Pulsed Bessel Beams Research Articles

Bessel beam fabrication of graphitic micro electrodes in diamond using laser bursts

We present the fabrication of conductive graphitic microelectrodes in diamond by using pulsed Bessel beams in the burst mode laser writing regime. The graphitic wires are created in the bulk of a 500 μm thick monocrystalline HPHT diamond (with (100) orientation) perpendicular to the sample surface, without beam scanning or sample translation. In particular, the role of different burst features in the resistivity of such electrodes is investigated for two very different sub-pulse durations namely 200 fs and 10 ps, together with the role of thermal annealing. Micro-Raman spectroscopy is implemented to investigate the laser-induced crystalline modification, and the results obtained by using two different laser repetition rates, namely 20 Hz and 200 kHz, are compared. A comparison of the micro-Raman spectra and of the resistivity of the electrodes fabricated respectively with 10 ps single pulses and with bursts (of sub-pulses) of similar total duration has also been made, and we show that the burst mode writing regime allows to fabricate more conductive micro electrodes, thanks to the heat accumulation process leading to stronger graphitization. Moreover, the microfabrication of diamond by means of the longest available bursts (~ 46.7 ps duration) featured by 32 sub-pulses of 200 fs duration, with intra-burst time delay of 1.5 ps (sub-THz bursts), leads to graphitic wires with the lowest resistivity values obtained in this work, especially at low repetition rate such as 20 Hz. Indeed, micro electrodes with resistivity on the order of 0.01 Ω cm can be fabricated by Bessel beams in the burst mode regime even when the bursts are constituted by femtosecond laser sub-pulses, in contrast with the results of the standard writing regime with single fs pulses typically leading to less conductive micro electrodes.

Effect of crystallographic orientation on the potential barrier and conductivity of Bessel written graphitic electrodes in diamond

Ultrafast laser micromachining can be used to promote diamond graphitization, enabling the creation of electrically conductive wires embedded in the diamond matrix. In this context, the presence of a potential barrier in the conductivity of transverse graphitic wires fabricated by pulsed Bessel beams without sample translation across 500 μm thick monocrystalline CVD diamond has been studied. In particular, the role of the crystallographic orientation has been analysed. The morphology and the conductivity of the obtained electrodes have been studied using optical microscopy and current-voltage measurements, while the structural changes have been investigated by means of micro-Raman spectroscopy. By using different laser writing parameters, we have explored the features of different electrodes in a (100) and a (110) oriented diamond crystal respectively. We show that in addition to the use of specific pulse energies and durations (in the fs and ps regimes), the crystallographic orientation of the sample plays an important role in reducing or eliminating the potential barrier height of the IV electrical characterization curves. In a (110) oriented sample, it is possible to eradicate the potential barrier completely even for graphitic wires fabricated at low pulse energy and in the fs pulse duration regime, in contrast to the (100) oriented-crystal case where the barrier is generally observed. The effect of thermal annealing of the diamond samples on the resistivity of the fabricated micro-electrodes has also been investigated. In (110) oriented diamond, resistivities lower than 0.015 Ω cm have been obtained.

Evaluation of microscale crystallinity modification induced by laser writing on Mn3O4 thin films

Defining microstructures and managing local crystallinity allow the implementation of several functionalities in thin film technology. The use of ultrashort Bessel beams for bulk crystallinity modification has garnered considerable attention as a versatile technique for semiconductor materials, dielectrics, or metal oxide substrates. The aim of this work is the quantitative evaluation of the crystalline changes induced by ultrafast laser micromachining on manganese oxide thin films using micro-Raman spectroscopy. Pulsed Bessel beams featured by a 1 μm-sized central core are used to define structures with high spatial precision. The dispersion relation of Mn3O4 optical phonons is determined by considering the conjunction between X-ray diffraction characterization and the phonon localization model. The asymmetries in Raman spectra indicate phonon localization and enable a quantitative tool to determine the crystallite size at micrometer resolution. The results indicate that laser-writing is effective in modifying the low-crystallinity films locally, increasing crystallite sizes from ∼8 nm up to 12 nm, and thus highlighting an interesting approach to evaluate laser-induced structural modifications on metal oxide thin films.

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.

Fabrication of conductive micro electrodes in diamond bulk using pulsed Bessel beams

High-quality, in-bulk conductive graphitic microelectrodes are fabricated perpendicular to the surface of a 500 μm thick monocrystalline CVD diamond sample using pulsed Bessel beams. With a 12o cone angle beam, different pulse parameters are explored to optimize the graphitic wires which are written without sample translation. The quality of the electrodes and their electrical and structural properties have been analysed through current-voltage characterization and micro-Raman spectroscopy. We have found that higher pulse duration favours better conductivity while pulse energy has an optimum value for the same. This trend is confirmed by the presence and the different amounts of graphitic-like sp2 bonded carbon revealed by the micro-Raman spectra in different configurations. Using suitable writing parameters, we are able to create electrodes with the resistivity of 0.04 Ω cm, which, to the best of our knowledge, is one of the lowest values ever reported in literature in the case of graphitic-like wires fabricated through laser micromachining.

Conductive graphitic wires generation in diamond by means of pulsed Bessel beam micromachining

We present the fabrication of transverse graphitic microelectrodes in a 500 μm thick synthetic diamond bulk by means of pulsed Bessel beams. By suitably placing the elongated focal length of the Bessel beam across the entire sample, the graphitic wires grow from the bottom surface up to the top during multiple shot irradiation. The morphology of the microstructures generated and the micro-Raman spectra are studied as a function of the laser parameters and the diamond crystal orientation. We show the possibility to generate high conductivity microelectrodes, which are crucial for the application of electric fields or current transport/collection in various chips and detectors.

Noise-tolerant single-photon imaging by using non-diffractive pulsed laser beams

Accurate optical imaging capable of reconstructing three-dimensional (3D) target objects with strong surrounding noises are highly demanding in diverse applications. For conventional imagers with narrow probe beams, the probe beams usually suffer from serious spreading and distortion due to the diffraction and surrounding noises. Here, we construct a 3D imager by using pulsed Bessel beams, enabling reconstruction of high-resolution 3D profiles with sub-millimeter transverse resolution and sub-centimeter depth resolution. We demonstrate that due to the outstanding diffraction-free and self-healing properties of Bessel beams, the present imager can provide high-quality reconstructed images in high-noise environments with opaque scatters. The method developed here for reducing the effects of surrounding noises can also be exploited in the light detection and ranging technology at single-photon level.

Study of Through-Hole Micro-Drilling in Sapphire by Means of Pulsed Bessel Beams.

Ultrashort Bessel beams have been used in this work to study the response of a 430-μm-thick monocrystalline sapphire sample to laser–matter interaction when injecting the beam orthogonally through the whole sample thickness. We show that with a 12° Bessel beam cone angle, we are able to internally modify the material and generate tailorable elongated microstructures while preventing the formation of surface cracks, even in the picosecond regime, contrary to what was previously reported in the literature. On the other hand, by means of Bessel beam machining combined with a trepanning technique where very high energy pulses are needed, we were able to generate 100 μm diameter through-holes, eventually with negligible cracks and very low taper angles thanks to an optimization achieved by using a 60-μm-thick layer of Kapton Polyimide removable tape.

Generation of long homogeneous plasma channels with high power long-wave IR pulsed Bessel beams.

Long-wave multi-joule ultrashort laser pulses are predicted to confine highly uniform electromagnetic energy and field intensities while sustaining high density uniform plasmas within nonlinear Bessel zones under extreme driving conditions in contrast to near-IR sources. This opens up novel applications in laser wakefield generation, radiofrequency/microwave guiding, and lightning control.

Micro-Hole Generation by High-Energy Pulsed Bessel Beams in Different Transparent Materials.

Micro-drilling transparent dielectric materials by using non-diffracting beams impinging orthogonally to the sample can be performed without scanning the beam position along the sample thickness. In this work, the laser micromachining process, based on the combination of picosecond pulsed Bessel beams with the trepanning technique, is applied to different transparent materials. We show the possibility to create through-apertures with diameter on the order of tens of micrometers, on dielectric samples with different thermal and mechanical characteristics as well as different thicknesses ranging from two hundred to five hundred micrometers. Advantages and drawbacks of the application of this technique to different materials such as glass, polymer, or diamond are highlighted by analyzing the features, the morphology, and the aspect-ratio of the through-holes generated. Alternative Bessel beam drilling configurations, and the possibility of optimization of the quality of the aperture at the output sample/air interface is also discussed in the case of glass.

Photonic crystal spatial filters fabricated by femtosecond pulsed Bessel beam.

We propose and experimentally demonstrate femtosecond direct laser writing with Bessel beams for the fabrication of photonic crystals with spatial filtering functionality. Such filters are mechanically stable, of small (of the order of a millimeter) size, do not require direct access to the far-field domain, and therefore are excellent candidates for intracavity spatial filtering applications in minilasers and microlasers. The proposed technique allows the fabrication of photonic crystal filters in inorganic glass, with a narrow angle (∼1°) nearly 100%-transmission passband between a broad angle (∼10°) nearly 0%-transmission angular stopbands.

Etching and drilling of through-holes in thin glass by means of picosecond Bessel beams

In this paper, we present a laser etching and drilling technique for thin glass materials based on the use of finite-energy pulsed Bessel beams orthogonally impinging on the sample surface. Thanks to the elongated focal zone of the quasi-non-diffractive beam, the laser processing can be performed without scanning the beam position along the thickness of the transparent material sample, but simply moving the sample in the plane orthogonal to the beam. We first present the results of single-shot glass microfabrication performed to identify the optimal laser parameters needed for an efficient internal material ablation. We then describe the micromachining technique used for etching the dielectric material at glass–air interfaces and for generating, without chemical etching, through-holes in thin glasses.

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

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.

Laser surface structuring of diamond with ultrashort Bessel beams

We investigate the effect of ultrafast laser surface machining on a monocrystalline synthetic diamond sample by means of pulsed Bessel beams. We discuss the differences of the trench-like microstructures generated in various experimental conditions, by varying the beam cone angle, the energy and pulse duration, and we present a brief comparison of the results with those obtained with the same technique on a sapphire sample. In diamond, we obtain V-shaped trenches whose surface width varies with the cone angle, and which are featured by micrometer sized channels having depths in the range of 10–20 μm. By laser writing crossed trenches we are also able to create and tailor on the diamond surface pillar-like or tip-like microstructures potentially interesting for large surface functionalization, cells capturing and biosensing.

Study of graphitic microstructure formation in diamond bulk by pulsed Bessel beam laser writing

The advantages of using Bessel beams for the generation of graphitic structures in diamond bulk are presented. We show that by irradiating the sample with a pulsed Bessel beam whose non-diffracting zone is of the same order of the sample thickness, it is possible to produce without any sample translation straight graphitic through-microstructures, whose size depends on the input pulse energy. The microstructure growth is investigated as a function of the number of irradiating pulses, and the femtosecond and picosecond regimes are contrasted.

Generation of Terahertz Radiation by Optical Rectification Using Femtosecond Bessel Beam

Optical rectification is one of the most widely used methods to generate broadband terahertz (THz) radiation using femtosecond laser pulses. In this work, we report the generation of THz radiation from GaP crystals based on optical rectification using a pulsed Bessel beam, i.e., a beam with a spatial profile described by a zeroth-order Bessel function of the first kind. It is shown that the nondiffracting feature of the central spot of the Bessel beam allowed an enhancement of the THz radiation generation efficiency. Compared with a Gaussian beam having the same beam size and pump power as its central spot, the Bessel beam enabled an average increase of the THz radiation power by factors of 1.37, 2.12, and 2.06 from GaP crystals of 1, 3, and 5 mm, respectively. Measurements showed that the spatial profile of the THz radiation followed a Gaussian distribution for both pump beams and increased linearly after the emitters.

Pulsed Bessel beam-induced microchannels on a diamond surface for versatile microfluidic and sensing applications

We present a laser machining method based on the use of pulsed Bessel beams to create, by single pass transverse writing, three-dimensional trench-like microstructures on a synthetic monocrystalline diamond substrate. By tuning the laser pulse energy and the writing speed, it is possible to control the features of the surface trenches obtained and to optimize the resulting high aspect-ratio and low roughness microstructures. This work confirms the potentialities of quasi-stationary beams in ultra-fast laser inscription technology. In particular the presented results show the possibility to fabricate deep and precise microfluidic channels on biocompatible diamond substrates, offering a great potential for biomedical sensing applications.

Experimental investigation of high aspect ratio tubular microstructuring of glass by means of picosecond Bessel vortices

We report on experiments on glass material modification using nondiffractive high-order picosecond pulsed Bessel beams, generated by a spatial light phase modulator and then suitably demagnified. We investigate the possibility to generate in single-shot tubular microstructures across 100-μm-thin borosilicate glass, when a suitable energy range is considered, and we highlight the effect of the unstable propagation regime for very high input energies, leading to a breakup of the tubular microstructure. The micromachined glass samples are observed on their top and bottom surfaces as well as longitudinally along their thickness. For the conical beam geometry used, we observe no internal material modification pattern with pulses in the femtosecond range. A comparison with glass machining by means of a focused ring-shaped beam is also presented. The results highlight the role of the conical energy flux for single-shot smooth high aspect ratio material modification in a regime where nonlinear Kerr effects are absent.

Spatiotemporal structure of femtosecond Bessel beams from spatial light modulators

We numerically investigate the spatiotemporal structure of Bessel beams generated with spatial light modulators (SLMs). Grating-like phase masks enable the spatial filtering of undesired diffraction orders produced by SLMs. Pulse front tilt and temporal broadening effects are investigated. In addition, we explore the influence of phase wrapping and show that the spatiotemporal structure of SLM-generated femtosecond Bessel beams is similar to Bessel X-pulses at short propagation distance and to subluminal pulsed Bessel beams at long propagation distance.

Adaptive Generation and Diagnostics of Linear Few-Cycle Light Bullets

Recently we introduced the class of highly localized wavepackets (HLWs) as a generalization of optical Bessel-like needle beams. Here we report on the progress in this field. In contrast to pulsed Bessel beams and Airy beams, ultrashort-pulsed HLWs propagate with high stability in both spatial and temporal domain, are nearly paraxial (supercollimated), have fringe-less spatial profiles and thus represent the best possible approximation to linear “light bullets”. Like Bessel beams and Airy beams, HLWs show self-reconstructing behavior. Adaptive HLWs can be shaped by ultraflat three-dimensional phase profiles (generalized axicons) which are programmed via calibrated grayscale maps of liquid-crystal-on-silicon spatial light modulators (LCoS-SLMs). Light bullets of even higher complexity can either be freely formed from quasi-continuous phase maps or discretely composed from addressable arrays of identical nondiffracting beams. The characterization of few-cycle light bullets requires spatially resolved measuring techniques. In our experiments, wavefront, pulse and phase were detected with a Shack-Hartmann wavefront sensor, 2D-autocorrelation and spectral phase interferometry for direct electric-field reconstruction (SPIDER). The combination of the unique propagation properties of light bullets with the flexibility of adaptive optics opens new prospects for applications of structured light like optical tweezers, microscopy, data transfer and storage, laser fusion, plasmon control or nonlinear spectroscopy.