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A Yb3+:KBaY(MoO4)3 (Yb3+:KBYM) crystal with dimensions of 51 mm × 27 mm × 10 mm was successfully grown by the TSSG method. The characteristics of the crystal structure and probability of good optical properties were analyzed. The absorption and emission spectra of Yb3+:KBYM crystal exhibit broadened bands, with the maximum absorption cross-sections of 1.17 × 10-20, 1.44 ×10-20 and 1.37 × 10−20 cm2 at 976 nm for X-, Y- and Z-polarizations, respectively. The corresponding absorption FWHMs are as wide as 77, 46 and 55 nm. The well-known re-absorption effect of Yb3+ in the crystal is discussed. Two methods, the Fuchtbauer-Ladenburg method (FL) and reciprocity method (RM) were adopted to compute the emission cross-sections and results show a certain discrepancy but the errors are allowable. The laser potentiality of the Yb3+:KBYM crystal was also evaluated by calculations of minimum inversion fraction β min , saturation pump intensity I sat , the minimum pump intensity I min and gain cross-sections spectra. Laser experiment was carried out and Watt-level continuous wave laser has been realized. Results indicate that the Yb3+:KBYM crystal with a disordered structure may be a potential laser media that can be used to generate tunable and ultrashort pulse laser emissions with high quality beam.

Nanofillers are added to polymeric insulating materials in order to modify the electrical, mechanical and thermal properties. This study investigates silicone rubber filled with hydrophilic and hydrophobic silica nanoparticles with respect to their resistance to laser ablation and to arc discharges, as well as their mechanical properties. The laser ablation is performed with single femtosecond laser pulses at wavelengths of 520 nm and 1040 nm. This way, the influence of the nanoparticles on the ablation by laser pulses with defined energy is investigated. The energy absorbed in the ablated volume is determined by taking into account the absorption for various fluences above the ablation threshold. For the silica nanocomposites, the energy specific ablation volume (ESAV), i.e., the ablated volume per absorbed energy, is significantly reduced compared to the unfilled polymer. The reduction is more pronounced for hydrophilic nanoparticles than for hydrophobic nanoparticles. Furthermore, the resistance to arc discharges and the mechanical strength of the nanocomposites are higher compared to the unfilled polymer. In particular, for nanocomposites with different types of silica filler particles the resistance to laser ablation, expressed by the ESAV, the resistance to arc discharges and the mechanical strength, expressed by Shore A hardness and the Young's modulus, show a strong correlation. From this, it is concluded that the modifications of the material properties are caused by interactions between the polymer and the nanoparticle surface, i.e., an interphase. The different extent of this interphase becomes obvious in the different degrees of impact on various electrical and mechanical properties.

Surface modification processes of any metallic structure with the help of laser irradiation are a universal practice. The profoundly focused laser beam is to irradiate on the substrate surface to modify the surface condition for the improvement of the tribological properties in heavy and rouged engineering applications. There is a wide application of pure or raw Ti in the field of the biomedical sector specifically in implants and artificial joint prosthesis. In the viewpoint of the above, the present work determines the thermal characteristic aspect in a pure Ti physical domain by developing a two-dimensional heat transfer approach with the dual-phase-lag (DPL) model under the influence of the ultrashort pulsed laser heating. The physical domain has a Ti nanofilm of the 4 nm length, 2 nm width, and 0.02 nm thickness. A DPL model is developed for analyzing the ultrafast heating, as it is the most potential heat transfer model. The present study has modelled a hybrid analytical analysis comprising of the Duhamel’s theorem and the finite integral transform method. This work highlights the essentiality of applications of the DPL heat conduction model over the conventional Fourier’s model based on the qualitative assessment. The selection of thermal relaxation time lags was carefully chosen from the existing experimental evidence due to the requirement to reach the melting point temperature of pure Ti. The peak temperature of laser irradiation declines with the increase in the optical penetration depth and laser pulse time. Finally, the competence of the present analysis is validated with the published numerical and experimental works from the engineering accuracy standpoint.

Sesquioxide ceramics (Y2O3, Lu2O3 and Sc2O3) along with their solid solutions show great potential as hosts for solid-state laser gain materials due to their great suitability for various rare-earth ions, comparably physical properties to single crystals and far lower preparation temperature than the melting point (above 2400 °C). In this work, the development of the fabrication of rare-earth ions doped-sesquioxide ceramics like synthesis of raw nano-powders and ceramic sintering is reviewed, which demonstrates that the ceramics fabricated by vacuum sintering plus hot isostatic pressing technology from well-dispersive nano-powders possess high transparency with uniform microstructures. And the stable synthesis of powders, optical quality and large-scale preparation of ceramics need further investigation. Meanwhile, the progress on laser performance of the corresponding transparent ceramics is also summarized, which reveals that rare-earth active ions doped sesquioxide ceramics have great potential in efficient high power and ultrashort pulse laser applications during 1∼3 μm.

Understanding the laser ablation mechanism is highly essential to find the effect of different laser parameters on the quality of the laser ablation. A mathematical model was developed in the current investigation to calculate the material removal rate and ablation depth. Laser cuts were created on the workpiece with different laser scan speeds from 1 to 10 mm s−1 by an ultrashort pulse laser with a wavelength of about 1000 nm. The calculated depths of laser cuts were validated via practical experiments. The variation of the laser power intensity on the workpiece’s surface during laser radiation was also calculated. The mathematical model has determined the laser-material interaction mechanism for different laser intensities. The practical sublimation temperature and ablated material temperature during laser processing are other data that the model calculates. The results show that in laser power intensities (IL) higher than 1.5 × 109 W cm−2, the laser-material interaction is multiphoton ionisation with no effects of thermal reaction, while in lower values of IL, there are effects of thermal damages and HAZ adjacent to the laser cut. The angle of incidence is an essential factor in altering incident IL on the surface of the workpiece during laser processing, which changes with increasing depth of the laser cut.

We investigate MeV-level attosecond electron bunches from ultrashort-pulse laser-solid interactions through similarities between experimental and simulated electron energy spectra. We show measurements of the bunch duration and temporal structure from particle-in-cell simulations. The experimental observation of such bunches favors specular reflection direction when focusing the laser pulse onto a subwavelength boundary of thick overdense plasmas at grazing incidence. Particle-in-cell simulation further reveals that the attosecond duration is a result of ultra-thin ($$\sim $$ ∼ tenth of a micron) gaps of zero electromagnetic energy density in the modulated reflected radiation, while the bunching (locally peaked electron concentration) comes from the highly-directional electron angular distribution acquired by the electrons in a grazing incidence setup. To isolate a single electron bunch, we perform simulations using 1-cycle laser pulses and analyze the effect of carrier-envelop phase with particle tracking. The duration of the electron bunch can be further decreased by increasing the laser intensity and the focal spot size, while its direction can be changed by tuning the preplasma density gradient.

We demonstrate significant temporal contrast improvement for a petawatt-class Ti:sapphire femtosecond laser by introducing a temporal pulse filter based on a self-diffraction process. A double chir...

An ultrashort pulse laser is being used at the Schlesinger center for compact accelerators in Ariel University. The pulse duration ranges from 10 ps to 35 fs and can be varied using a grating, controlled by a remote control. However, there is no real-time indication of its duration while changing the grating position. This laser hits a copper cathode which results in ejection of electrons that serve as free electrons for a THz free electron laser (FEL). Extracting electrons from a copper cathode requires more energetic photons than IR, and therefore the laser’s frequency is tripled (from 800 to 266 nm) using non-linear crystals. The conversion efficiency of the third harmonic generation setup is greater for pulses shorter than 50 fs. In order to properly tune the pulse duration a method to measure the pulse duration in real-time is required. Generally, in order to measure an event, one should use a shorter event than the one being measured. For ultra-short laser pulses, using shorter events to measure the pulse is impossible. Hence, we use the method of auto-correlation which means we harness the pulse itself in order to measure its duration. We used 3DOptix components to build an autocorrelator setup, resulting with a significantly more compact set-up, and very easy to align. We controlled the measurement by a dedicated software we developed for this purpose. Methods and information of the elements in the autocorrelator system are presented, and the necessary requirements to simplify the alignment and measuring procedures are outlined in this paper.

An ultrashort pulse laser, capable of varying the pulse duration from 0.2 ps up to 10 ps, is used to study the ablation characteristics of stainless steel and cemented tungsten carbide. In addition to the influence of pulse duration, the number of pulses and the wavelength are examined for their influence on the ablation process. By determining the ablation diameter of generated cavities, the ablation threshold of the materials is calculated as a function of the number of pulses, the pulse duration, and the wavelength. The experimentally determined ablation thresholds tend to agree with calculated values. Due to the incubation effect, the ablation threshold decreases with increasing the number of pulses. In this context, the incubation factor (0.81@1030 nm and 0.80@515 nm for stainless steel, 0.90@1030 nm and 0.77@515 nm for cemented tungsten carbide) for the investigated materials is determined. On the basis of the measured ablated volume, the effective penetration depth (a reduction in a range from about 12 nm and 15 nm to 6 nm for stainless steel and in a range from 22 nm and 32 nm to 11 nm and 13 nm for cemented tungsten carbide by increasing the pulse duration from 0.2 ps to 10 ps) of the energy is calculated and it is proven that in the femtosecond regime the penetration depth increases compared with the picosecond regime. In consequence, the efficiency of the ablation process is increased by using shorter laser pulses.

The multi-foci division of through thickness nonlinear pulse energy absorption on ultrashort pulse laser singulation of single side polished sapphire wafers has been investigated. Firstly, it disclosed the enhancement of energy absorption by the total internal reflection of the laser beam exiting from an unpolished rough surface. Secondly, by optimizing energy distribution between foci and their proximity, favorable multi-foci energy absorption was induced. Lastly, for effective nonlinear energy absorption for wafer separation, it highlighted the importance of high laser pulse energy fluence at low pulse repetition rates with optimized energy distribution, and the inadequacy of increasing energy deposition through reducing scanning speed alone. This study concluded that for effective wafer separation, despite the lower pulse energy per focus, energy should be divided over more foci with closer spatial proximity. Once the power density per pulse per focus reached a threshold in the order of 1012 W/cm2, with approximately 15 μm between two adjacent foci, wafer could be separated with foci evenly distributed over the entire wafer thickness. When the foci spacing reduced to 5 μm, wafer separation could be achieved with pulse energy concentrated only at foci distributed over only the upper or middle one-third wafer thickness.

A hundred-watt chirped pulse amplification (CPA) fiber laser system based on Yb-doped rod-type photonic crystal fiber (PCF) is demonstrated. The seed was an all-fiber dispersion-managed mode-locked oscillator based on chirped fiber bragg grating (CFBG) and semiconductor saturable absorber mirror (SESAM). The oscillator delivers 1.5 ps pulses at a repetition rate of 45 MHz. The output power from the oscillator was 0.9 mW and the central wavelength was 1032 nm. After pulse stretching, pulse picking and amplification, 151 W output power with repetition rate of 1 MHz was achieved by employing a home-made 85/260 μm PCF (Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences). After compression, laser pulses with duration of 616 fs and average power of 102.9 W were obtained. The single pulse energy and peak power were calculated to be 102.9 μJ and 167 MW respectively.

The interaction of ultrashort pulsed laser radiation with intensities of 1013 W cm−2 and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 1012 W∙cm−2 &lt, I0 &lt, 5.2 × 1016 W∙cm−2, up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as H˙′&nbsp, (0.07) &gt, 45 mSv h−1 can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.

This paper presents the results of an investigation, performed using a 300 W picosecond laser, for deep (~4 mm) machining of tungsten carbide. The influence of picosecond laser parameters on the precision and quality were studied and discussed. Laser pulse frequency and scanning speed have minimal effect on ablation rate at high power levels. The surface roughness of the ablated area increases with the ablation depth. Optimal fluence of 6 J/cm2 results in an energy specific ablation volume (ESAV) of 2.2E-3 mm3/J and surface roughness Sa of 2.2µm. At optimal conditions, no thermal defects were observed, even at a high average power of 300 W. The material removal rate (MRR, mm3/min) seems to be proportional to the average power of the laser, and a MRR of around 45 mm3 per minute can be achieved at a 300 W power level.

Phase-only spatial light modulators are ideal for the generation of beam splitter profiles to parallelize a variety of laser processes. A novel approach for the calculation of phase holograms is proposed to achieve a highly accurate power distribution over all spots. The Iterative Fourier Transform Algorithm (IFTA) is extended by the use of different machine learning methods, which are trained in an open camera-feedback loop. After the training phase, improvement of the beam splitting accuracy is then validated experimentally. The advantage of the presented approach is shown by comparing it to the standard IFTA algorithm. Finally, use of the approach is demonstrated through metal marking with an ultrashort pulse laser.

We present a way to prevent stray light generated from draft facets of Fresnel lens. Fresnel lens has distinctive shape that induces stray light. The draft facets can generate as much as 8% of stray light, deteriorating performance of optics. To prevent such stray light, we fabricated light-shielding film on draft facets and edges of Fresnel lens. First, we investigated structure and material of the shielding film. We then studied optimal laser source which can perform the precise processing, and found that laser ablation by picosecond laser works the best. Finally, we demonstrated the fabrication of light-shielding film on draft facets of Fresnel lens by experiment, and verified the elimination of stray light.

Measuring the complex field of ultrashort pulse lasers plays a fundamental role in light wavefront manipulation and nonlinear phenomena investigation; yet, it still constitutes a challenge for both full-field and high-resolution characterization due to the short coherent length. We proposed and implemented an off-axis interference system with a delay-line sweeping technique to overcome the fringe contrast degradation caused by the envelope mismatch between interfering pulses, resulting in an increased effective analysis area. The effectiveness of the proposed method was first demonstrated by measuring a complex field generated by a phase-only spatial light modulator, where a four-pixel binning technique was adopted for both amplitude and phase modulation; then it was used for the measurement of the second harmonic generation signal of a urea crystal sample. The experimental results show that the proposed method is capable of measuring complex fields having fine features within the full field. The proposed technique can be applied for strongly scattering medium refocusing and adaptive optics, where measuring the complex field of ultrashort pulse lasers is essential.

We investigated GHz pulse bursts ablation on metals, silicon, zirconium dioxide, soda-lime glass and sapphire for surface structuring applications with a commercial laser system providing a burst pulse frequency of 5.4 GHz and a maximum of 25 pulses per burst pulse train. The results on metal show dramatic decrease of the ablation efficiency and a reduction of the machining quality. For silicon we also observed a reduction of the ablation efficiency for GHz pulse bursts but found a strong increase for MHz pulse bursts using a 10 ps laser system. On glass an increase of the ablation rate for GHz pulse bursts was observed, however with pure machining quality indicated by crakes in the surface and boarders. Zirconium oxide was the only investigated material, where a GHz pulse bursts induces a moderate higher ablation efficiency with comparable surface qualities, however a 10% higher ablation rate was obtained with a 10 ps laser system.

Glass-to-glass welding using ultrashort pulse laser is attracting attention. However, the low processing speed and the requirement of the small air gap between glass substrates have impeded its use in industry. In our study, we achieved rapid welding of glass substrates by coaxially focusing a single femtosecond laser pulse and a continuous-wave (CW) laser with a wavelength that transmits through glass. The selective absorption of the CW laser into the excited electrons increased the processing speed by a factor of 500 compared to the conventional method, while the allowable gap increased by a factor of 4.

Laser structuring with ultrashort laser pulses of electrodes for lithium ion batteries is a promising technology to further enhance their application for energy storage in electric vehicles. We investigated the influence of laser parameters, such as average power, pulse repetition frequency and pulse number when processing electrodes of different thicknesses. Using an ultrashort pulse laser, two graphite anodes with a thickness of 136 μm and 248 μm, were structured with a dot pattern and then analyzed, with respect to their shape and size of the created structure units. Whereas the electrodes individually show similar ablation behavior, the higher coating thickness of 248 μm compared to a coating thickness of 136 μm results in deeper hole structures with a lower ablation volume when processed with the same laser parameters. Thus, the objective of creating electrodes with a three-dimensional structure and reducing material loss can be achieved more efficiently with electrodes with a higher coating thickness.

In this work, 2D piezoelectrically driven MEMS circular scanners have been designed, fabricated and tested. These mirrors own large optical apertures of 7 mm, 10 mm and 20 mm for good beam shapes. Also HR-coating layers for 515 nm and 1050 nm reaching up to 99.99% reflexion and 0.1% transmission were applied onto the mirror surface for the suitability of high power laser, where the wavelengths were specified according to the laser source development demands. Based on piezoelectric position sensing elements integrated on the MEMS mirrors a closed-loop control was developed. In this paper the design efforts, realizing circular-scanning and eliminating non-linearity during mode superposition, and fabrication efforts will be reported. Characterization results focusing on mechanical behaviors, position sensing signal, HR-coating will be also important parts of this work.

Micro/nanoscale surface patterning of zirconia ceramic is needed for surface functionalization and performance enhancement, such as improved biocompatibility of medical devices, as well as device miniaturization. Therefore, formation of laser-induced periodic surface structures (LIPSS) with periods shorter than the laser wavelength on a zirconia ceramic was carried out using an ultrashort pulsed laser. In this case, it is important to shorten the processing time required for forming the LIPSS without deteriorating the processing quality. Using the various-parameters-controlled laser processing and observation system, we optimized LIPSS formation by changing parameters such as pulse duration, repetition frequency, number of shots, and fluence.

Gallium nitride GaN film delamination is an important process during the fabrication of GaN light emitting diodes LEDs and laser diodes. Here, we utilize 520 nm femtosecond laser pulses, exploiting nonlinear absorption rather than single photon absorption such as in conventional laser lift off LLO employing excimer or Q switched laser sources. The focus of this study is to investigate the influence of laser scanning speed and integrated fluence corresponding to laser energy per area during the LLO processing of GaN LED chips and their resulting structural properties. Because both the sapphire substrate and InGaN GaN heterostructures are fully transparent to the emission of the laser system, a key question is related to the impact of laser pulses on the quality of a thin film structure. Therefore, several characterization methods i.e., scanning electron microscopy, atomic force microscopy, X ray diffraction, Raman spectroscopy, and electroluminescence spectroscopy were employed to understand the material modifications made by femtosecond LLO fs LLO . We demonstrated that by adjusting the laser scanning speed, smooth GaN surfaces and good crystal quality could be obtained regardless of the existing delamination of metal contact, which then slightly downgraded the LED performance. Here, the integrated fluence level was set in the range of 2.6 4.4 J cm2 to enable the fs LLO process. Moreover, two mitigation strategies were developed and proven to improve the optoelectrical characteristics of the lifted off LEDs i.e., modification of the processing step related to the metal creation and reduction of laser energy

Crystalline-Si (c-Si) based solar cells (SC) efficiency remains one of the most challenging part in photovoltaic industry. Besides of the thermodynamical limits, the optical losses due to indirect band-gap structure of the material require additional treatment which known as a photonic design of the SC surface [1] . Numerous manipulations are performed to increase the efficiency of commercial solar cells. In most of these methods, the main concept is increasing light interaction by the cell through chemical modification of the morphology of the surface. Creation of pyramid-like structure on SC surface through the chemical etching by KOH solution is the most common industrial method nowadays. Recently we proposed Nonlinear Laser Lithography (NLL) as an alternative method for the traditional chemical etching [2] . The method is based on the well known phenomenon Laser Induced Periodic Surface Structuring (LIPSS), allowing creation of wellordered, periodic ablation and/or oxidation lines on the material surface with subwavelength period under ultrashort pulse laser illumination. In comparison with the traditional chemical treatment, the method is cheap, single-step and chemically free. However, the damage of the crystalline structure of the SC surface during ablation limiting the final efficiency of the device. In the current work, we demonstrate the new achievement in the efficiency of c-Si solar cell based on NLL treated surface. By proper design of the laser parameters and the scanning geometry during the NLL process, as well as proper post-passivation of the SC surface, we demonstrate more than 16% efficiency of the final device. To the best of our knowledge, this is the highest efficiency demonstrated so far on a laser treated c-Si solar cell without any chemical texturing.

Zettawatt Equivalent Ultrashort pulse laser System (ZEUS): The National Science Foundation’s mid-scale 3PW laser facility for exploration of relativistic plasmas, nonlinear quantum electrodynamics, and other extreme high-field phenomena is described.

JPhys photonics 3(2), 021004 (2021). doi:10.1088/2515-7647/abf24f, Published by IOP Publishing, Bristol

For ultrashort pulse laser processing of a sapphire wafer, nonlinear absorption of laser energy at sufficiently high laser intensity is essential. This study found that in ultrashort pulse laser processing of a sapphire wafer, its surface morphology can have a significant effect on nonlinear absorption and its threshold. Interestingly, for a single side polished wafer, the orientation of the rough or smooth surface towards or away from the laser source (i.e. if the laser is an incident or exit beam) is a critical parameter on the processing outcomes. The orientation of the grinded rough surface affects the amount of light internally reflected within the material; this has a direct impact on the laser energy absorbed and nonlinear absorption threshold. Indeed, the presence of a grinded rough exit surface increases the possibility of total internal reflection of the laser beam at the exit surface; this traps the laser's energy within the material more efficiently and thus promoting nonlinear absorption. This unique characteristic of having total internal reflection at the rough exit surface can result in laser scribing at both the smooth entry and rough exit surfaces simultaneously. This mechanism can be exploited to improve the laser processing of sapphire sample. Agency for Science, Technology and Research (A*STAR) Accepted version The research work was supported by A-Star Research Agency, Singapore Institute of Manufacturing Technology (SIMTech) under SIMTech-NTU Joint Laboratory with project No. U12-M- 007JL.

Ultrakrátké pulzní laserové mikroobrábění je ovlivněno akumulací tepla, která je výsledkem zbytkového tepla z předchozích laserových pulzů na povrchu vzorku. Až dosud většina prací analyzovala akumulaci pomocí numerického modelování. Předkládaná práce se zaměřila na vývoj a první použití systému měření teploty akumulace tepla přímo během procesů v časových rozsazích nanosekund a mikrosekund. Měřicí systém byl založen na infračervené radiometrii a obsahoval rychlý fotodetektor HgCdTe chlazený kapalným dusíkem a paraboloidní zrcadla. Mikroobrábění drážek bylo prováděno pomocí 14 W pikosekundového laseru s různými energiemi v pulzu, opakovacími frekvencemi a rychlostmi skenování. Kalibrace měřicího systému byla provedena za účelem získání teplot z měřeného signálu. Kalibrace nebyla jednoduchá kvůli velmi malé laserové stopě (25 μm), malému signálu a změně velikosti vyhřívané oblasti pro nízké rychlosti skenování. Získaná teplota akumulace tepla se pohybovala od 300°C do 2600°C pro rychlosti skenování od 8 m/s do 0,07 m/s a pulzní energie od 0,1 µJ do 100 µJ. Podle obrázků ze skenovacího elektronového mikroskopu (SEM) byl materiál již částečně roztaven (malé kapičky na hranicích) pro nízkou rychlost skenování. Drsnost povrchu a účinnost ablace byly stanoveny pomocí 3D konfokálního laserového mikroskopu. Byla nalezena dobrá korelace mezi drsností a teplotou akumulace tepla, což potvrdilo platnost kalibrace. Naměřená teplota akumulace tepla byla překvapivě nejvyšší pro nejúčinnější ablační parametry a současně byla dosažena nízká drsnost povrchu. Ultrashort pulse laser micromachining is affected by the heat accumulation resulting from the residual heat from previous laser pulses on the sample surface. Up to now, most of the works analysed the accumulation by numerical modelling. The present work focussed on development and application for the first time of a measurement system of heat accumulation temperature directly during the processes in nanosecond and microsecond time ranges. The measurement system was based on the infrared radiometry and contained liquid nitrogen cooled fast HgCdTe photodetector and paraboloid mirrors. Micromachining of grooves was done using a 14 W picosecond laser with different pulse energies, repetition frequencies and scanning speeds. Calibration of the measurement system was done in order to obtain temperatures from the measured signal. The calibration was not straightforward due to very small laser spot (25 μm), small signal and changing of the size of the heated area for low scanning speeds. Obtained heat accumulation temperature ranged from 300°C to 2600°C for scanning speeds from 8 m/s to 0.07 m/s and pulse energies from 0.1 µJ to 100 µJ. According to the scanning electron microscope (SEM) images, the material was already partially melted (small droplets on boarders) for low scanning speeds. Surface roughness and ablation rate were determined by 3D confocal laser microscope. Good correlation was found between the roughness and the heat accumulation temperature, thus confirming the validity of calibration. Measured heat accumulation temperature was surprisingly the highest for the most efficient ablation parameters and at the same time low surface roughness was achieved.

Using an analytical model and computer simulation, we show that the wakefield driven by an ultrashort laser pulse in high-density plasma periodically reverses its polarity due to the carrier-envelope phase shift of the driver. The wakefield polarity reversal occurs on spatial scales shorter than the typical length considered for electron acceleration with the laser-wakefield mechanism. Consequently, the energies of accelerated electrons are significantly affected. The results obtained are important for the laser-wakefield acceleration under the conditions relevant to present-day high-repetition-rate laser systems.

BACKGROUND AND OBJECTIVES Light delivery is an essential part of therapy forms like photodynamic therapy (PDT), laser-induced thermotherapy, and endovenous laser therapy. While there are approaches to the light application for all three therapies, there is no diffuser that can be used for all three approaches. This diffuser must meet the following criteria: Homogeneous radiation profile over a length of 40 mm, efficient light extraction in the diffuser area, mechanical breakage resistance as well as thermal stability when applying high power. STUDY DESIGN/MATERIALS AND METHODS An ultrashort pulse laser was used to inscribe inhomogeneities into the core of a fused-silica fiber core while scanning the laser focus within a linear arrangement of cuboids centered around the fiber axis. The manufactured diffuser was optically and mechanically characterized and examined to determine the maximum power that can be applied in a tissue environment. RESULTS Based on the analysis of all examined diffusers, the manufactured diffuser exhibits an emission efficiency e = (81.5 ± 5.9)%, an intensity variability of (19 ± 5)% between distal and proximal diffuser end, and a minimum bending radius Rb = (15.4 ± 1.5) mm. It was taken advantage of the fact that the outer areas of the fiber core do not undergo any structural changes due to the machining and therefore do not suffer a major loss of stability. Tissue experiments revealed that a maximal power of 15 W was deliverable from the diffuser without harming the diffuser itself. CONCLUSIONS It could be shown that a diffuser manufactured by ultrafast-laser processing can be used for low power applications as well as for high power applications. Further tests have to show whether the mechanical stability is still maintained after the application of high power in a tissue environment. Lasers Surg. Med. © 2020 Wiley Periodicals LLC.

We report on the stress induced birefringence of 10 mm BK7 cubes bonded to 15 mm x 15 mm x 5 mm aluminium coupons using ultrashort pulse laser welding, hydroxide catalysis bonding, and an optical adhesive using a standard approach used in industry. It was observed that ultrashort pulse laser welding results in a low level of stress induced birefringence within an 85% optical aperture of the 10 mm cube. These levels are suitable for use in photography and microscopy applications as defined by the relevant ISO standard for permissible stress induced birefringence limits in optics. Hydroxide catalysis bonding was shown to approach and possibly exceed this limit, however, it must be noted that the bonding area of the hydroxide catalysis samples greatly exceeds that of the welding and adhesive bonded samples by a factor of 20 and thus will likely experience grater mechanical stress from defects on the metal surface. The induced stress in the adhesively bonded samples was found to be well-confined, with the retardation falling below the maximum acceptable limits for microscopy type applications of 10 nm/cm retardation within 0.5 - 1 mm of the glass-metal interface for the majority of samples tested. The hydroxide catalysis and ultrashort pulse laser welded samples have a significant increase in retardation in the region directly above the bonded surface compared to the adhesively bonded samples. This retardation decays rapidly as a function of distance from the bonded interface. In the case of ultrashort pulse laser welded samples, optimisation of the welding parameters has been shown to control the level of retardation within the interface region, thus reducing the overall effect of the weld on the bulk optic.

The laser scribing is one of the crucial technological steps in the industrial processing of solar cells, while the mechanical scribing is generally used for practical reasons on the laboratory scale. Using a picosecond laser, preliminary results on the scribing of CuInGaSe2 thin film solar cells deposited by the low temperature pulsed electron technique, are reported. The complete comparison between laser- and mechanical-scribing is still in progress, however SEM imaging and EDX analysis confirm the excellent structural and morphological quality of the laser scribed samples.

The excellent performance of PMN–PT has received increasing attention and applications in high-frequency (>20 MHz) ultrasonic array transducers. However, due to its brittleness and low phase-transition temperature of PMN–PT, a decrease of piezoelectric properties in the fabrication of small-scale array elements is still an urgent problem and would substantially determine the performance of transducers. Here, in this study, ultrashort pulse laser technique is proposed to fabricate PMN–PT array transducers with outstanding performance. The effect of the laser parameters on domain structures has been studied systematically, and it was found that the ultrahigh piezoelectric properties of micro-sized elements still maintained after optimizing the laser parameters. A Fresnel array transducer with center frequency of 40.84 MHz and −6dB bandwidth as high as 122.28% was also designed and fabricated. This work demonstrates that it is feasible and promising to fabricate superior PMN–PT and other piezoelectric ultrasonic array transducers based on ultrashort pulse laser techniques.

The unique electron distribution, timescales and geometry of ultrashort pulse laser generated plasmas are a novel source of radio-frequency electromagnetic emission, with a bandwidth larger than 70 GHz. To identify the emission mechanism, we characterize the RF signal for a wide variety of experimental conditions.

Molybdenum oxide is an attractive material for application in several technology fields such as sensors, displays, and batteries, among many others. In this work, we present a reliable laser direct writing (LDW) method for synthesizing multi-phase molybdenum oxide (MoOx) on a single processing step. We use femtosecond laser pulses to produce up to five distinct crystalline phases of molybdenum oxide at once. We demonstrate how the laser irradiation conditions determine the MoOx stoichiometry, phase, and morphology. We show that by conveniently adjusting either the per-pulse laser fluence or the exposure time, MoOx can be obtained in nano or micro-structured form. We found that this ultrashort pulse laser processing method allows for the formation of unusual MoOx phases such as o-Mo18O52, which is rarely reported in the literature. In addition, it is possible to synthesize other sub-stoichiometric molybdenum oxide phases such as o-Mo4O11 and m-Mo8O23 all at atmospheric air conditions, with no need for demanding oxygen pressure precautions.

Results on the manipulation of the wetting properties of stainless steel alloy surface by ultrashort pulse laser texturing are presented. The wide range of water droplet contact angles from highly-hydrophilic to super-hydrophobic was achieved by generation of laser-induced periodic surface structures (LIPSS) and nanospikes. In particular, the wetting state was controlled by accumulated laser fluence, which determines the carbon/oxygen content and nano-texture type of the surface after laser treatment. A super-hydrophobic water-repelling surface was generated. The simple, single-step laser processing technology was demonstrated as a promising tool for the large-scale industrial production of self-cleaning stainless steel.

Bismuthene, as a novel two-dimensional (2D) material, has attracted extensive attention because of its outstanding properties including narrow band gap, stability at room temperature, nonlinear optical transmission, and so on. In this paper, the physical characteristic, nonlinear optical response, and ultrafast photonics application of few-layer bismuthene are studied experimentally. By the balanced twin-detector measurement method, the saturable absorption property of few-layer bismuthene with a modulation depth of 2.5% and saturable intensity of 110 MW/cm2 at the optical communication band (C-band) is illustrated. Dependent on a few-layer bismuthene saturable absorber, an all-fiber ultrashort pulse laser is fabricated and the proposed fiber laser can operate with coexistence of harmonic mode-locking and dual-wavelength mode-locking. The different laser generations of harmonic and dual wavelength depend on the saturable absorption of few-layer bismuthene, the suitable birefringence and nonlinearity strength in the laser cavity. The results suggest that the ultrashort pulse laser obtained based on few-layer bismuthene could be applied to the field of pump-probe experiments and tunable terahertz radiation generation potentially.

Two-dimensional materials, such as graphene and semiconductor transition metal dichalcogenides (TMDCs), exhibit remarkable optical properties which are of great potential for applications in modern electronics. The first part of this dissertation focuses on the dispersion of the second order resonant nonlinearity (χ(2)) in the single layer TMDC. We begin with the study of the nonlinear optical properties of monolayer TMDC, WSe2. We experimentally obtain the χ(2) dispersion data from the single layer sample of WSe2 by using broadband ultrashort pulse laser sources. The broadband pulse is generated by specially designed photonic crystal fiber (PCF). This PCF fiber is pumped by TiS mode-locked laser to generate continuum pulse that spans from visible to near-infrared. This continuum broadband pulse is used as a fundamental beam to generate signal at the second harmonic frequency in 2D semiconductor material. We detect the signal generated in the sample by using monochrometer and charge-coupled device (CCD), which provide the spectrum of the second harmonic signal that carries the signature of the materials. To get the images of these materials, we employ an optical parametric oscillator (OPO) tuning at reasonable wavelengths. Then we shine the beam on the sample, and after the signal has been generated in the sample, it gets reflected and this beam is then collected by photomultiplier (PMT) before angle scanned using galvo-mirror scanner to provide 200x200 µm2 imaging area. The dispersion obtained with better than 3 meV photon energy resolution showed peak value being within 6.3-8.4 x 10-19 m2/V range. We estimate the fundamental bandgap to be at 2.2 eV. Sub-structure in the χ(2) dispersion reveals a contribution to the nonlinearity due to exciton transitions with exciton binding energy estimated to be at 0.7 eV. In the second half of this work, we study two other materials. First, we show resolution of fine spectral features within several Raman active vibrational modes in potassium titanyl phosphate (KTP) crystal. Measurements are performed using a femtosecond time-domain coherent anti-Stokes Raman scattering spectroscopy technique that is capable of delivering equivalent spectral resolution of 0.1 cm-1. The Raman spectra retrieved from our measurements show several spectral components corresponding to vibrations of different symmetry with distinctly different damping rates. In particular, linewidths for unassigned optical phonon mode triplet centered at around 820 cm-1 are found to be 7.5 ± 0.2 cm-1, 9.1 ± 0.3 cm-1, and 11.2 ± 0.3 cm-1. Second, we demonstrate the quantitative spectroscopic characterization and imaging of biological tissue using coherent time-domain microscopy with femtosecond resolution. We identify tissue constituents and perform dephasing time (T2) measurements of characteristic Raman active vibrations. This was shown in subcutaneous mouse fat embedded within collagen rich areas of the dermis and the muscle connective tissue. The demonstrated equivalent spectral resolution (

A shifted laser surface texturing method (sLST) was developed for the improvement of the production speed of functional surface textures to enable their industrial applicability. This paper compares the shifted method to classic methods using a practical texturing example, with a focus on delivering the highest processing speed. The accuracy of the texture is assessed by size and circularity measurements with the use of LabIR paint and by a depth profile measurement using a contact surface profiler. The heat accumulation temperature increase and laser usage efficiency were also calculated. The classic methods (path filling and hatch) performed well (deviation &le, 5%) up to a certain scanning speed (0.15 and 0.7 m/s). For the shifted method, no scanning speed limit was identified within the maximum of the system (8 m/s). The depth profile shapes showed similar deviations (6% to 10%) for all methods. The shifted method in its burst variant achieved the highest processing speed (11 times faster, 146 mm2/min). The shifted method in its path filling variant achieved the highest processing efficiency per needed laser power (64 mm2/(min&middot, W)), lowest heat accumulation temperature increase (3 K) and highest laser usage efficiency (99%). The advantages of the combination of the shifted method with GHz burst machining and the multispot approach were described.

A mathematical model is developed to find the depth of laser ablation and laser-material interaction in laser processing of Si3N4 (SL200-BG) by an ultrashort (picosecond) pulse laser. The model is verified with experiments which were carried out at different laser scan speeds from 1 mm/s to 100 mm/s. Experimental validation shows a model accuracy of 85%. Additionally, the results show that in laser intensities (IL) higher than 1.5 × 109 W/cm2, the laser-material interaction is “Multi Photon Ionization” (MPI) with no effects of thermal reaction while in lower values of IL, there are effects of thermal damages adjacent to the laser cut.

The spatiotemporal plasma-lens filter proposed here enhances the temporal contrast of the ultrashort pulse laser by combining plasma optics and spatial filtering. Experimentally, the spatiotemporal plasma-lens filter has improved the temporal contrast by 2 orders of magnitude with 80% laser transmission efficiency under a 1 Hz repetitive laser operation. Not only were the pre-pulse and the pedestal cleaned out, but also the rising edge of the main pulse was sharpened.

When the femtosecond laser pulse is comparable to the electron relaxation time, the hyperbolic effect cannot be neglected in heat transfer analysis. The non-Fourier effect is considered for heat transfer analysis assuming finite delay in the development of temperature within the body. This delay is represented in terms of two relaxation times connected to heat flux and temperature gradient. In the present work, a 3D finite element-based heat transfer model is developed using a dual-phase-lag effect. Since the experimental basis of transient temperature distribution in ultrashort pulse laser is extremely difficult or nearly impossible, the model results have been validated with the literature reported results. Furthermore, the simulation of dissimilar fusion welding system treated by an ultrashort pulse laser is demonstrated. The typical characteristic of thermal behavior with the application of femtosecond fiber laser on welding of dissimilar aluminum alloy and stainless steel is presented. The model results in the form of computed isotherm are compared with the literature reported weld pool geometry for dissimilar materials. The feasibility of characteristic parameters like pulse frequency, pulse width, and relaxation times are assessed in this study. A clear guideline of the geometric shape and size of weld pool geometry and the peak temperature of the welding system corresponding to predictable laser parameters is the effective output from this study. Peak temperature reached in a very short interval of time (∼ nanosecond) is analogous to a high rate of heating or cooling that affects the microstructural changes, specifically the formation of intermetallic for dissimilar welding.

The recent developments in high-speed laser machining combine high-average power lasers with ultrafast beam deflection systems.1 However, when using ultrashort pulses at megahertz pulse repetition frequencies the incoming laser beam may interact with the plasma/particle ablation plume that is due to the very short time intervals between the impinging megahertz-repetitive pulses. Thereby, the subsequently irradiated pulses will either be reflected, absorbed and/or scattered by the still existent plasma/particle plume as induced by the previously irradiated pulse(s). This shielding of the incoming laser beam is adversely affecting material ablation and can potentially be avoided when the beam is ultrafast moving in front of the emerging plasma/particle plume. For the real-time analysis of such fairly unknown processing regimes, a pump-probe shadowgraph imaging technique was used to visualize laser materials interaction in high-speed ultrashort pulse laser machining. The pump laser source supplied a beam of λ=1030 nm, ƬH =600 fs and 48MHz maximum pulse repetition frequency for material ablation. A diode laser system provided the probe beam of λ=688 nm and ƬH =13 ns. The pump and the probe beam could be electronically delayed within a time frame of up to 10 µs. Sequences of shadowgraphs will be presented to visualize plasma/ablation plume expansion for providing unique insights into high-pulse repetition frequency ultrashort pulse laser materials interactions at ultrafast laser beam movements above 250m/s. The shadowgraphs reveal that the form of the emerging plasma/particle plumes is influenced by the laser beam moving speed. The plumes vary in their density and structure from clearly distinguishable at highest speed, through smoothly detached to be turbulent. In addition, it is shown on the shadowgraphs material ejection takes place at the time when the ultrafast scanned laser beam moved away from the initial irradiation area.

Quality and throughput of laser material processing are influenced by multiple laser parameters as well as material properties. Since material degradation due to laser processing is generally related to heat-affected zone, quick optimization and realization of nonthermal process is one of the most important issues for higher-grade laser processing. In this work, we developed various parameter-controlled ultrashort pulse laser system that can control various laser parameters over a wide range, such as pulse duration over three orders of magnitude. Using this laser system (wavelength: 1033 nm, pulse duration: 0.4 to 400 ps, repetition: up to 1 MHz), we have investigated nonthermal–thermal processing boundary in drilling of ceramics: aluminum nitride (AlN) and yttria-stabilized zirconia (YSZ), with high and low thermal conductivity, respectively. As a result, the AlN ceramic exhibited strong dependence of ablation rate on fluence: low-, mid- and high-fluence regimes appeared with different logarithmic correlations. Further, the nonthermal–thermal process boundaries appeared, depending on the pulse duration. On the other hand, the ablation behavior of YSZ was much different from AlN; the ablation rate did not show distinct three regimes. Further, there were no nonthermal process windows for the YSZ. Therefore, the nonthermal–thermal process boundaries were governed by the laser parameters like pulse duration as well as the material thermal conductivity.

As a result of laser ablation, a surface roughness Sa between 0.4 μm and 2μm is obtained on 3D structures that have to be polished afterwards to meet customer requirements. For that reason a laser polishing process using ultrashort pulse laser sources is investigated. Applying the polishing process immediately after laser structuring in the same setting simplifies the process chain and saves both time and money. The results reveal an improvement of the surface roughness from an initial grinded surface with 0.4 μm to 0.2 μm by ultrashort pulse laser polishing. The productivity measured by the area that can be processed per time (polishing rate) is with 12.15 cm2/min one order of magnitude higher than state of the art laser polishing using nanosecond pulsed lasers.

We present a new fabrication method to realize smooth structures in lithium niobate. Therefore a detailed study on laser micro-polishing using ultrashort laser pulses is carried out by separating the effects of spatial pulse overlap and temporal pulse overlap. The adventage of this approach is the smoothing of the processed area by a simultaneous ablation. That will allow ablation depths between 1 µm and 4 µm with rms-roughness values of ≈20 nm.

We present a laser beam source with average output powers of up to 15 W and pulse energies of up to 50 μJ at a wavelength of 3.4 μm based on a frequency converted ultrashort pulse laser. The laser beam source consists of a commercially available ultrashort pulse laser with a pulse duration of 10 ps and a downstream multi-stage optical parametric frequency converter based on large-aperture PPLN crystals. Different output powers, pulse energies, conversion efficiencies and beam qualities are achieved depending on the design of the multi-stage setup. In addition, we present some results of processing experiments on different polymer films obtained with the laser beam source presented here.

Experimental evidence for near-surface micro- and nanoscopic processes at ultrashort pulsed laser irradiation (wavelength 775 nm, pulse duration 150 fs) of soda-lime glass near the laser ablation threshold is acquired by transmission electron and secondary electron microscopy (SEM, TEM) as well as by energy disperse x-ray spectroscopy (EDX) investigations of laser-induced subsurface bubbles. Well separated laser irradiations result in individual micrometre bubbles while submicron bubble groups or asymmetrically damaged bubbles appear for overlapping pulses. The overlapping laser irradiations result in a direction dependent damage for the successive bubbles when the inter-pulse distance is small. TEM/SEM studies of cross sectioned bubble prove that precipitations and jets are formed in the centre of the micron bubbles’ bottom. This nano-jets inside the bubble can be related to the self-focusing of the femtosecond laser. A modified layer with a composition deviation from the bulk glass can be a key factor for the formation of the bubble with a thin shell about 100 nm. Moreover, next to the central main bubble, nanometre sized bubbles appear below the surface. Thermal processes inside the bubble cause a smooth inner surface as well as redeposited material mainly opposite to the jet formed at the bottom. The different glass composition near the surface strongly suggests that the altered glass surface composition contributes to the forming mechanism of these near-surface features. Implications of these sub-surface bubbles and jet formation for laser ablation related applications comprise incubation as well as roughness development that limits the achievable precision.

Novel transparent Ho-doped sesquioxide ceramics are synthesized by high-temperature vacuum sintering and hot isostatic pressing. The spectroscopic properties of 1.4 at% and 3 at% Ho:(Lu,Sc)2O3 ceramics are studied within Judd-Ofelt theory, yielding the intensity parameters Ω2 = 7.863, Ω4 = 1.843 and Ω6 = 0.531 [10−20 cm2]. The radiative lifetime of the 5I7 state is 10.63 ms. Absorption, stimulated-emission and gain cross-sections for the 5I7 → 5I8 Ho3+ transition are calculated. At > 2 µm, the maximum σSE = 3.41× 10−21 cm2 at 2.101 µm. Under GaSb diode-pumping at 1.929 µm, the 1.4 at% Ho:(Lu,Sc)2O3 ceramic laser generated 187 mW of quasi-continuous wave (CW) output at 2.114–2.135 µm with a slope efficiency η of 7.6%. When pumped at 1.946 µm with a Tm laser, η reached 25% while the CW output power was 20.5 mW. The Ho:(Lu,Sc)2O3 ceramics are promising for broadband tunable and ultrashort pulse laser operation at ~ 2.1 µm.

The advanced technology of ultrashort pulse laser has given many useful biomedical applications such as micro- and nanosurgery on a cellular and subcellular level, excimer laser refractive surgery (LASIK), protein or cell organelles activation or inactivation, and optogenetics. Here, we argue that the same technology can also be applied to stimulate the neurons in the retina mechanically with the pressure (pressure stimulation) generated from Laser Induced Breakdown (LIB) regime using femtosecond laser. Stimulating retinal cell with pressure is rarely discussed because of considerable difficulties to invoke pressure waves precisely, effectively, and safely into the eye. We investigated these problems theoretically to show that the generated pressure waves could be used to stimulate the neurons in the retina. This new technique might provide a way to understand deeper the roles of mechanical cues in the human visual system and to create an alternative phosphene-based retinal prosthetic.

The setup of nonlinear pulse compression with hollow-core photonic crystal fiber (HC-PCF) is demonstrated. In our experiment, the HC-PCF is just put in the air with no extra gas fill. The spectrum is effectively broadened to more than 60 nm by delivering the ultrashort laser in the HC-PCF and compressing the pulse with Gires–Tournois interferometer-type dispersion mirrors. The initial pulse duration of 279 fs has been shortened to 36 fs with $20\mu \text{J}$ of pulse energy, which corresponds to a peak power of about 556 MW. To our best knowledge, it is the highest peak power obtained from the solid-state laser at the wavelength around $1~\mu \text{m}$ by using Kagome-type HC-PCF without extra gas fill. With precise dispersion compensation and higher input pulse energy, even shorter pulse width and higher peak power can be obtained.