Femtosecond Vortex Beam Research Articles

Generation of a mid-infrared femtosecond vortex beam from an optical parametric oscillator.

Mid-infrared femtosecond vortex beams generated by optical parametric oscillators (OPO) are reported for the first time, to the best of our knowledge. Order-tunable femtosecond Hermite-Gauss beams from the first to sixth order are produced from a synchronously pumped OPO and then converted into the corresponding first through sixth-order femtosecond vortex beams by a cylindrical lens mode converter. By slightly tuning the cavity length, the wavelength of the vortex beam can be continuously tunable in the range from 2323 to 2382 nm, and the pulse duration can be changeable from ${\sim}{400}\;{\rm fs}$∼400fs to ${\sim}{1.1}\;{\rm ps}$∼1.1ps. The work provides a flexible and reliable way to generate mid-infrared femtosecond vortex beams, and is of special significance for expanding the wavelength range of femtosecond vortex beams and their application fields.

Differential Nonlinear Absorption of an Elliptically Polarized Femtosecond Vortex Beam in Tellurite Glass-=SUP=-*-=/SUP=-

We present differential nonlinear absorption of an elliptically polarized femtosecond laser vortex beam carrying an angular momentum of l = -1 in tellurite glass. Transmission measurements were utilized to measure the nonlinear absorption of the glass on a shot-by-shot basis, and to investigate the modification threshold for the beam. Additionally, the effect of pulse energy on the polarization ellipse was investigated. The results revealed a reduction in the ellipticity as the incident pulse energy was increased. Non-rotation of the ellipsoid indicated that there is no change in the third order nonlinear susceptibility (chi(3)) of the telluride glass upon irradiation by the femtosecond beam. The transmission of multiple pulses through the same spot revealed that although an effect was evident for the first ten pulses, the subsequent pulses had no effect. The transmission of a complete cycle of increasing and decreasing pulse energies revealed no hysteresis of the femtosecond laser vortex beam in tellurite glass in contrast to that reported for Gaussian beams in silica glass. Keywords: vortex beam, tellurite glass, nonlinear absorption.

Formation of Annular Light Bullets in a Femtosecond Vortex Beam

The formation of annular light bullets during the filamentation of femtosecond optical vortices in a LiF crystal under conditions of anomalous group velocity dispersion is considered. Features of the spatiotemporal dynamics of pulses are described. Quantitative estimates of the parameters of light bullets are given. The mechanism of the formation of a light bullet in the first nonlinear focus is analyzed. Models of a stationary optical vortex and a slit beam are considered.

Inverted field interferometer for measuring the topological charges of optical vortices carried by short pulses

In this work, we present an improved technique for measuring both the magnitude and sign of the topological charge of input optical vortex beams carried by short laser pulses. Numerical simulations and experimental evidences for the interference signal obtainable at the output of an inverted field interferometer (IFI) valid for both continuous wave and femtosecond optical vortex beams and pulses with an eventual pulse front tilt are in an excellent agreement. An IFI also appears to be a valuable tool for calibrating a built-in variable delay line and for estimating an eventual pulse front tilt of the input ultrashort laser pulses without any realignments. As a trivial side effect, by blocking alternatively one of the two arms of the interferometer, one can use OV beams/pulses of opposite TCs, which are well located in the plane of a desired target, eventually — at precisely known time delays.

Conical Hollow Microhelices with Superior Swimming Capabilities for Targeted Cargo Delivery.

Inspired by flagellate microorganisms in nature, the microhelix is considered as an ideal model for transportation in fluid environment with low Reynolds number. However, how to promote the swimming and loading capabilities of microhelices with controllable geometries remains challenging. In this study, a novel kind of conical hollow microhelices is proposed and a method is developed to rapidly fabricate these microhelices with controllable parameters by femtosecond vortex beams generated from spatial light modulation along helical scanning. Conical hollow microhelices with designable heights (H = 45-75 µm), diameters (D = 6-18 µm), pitch numbers (Pi = 2-4), taper angles (T = 0.1-0.6 rad), and pitch periods (ΔP = 10-30 µm) are efficiently fabricated. In addition, compared with straight microhelices, the forward swimming capability of conical microhelices increases by 50% and the lateral drift of the conical hollow microhelices is reduced by 70%. Finally, the capabilities of these conical hollow microhelices for nanocargo loading and release by the inner hollow core, as well as transportation of neural stem cells by the outer surface are demonstrated. This work provides new insights into faster and simultaneous transportation of multicargoes for hybrid drug delivery, targeted therapy, and noninvasive surgery in vivo.

Direct generation of femtosecond vortex beam from a Yb:KYW oscillator featuring a defect-spot mirror

We have demonstrated the first direct femtosecond vortex beam generation from a semiconductor saturable absorber mirror (SESAM) mode locked Yb:KYW oscillator, featuring a homemade defect-spot mirror. Experimentally, a Laguerre Gaussian LG01 vortex beam with a repetition rate of ∼103.5 MHz and a pulse duration of 298 fs has been obtained. Both the defect mirror induced spatial loss modulation and intra-cavity astigmatism are responsible for the doughnut mode generation. Further, the chirality of the vortex beam is controlled by slightly tilting the angle of the Yb:KYW crystal, which hence breaks the azimuthal symmetry inside the laser cavity and leads to differential losses for LG0,+1 and LG0,-1 modes. The potential of the vortex beam’s amplification by a 2-m long Yb-doped double cladding fiber (DCF) amplifier has also been demonstrated.

Simultaneously spatially and temporally focused femtosecond vortex beams for laser micromachining

We combine conventional beam shaping with simultaneous spatial and temporal focusing (SSTF) to generate femtosecond SSTF vortex beams. We demonstrate the utility of these beam structures by producing single-shot encryption of doughnut-shaped ablation marks on the back surfaces of glass plates. The SSTF enables this type of machining by significantly reducing the nonlinear beam distortions on propagation through the glass.

Nonlinearity-Induced Multiplexed Optical Trapping and Manipulation with Femtosecond Vector Beams.

Optical trapping and manipulation of atoms, nanoparticles, and biological entities are widely employed in quantum technology, biophysics, and sensing. Single traps are typically achieved with linearly polarized light, while vortex beams form rotationally unstable symmetric traps. Here we demonstrate multiplexed optical traps reconfigurable with intensity and polarization of the trapping beam using intensity-dependent polarizability of nanoparticles. Nonlinearity combined with a longitudinal field of focused femtosecond vortex beams results in a stable optical force potential with multiple traps, in striking contrast to a linear trapping regime. The number of traps and their orientation can be controlled by the cylindrical vector beam order, polarization, and intensity. The nonlinear trapping demonstrated here on the example of plasmonic nanoparticles opens up opportunities for deterministic trapping and polarization-controlled manipulation of multiple dielectric and semiconductor particles, atoms, and biological objects since most of them exhibit a required intensity-dependent refractive index.

Temporally shaped Laguerre–Gaussian femtosecond laser beams

Femtosecond vortex beams with adjustable temporal pulse shapes are generated. These shaped laser pulses are characterized in the spectral domain by determination of the spectral amplitude and phase as well as in the spatial domain by expansion of the beam profile in a superposition of Laguerre-Gaussian transversal laser modes. The experiments demonstrate that the temporal pulse shapes impressed with a pulse shaper based on a programmable liquid-crystal spatial light modulator are basically unaltered by subsequent transmission through a spiral phase plate, while a high-quality optical vortex is imposed. The combination of programmable pulse shapes and optical vortices in femtosecond laser beams opens new possibilities for applications in micromachining, high harmonic generation, and microscopy.

Sub-diffraction-limited fluorescent patterns by tightly focusing polarized femtosecond vortex beams in a silver-containing glass.

We report that the shape and size of fluorescent patterns can be controlled by the focused laser intensity distribution, which depends on irradiation conditions as well as on the spin and orbital angular momenta being carried by light, inducing the formation of silver cluster patterns in a silver-containing zinc phosphate glass. In particular, we demonstrate that sub-diffraction-limited inner structures of fluorescent patterns can be generated by direct laser writing (DLW) with tightly focused femtosecond laser vortex beams as Laguerre-Gauss modes (LG0l) with linear and left-handed circular polarizations. We believe this technique, further combined with dual-color DLW, can be useful and powerful for developing structured light enabled nanostructures.

Generation of Femtosecond Optical Vortex Beams in All-Fiber Mode-Locked Fiber Laser Using Mode Selective Coupler

We experimentally demonstrated a high-order optical vortex pulsed laser based on a mode selective all-fiber fused coupler composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity not only acts as mode converter from LP01 mode to LP11 or LP21 modes with a broadband width over 100 nm, but also directly delivers femtosecond vortex pulses out of the mode locked cavity. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in mode-locked fiber laser. The generated 140 femtosecond vortex beam has a spectral width of 67 nm centered at 1544 nm.

Femtosecond Laser High Precision Fabrication for Novel Applications

Background: Femtosecond laser provides a versatile tool for microand nanofabrication, such as periodic surface structures with a periodicity much less than the wavelength, microfluidic channels for biological uses, photonic metamaterials and optical integrated circuits for quantum applications. These facts prove that fabrication methods based on femtosecond laser have provided convenience for novel applications in the fields of high precision fabrication, experimental quantum physics, functional materials, etc. Methods: We focus on the novel applications of the femtosecond laser high precision fabrication, such as quantum integrated circuits, biological and medical devices, multifunctional surfaces and functional periodic arrays. These applications are mostly undertook this 5 years, especially the quantum integrated circuits which were studied since 2013. Moreover, case studies were shown to illustrate the representative results and study methods. Results: We have reviewed 128 papers that are associated with the novel applications and case studies. Approximate 50% papers introduced the novel applications in the fields of quantum integrated circuits, biological and medical devices, multifunctional surfaces and functional periodic arrays. Other papers presented illustrated the case studies including femtosecond laser-induced forward transfer, characterization of femtosecond laser-induced periodic surface structures and results led by femtosecond vortex beam generated by a q-plate. Conclusion: The applications of the technology of the femtosecond laser direct writing waveguides in the fields of integrated quantum optics, photonic metamaterials, biological and medical circuits will surely pave a new way to fabricate full-size quantum computer and provide enormous kinds of biomimetic and medical finer devices. These state-of-the-art microdevices will provide much more convenience than before and even probably overturn the structure of the modern industry society. With the development of the relative technologies, femtosecond laser will prominently enhance the level of the fabrication in particular fields. Keywords: Direct laser writing, femtosecond laser, integrated quantum circuits, LIFT, LIPSS, microfluidic channels, optical vortex beam, precision fabrication.

White light generated by femtosecond optical vortex beams

In this work we report detailed experimental and numerical investigation of the white light generation by singly and doubly charged optical vortices propagating in a Kerr medium, where spectral broadening and transfer of topological charge (TC) into emerging spectral satellites take place due to self-phase modulation and degenerate four-wave frequency mixing (FWFM). Experiments performed with different pump beams show excellent agreement with theory. Singly and doubly charged white light vortices are observed within more than ±200 nm bandwidth after nonlinear propagation in Argon gas. Our experiment and theory data confirm that the TC transformation of the newly generated spectral components follows a law analogous to the one for energy conservation in the FWFM process. We also present results on the white light vortex stability.

Laser ablation of silicon induced by a femtosecond optical vortex beam.

We investigate laser ablation of crystalline silicon induced by a femtosecond optical vortex beam, addressing how beam properties can be obtained by analyzing the ablation crater. The morphology of the surface structures formed in the annular crater surface allows direct visualization of the beam polarization, while analysis of the crater size provides beam spot parameters. We also determine the diverse threshold fluences for the formation of various complex microstructures generated within the annular laser spot on the silicon sample. Our analysis indicates an incubation behavior of the threshold fluence as a function of the number of laser pulses, independent of the optical vortex polarization, in weak focusing conditions.

Direct femtosecond laser ablation of copper with an optical vortex beam

Laser surface structuring of copper is induced by laser ablation with a femtosecond optical vortex beam generated via spin-to-orbital conversion of the angular momentum of light by using a q-plate. The variation of the produced surface structures is studied as a function of the number of pulses, N, and laser fluence, F. After the first laser pulse (N = 1), the irradiated surface presents an annular region characterized by a corrugated morphology made by a rather complex network of nanometer-scale ridges, wrinkles, pores, and cavities. Increasing the number of pulses (2 < N < 100), the surface texture progressively evolves towards larger structures, while the central, non-ablated area is gradually decorated by nanoparticles produced during laser ablation. At large number of pulses (200 < N < 1000), a micro-tip with a nanostructured surface forms in the center of the irradiated area, which eventually disappears at still larger number of pulses (N > 1000) and a deep crater is formed. The nanostructure variation with the laser fluence, F, also evidences an interesting dependence, with a coarsening of the structure morphology as F increases. Our experimental findings demonstrate that direct femtosecond laser ablation with optical vortex beams produces interesting patterns not achievable by the more standard beams with a Gaussian intensity profile. They also suggest that appropriate tuning of the experimental conditions (F, N) can allow generating micro- and/or nano-structured surface for any specific application.

Analysis of femtosecond optical vortex beam generated by direct wave-front modulation

The generation of femtosecond optical vortex beam based on direct wave-front modulation with phase-only liquid crystal spatial light modulator is demonstrated. The spatial and temporal properties of the generated femtosecond vortices are investigated in detail. The experimental results show remarkable agreement with the results of the theoretical analysis and simulations, and indicate that the method we utilized can efficiently generate femtosecond optical vortex beam of arbitrary topological charge. The temporal and spectral properties of the femtosecond pulsed beam are hardly affected by the phase dislocation imposed on the wave-front.

Self-Focusing of Ultraintense Femtosecond Optical Vortices in Air

Our experiments show that the critical power for self-focusing collapse of femtosecond vortex beams in air is significantly higher than that of a flattop beam and grows approximately linearly with the vortex order. With less than 10% of initial transverse intensity modulation of the beam profiles, the dominant mode of self-focusing collapse is the azimuthal breakup of the vortex rings into individual filaments, the number of which grows with the input beam power. The generated bottlelike distributions of plasma filaments rotate on propagation in the direction determined by the sense of vorticity.

Single-step fabrication of stressed waveguides with tubular depressed-cladding in phosphate glasses using ultrafast vortex laser beams

We report on the fabrication of the stressed optical waveguide with tubular depressed- refractive-index cladding in phosphate glasses by use of femtosecond vortex beam. Strained regions were emerged in domains surrounding the tubular track. Waveguiding occurs mainly within the tube induced by femtosecond laser.

Self-focusing of femtosecond diffraction-resistant vortex beams in water

We report experiments on self-focusing of femtosecond diffraction-resistant vortex beams in water. These beams are higher-order Bessel beams with weak azimuthal modulation of the transverse intensity patterns. The modulation overrides the self-focusing dynamics and results in the formation of regular bottlelike filament distributions. The peak-power thresholds for filamentation, at a particular distance, are relatively accurately estimated by the adaptation of the Marburger formula derived earlier for Gaussian beams. The nonlinear conversion of the incident conical waves into the localized spatial wave packets propagating near the beam axis is observed.

Dielectric Materials Volume and Surface Processing Using Femtosecond Vortex Beam

We demonstrate the possibility to modify dielectric materials (borosilicate glass) surface and volume using tightly focused femtosecond vortex beams (with different topological charge m = 1; m = 2; m = 3; m = 5; m = 10; m = 25) formed by computer generated amplitude holograms. By controlling femtosecond vortex beam (λ = 1030nm) waist position Δz, number of incident pulses N, topological charge m and pulse energy EP we get different structures induced: from micron-size ring-shaped to nanosized structures.