Femtosecond Bessel Beam Research Articles

Femtosecond Bessel laser beam induced concentric rings on SiC for circular symmetry wide-viewing angle structural color

The laser-induced structural coloring technique holds significant potential for various applications due to its precision, controllability, and versatility across different materials. However, laser-induced structural colors face the problems of durability, homogeneity and indistinguishability, which are not conducive to their application as large-scale identifiable markers. In this paper, an innovative approach has been developed to achieve a circular-symmetric structural color display on durable SiC surfaces by fabricating periodic concentric ring structure arrays using the sidelobe light field of a femtosecond zero-order Bessel beam. The research involved a systematic study of the effects of laser power and pulse number on SiC, along with a detailed analysis of the micro-nanostructures evolving on the SiC surface after laser scanning. Additionally, an examination of the characteristics of circular-symmetric structural color displays and their wide viewing angles in periodic circular structures was conducted. Finally, a comparison was made between the structural color display effects of a concentric ring structure generated by a single pulse and the Laser-Induced Periodic Surface Structures (LIPSS) generated by three pulses. This comparison underscores the potential application of alternating between the high saturation color of the concentric ring structure and the low saturation dark light of the LIPSS for anti-counterfeit coding purposes. The findings of this research present promising opportunities for low-cost mass manufacturing in anti-counterfeit labeling and the advanced processing of photonic devices using SiC.

Preparation of large-area superhydrophobic and anti-icing 3D micro-nano-structures using femtosecond Bessel beams with fluorination treatment

This study proposes a simple strategy for overcoming the adverse effects of icing on material surfaces and improving the anti-icing performance of these surfaces. First, a femtosecond Gaussian beam is transformed into a femtosecond Bessel beam through spatial shaping to improve its energy distribution in the propagation direction while avoiding the adverse effects of defocusing during processing, thereby achieving large-area, consistent micro-nano-fabrication. Subsequently, the surface wettability of the considered material is controlled through the preparation of a functionalized three-dimensional micro-nano-structure using fluorination, transforming the material surface from superhydrophilic to superhydrophobic, thereby directly preventing the adhesion of subcooled droplets. Static-droplet condensation tests and dynamic anti-icing tests in simulated freezing-rain environments demonstrated that the proposed strategy can considerably increase the droplet condensation time and reduce the amount of surface icing. Overall, the strategy provides an efficient, high-quality, highly reproducible solution to the problem of surface icing.

Wide-Size Range and High Robustness Self-Assembly Micropillars for Capturing Microspheres.

Capillary force driven self-assembly micropillars (CFSA-MP) holds immense promise for the manipulation and capture of cells/tiny objects, which has great demands of wide size range and high robustness. Here, we propose a novel method to fabricate size-adjustable and highly robust CFSA-MP that can achieve wide size range and high stability to capture microspheres. First, we fabricate a microholes template with an adjustable aspect ratio using the spatial-temporal shaping femtosecond laser double-pulse Bessel beam-assisted chemical etching technique, and then the micropillars with adjustable aspect ratio are demolded by polydimethylsiloxane (PDMS). We fully demonstrated the advantages of the Bessel optical field by using the spatial-temporal shaping femtosecond laser double-pulse Bessel beams to broaden the height range of the micropillars, which in turn expands the size range of the captured microspheres, and finally achieving a wide range of capturing microspheres with a diameter of 5-410 μm. Based on the inverted mold technology, the PDMS micropillars have ultrahigh mechanical robustness, which greatly improves the durability. CFSA-MP has the ability to capture tiny objects with wide range and high stability, which indicates great potential applications in the fields of chemistry, biomedicine, and microfluidics.

Rapid Fabrication of Yttrium Aluminum Garnet Microhole Array Based on Femtosecond Bessel Beam

High-aspect-ratio microholes, the fundamental building blocks for microfluidics, optical waveguides, and other devices, find wide applications in aerospace, biomedical, and photonics fields. Yttrium aluminum garnet (YAG) crystals are commonly used in optical devices due to their low stress, hardness, and excellent chemical stability. Therefore, finding efficient fabrication methods to produce high-quality microholes within YAG crystals is crucial. The Bessel beam, characterized by a uniform energy distribution along its axis and an ultra-long depth of focus, is highly suitable for creating high-aspect-ratio structures. In this study, an axicon lens was used to shape the spatial profile of a femtosecond laser into a Bessel beam. Experimental verification showed a significant improvement in the high aspect ratio of the microholes produced in YAG crystals using the femtosecond Bessel beam. This study investigated the effects of the power and defocus parameters of single-pulse Bessel beams on microhole morphology and size, and microhole units with a maximum aspect ratio of more than 384:1 were obtained. Based on these findings, single-pulse femtosecond Bessel processing parameters were optimized, and an array of 181 × 181 microholes in a 400 μm thick YAG crystal was created in approximately 13.5 min. The microhole array had a periodicity of 5 μm and a unit aspect ratio of 315:1, with near-circular top and subface apertures and high repeatability.

Nanotubular Arrayed LIPSS Achieved with Femtosecond Bessel Beam on Silver Surface for SERS-Based Multiple Explosive Sensing

Nanotubular Arrayed LIPSS Achieved with Femtosecond Bessel Beam on Silver Surface for SERS-Based Multiple Explosive Sensing

Bessel beams as a versatile tool to generate embedded devices in optical glasses

Beam shaping offers many opportunities to both streamline and accelerate laser fabrication processes, closing the gap between lab research and large-scale manufacturing. In the field of embedded devices, a Bessel beam configuration allows for large volume engraving of transparent materials in a single laser pass. In this work, we take advantage of femtosecond Bessel beams with a length of 340 μm and a diameter of 1 μm to generate diffractive elements within three distinct optical glasses. This strategy represents a chemical-free and cost-effective alternative to the conventional manufacturing of holographic elements based on photoresist micro-patterning. Despite the different nonlinear effects taking place in each material, we report the successful fabrication of diffraction gratings operating at either normal or Bragg incidence, with efficiencies in the first combined order of up to 70%. Our experimental results show the potential of Bessel beams as a tool to produce buried light devices in a wide variety of glass materials.

Local and high‐strength joining of transparent glass and glass‐ceramics by femtosecond Bessel beam

AbstractThe creation of robust joint among different materials with notably different coefficients of thermal expansion (CTE) is an urgent task for the development of the engineering, laser technology, and aerospace industry. In this regard, femtosecond laser welding is a promising technique for efficient adhesion‐free bonding of transparent materials. This paper reports on the laser welding of laser phosphate glass (CTE = 110 × 10−7 K−1) and spinel‐based glass–ceramic (CTE = 50 × 10−7 K−1) by the Bessel beam. Dependencies of weld geometric parameters on the pulse energy and scanning speed were investigated, and the laser exposure conditions were optimized for durable welding. The weld joints were tested on shear strength, which was shown to be as record high as up to 147 MPa. Atomic force microscopy, Raman spectroscopy, and elemental analysis were used to investigate the structure and morphology of the weld after its fracture, which revealed the laser‐induced amorphization of glass–ceramics and the formation of strong interconnection by laser‐driven mutual diffusion of two glassy phases. The results demonstrate the prospects for using femtosecond lasers for efficient industrial welding of a wide range of materials.

Nonlinear-Compensation High-Uniformity Pulsed Bessel Beam in the Transparent Material

Ultrashort pulse Bessel beams have gained attention for their ability to process high aspect ratio structures with precision drilling and cutting. However, focusing the beam inside transparent materials can lead to local stretching or compression due to material-air interface mismatch and nonlinear absorption, causing issues with uniformity of focus and processing depth. In this study, we propose a nonlinear compensation scheme for femtosecond pulsed Bessel beam stretching that modulates the optical phase of the pulse beam ring, achieving high-uniformity longitudinal distribution. This scheme enabled the creation of a highly uniform damaged structure measuring 540 μm in length and with a width standard deviation of only 145 nm inside fused silica. Our findings offer a new solution for ultrashort pulse laser applications in high aspect ratio drilling and fast precision cutting of transparent materials.

Bioinspired Near-Full Transmittance MgF2 Window for Infrared Detection in Extremely Complex Environments

Due to the extreme complexity of the anti-reflective subwavelength structure (ASS) parameters and the drastic limitation of Gaussian beam manufacturing accuracy, it remains a great challenge to manufacture ASS with ultrahigh transmittance on the surface of infrared window materials (such as magnesium fluoride (MgF2)) directly by femtosecond laser. Here, a design, manufacturing, and characterization method that can produce an ultrahigh-performance infrared window by femtosecond laser Bessel beam is proposed. Inspired by the excellent anti-reflective and hydrophobic properties of the special structure of dragonfly wings, a similar structural pattern with grid-distributed truncated cones is designed and optimized for its corresponding parameters to achieve near-full transmittance. The desired submicron structures are successfully fabricated by a Bessel beam after effectively shaping the beam. As a practical application, the bioinspired ASS is manufactured on the surface of MgF2, achieving an ultrahigh transmittance of 99.896% in the broadband of 3–5 μm, ultrawide angle of incidence (over 70% at 75° incidence), and good hydrophobicity with a water contact angle of 99.805°. Results from infrared thermal imaging experiments show that the ultrahigh-transmittance MgF2 window has superior image acquisition and anti-interference performance (3.9–8.6% image contrast enhancement and more accurate image edge recognition) in an environment with multiple interfering factors, which may play a significant role in facilitating applications of infrared thermal imaging technologies in extremely complex environments.

Femtosecond Bessel beam induced ladder-like LIPSS on trimetallic surface for SERS-based sensing of Tetryl and PETN

Laser-induced periodic surface structures (LIPSS) have emerged as a potential branch of nanophotonics, offering unparalleled applications in numerous other research fields such as on-chip microelectronics, hydrogen evolution, solar-energy conversion, photodetectors, photocatalysis, and sensing. In this study, for the first time, we have created novel, Ag-Au-Cu alloyed trimetallic, ladder-like LIPSS nanostructures engaging an axicon-generated femtosecond Bessel beam. The LIPSS formation mechanism through modulation of ablation fluence profile, involving interference of photon-induced surface plasmon polariton with incident beam, has been meticulously divulged. The overlapping feature of Bessel beam profiles has been considered to improve the coupling of electromagnetic modes with the pre-patterned surfaces. The period (Λ) of the fabricated low spatial frequency LIPSS has been estimated to be 540 ± 12 nm. Also, as a comparative study, the distinct topographical features of nanostructures generated under a Gaussian beam with an identical fluence has also been presented. Additionally, these trimetallic plasmonic nanostructures have demonstrated outstanding excellence as active surface-enhanced Raman scattering substrates for sensing applications. Traces (200 nM) of real-time explosives such as Tetryl and pentaerythritol tetranitrate (PETN) have been detected, while achieving enhancement factors in the range of 105, making these viable for practical applications.

Two-photon polymerization of femtosecond high-order Bessel beams with aberration correction

Two-photon polymerization of femtosecond high-order Bessel beams with aberration correction

Rapid Fabrication of Wavelength-Scale Micropores on Metal by Femtosecond MHz Burst Bessel Beam Ablation.

The preparation of the wavelength-scale micropores on metallic surfaces is limited by the high opacity of metal. At present, most micropores reported in the literature are more than 20 µm in diameter, which is not only large in size, but renders them inefficient for processing so that it is difficult to meet the needs of some special fields, such as aerospace, biotechnology, and so on. In this paper, the rapid laser fabrications of the wavelength-scale micropores on various metallic surfaces are achieved through femtosecond MHz burst Bessel beam ablation. Taking advantage of the long-depth focal field of the Bessel beam, high-density micropores with a diameter of 1.3 µm and a depth of 10.5 µm are prepared on metal by MHz burst accumulation; in addition, the rapid fabrication of 2000 micropores can be achieved in 1 s. The guidelines and experimental results illustrate that the formations of the wavelength-scale porous structures are the result of the co-action of the laser-induced periodic surface structure (LIPSS) effect and Bessel beam interference. Porous metal can be used to store lubricant and form a lubricating layer on the metallic surface, thus endowing the metal resistance to various liquids' adhesion. The microporous formation process on metal provides a new physical insight for the rapid preparation of wavelength-scale metallic micropores, and promotes the application of porous metal in the fields of catalysis, gas adsorption, structural templates, and bio-transportation fields.

Reverse wavefront engineering for remote generation of a near-infrared femtosecond Bessel beam filament in air

Precise control of the filamentation of an ultrashort and intense near-infrared laser pulse in air is crucial for many applications but remains challenging. By combining inverse design for shaping of a pulse at the output of a laser with simulation of the nonlinear propagation of the pulse in air, we numerically investigate the way to simultaneously control the length and position of a plasma filament at an arbitrary remote distance for the first time. An intermediate state, with a Bessel-Gauss beam profile, is introduced between the laser output and the filament. Through forward and backward propagations of the intermediate state, an end-to-end design of the initial laser pulse is achieved to match the desired filament characteristics. Our calculations show that a properly engineered torus pulse can project such a Bessel-Gauss beam at remote distance and generate a long plasma filament without requiring any feedback loop. We also proposed an experimental arrangement to project such an intermediate state and generate the corresponding filament remotely. The method can be further extended to other types of filaments or targets that may be reached from different intermediate states.

Femtosecond Laser Bessel Beam Fabrication of a Supercapacitor with a Nanoscale Electrode Gap for High Specific Volumetric Capacitance

Supercapacitors are widely used in electronic systems as energy storage devices. The fabrication of a miniaturized supercapacitor with high specific capacitance has attracted much attention in recent years. Here, we propose a new method to fabricate supercapacitors with a nanoscale electrode gap by using a femtosecond laser. The original femtosecond laser was converted to a nondiffraction Bessel light field with nanoscale beam width and microscale focal depth. Nanoscale processing precision was achieved by regulating the Bessel beam. We fabricated graphene supercapacitors with different electrode gap widths (varying from the microscale to the nanoscale) using this method. Supercapacitors fabricated by this method have advantages in both size miniaturization (electrode gap width down to ∼500 nm) and electrochemical performance improvement (a specific volumetric capacitance of 195 F/cm3). This work demonstrates that the femtosecond laser Bessel beam processing method provides a reliable pathway to fabricate miniaturized supercapacitors with high specific capacitance and other nanoscale electronic devices.

Insight on the regulation mechanism of the nanochannels in hard and brittle materials induced by sparially shaped femtosecond laser.

The efficient fabrication of nanochannels on hard and brittle materials is a difficult task in the field of micro and nano processing. We have realized nanochannel arrays on silica with characteristic scales varying from 50–230 nm using a single femtosecond Bessel beam pulse of 515 nm. By characterizing the surface openings, we found that the characteristic scales of the nanopore openings are inextricably linked to the surface energy deposition effect. We achieved not only three asymmetric channel profiles by adjusting the laser-sample interaction region, but also high aspect ratio nanochannels with characteristic scales about 50 nm and aspect ratios over 100. These results on hard and brittle materials provide a broader platform and application scenarios for smart particle rectifiers, DNA molecular sequencing, biosensors, and nanofluidic devices, which are also more suitable for future practical applications due to their low cost, good durability, and high productivity.

Antireflective array on zinc sulfide fabricated by femtosecond laser

Surface antireflection of zinc sulfide (ZnS) is crucial to improve the performance of infrared windows. In this study, femtosecond laser Bessel beam micromachining is proposed to fabricate antireflective microstructures (ARMs) on ZnS to suppress surface Fresnel reflection. The ARM consisting of approximately one hundred thousand holes within 30 × 30 mm2 is uniformly spread on the ZnS surface. The Fabry-Perot (FP) and Mie resonances in the multi-resonant surface work together to reduce reflection and are enhanced with increasing ARM depth. The ARM depth is tunable by adjusting the pulse energy and is utilized to facilely regulate the reflectance spectra as well as wettability. Finally, the fabricated ARM with wide-angle antireflection and hydrophobicity proved the effectiveness of the proposed method.

Femtosecond laser-induced sub-wavelength plasma inside dielectrics: I. Field enhancement

The creation of high-energy-density (≳106 joules per cm3) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with sub-wavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two limiting cases are considered: one is a homogeneous step-like profile that can sustain plasmon formation, and the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case, an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region.

Femtosecond laser-induced sub-wavelength plasma inside dielectrics. II. Second-harmonic generation

Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser–plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two-surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons.

Broadband antireflective subwavelength structures for infrared detection

Enhancing infrared window transmittance of zinc sulfide (ZnS) is an effective way to improve the infrared detection imaging quality. In this study, the antireflective subwavelength structure (ASS) is designed and fabricated on ZnS substrate, aiming to suppress surface Fresnel reflection and achieve high transmittance within the far-infrared band. The structure parameters of ASS such as height, period, and fill factor are initially set based on the effective medium approximation theory, and are optimized via the rigorous coupled-wave analysis (RCWA) method. Moreover, the antireflective ASS mechanism is explored by using the local light field selective enhancements in structures for capturing incident photons. In addition, a femtosecond laser Bessel beam is used to experimentally fabricate the designed ASS on the ZnS substrate for obtaining large array height, resulting in transmittance enhancement in the far-infrared region. Ultimately, the broadband and wide-angle ASS with the anti-laser damage properties and hydrophobicity is achieved on the optical window. Hence, the clarity of the infrared image captured by the infrared detection system is improved.

Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

As an important mid-infrared to far-infrared optical window, ZnS is extremely important to improve spectral transmission performance, especially in the military field. However, on account of the Fresnel reflection at the interface between the air and the high-strength substrate, surface optical loss occurs in the ZnS optical window. In this study, the concave antireflective sub-wavelength structures (ASS) on ZnS have been experimentally investigated to obtain high transmittance in the far-infrared spectral range from 6 μm to 10 μm. We proposed a simple method to fabricate microhole array ASS by femtosecond Bessel beam, which further increased the depth of the microholes and suppressed the thermal effects effectively, including the crack and recast layer of the microhole. The influence of different Gaussian and Bessel beam parameters on the microhole morphology were explored, and three ASS structures with different periods were prepared by the optimized Bessel parameters. Ultimately, the average transmittance of the sample with the ASS microhole array period of 2.6 μm increased by 4.1% in the 6 μm to 10 μm waveband, and the transmittance was increased by 5.7% at wavelength of 7.2 μm.