same excitation. The results proved both the efficacy of the suggested strategy and the dominance of DE over GA as well as FA.

Thermal-induced effects on ultrafast laser filamentation in ethanol

Fs laser filamentation in ethanol is studied for bottom-up nanomaterials synthesis at different laser repetition rate. Measurements of energy loss, visible conical images, transmitted beam profile and the optical emission spectra show that laser repetition rate affects ultrafast laser filamentation in ethanol significantly because of the induced thermal processes. Firstly, near the threshold laser energy for self-focusing, high laser repetition rate could interfere with the process, via thermal defocusing that inhibits the onset of filamentation. An enlarged, near flat-top laser beam is produced for the conditions that favor thermal lensing. Secondly, at high laser energy, thermal defocusing effect is saturated as convective heat dissipation occurs. Thus, self-focusing prevailed and led to filamentation at all repetition rates. The strong convection at high laser repetition rate increases the rate of reactions for nanodiamonds synthesis, as the flow of fresh ethanol precursor to the localized filamentation zone is enhanced.

Sidelobe-Level Minimization With Power Gain Constraint via a Wide-Beam Antenna Array

To minimize the sidelobe level (SLL) of a wide-beam antenna array, the conventional approaches usually solve the shaped-beam pattern synthesis (SBPS) problem. These SBPS-based approaches focus on synthesizing a beam pattern that is similar to the desired one. However, the power gain except the beam shape is also an important concerning factor in array applications. This letter proposes an SLL minimization approach to obtain a lower SLL while achieving the similar minimum power gain in the wide-beam mainlobe as being obtained via the SBPS-based approaches. The proposed approach takes the power gain instead of the beam shape as the cost function and set up the synthesis optimization problem. An auxiliary variable is introduced to relax this nonconvex formulated problem into convex form. As a result, the original optimization problem can be solved iteratively. Even though the optimal solution cannot be theoretically guaranteed, numerical examples demonstrate that the proposed approach is able to further suppress the SLL by more than 10 dB for the flat-topped beam pattern and more than 3 dB for the cosecant squared beam pattern.

Microexplosions in bulk sapphire driven by simultaneously spatially and temporally focused femtosecond laser beams.

We report experimental results on the formation of microvoids in bulk sapphire produced through the single-shot illumination of the sample by near-infrared, simultaneously spatially and temporally focused (SSTF) laser beams. Compared with the cases of tightly focused femtosecond Gaussian and flattop beams, the SSTFs produce internal microvoids with significantly larger volumes and without cracks between the interaction sites, which may be beneficial for applications in volumetric data storage and for the studies of exotic, super-dense elemental phases contained in the densified shells surrounding the microvoids.

Limitation of laser radiation power by carbon materials with a nonlinear optical threshold effect at a flat-top pulse shape

The possibilities of using multivariate determination of carbon nanotubes properties based on the dependence data of the normalized transmission logarithm on the sample displacement in the case of Z-scan with an open aperture as well as on the total pulse energy in the case of fixed sample position measurements are studied. The radiation transfer equation for the threshold dependence of the absorption coefficient on the intensity of a flat-top laser beam is used. The data of physical and computational experiments showed the sensitivity of the measured curves with respect to the values of the constants of the optical and threshold energy fluence, as well as the beam waist radius. The possibility of multivariate determination of the properties of liquid carbon nanotube-based dispersed media with a nonlinear optical threshold effect and beam waist radius inside such samples has been established. Keywords: optical limiting, multivariate analysis, carbon nanotubes, Z-scan

Focus on lasers, imaging, microscopy, and nanoscience

Focus on lasers, imaging, microscopy, and nanoscience

Ограничение мощности лазерного излучения углеродными материалами с нелинейным оптическим пороговым эффектом при форме импульса с плоской вершиной

The possibilities of using multivariate determination of carbon nanotubes properties based on the dependence data of the normalized transmission logarithm on the sample displacement in the case of Z-scan with an open aperture as well as on the total pulse energy in the case of fixed sample position measurements are studied. The radiation transfer equation for the threshold dependence of the absorption coefficient on the intensity of a flat-top laser beam is used. The data of physical and computational experiments showed the sensitivity of the measured curves with respect to the values of the constants of the optical and threshold energy fluence, as well as the beam waist radius. The possibility of multivariate determination of the properties of liquid carbon nanotube-based dispersed media with a nonlinear optical threshold effect and beam waist radius inside such samples has been established.

A method of generating arbitrary uniform fields based on angular spectrum domain and time inversion

Existing uniform fields are usually based on the special arrangement of the array antenna. The uniform fields generated by flat-top beam shaping in angular far-field area or by point focusing in near-field area are directly subject to the array configuration and cannot be flexibly controlled. This paper presents a method of generating uniform field based on the combination of angular spectral domain and improved time reversal technique. This method is not limited by the array arrangement. It can generate a uniform field of specified size, shape and deflection angle in the same array arrangement at any position, including the near-field region. In this work, the reason why this method is not limited by array arrangement is explained theoretically. Secondly, the ability of the fixed array configuration to generate multiple uniform fields is validated numerically. Finally, the time-reversal technique of reversal signal amplitude reciprocal weighting is introduced. The problem of deterioration of uniform field flatness, caused by amplitude decay and phase delay during the generation of uniform field, is solved through this technology. The results show that the quality of the synthesized field is related to the main lobe and sidelobe information of its corresponding angular spectrum domain envelope, and the generated any uniform field must contain at least half of the angular spectrum domain main lobe information and half of the sidelobe information. This method can flexibly control the position, size, shape and deflection angle of one-dimensional and two-dimensional uniform field, which provides a new way to flexibly generating uniform fields.

Spot size estimation of flat-top beams in space-based gravitational wave detectors

Motivated by the necessity of a high-quality stray light control in the detection of the gravitational waves in space, the spot size of a flat top beam generated by the clipping of the Gaussian beam (GB) is studied. By adopting the mode expansion method (MEM) approach to simulating the beam, a slight variant of the definition of the mean square deviation (MSD) spot size for the MEM beam is proposed. This enables us to quickly estimate the spot size for arbitrary propagation distances. Given that the degree of clipping is dependent on the power ratio within the surface of an optical element, the power ratio within the MSD spot range is used as a measure of spot size. The definition is then validated in the cases of simple astigmatic Gaussian beam and nearly-Gaussian beam profiles. As a representative example, the MSD spot size for a top-hat beam in a science interferometer in the detection of gravitational waves in space is then simulated. As in traditional MSD spot size analysis, the spot size is divergent when diffraction is taken into account. A careful error analysis is carried out on the divergence and in the present context, it is argued that this error will have little effect on our estimation. Using the results of our study allows an optimal design of optical systems with top-hat or other types of non-Gaussian beams. Furthermore, it allows testing the interferometry of space-based gravitational wave detectors for beam clipping in optical simulations. This work will serve as a useful guide in the future system design of the optical bench and the sizes of the optical components.

Microstructure characteristics of a René N5 Ni-based single-crystal superalloy prepared by laser-directed energy deposition

This work focused on the microstructure and mechanical properties of nickel-based single crystal (SX) superalloy René N5 prepared by laser-directed energy deposition (L-DED) using a flat-top laser beam. A comparison was conducted with the conventional directional solidification technique. Results show that the crack-free single-crystal superalloys with a fraction of low-angle grain boundary (LAGB) higher than 98.5% were obtained by epitaxial growth under different laser powers. The flat fusion line using a flat-top laser beam can suppress the formation of stray grain and high-angle grain boundaries (LAGB) during the L-DED process. Microstructure observation shows the extremely refined dendrites in L-DED SX superalloys with primary dendrite arm spacing (PDAS) values around 20–30 µm, which is significantly lower than as-cast ones (∼350 µm). PDAS increases with the increasing laser power. Segregation of refractory elements of Re, W, and Ta increases with increasing laser power, and the SX specimens with laser power of 1100 W and 1300 W have lower elemental segregation compared with as-cast specimens. The SX specimens exhibit a uniform square γ′ phase at the dendrite core, with an average size of 79–101 nm and a volume fraction of 57–68%. Besides, the carbides with discontinuous and refined shapes show a uniform distribution in the L-DED specimens in comparison with as-cast specimens. The microhardness of as-deposited specimens increases with the increasing laser power, which is higher than that of as-cast ones. This work can provide potential guidance for the microstructure control in the laser additive manufacturing of Ni-based SX superalloys and a further improvement in high-temperature mechanical performance.

Manufacturing single crystals of pure nickel via selective laser melting with a flat-top laser beam

• Fabrication of single-crystal structure using selective laser melting was studied. • A flat-top profile was utilized without the implementation of single-crystal seeds. • Parametric optimization resulted in planar melt pool formation. • A homogeneous near-{001}<100> texture was achieved in high building heights. • Suppressing strain accumulation was necessary to avoid high-angle grain boundary. The exploration of flat-top laser profile in fabricating a single crystal (SX) structure using selective laser melting (SLM) in pure Ni was investigated. Optimization of the parameters led to the formation of a planar melt pool. A homogeneous near-{001}<100> texture with suppressed high-angle grain boundary (HAGB) in high building heights of >20 mm was achieved without an SX seed. In addition, the planar melt pool suppressed the geometrically necessary dislocation accumulation and prevented strain-induced continuous dynamic recrystallization that could cause HAGB formation. Thus, an SX structure with homogeneous near-{001}<100> texture and suppressed HAGB was successfully achieved without an SX seed.

Propagation characteristics of radial phase-locked discrete vector beam generated from a Gaussian Schell-Model laser array in atmospheric turbulence

In this paper, combining the advantages of partial coherence, vector beam, and laser array in better resistance to atmospheric turbulence, high capacity, and high output power, we proposed a radial phase-locked discrete vector beam generated from a Gaussian Schell-Model (GSM) laser array (i.e., phase-locked GSM discrete vector beam array) and theoretically studied its propagation characteristics in atmospheric turbulence. The analytical expressions for spectral intensity, spectral degree of coherence (DOC), and spectral degree of polarization (DOP) of a phase-locked GSM discrete vector beam array were derived based on the extended Huygens-Fresnel principle. And also, based on numerical simulation, we investigated the influence of atmospheric and laser parameters on the propagation properties in detail. The results show that the beam profile will gradually evolve from hollow structure distribution, then form a flat-topped beam and finally present the Gaussian-like distribution; the spectral DOC exhibits decaying oscillation analogous to under-damping vibration; a dip appears in the distribution of the spectral DOP. The evolution rate of spectral intensity, oscillation amplitude, and period of spectral DOC and dip width of spectral DOP are determined by propagation distance, refraction structure constant, beam waist, spatial correlation length, and the number of beamlets. More importantly, by analyzing Stokes parameters, different initial polarization distributions correspond to different crosslines of S1/S0, and even with the increase of the propagation distance, the crosslines of S1/S0 still show similar performance. This work may provide novel thoughts and methods to improve propagation performance and benefit potential applications in free-space optical communication.

Propagation properties of hollow rectangular quad-Airy beams

The hollow rectangular quad-Airy beam (HRQAB) is proposed in this paper based on the Fourier transform of the hollow flat-topped Gaussian beam with a cubic phase. The propagation characteristics of HRQAB are investigated in detail. Compared with common quad-Airy beam (QAB), the four focusing property and hollow rectangular profile of this beam are found. In the four-focusing plane, the positions and intensities of the four light spots could be controlled by the initial radius r0, the order of hollow Gaussian beam N, and the coefficient in the cubic phase α. After propagation some distance, a hollow rectangular shape is formed; the area of the hollow region increases significantly with the increase of r0; its intensity of the hollow rectangular light spot would become stronger for a larger α. Finally, the HRQAB is generated in experiment and its special properties are demonstrated.

Creating stable trapping force and switchable optical torque with tunable phase of light.

Light-induced rotation of microscopic objects is of general interest as the objects may serve as micromotors. Such rotation can be driven by the angular momentum of light or recoil forces arising from special light-matter interactions. However, in the absence of intensity gradient, simultaneously controlling the position and switching the rotation direction is challenging. Here, we report stable optical trapping and switchable optical rotation of nanoparticle (NP)-assembled micromotors with programmed phase of light. We imprint customized phase gradients into a circularly polarized flat-top (i.e., no intensity gradient) laser beam to trap and assemble metal NPs into reconfigurable clusters. Modulating the phase gradients allows direction-switchable and magnitude-tunable optical torque in the same cluster under fixed laser wavelength and handedness. This work provides a valuable method to achieve reversible optical torque in micro/nanomotors, and new insights for optical trapping and manipulation using the phase gradient of a spatially extended light field.