Ultrashort Laser System Research Articles

Lens Fragmentation with Picosecond Laser Pulses After Artificial Cataract Induction with Microwaves.

Objectives: In this work we demonstrate the first laboratory study results of lens fragmentation with low-energy picosecond ultrashort laser pulses after artificial induction of cataract with microwave radiation on an ex vivo animal model. Background: This method will be evaluated with regard to the further development of lens fragmentation with novel ultrashort picosecond laser systems instead of ultrasonic phacoemulsification or the significantly more complex femtosecond laser fragmentation. Methods: As samples we used postmortem porcine eyes. The lenses were dissected and then irradiated in a microwave oven for artificial cataract induction. Subsequent computer-driven lens fragmentation was performed with a 12 ps, 1064 nm pulsed laser source with 100 µJ pulse energy, and 10 kHz pulse repetition rate. Results: Both the artificial cataract induction and the lens fragmentation were demonstrated. When inducing cataract, different degrees/stages of opaqueness and hardness could be achieved with different irradiation times and methods. The fragmentation with 12 ps pulses led to good results with regard to ablation depth and rate, especially for the softer lenses. Conclusions: As could be shown, low-energy picosecond ultrashort laser pulses are feasible for cataractous lens fragmentation on an ex vivo animal model with artificial cataract induction. Thus, this technique may influence future cataract surgeries by possibly being an alternative or extension to state-of-the-art methods. This will be evaluated with further tests and studies.

A Platform for Ultra-Fast Proton Probing of Matter in Extreme Conditions.

Recent developments in ultrashort and intense laser systems have enabled the generation of short and brilliant proton sources, which are valuable for studying plasmas under extreme conditions in high-energy-density physics. However, developing sensors for the energy selection, focusing, transport, and detection of these sources remains challenging. This work presents a novel and simple design for an isochronous magnetic selector capable of angular and energy selection of proton sources, significantly reducing temporal spread compared to the current state of the art. The isochronous selector separates the beam based on ion energy, making it a potential component in new energy spectrum sensors for ions. Analytical estimations and Monte Carlo simulations validate the proposed configuration. Due to its low temporal spread, this selector is also useful for studying extreme states of matter, such as proton stopping power in warm dense matter, where short plasma stagnation time (<100 ps) is a critical factor. The proposed selector can also be employed at higher proton energies, achieving final time spreads of a few picoseconds. This has important implications for sensing technologies in the study of coherent energy deposition in biology and medical physics.

Effect of wavelength on propagation of high-power femtosecond laser pulses in diamond

Numerical modeling of the propagation of high-power femtosecond laser beams in a diamond crystal in the self-focusing mode is carried out. The wavelength of the modeled beams is varied in a wide range (400 nm, 532 nm, 800 nm, and 1064 nm), which covers the most common ultrashort laser systems. For all the wavelengths analyzed, a limitation in the growth of laser energy density inside the diamond crystal with increasing pulse energy is found. This effect has a certain similarity to the effect of optical limitation in laser filaments, but manifests itself outside the filament formation zone and is capable of equalizing the laser energy density, as well as the electron density, inside the extensive pre-focal region.

Modeling of the femtosecond pulsed laser-induced damage of multi-layer dielectric gratings

The laser damage resistance of multi-layer dielectric gratings (MDGs) and metal multi-layer dielectric gratings (MMDGs) for femtosecond lasers directly determines the performance and life span of the high-energy laser system. It remains a significant challenge to predict the laser-induced damage threshold (LIDT) of MDGs and MMDGs for femtosecond lasers. Based on the nonlinear ionization and electric field intensity model, an effective method is proposed to calculate the LIDT of MDGs and MMDGs by introducing the dual temperature model. We take into account the evolutions of thermophysical parameters including the absorption coefficient, heat capacity, and thermal conductivity of the dielectric material as a function of the free-electron density and temperature. The free electron density of the conduction band combined with the electric field strength inside the grating can be calculated by the laser-induced ionization theory, more importantly, the transient distribution of the lattice temperature field generated by femtosecond laser irradiated grating excitation is obtained by the two-temperature model (TTM). This work is beneficial for guiding the design and fabrication of MDGs and MMDGs with high-LIDTs for ultra-short ultra-intense laser systems.

Unraveling the ultrafast dynamics of thermal-energy chemical reactions.

In this perspective, we discuss how one can initiate, image, and disentangle the ultrafast elementary steps of thermal-energy chemical dynamics, building upon advances in technology and scientific insight. We propose that combinations of ultrashort mid-infrared laser pulses, controlled molecular species in the gas phase, and forefront imaging techniques allow to unravel the elementary steps of general-chemistry reaction processes in real time. We detail, for prototypical first reaction systems, experimental methods enabling these investigations, how to sufficiently prepare and promote gas-phase samples to thermal-energy reactive states with contemporary ultrashort mid-infrared laser systems, and how to image the initiated ultrafast chemical dynamics. The results of such experiments will clearly further our understanding of general-chemistry reaction dynamics.

Generation of coherent XUV radiation at different interaction lengths using the ultrashort laser system

Generation of short wavelengths is obtained using the gas cells having different lengths. A Ti:sapphire laser system at 1[Formula: see text]kHz or 10[Formula: see text]Hz repetition rate is used to produce high harmonic generation (HHG). The gas cell producing optimum harmonic spectrum is determined using the 1[Formula: see text]kHz laser system. The optimum harmonic signal is obtained under the phase-matched condition. The total harmonic yield is increased by adjusting argon gas pressure in the different gas cell lengths (length of the interaction mediums). Harmonics up to the 27th order are obtained. The 1D model is used for the calculation of harmonic yield at different interaction lengths. The high-power laser system at the 10[Formula: see text]Hz repetition rate is used to generate high-order harmonics. The harmonic orders are extended to the 35th harmonic in argon gas. A harmonic spectrum in H2 gas is generated using different pump laser power. The power dependence of the harmonic signal from H2 gas is obtained. The variation of the harmonic spectrum for the different cell parameters is explained as a constructive or destructive buildup of generated harmonics while they propagate through the gas cell.

Effect of femtosecond laser interaction with human fibroblasts: a preliminary study

In in vitro methods and cell culture models, femtosecond (fs) laser interaction has been employed to assess its effect on the proliferation and morphology of human skin fibroblasts. We cultured a primary human skin fibroblast cell line on a glass plate, passages 17–23. The cells were irradiated with a 90-fs laser at a wavelength of 800 nm and a repetition rate of 82 MHz. The target received an average power of 320 mW for 5, 20, and 100 s, corresponding to the radiation exposures of 22.6, 90.6, and 452.9 J/cm2, respectively. Using a laser scanning microscopy technique, the photon densities were measured to be 6.4 × 1018, 2.6 × 1019, and 1.3 × 1020 photons/cm2 in a spot area of 0.07 cm2; the recorded spectra were obtained from the laser interaction after 0.00, 1.00, 25.00, and 45.00 h. The cell count and morphological changes showed that the cultured cells were affected by laser irradiation under photon stress; some fibroblasts were killed, while others were injured and survived. We discovered evidence of the formation of several coenzyme compounds, such as flavin (500–600 nm), lipopigments (600–750 nm), and porphyrin (500–700 nm). This study is motivated by the future development of a novel, ultra-short fs laser system and the need to develop a basic in vitro understanding of photon–human cell interaction. The cell proliferation indicated that cells are partly killed or wounded. The exposure of fibroblasts to fs laser fluence up to 450 J/cm2 accelerates cell growth of the viable residual cell.

Use of Green Fs Lasers to Generate a Superhydrophobic Behavior in the Surface of Wind Turbine Blades.

Ice generation on the surface of wind generator blades can affect the performance of the generator in several aspects. It can deteriorate sensor performance, reduce efficiency, and cause mechanical failures. One of the alternatives to minimize these effects is to include passive solutions based on the modification of the blade surfaces, and in particular to generate superhydrophobic behavior. Ultra-short laser systems enable improved micromachining of polymer surfaces by reducing the heat affected zone (HAZ) and improving the quality of the final surface topography. In this study, a green fs laser is used to micromachine different patterns on the surface of materials with the same structure that can be found in turbine blades. Convenient optimization of surface topography via fs laser micromachining enables the transformation of an initially hydrophilic surface into a superhydrophobic one. Thus, an initial surface finish with a contact angle ca. 69° is transformed via laser treatment into one with contact angle values above 170°. In addition, it is observed that the performance of the surface is maintained or even improved with time. These results open the possibility of using lasers to control turbine blade surface microstructure while avoiding the use of additional chemical coatings. This can be used as a complementary passive treatment to avoid ice formation in these large structures.

Online monitoring and active control of alignment errors in a tiled grating assembly using single wedge plate

With reference to application in chirped pulse amplification (CPA) based high-energy, ultra-short laser systems, online monitoring of alignment errors and active coherent locking of gratings in a tiled grating assembly (TGA) consisting of two gratings, each of size 140 mm x 120 mm and groove density 1740 grooves/mm, is presented. All the tiling alignment errors present in the TGA have been monitored simultaneously using a single wedge plate based lateral shearing interferometry and corrected using piezo motor driven 6-axis aligner stage. As a novelty, to have further advantages in the wedge plate based method for TGA alignment, the He-Ne laser beam diffracted/ reflected from the TGA has been folded back towards the source, which facilitated measurement sensitivity enhancement by a factor of two, and also generation and simultaneously capturing of both reflected and diffracted beam's interferograms using comparatively smaller dimension single wedge plate and CCD camera. The angular and translational alignment errors of the TGA have been actively locked in the range of ± 0.5 µrad and ± 50 nm respectively.

Four-Beam Tiled-Aperture Coherent Beam Combining of High-Power Femtosecond Laser With Two Compressors

The former reports of coherent beam combining (CBC) in the ultra-intense ultra-short laser field focus on the phase control technology and two-beam CBC. To study the four-beam tiled-aperture CBC in the ultra-intense and ultra-short laser field, we demonstrate four-beam CBC of femtosecond laser pulses based on chirped pulse amplification (CPA) scheme. In this experiment, the four beams were compressed using two grating compressors to simulate the situation in ultra-intense and ultra-short laser CBC systems. To achieve real-time measurement of the phase error between the four beams, we introduced a continuous reference laser and the phases of beams 2, 3, and 4 were locked to beam 1.A combined efficiency of 57% was achieved. Although the coherent combining efficiency is not very high, the possibility of a four-beam CBC based on the CPA scheme was confirmed, especially with different grating compressors which can reduce the limitation of gratings in single beam petawatt lasers. Finally, the remaining difficulties in the implementation of CBC of four beams and the methods to improve its efficiency were analyzed. The use of CBC in ultra-intense and ultrashort laser systems is of considerable significance.

Survey of spatio-temporal couplings throughout high-power ultrashort lasers.

The investigation of spatio-temporal couplings (STCs) of broadband light beams is becoming a key topic for the optimization as well as applications of ultrashort laser systems. This calls for accurate measurements of STCs. Yet, it is only recently that such complete spatio-temporal or spatio-spectral characterization has become possible, and it has so far mostly been implemented at the output of the laser systems, where experiments take place. In this survey, we present for the first time STC measurements at different stages of a collection of high-power ultrashort laser systems, all based on the chirped-pulse amplification (CPA) technique, but with very different output characteristics. This measurement campaign reveals spatio-temporal effects with various sources, and motivates the expanded use of STC characterization throughout CPA laser chains, as well as in a wider range of types of ultrafast laser systems. In this way knowledge will be gained not only about potential defects, but also about the fundamental dynamics and operating regimes of advanced ultrashort laser systems.

Ultrashort pulse duration and broadband dual-comb laser system based on a free-running passively mode-locked Er-fiber oscillator

We have demonstrated pulse duration compression and wavelength extension to a dual-comb free-running passively mode-locked Er-fiber oscillator. At first, the pulse duration of the dual-comb ultrashort pulses from the oscillator is nonlinearly compressed to ∼50 fs by fiber amplifiers. Then, a dual-comb supercontinuum laser source with broadband wavelength band from 1.0 to more than 1.75 µm is achieved when the compressed pulses are used to pump highly nonlinear fibers. At last, utilizing the dual-comb supercontinuum source as the seed for spectral-selected Yb-fiber amplifiers, sub-100 fs ultrashort pulses are obtained at 1.0 µm. This free-running dual-comb fiber laser system can provide ultrashort pulses with sub-100 fs pulse duration at 1.0 µm, 1.5 µm and broadband supercontinuum covering 1.0 to more than 1.75 µm, so it can be potentially deployed in dual-comb applications such as absolute distance measurement, coherent anti-stokes Raman spectroscopy, and mid-infrared differential frequency generation.

Towards Rapid Fabrication of Superhydrophobic Surfaces by Multi-Beam Nanostructuring with 40,401 Beams.

Superhydrophobic surfaces attract a lot of attention due to many potential applications including anti-icing, anti-corrosion, self-cleaning or drag-reduction surfaces. Despite a list of attractive applications of superhydrophobic surfaces and demonstrated capability of lasers to produce them, the speed of laser micro and nanostructuring is still low with respect to many industry standards. Up-to-now, most promising multi-beam solutions can improve processing speed a hundred to a thousand times. However, productive and efficient utilization of a new generation of kW-class ultrashort pulsed lasers for precise nanostructuring requires a much higher number of beams. In this work, we introduce a unique combination of high-energy pulsed ultrashort laser system delivering up to 20 mJ at 1030 nm in 1.7 ps and novel Diffractive Laser-Induced Texturing element (DLITe) capable of producing 201 × 201 sub-beams of 5 µm in diameter on a square area of 1 mm2. Simultaneous nanostructuring with 40,401 sub-beams resulted in a matrix of microcraters covered by nanogratings and ripples with periodicity below 470 nm and 720 nm, respectively. The processed area demonstrated hydrophobic to superhydrophobic properties with a maximum contact angle of 153°.

Generation of a 59 fs pulse with a 550 nm spectral range in a mode-locked Er-doped single-mode fiber laser system

We demonstrated a compact broad spectrum ultrashort fiber laser system that employed a similariton laser as a seed and fiber-optic nonlinearities in an erbium-doped fiber amplifier to broaden the optical spectrum. The system generated a stable 59 fs ultrashort pulse with a spectral bandwidth beyond 550 nm (i.e. 1490–2030 nm). The mode-locked oscillator produced an 85.9 fs similariton pulse at the wavelength of 1553 nm. The maximum average output power of the laser system was 251 mW. This new type of standard single-mode fiber-format energetic ultrashort system was suitable as a source for biological tissue imaging, broad-band spectroscopy, and optical frequency combs.

Angle- and polarization-dependent reflection and backscattering from FS-LIPSS covered stainless steel surfaces: experimental study

Large double-scale random rough surfaces covered by femtosecond laser-induced periodic structures are produced experimentally. Irradiation circumstance is considered as the main approach for manufacturing various samples. Fabricated surfaces on the steel substrate are full of high spatial frequency ripples which are crossed by deeper grooves. A polarized He–Ne laser beam is utilized to illuminate the complex surfaces and probe their optical features. Intensities of the reflected and backscattered beams at various incident angles (from small values to large amounts) are monitored in this regard. Results show that the linearly polarized probe beam is scattered diffusely and also de-polarized significantly after striking the irradiated surfaces. However, the amounts of de-polarization of the reflected/backscattered lights are dependent on characteristics of the surface as well as the initial polarization and incident angle of the probe beam. Besides, a relation between the optical response and orientation of the samples is also observed. Finally, at last section, discussions concerning the physical phenomena that are playing a role in the optical response of the samples are presented in detail. Our experimental results reveal that photo-ablation with ultrashort laser systems is highly sensitive and fast enough method for fabrication of surfaces with applications in industry and medical fields in which controlled light reflections or backscattering are needed.

Generation and imaging of a tunable ultrafast intensity-rotating optical field with a cycle down to femtosecond region

A tunable ultrafast intensity-rotating optical field is generated by overlapping a pair of 20 Hz, 800 nm chirped pulses with a Michelson interferometer (MI). Its rotating rate can be up to 10 trillion radians per second ( $\text{Trad}/\text{s}$ ), which can be flexibly tuned with a mirror in the MI. Besides, its fold rotational symmetry structure is also changeable by controlling the difference from the topological charges of the pulse pair. Experimentally, we have successfully developed a two-petal lattice with a tunable rotating speed from $3.9~\text{Trad}/\text{s}$ up to $11.9~\text{Trad}/\text{s}$ , which is confirmed by our single-shot ultrafast frame imager based on noncollinear optical-parametric amplification with its highest frame rate of 15 trillion frames per second (Tfps). This work is carried out at a low repetition rate. Therefore, it can be applied at relativistic, even ultrarelativistic, intensities, which usually operate in low repetition rate ultrashort and ultraintense laser systems. We believe that it may have application in laser-plasma-based accelerators, strong terahertz radiations and celestial phenomena.

Discovery of Several New Families of Saturable Absorbers for Ultrashort Pulsed Laser Systems

Saturable Absorber (SA) is a key element of any passive mode-locked laser system to provide ultrashort laser system. So far various materials have been proposed that could be used for this purpose. However, the field is still looking for new ways to make the fabrication process easier and cost-effective. Another challenge in testing mode-locked laser systems using various SA samples is the lack of knowledge in preparing these by laser physicists given this is outside their remit of expertise. In this study, we have proposed a novel method to produce these SAs from plastic materials and glycol. Our new method relies upon increase in thickness up to a value where the modulation depth is enough to give stable ultrashort pulses. Although we have shown this method for four materials; similar approach could be applied to any material. This will open the door of unlimited families of SAs that could be easily prepared and applied without any prior knowledge in material sciences.

Antireflective coatings with high damage threshold prepared by laser ablation

Latest developments in the field of high power ultra-short pulse lasers have led to intensive studies dedicated to the fabrication possibility of new antireflective coatings which exhibit high damage threshold. Therefore, this study is focused on the deposition and characterization of metal oxide heterostructures followed by laser-induced damage threshold tests which evidence their application in high power laser optics. Al2O3, SiO2, and HfO2 layers are combined to obtain different heterostructures, i.e. HfO2/Al2O3/HfO2/Al2O3/HfO2 and HfO2/SiO2/HfO2/SiO2/HfO2. The metal oxide heterostructures are deposited in a controllable oxygen atmosphere, either at room temperature or high temperatures (600 °C) by pulsed laser deposition (PLD). The morphological, structural and optical properties of the as-deposited heterostructures are first investigated. Atomic force microscopy and spectroscopic ellipsometry investigations reveal a lower roughness of the heterostructures based on HfO2/Al2O3 layers grown at 600 °C as compared to those grown at room temperature. Furthermore, following the laser-induced damage threshold (LIDT) tests carried out with a Ti–Sapphire laser, higher LIDT values are obtained for the HfO2/Al2O3-based heterostructures than for the HfO2/SiO2-based heterostructures. The ability to control the morphological and structural properties of the antireflective coatings by modifying the deposition parameters of the metal oxide heterostructures demonstrates that PLD is a suitable technique for the manufacturing of antireflective coatings for high power ultra-short laser systems.

Dynamic chromatic aberration pre-compensation scheme for ultrashort petawatt laser systems.

A novel chromatic aberration pre-compensation scheme for ultrashort petawatt laser systems was proposed. The pre-compensation scheme consists of a convex lens, group of concave lenses, and a spherical reflector combined with a conventional vacuum chamber. It provides a versatile method to accurately compensate the chromatic aberration of an entire laser system via controlling the amount of propagation time delay (PTD) induced by the compensator without changing the input and output beam size. A compensator, tailored based on the proposed scheme, was designed and experimentally evaluated for the Shen-Guang-II 5PW (SG-II 5PW) laser system at Shanghai Institute of Optics and Fine Mechanics (SIOM). The experimental results verified that chromatic aberration in the laser system was almost fully compensated: the size of laser beam focused by an f/2.42 off-axis parabolic mirror (OAP) was reduced tremendously from 32×18μm2to about 4×4μm2at full width at half maximum (FWHM). The proposed scheme provides the flexibility to accurately correct chromatic aberration in high-power laser systems within a wide dynamic range.

Influence of nodular defect size on metal dielectric mixed gratings for ultra-short ultra-high intensity laser system

With the development of the high power laser facilities, laser resistance of pulse compression grating is getting increased attention. The metal dielectric mixed grating (MDMG) is widely studied, because it has wide diffraction bandwidth, low surface stress and potential high threshold. The effect of nodular defect size on damage of MDMG is studied by experiments and theoretical analysis in two typical cases. The initial damages appear in the district of nodular defect due to local electric field enhancement induced by nodular defect, which is different with the initial damage located in grating pillar of the perfect MDMG. Laser induced ionization theory is introduced to describe the damage process and give a quantitative analysis on the relationship of laser damage and nodular defect size. It is concluded that local electric field enhancement in nodular defect area would cause the electron density to reach the critical value and lead to the damage occurrence, and nodule damage critical diameters are 1400 nm and 1500 nm respectively in case 1 (seed under the grating pillar) and case 2 (seed under the grating groove) which will lead to nodule damage initiation.