High-power Ultrashort Laser Pulses Research Articles

Length Dependence on Broadband Microwave Emission From Laser-Generated Plasmas

A high-power ultrashort laser pulse focused in air generates a plasma that radiates broadband electromagnetic waves. The transient current source responsible for the radiation remains an open area of study. To better understand how the laser drives currents in the plasma, we investigate the dependence of radiation at microwave frequencies on the length of the plasma column. Radiation patterns of the microwaves are consistent with currents that flow longitudinally (in the direction of laser propagation). We measure the angular dependence of the microwave frequency spectrum to determine the microwave radiation source characteristics. We also model the plasma evolution using a PIC code to provide initial conditions for a finite-difference time domain (FDTD) simulation of the plasma currents responsible for the microwaves. The experiments and simulations show that the plasma length determines the shape of the radiation pattern, and also weakly influences the microwave frequency content. Our research demonstrates that the radiation mechanism is coupled with variation in the current along the entire plasma length.

Interference-based laser-induced micro-plasma ablation of glass

AbstractGlass is one of the most important technical surfaces for numerous applications in automotive, optical, and consumer industries. In addition, by producing textured surfaces with periodic features in the micrometre range, new functions can be created. Although laser-based methods have shown to be capable to produce structured materials in a wide amount of materials, due to its transparency large bandgap dielectrics can be only processed in a controlled manner by employing high-power ultra-short pulsed lasers, thus limiting the employable laser sources. In this article, an interference-based method for the texturing of soda-lime glass using a 15 ns pulsed (1 kHz repetition rate) infrared (1053 nm) laser is proposed, which allows fabricating different periodic patterns with micrometre resolution. This method consists on irradiating a metallic absorber (stainless steel) put in direct contact with the glass sample and inducing locally an etching process on the backside of the glass. Then, the produced plasma at the interference maxima positions leads to the local fabrication of well-defined periodic line-like and dot-like surface patterns. The produced patterns are characterised using white light interferometry and scanning electron microscopy.

High-throughput micromachining with ultrashort pulsed lasers and multiple spots

A one step functionalization of large surfaces and layers could be enabled by high power ultrashort pulsed lasers. Despite today's availability of high power ultrashort pulsed lasers (up to several hundred watts), it is still a challenge to structure large surface areas, which is required on embossing rollers, within an acceptable processing time for industrial roll-to-roll production. Furthermore, fluencies near the ablation threshold are necessary to provide the highest depth resolution with minor side effects. So, it is a challenge to convert the increasing high average laser power into a high processing speed by maintaining the well-known quality of ultrashort pulsed lasers constant. One approach is the combination of a high-speed application and several parallel ablating laser spots. In this contribution, a newly developed high compact picosecond laser with high pulse repetition rates and an average power of 500 W was distributed into 8 and 16 parallel beamlets by a diffractive optical element. The power was controlled by single acousto-optical modulators per beamlet. Different functional surface geometries have been realized on an embossing roller as a master, which is used for the replication of the structures in a roll-to-roll or a roll-to-plate process. Feature sizes from 2 μm to 10 μm in areas of 2 m2 could be processed. With this approach, functional structures such as reduction in friction, improved soft touch, or light guiding elements can be generated on large surfaces within short processing times.

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.

Quantum theory of scattering of ultrashort electromagnetic field pulses by polyatomic structures.

In the theoretical description of the scattering of ultrashort electromagnetic field pulses (USP), the semiclassical approach is usually used, where the electromagnetic field is classical and the atomic system is quantum. This article shows the need to take into account the quantum properties of scattered photons, it is that if we take into account the interaction of an USP with a system of atoms, then with the scattering of the pulse it is possible to generate a given number of n photons with a probability P n. The main equations for the probability P n of the production of n photons and their average energy E n are found in an analytical form. It is shown that only for a certain number of atoms in the system can multiphoton scattering of ultrashort electromagnetic field pulses occur, where it is necessary to take into account the obtained basic equations for P n and E n. Various biomolecules, nanosystems and polyatomic structures can consist of such a number of atoms. This is especially important because experiments are currently being conducted with such structures at the present time using high-power ultrashort laser pulses. It is shown that the developed theory in limiting cases turns into well-known approaches of single-photon and multi-photon theories.

Strong nonlinear optical effects in micro-confined atmospheric air

Historically, nonlinear optical phenomena such as spectral broadening by harmonic generation have been associated with crystals owing to their strong nonlinear refractive indices, which are in the range of ∼10−14 cm2/W. This association was also the result of the limited optical power available from early lasers and the limited interaction length that the laser–crystal interaction architecture could offer. Consequently, these limitations disqualified a large number of materials whose nonlinear coefficient is lower than n2∼10−16 cm2/W as suitable materials for nonlinear optics applications. For example, it is a common practice in most of optical laboratories to consider ambient or atmospheric air as a “nonlinear optically” inert medium due to its very low nonlinear coefficient (∼10.10−19 cm2/W) and low density. Today, the wide spread of high-power ultra-short pulse lasers on one hand, and low transmission loss and high-power handling of Kagome hollow-core photonic crystal fiber on the other hand, provide the necessary ingredients to excite strong nonlinear optical effects in practically any gas media, regardless of how low its optical nonlinear response is. By using a single table-top 1 mJ ultra-short pulse laser and an air exposed inhibited-coupling guiding hollow-core photonic crystal fiber, we observed generation of supercontinuum and third harmonic generation when the laser pulse duration was set at 600 fs and Raman comb generation when the duration was 300 ps. The supercontinuum spectrum spans over ∼1000 THz and exhibits a typical spectral-density energy of 150 nJ/nm. The dispersion profile of inhibited-coupling hollow-core fiber imprints a distinctive sequence in the supercontinuum generation, which is triggered by the generation of a cascade of four-wave mixing lines and concluded by solitonic dynamics. The Raman comb spans over 300 THz and exhibits multiple sidebands originating from N2 vibrational and ro-vibrational Raman transitions. With the growing use of hollow-core photonic crystal fiber in different fields, the results can be applied to mitigate air nonlinear response when it is not desired or to use ambient air as a convenient nonlinear medium.

Uniform long focal depth with centimeter-scale range produced by an aspherical mirror

An aspherical mirror used to produce light fields with uniform long focal depth is first proposed by the energy conservation equation and the principle of equal optical path. The focusing characteristics of the novel mirror are studied in detail by numerical simulations based on scalar diffraction theory. Compared with parabolic mirror with similar parameters, the largest difference of surface profile is only at the micron level and the length of focal depth is stretched about 100 times. By using the apodization function, the oscillation of the axial intensity can be eliminated effectively and the axial inhomogeneity can be less than 0.5%. The effect of pulse shape on the focusing properties is also discussed and it is found that the pulse shape must be flat-top to obtain uniform long focal depth for a picosecond pulse. The novel mirror presented in this paper can be used for focusing high power ultrashort pulse laser with pulse width of picoseconds or femtoseconds, and there are many potential applications in physics of strong field.

Laser surface texturing with shifted method—Functional surfaces at high speed

Laser surface texturing is a promising technology for future wide applications of functional surfaces with specific properties like hydrophobic, antibacterial, adhesive, self-cleaning, anticorrosion, light absorbing, low friction, etc. Great advancements have been made in this field in the last few years, but in most cases, it takes from minutes up to 1 h to produce 1 cm2 of a functional surface. Even the availability of high-power ultrashort pulsed lasers in the last few years did not dramatically increase productivity, because there are physical limitations of current processing methods: heat accumulation and oxidation, plasma shielding effect, and precision at high speeds. In order to solve these limitations, there have been developed a new method called a shifted laser surface texturing (sLST) method. The new method has a potential to be at least 100 times more productive with no heat accumulation effect and virtually unlimited number of complex shape objects produced with high precision on the surface. In the present work, the principle and advantages of the method are described. The results of the method are compared with two standard methods (path filling of objects and hatch over all objects). The sLST method is presented in both single pulse and burst variants. Examples of its application on different materials for increased adhesion of surface coatings are shown.

Investigation of heat accumulation effects during deep hole percussion drilling by high power ultrashort pulsed laser radiation

In the last few years, commercially available ultrashort pulsed (usp) laser systems have reached average powers of several 100 W, which makes them also interesting for enhancing traditional applications. For example, laser drilling, a conventionally melt-dominated process, would benefit from the advantages of an usp ablation process. Due to the small processing area in laser drilling, substantial heat accumulates already at a few Watts of average power. This heat accumulation creates melt but also increases the mean ablation rate at least 1 order of magnitude and could lead to a productive drilling process. In this study, the increase of the mean ablation rate in percussion drilling due to heat accumulation is examined for various metals and sample thicknesses for high average powers of up to 300 W and a pulse energy in the milliJoule range. Those investigations have not yet been performed in such detail. It is shown that by doubling the pulse energy the drilling time can be decreased by 2 orders of magnitude due to heat accumulation. This behavior is valid for various metals like steel or aluminum, despite their varying material parameters. By analyzing the influence of different repetition rates and focal diameters, it is shown that the predominant parameter to characterize the heat accumulation is the average power. No significant difference is observed if the average power starting from 20 W is set up by a high pulse energy or repetition rate. Stainless steel has a different behavior compared to the other investigated metals when the pulse duration is changed from 2 to 20 ps, which is caused by a modified plasma generation. For stainless steel, the drilling time is found to be describable with one empirical formula for the entire range of examined average power and sample thickness.

Self-phase modulation and spectral broadening in millimeter long self-written polymer waveguide integrated with single mode fibers

Polymer waveguide bridges up to 1.2 mm in length were optically written between two single-mode optical fibres. Optical Nonlinearity of polymer waveguide was measured by coupling a high power ultra-short pulsed laser. Before waveguide fabrication fiber ends were treated by adhesion promoter which covalently bond to both the silica fibre and the photopolymer, also photopolymerization was conducted from both fibres. The new techniques improve both the mechanical properties and optical transmission of polymer bridges. The optical insertion loss was 1.1 dB for the longest waveguides. A considerably long interaction length of polymer waveguide allows spectral broadening and self-phase modulation features to occur in response to the high power laser propagation through the polymer bridges. The spectral broadening in polymer waveguide was much broader than that of 1.5 m plain fibre. The ultrafast nonlinearity of the polymer waveguides was determined to be of the order of 103 times the nonlinearity of silica.

Determination of the thermally induced focal shift of processing optics for ultrafast lasers with average powers of up to 525 W.

The continuous increase of the average laser power of ultrafast lasers is a challenge with respect to the thermal load of the processing optics. The power which is absorbed in an optical element leads to a temperature increase, temperature gradients, changing refractive index and shape, and finally causes distortions of the transmitted beam. In a first-order approximation this results in a change of the focal position, which may lead to an uncon-trolled change of the laser machining process. The present study reports on investigations on the focal shift induced in thin plano-convex lenses by a high-power ultra-short pulsed laser with an average laser power of up to 525 W. The focal shift was determined for lenses made of different materials (N-BK7, fused silica) and with different coatings (un-coated, broadband coating, specific wavelength coating).

Intense laser filament-solid interactions from near-ultraviolet to mid-infrared.

Studies of high-power ultrashort laser pulse interaction with matter are not only of fundamental scientific interest, but are also highly relevant to applications in the domain of remote sensing. Here, we investigate the effect of laser wavelength on coupling of femtosecond laser filaments to solid targets. Three central wavelengths have been used to produce filaments: 0.4, 0.8, and 2.0 µm. We find that, unlike the case of conventional tight focusing, use of shorter wavelengths does not necessarily produce more efficient ablation. This is explained by increased multi-photon absorption arising in near-UV filamentation. Investigations of filament-induced plasma dynamics and its thermodynamic parameters provide the foundation for unveiling the interplay between wavelength-dependent filament ablation mechanisms. In this way, strategies to increase the sensitivity of material detection via this technique may be better understood, thereby improving the analytical performance in this class of applications.

Revealing the nature of morphological changes in carbon nanotube-polymer saturable absorber under high-power laser irradiation

Composites of single-walled carbon nanotubes (SWNTs) and water-soluble polymers (WSP) are the focus of significant worldwide research due to a number of applications in biotechnology and photonics, particularly for ultrashort pulse generation. Despite the unique possibility of constructing non-linear optical SWNT-WSP composites with controlled optical properties, their thermal degradation threshold and limit of operational power remain unexplored. In this study, we discover the nature of the SWNT-polyvinyl alcohol (PVA) film thermal degradation and evaluate the modification of the composite properties under continuous high-power ultrashort pulse laser operation. Using high-precision optical microscopy and micro-Raman spectroscopy, we have examined SWNT-PVA films before and after continuous laser radiation exposure (up to 40 hours) with a maximum optical fluence of 2.3 mJ·cm−2. We demonstrate that high-intensity laser radiation results in measurable changes in the composition and morphology of the SWNT-PVA film due to efficient heat transfer from SWNTs to the polymer matrix. The saturable absorber modification does not affect the laser operational performance. We anticipate our work to be a starting point for more sophisticated research aimed at the enhancement of SWNT-PVA films fabrication for their operation as reliable saturable absorbers in high-power ultrafast lasers.

Femtosecond laser-induced damage threshold in snow micro-structured targets

Enhanced acceleration of protons to high energy by relatively modest high power ultra-short laser pulses, interacting with snow micro-structured targets was recently proposed. A notably increased proton energy was attributed to a combination of several mechanisms such as localized enhancement of the laser field intensity near the tip of $1~\unicode[STIX]{x03BC}\text{m}$ size whisker and increase in the hot electron concentration near the tip. Moreover, the use of mass-limited target prevents undesirable spread of absorbed laser energy out of the interaction zone. With increasing laser intensity a Coulomb explosion of the positively charged whisker will occur. All these mechanisms are functions of the local density profile and strongly depend on the laser pre-pulse structure. To clarify the effect of the pre-pulse on the state of the snow micro-structured target at the time of interaction with the main pulse, we measured the optical damage threshold (ODT) of the snow targets. ODT of $0.4~\text{J}/\text{cm}^{2}$ was measured by irradiating snow micro-structured targets with 50 fs duration pulses of Ti:Sapphire laser.

Crystal growth and properties of the disordered crystal Yb:SrLaAlO4: a promising candidate for high-power ultrashort pulse lasers

A high quality Yb3+-doped strontium lanthanum aluminate crystal, Yb:SrLaAlO4 is grown by the Czochralski method.

Yb3+-doped CaF2-LaF3 ceramics laser.

Highly transparent ceramic is an attractive gain medium for high-power lasers due to its high fracture toughness, homogeneity, and size scalability. Here we report the first Yb3+-doped CaF2-LaF3 ceramics laser. Codoping of La3+ ion can reduce the formation of Yb2+ ions and enhance the laser efficiency. In the laser experiment, the maximum output power of 4.36 W and the maximum slope efficiency of 69.5% were obtained with a 3% La, 2% Yb sample and a 2% La, 1% Yb sample, respectively. Due to the combined properties of Yb:CaF2 and a ceramic laser gain medium, Yb:CaF2-LaF3 ceramic is a promising gain medium for a high-power ultrashort pulse laser and amplifier.

Laser-driven plasma photonic crystals for high-power lasers

Laser-driven plasma density gratings in underdense plasma are shown to act as photonic crystals for high power lasers. The gratings are created by counterpropagating laser beams that trap electrons, followed by ballistic ion motion. This leads to strong periodic plasma density modulations with a lifetime on the order of picoseconds. The grating structure is interpreted as a plasma photonic crystal time-dependent property, e.g., the photonic band gap width. In Maxwell–Vlasov and particle-in-cell simulations it is demonstrated that the photonic crystals may act as a frequency filter and mirror for ultra-short high-power laser pulses.

Нелинейная динамика сверхмощных ультракоротких лазерных импульсов: эксафлопные вычисления на лабораторном компьютере и субпериодные световые пули

Нелинейная динамика сверхмощных ультракоротких лазерных импульсов: эксафлопные вычисления на лабораторном компьютере и субпериодные световые пули, Воронин А.А., Желтиков А.М.

Hollow-core fibers for high power pulse delivery.

We investigate hollow-core fibers for fiber delivery of high power ultrashort laser pulses. We use numerical techniques to design an anti-resonant hollow-core fiber having one layer of non-touching tubes to determine which structures offer the best optical properties for the delivery of high power picosecond pulses. A novel fiber with 7 tubes and a core of 30µm was fabricated and it is here described and characterized, showing remarkable low loss, low bend loss, and good mode quality. Its optical properties are compared to both a 10µm and a 18µm core diameter photonic band gap hollow-core fiber. The three fibers are characterized experimentally for the delivery of 22 picosecond pulses at 1032nm. We demonstrate flexible, diffraction limited beam delivery with output average powers in excess of 70W.

The Application of Cryogenic Laser Physics to the Development of High Average Power Ultra-Short Pulse Lasers

Ultrafast laser physics continues to advance at a rapid pace, driven primarily by the development of more powerful and sophisticated diode-pumping sources, the development of new laser materials, and new laser and amplification approaches such as optical parametric chirped-pulse amplification. The rapid development of high average power cryogenic laser sources seems likely to play a crucial role in realizing the long-sought goal of powerful ultrafast sources that offer concomitant high peak and average powers. In this paper, we review the optical, thermal, thermo-optic and laser parameters important to cryogenic laser technology, recently achieved laser and laser materials progress, the progression of cryogenic laser technology, discuss the importance of cryogenic laser technology in ultrafast laser science, and what advances are likely to be achieved in the near-future.