Femtosecond Vortex Beam Research Articles

Spectrally-broadened femtosecond vortex beams in air with longitudinal pressure distribution.

The spectral broadening of femtosecond vortex beams in air with different longitudinal pressure distributions is studied numerically. By introducing a symmetrical pressure distribution, a sufficiently broadened spectrum while preserving vortex characteristics of the beam for different input energies can be generated. The proposed pressure distribution involves an increase during the self-focusing stage, followed by a flat-top and symmetric decrease. This approach takes advantage of the mechanism that the strong Kerr nonlinearity in the high-pressure filamentation region results in a broader spectrum towards the blue side through the self-phase modulation and ionization, while the weak Kerr nonlinearity in the low-intensity regions before and after filamentation with low pressure leads to the decrease of the intensity fluctuation and the preservation of vortex characteristics due to weak modulation instability. Consequently, the resulting vortex beam exhibits a broad enough spectrum, and the transform-limit duration reaches a single cycle. This study provides a valuable approach for generating single-cycle vortex beams.

Numerical simulation study on distinguishing nonlinear propagation regimes of femtosecond pulses in fused silica

We perform numerical simulations to investigate the nonlinear propagation dynamics of femtosecond Gaussian and vortex beams in fused silica. By analyzing the extent of spectral broadening, we are able to distinguish between the linear, self-focusing, and filamentation regimes. Additionally, the maximum intensity and fluence distribution within the cross-section of the vortex beams are analyzed for different incident laser energies. The results demonstrate a direct correlation between the spectral broadening and the peak intensity of the femtosecond laser pulse. As a result, this provides a theoretical foundation for distinguishing different propagation regimes, and determining critical powers for self-focusing and filamentation of both femtosecond Gaussian and structured beams.

An Experimental Determination of Critical Power for Self-Focusing of Femtosecond Pulses in Air Using Focal-Spot Measurements

The filamentation of femtosecond pulses has attracted significant attention, owing to its unique characteristics and related applications. The self-focusing critical power of femtosecond pulses is one of the key parameters in the filamentation process and its application. However, the experimental determination of this power remains a challenging task. In this study, we propose an experimental approach to investigating the critical power for self-focusing of both femtosecond Gaussian and vortex beams with relatively low topological charges by analyzing the changes in the focal spot at different propagation distances. Our work offers a practical and convenient method for determining the self-focusing critical power of femtosecond pulses.

Conical Emission Induced by the Filamentation of Femtosecond Vortex Beams in Water

Conical emission is a typical nonlinear phenomenon that occurs during the filamentation of femtosecond laser pulses in transparent media. In this work, the conical emission induced by two kinds of typical vortex beams (i.e., Laguerre–Gaussian (LG) and Bessel–Gaussian (BG) beams) in water is experimentally studied. By recording the light spots of different spectra components from the supercontinuum induced by the vortex beams, the characteristics of the conical emission induced by femtosecond vortex beams are studied. It is found that the spots of the supercontinuum induced by the two kinds of vortex beams differ greatly from each other. The spots of the supercontinuum induced by the BG beams are a set of concentric rings like a rainbow with a white center, while the white light spots in the case of the LG beams are circular white disks, which are different from the commonly observed white light spots. By measuring the maximum divergence angle, it is observed that the divergence angle increases with a decrease in the wavelength, while it is merely affected by the topological charge, which is explained by the formation mechanism of conical emission in terms of self-phase modulation. Based on the observed results, we discuss the transfer of optical angular momentum during the supercontinuum induced by the filamentation of femtosecond vortex beams. This work may help to better understand the transfer of optical angular momentum in non-optical parametric processes as well as the interaction of high-intensity pulses with matter.

High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes, enabling multi-dimensional quantum control of light fields. Until now, generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration, high power, high vortex order, and a low cost and compact laser source has been very challenging due to the limitations of available generation methods. Here, we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques. This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order, which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator. The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and <650 fs, respectively. In particular, 424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W, several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

Nitrogen fluorescence emission induced by femtosecond vortex beams in air

Fluorescence emission is a typical nonlinear phenomenon occurring during the femtosecond laser filamentation in air. The characteristics of nitrogen fluorescence emission induced by femtosecond vortex beams in air are experimentally studied. It is found that the topological charge has a great influence on the nitrogen fluorescence emission induced by a femtosecond filament: the fluorescence signals from nitrogen molecules (e.g., 337, 357 and 380 nm spectral lines) and molecular nitrogen ions (e.g., 391 and 428 nm spectral lines) differ much in the intensity distribution along the propagation direction. The discrepancy is explained by revisiting the generation mechanisms for nitrogen fluorescence, and it is found that the intersystem crossing scheme can account for it, indicating that the intersystem crossing scheme plays the main role in the formation of This work is helpful to the understanding of the physical mechanism of fluorescence emission induced by the filamentation of femtosecond vortex beams in air.

Nitrogen fluorescence emission pumped by femtosecond optical vortex beams

An experimental investigation on nitrogen fluorescence emissions pumped by a single 800 nm fs optical vortex (OV) beam with different topological charges (TC) is reported. The intensity of the two prototype emission lines from neutral nitrogen molecules (N2) and molecular nitrogen ions (N2+), i.e., 337 nm and 391 nm, respectively, shows different TC dependences: the former slightly decreases as the TC increases, while the latter sharply decreases as the TC changes from zero to a non-zero value. The dependences of the 337-nm and 391-nm emission intensity on pulse energy are also different: the former shows a linear variation for different TCs, while the latter has an abrupt change in the slope when changing the TC. Furthermore, the dependence of the 337-nm emission intensity on gas pressure exhibits a plateau which is universal for different TCs. In contrast, the dependence of the 391-nm emission intensity on gas pressure shows an apparent hump which is respective of the TC. These actual new results will facilitate further theoretical study on the formation dynamics of the nitrogen fluorescence emissions induced by the OV beam, and inspire that such beam can been taken as a unique pure optical tool to manipulate the transitions between different electronic-vibrational states.

Experimentally determined critical power for self-focusing of femtosecond vortex beams in air by a fluorescence measurement.

The filamentation of the femtosecond vortex beam has attracted much attention because of the unique filamentation characteristics, such as annular distribution and helical propagation, and related applications. The critical power for self-focusing of the femtosecond vortex beams is a key parameter in the filamentation process and applications. But until now, there is no quantitative determination of the critical power. In this work, we experimentally determine the self-focusing critical power of femtosecond vortex beams in air by measuring fluorescence using a photomultiplier tube. The relation between the self-focusing critical power and the topological charge is further obtained. Our work provides a simple method to determine the self-focusing critical power not only for vortex beams but also for Airy, Bessel, vector, and other structured laser beams.

Few-cycle vortex beam generated from self-compression of midinfrared femtosecond vortices in thin plates

We demonstrate theoretically that few-cycle vortex beam with subterawatt peak power can be generated by self-compression of mid-infrared femtosecond vortex beam using the thin-plate scheme. The 3 {\mu}m femtosecond vortex beam with input duration of 90 fs is compressed to 15.1 fs with the vortex characteristics preserved. The conversion efficiency is as high as 91.5% and the peak power reaches 0.18 TW. The generation of the high-peak-power few-cycle vortex beam is owing to the proper spatiotemporal match by this novel scheme, where the spectrum is broadened enough, the negative group velocity dispersion can compensate the positive chirp induced by nonlinear effects, and multiple filamentation is inhibited for the keeping of the vortex characteristics. Our work will help to generate isolated attosecond vortices, opening a new perspective in ultrafast science.

Simultaneous spatial and temporal focusing optical vortex pulses for micromachining through optically transparent materials.

We introduce the optical vortex beam into simultaneous spatial and temporal focusing (SSTF) technique, and theoretically and experimentally demonstrate the local control of peak intensity distribution at the focus of a simultaneous spatiotemporally focused optical vortex (SSTF OV) beam. To avoid nonlinear self-focusing in the conventional focusing scheme, a spatiotemporally focused femtosecond laser vortex beam was employed to achieve doughnut-shaped ablation and high aspect ratio (∼28) microchannels on the back surface of 3 mm thick soda-lime glass and fused silica substrates.

Femtosecond filamentation of optical vortices for the generation of optical air waveguides.

We study the filamentation in air of multi-millijoule optical vortices and compare them with the classical filamentation regime. The femtosecond vortex beam generates multiple plasma filaments organized in a cylindrical geometry. This plasma configuration evolves into a meter-scale tubular neutral gas column that can be used as a waveguide for nanosecond laser pulses at 532 nm. It appears that optical vortices produce a more uniform heating along the propagation axis, when compared with Gaussian or super-Gaussian beams, and that the resulting low-density channel is poorly sensitive to the laser input power thanks to the combination of filamentation intensity clamping and phase vorticity.

High-power, femtosecond vortex beams generation in the visible and near-infrared region

Ultrafast vortex beams have been successfully utilized in a wide variety of applications. Here, we report on the generation of the visible and near-infrared cylindrical vector beams with femtosecond pulse duration and tens of Watts average output power. Based on an optimized fiber chirped-pulse amplification system and second harmonic generation technology, Gaussian beams are generated at the visible and near-infrared wavelength, respectively. After mode conversion operation, the high-quality cylindrical vector beams can be obtained with a central wavelength of 1036.13 nm and maximum output power and ultrafast pulse duration of 60.49 W/824 fs for radial vector beams and 59.67 W/783 fs for azimuthal vector beams. In the visible spectral region, the output cylindrical vector beams have a maximum output power of 30.78 W (radial vector beams) and 27.46 W (azimuthal vector beams) at 515.75 nm central wavelength. The pulse durations are measured to be 262 fs for radial vector beam and 273 fs for azimuthal vector beam at the maximum output power. The results demonstrate that the proposed laser systems may pay a promising way to develop high-power femtosecond cylindrical vector beams sources in the visible and near-infrared wavelength for optical communication, super-resolution imaging and laser processing.

Emission enhancement of femtosecond laser-induced breakdown spectroscopy using vortex beam

This paper used femtosecond Gaussian and vortex beams to ablate a Cu target and generate Cu plasmas. A Gaussian beam pulse is shaped into a vortex beam by a vortex wave plate (topological charge m = 1). The spectral intensity of Cu plasmas produced by the two lasers was measured, finding that the spectra with vortex beam were stronger than that with Gaussian beam. The spectral intensity was doubled by simply changing the Gaussian beam into the vortex beam. In addition, the position for maximum Cu atomic line emission along with the laser path was closer to the position of focusing-lens with increasing laser energy. Finally, the Boltzmann plot calculated the plasma temperature, finding that the plasma temperature with the vortex beam was also higher than that with the Gaussian beam. The results indicated that vortex beams could improve the spectral intensity of the femtosecond laser-induced plasmas.

Powerful supercontinuum vortices generated by femtosecond vortex beams with thin plates

We demonstrate numerically and experimentally the generation of powerful supercontinuum vortices from femtosecond vortex beams by using multiple thin fused silica plates. The supercontinuum vortices are shown to preserve the vortex phase profile of the initial beam for spectral components ranging from 500 nm to 1200 nm. The transfer of the vortex phase profile results from the inhibition of multiple filamentation and the preservation of the vortex ring with relatively uniform intensity distribution by means of the thin-plate scheme, where the supercontinuum is mainly generated from the self-phase modulation and self-steepening effects. Our scheme works for vortex beams with different topological charges, which provides a simple and effective method to generate supercontinuum vortices with high power.

The generation of femtosecond optical vortex beams with megawatt powers directly from a fiber based Mamyshev oscillator

Abstract Numerous approaches have been developed to generate optical vortex beams carrying orbital angular momentum (OAM) over the past decades, but the direct intracavity generation of such beams with practical output powers in the femtosecond regime still remains a challenge. Here we propose and experimentally demonstrate the efficient generation of high-peak-power femtosecond optical vortex pulses from a Mamyshev oscillator (MO) based on few-mode polarization-maintaining (PM) ytterbium-doped fibers (YDFs). By employing an appropriate intracavity transverse spatial mode selection technique, ultrafast pulses carrying OAM with selectable topological charge of l = ±1 are successfully generated with an average output power of ∼5.72 W at ∼24.35 MHz repetition rate, corresponding to a single pulse energy of ∼235 nJ. The chirped pulses can be compressed to ∼76 fs outside the cavity, leading to a pulse peak power of ∼2.2 MW. To the best of our knowledge, this is by far the highest pulse energy and peak power for optical vortex pulses ever generated directly from a fiber oscillator. This unprecedented level of performance should be of great interest for a variety of applications including materials processing and imaging.

Rapid Fabrication of 3D Chiral Microstructures by Single Exposure of Interfered Femtosecond Vortex Beams and Capillary‐Force‐Assisted Self‐Assembly

AbstractThe rapid fabrication of 3D chiral microstructures is of great significance in the fields of optics and mechanics. Here, a hybrid strategy is presented that combines spatial‐light‐modulator‐assisted two‐photon polymerization (SATPP, top‐down) and capillary‐force‐assisted self‐assembly (bottom‐up) for efficiently yielding chiral microstructures. Based on SATPP, the pre‐subunit can be efficiently fabricated via 3D chiral discrete vortex beam (CDVB), which is generated by interfering multiple parallel vortex beams. Then, the 3D chiral microstructures are assembled by subunits with the aid of meniscus‐directed capillary force. This strategy can fabricate stable 3D chiral microstructures and improve the manufacturing efficiency by more than 100 times. Furthermore, the height, diameter, rotation angle, and handedness of chiral microstructure can be flexibly regulated. Because the obvious chiral feature of the manufactured assembly, it can exhibit strong vortical dichroism when excited by the light carrying orbital angular momentums. Also, it can be used to prepare functional microrobots base on the chiral body rotation. This hybrid strategy can rapidly manufacture 3D chiral microstructures, which can be utilized as multifunctional scalable platform with promising prospect in micro/nano robotics, optical sensors, and advanced functional devices.

High-order femtosecond vortices converted from a tunable high-order Hermite–Gaussian Yb:CALGO laser

Femtosecond optical vortices open a variety of applications, ranging from femtosecond micro-processing to vortex strong-field physics. Thus far, the generation of high-order femtosecond vortices remains a challenge. The 1st to 4th order tunable femtosecond vortices generated by a z-type cavity semiconductor saturable absorber mirror mode-locked Yb:CALGO laser are reported in this study. The anti-reflection coated Yb:CALGO crystal is used as the gain material and is positioned perpendicular to the laser beam to avoid an angle dispersion, which results in multi-transverse-mode running. The astigmatism caused by the tilted curved mirrors is compensated by elevating the folding mirrors adjacent to the curved mirrors. The 1st to 5th order tunable femtosecond Hermite–Gaussian (HGn0) modes are realized by employing the off-axis pumping technique. The mode-locked pulse width is in the range of 220–580 fs for various HG modes. The mode-locked HG modes are converted into femtosecond vortices using a cylindrical-lens mode converter. The work provides a reliable way to generate high-order femtosecond vortex beams.

Helical filaments array generated by femtosecond vortex beams with lens array in air

When the peak power exceeds many times the critical power of self-focusing, femtosecond vortex beams will break up into multiple filaments with random distribution because of the initial random noise, which is adverse to the applications. By numerical simulation we demonstrate that the use of the lens array can resist the influence of initial noise, leading to a regular distribution of multiple filaments. During the propagation, the filaments are symmetrically distributed on a ring and rotate around the beam center, thus forming helical filaments array. Besides, the rotation rate of the filaments can be controlled by changing the vortex radius, topological charge, and focusing condition.

Study on Detection Method of Topological Charge of Femtosecond Vortex Beams

Study on Detection Method of Topological Charge of Femtosecond Vortex Beams

Differential Nonlinear Absorption of an Elliptically Polarized Femtosecond Vortex Beam in Tellurite Glass

Differential Nonlinear Absorption of an Elliptically Polarized Femtosecond Vortex Beam in Tellurite Glass