Femtosecond Vortex Research Articles

Femtosecond vortices generated from a self-started Kerr-lens mode-locked Hermite-Gaussian thin-disk oscillator with a defective mirror

Femtosecond vortices generated from a self-started Kerr-lens mode-locked Hermite-Gaussian thin-disk oscillator with a defective mirror

Ultrafast thermal modulation dynamics during ripple formation induced by a femtosecond laser multi-pulse vortex beam

Abstract This work theoretically investigated the ultrafast thermal modulation dynamics during early formation of ripples on an Au film induced by femtosecond laser multi-pulse vortex beam irradiation. An extended two-temperature dynamics model that comprehensively considers optical interference modulation for the formation of seed ripples, transient reflectivity and non-equilibrium thermal transfer was self-consistently built to predict high-contrast ripple formation. The two-dimensional evolution of electron and phonon temperature modulations during ripple formation in a high non-equilibrium state of Au film were obtained via femtosecond laser multi-pulse vortex beam irradiation. It was revealed that ripple contrast can be significantly amplified by shortening the laser wavelength, increasing the pulse number, or enlarging the laser fluence of the vortex beam. Moreover, the electron–phonon coupling time during ripple formation is fully explored in detail. This study provides valuable insights into optimizing laser parameters for controlled high-contrast ripple formation on Au films.

Generation of sub-100 fs vortices from a Kerr-lens mode-locked Hermite-Gaussian Yb:CALGO oscillator.

Mode-locked oscillators with high-order transverse modes are excellent platforms for generating femtosecond optical vortices with high average power and good propagation stability. These have important applications in diverse fields such as optical communication, strong-field physics, and laser processing. So far, generating vortex pulses with ultrashort pulse duration remains a challenge. In this Letter, we report a Kerr-lens mode-locked Yb:CALGO laser oscillator delivering Hermite-Gaussian (HG) pulses with a pulse duration of 86 fs using a non-collinear pumping technique. 91 fs optical vortex pulses were generated by using a cylindrical-lens mode converter. To the best of our knowledge, this is the shortest pulse duration ever obtained from a diode pumped solid-state mode-locked oscillator with a pure high-order Hermite-Gaussian mode. The phase structures of the generated femtosecond vortices are characterized.

Attosecond vortex pulse trains

The landscape of ultrafast structured light pulses has significantly advanced thanks to the ability of high-order harmonic generation (HHG) to translate the spatial properties of infrared laser beams to the extreme-ultraviolet (EUV) spectral range. In particular, the up-conversion of orbital angular momentum (OAM) has enabled the generation of high-order harmonics whose OAM scales linearly with the harmonic order and the topological charge of the driving field. Having a well-defined OAM, each harmonic is emitted as an EUV femtosecond vortex pulse. However, the order-dependent OAM across the harmonic comb precludes the synthesis of attosecond vortex pulses. Here we demonstrate a method for generating attosecond vortex pulse trains, i.e., a succession of attosecond pulses with a helical wavefront, resulting from the coherent superposition of a comb of EUV high-order harmonics with the same OAM. By driving HHG with a polarization tilt-angle fork grating, two spatially separated circularly polarized high-order harmonic beams with order-independent OAM are created. Our work opens the route towards attosecond-resolved light-matter interactions with two extra degrees of freedom, spin and OAM, which are particularly interesting for probing chiral systems and magnetic materials.

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.

Influences of turbulence on the filamentation of femtosecond Gaussian and vortex pulses in water

We report an experimental study on the behaviour of femtosecond Gaussian and vortex laser pulses during filamentation in turbulent water. The transverse and longitudinal wandering, intensity, and spectral broadening of the filamentation are evaluated. The results show that the filamentation of femtosecond vortex pulses is less affected by water turbulence than that of Gaussian pulses. Furthermore, the filamentation is less affected when the turbulence is located close or within the filamentation region, in contrast to the self-focusing region. These findings highlight the potential benefits of using femtosecond vortex pulse filaments in liquid-based applications.

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.

High-power femtosecond vortices generated from a Kerr-lens mode-locked solid-state Hermite-Gaussian oscillator.

We report the generation of high-order transverse modes from a Kerr-lens mode-locked femtosecond laser. Two different orders of Hermite-Gaussian modes were realized by non-collinear pumping, which were converted into the corresponding Laguerre-Gaussian vortex modes using a cylindrical lens mode converter. The mode-locked vortex beams, with an average power of 1.4 W and 0.8 W, contained pulses as short as 126 fs and 170 fs at the first and second Hermite-Gaussian mode orders, respectively. This work demonstrates the possibility of developing Kerr-lens mode-locked bulk lasers with various pure high-order modes and paves the way for generating ultrashort vortex beams.

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.

Curve-shaped ultrashort laser pulses with programmable spatiotemporal behavior

Structured ultrashort laser pulses with controlled spatiotemporal properties are emerging as a key tool for the study and application of light–matter interactions in different fields such as microscopy, time-resolved imaging, laser micro-machining, particle acceleration, and attosecond science. In practice, a structured ultrashort pulse focused along a target trajectory with controlled pulse dynamics is required, e.g., to set the trajectory and velocity of the resulting intensity peak. Here, to address this challenging problem, we present a technique and experimental setup that allows straightforward engineering of structured ultrashort laser pulses with control of their spatiotemporal properties enabling tailored pulse propagation dynamics along the target trajectory. Our theoretical framework describes the design and control of this kind of curve-shaped laser pulse in terms of the curve geometry and phase prescribed along it. We have derived a closed-form expression that describes the interplay between the curve geometry and prescribed phase governing the pulse dynamics, including the temporal behavior of the pulse peak intensity while preserving the pulse duration. The theoretical results and the corresponding numerical simulations allow us to analyze the pulse dynamics on the example of femtosecond curve-shaped vortex pulses, including contour-shaped pulses created to follow the outline of objects at micrometer scale. The experimental results demonstrate the generation of these structured ultrashort pulses. These findings could pave the way for the next generation of ultrashort laser-based optical tools for the study and control of light–matter interactions.

Tunable Mid‐Infrared Detail‐Enhanced Imaging With Micron‐Level Spatial Resolution and Photon‐Number Resolving Sensitivity

AbstractThe underdevelopment of mid‐infrared (MIR) components and detectors greatly limits the spatial resolution and sensitivity of MIR imaging. To overcome these limitations, MIR detail‐enhanced imaging is enhanced via non‐degenerate optical parametric amplification (OPA) pumped by a femtosecond vortex pulse. This design renders MIR illumination into a visible image by nonlinear wavelength‐conversion, together with a high OPA gain, large spatial bandwidth, and remarkable sensitivity. These experiments show that the design can realize MIR imaging with a spatial resolution of up to 114 line pairs per millimeter and a 2D spatial bandwidth product of up to 62 900, over a spectral region tunable from 2.0 to 3.0 µm. Equally important, this setup simultaneously achieves excellent imaging sensitivity of 25 photons at room temperature. It is thought that this work provides a powerful way to realize effective real‐time MIR imaging with an excellent spatial resolution even in very weak illumination environments, which can benefit many applications from semiconductor material characterization and biomedical imaging to security.

Ultrafast thermalization dynamics in Au/Ni film excited by femtosecond laser double-pulse vortex beam

We theoretically investigated ultrafast thermalization dynamics of spatio-temporal evolutions of temperature fields in typical Au/Ni film excited by femtosecond laser double-pulse vortex beam. It is proposed that the enhanced energy deposition in the layered Au/Ni film can be unprecedentedly facilitated via applying femtosecond laser double-pulse vortex beam, preferentially giving rise to vortical thermalization for the phonon system of bottom Ni layer on picosecond timescale. It can be explained as the intensified dynamics of double-pulse vortex beam interacting with the excited intermediate state of Au/Ni film and the following energy competitive processes of the electron thermal diffusion and the electron-phonon coupling in respective layers. Moreover, the electron-phonon coupling period of Au/Ni film with respect to the crucial laser parameters of pulse separation and vortex beam fluence are explored in details. The results can be basically helpful for understanding the ultrafast dynamics of vortical thermalization for advancing the potential applications of compound-cavity fabrication, stimulated emission depletion (STED) imaging and ultrahigh time-resolved spectroscopy, etc.

How to compress ultrashort vortex pulses.

Potential applications of ultrashort vortex pulses (ultrashort pulses with helical wavefronts) require compact tools to compensate for dispersion during their generation and propagation. In this work, a global simulated-annealing optimization algorithm, based on analysis of the temporal properties and waveforms of femtosecond vortex pulses, is used to design and optimize chirped mirrors. Performances of the algorithm through different optimization approaches and chirped mirror designs are presented.

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.