On the Melting Thresholds of Semiconductors under Nanosecond Pulse Laser Irradiation

Jiří Beránek, Alexander V. Bulgakov, Nadezhda M. Bulgakova

2023 — Applied Sciences — DOI: —

In this work, a unified numerical model is used to determine the melting thresholds and to investigate the early stages of melting of several crystalline semiconductors (Si, Ge, GaAs, CdTe and InP) irradiated by nanosecond laser pulses. A molten fraction approach is used for continuous transition over the melting point. The results are compared with previously published theoretical and experimental data. A survey on the thermophysical and optical properties of the selected materials has been carried out to gather the most relevant data on temperature dependent properties for the solid and liquid states of these semiconductors where such data are available. A generalization of the obtained results is established that enables evaluation of the melting thresholds for different semiconductors based on their properties and irradiation conditions (laser wavelength, pulse duration).

A multi-MeV alpha particle source via protonboron fusion driven by a 10-GW tabletop laser

Valeriia Istokskaia, Marco Tosca, Lorenzo Giuffrida, Jan Psikal, Filip Grepl, Vasiliki Kantarelou, Stanislav Stancek, Sabrina Di Siena, Arsenios Hadjikyriacou, Aodhan McIlvenny, Yoann Levy, Jaroslav Huynh, Martin Cimrman, Pavel Pleskunov, Daniil Nikitin, Andrei Choukourov, Fabio Belloni, Antonino Picciotto, Satyabrata Kar, Marco Borghesi, Antonio Lucianetti, Tomas Mocek, Daniele Margarone

2023 — Communications Physics — DOI: —

Nuclear fusion between protons and boron-11 nuclei has undergone a revival of interest thanks to the rapid progress in pulsed laser technology. Potential applications of such reaction range from controlled nuclear fusion to radiobiology and cancer therapy. A laser-driven fusion approach consists in the interaction of high-power, high-intensity pulses with H- and B-rich targets. We report on an experiment exploiting proton-boron fusion in CN-BN targets to obtain high-energy alpha particle beams (up to 5 MeV) using a very compact approach and a tabletop laser system with a peak power of ~10 GW, which can operate at high-repetition rate (up to 1 kHz). The secondary resonance in the cross section of proton-boron fusion (~150 keV in the center-of-mass frame) is exploited using a laser-based approach. The generated alpha particles are characterized in terms of energy, flux, and angular distribution using solid-state nuclear-track detectors, demonstrating a flux of ~105 particles per second at 10 Hz, and ~106 per second at 1 kHz. Hydrodynamic and particle-in-cell numerical simulations support our experimental findings. Potential impact of our approach on future spread of ultra-compact, multi-MeV alpha particle sources driven by moderate intensity (1016-1017 W/cm2) laser pulses is anticipated.

Single-shot selective femtosecond and picosecond infrared laser crystallization of an amorphous Ge/Si multilayer stack

V.A.Volodin, Yuzhu Cheng, A.V.Bulgakov, Y.Levy, J.Beránek, S.S.Nagisetty, M.Zukerstein, A.A.Popov, N.M.Bulgakova

2023 — Optics & Laser Technology — DOI: —

Pulsed laser crystallization is an efficient annealing technique to obtain polycrystalline silicon or germanium films on non-refractory substrates. This is important for creating “flexible electronics” and can also be used to fabricate thin-film solar cells. In this work, near- and mid-infrared femtosecond and picosecond laser pulses were used to crystallize a Ge/Si multilayer stack consisting of alternating amorphous thin films of silicon and germanium. The use of infrared radiation at wavelengths of 1030 and 1500 nm with photon energies lower than the optical absorption edge in amorphous silicon allowed obtaining selective crystallization of germanium layers with a single laser shot. The phase composition of the irradiated stack was investigated by the Raman scattering technique. Several non-ablative regimes of ultrashort-pulse laser crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of single- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are discussed. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point.

Double-pass optical parametric generator pumped by Yb thin-disk laser for efficient 1.4–2.9 µm mid-IR radiation generation

Bianka Csanaková, Ondřej Novák, Lukáš Roškot, Jiří Mužík, Martin Cimrman, Jaroslav Huynh, Martin Smrž, Helena Jelínková, Tomáš Mocek

2023 — Laser Physics — DOI: DOI: 10.1088/1555-6611/acb353 —

Many different fields benefit from the usage of light sources emitting in the mid-infrared wavelength range (2–10 µ m). A rising need for precise and fast sources in the mid infrared (mid-IR) is reflected in the development of a high-power, picosecond mid-IR source capable of generation at high repetition rates. In this work, we present the optimization of an optical parametric generator, pumped by a 3 W portion of total power of the Yb:YAG thin-disk laser (1.3 ps, 90 kHz, 90 W) by comparing a single-pass and double-pass arrangement output parameters in terms of output power dependences on input power, efficiency, beam profiles, stability, and spectra. The output tunability of both arrangements spanned from 1459 nm to 2891 nm, with the upper limit being influenced by the limited transmission of the dichroic components used in the setup above 2700 nm. It was shown that the double-pass arrangement increases the output power, from 17 mW in the single-pass arrangement to 193 mW in the double-pass arrangement at 1459 nm, resulting in over ten-fold output power increase.

Enhanced tribological performance and nanostructuring speed on AlTiN by beamshaping technology

T. Primus, P. Hauschwitz, T. Vitu, R. Bičišťová, P. Zeman, M. Cimrman, J. Brajer, T. Mocek, M. Smrž

2023 — Surface Engineering — DOI: —

For the first time, a dynamic beamshaping technology has been utilized for the efficient production of periodic nanostructures on top of AlTiN coating to enable dry machining without costly and environmentally hazardous cutting fluids. First, a variety of periodic nanostructures with periods in a range of 740–273 nm were produced utilizing different wavelengths. Additionally, beamshaping technology increased productivity by 4008% up to 105 cm2 min−1 by shaping the Gaussian beam into a rectangular beam of 500 × 30 µm. To simulate the application load and resulting heat production during manufacturing, friction analysis was performed at room and elevated temperature to 500°C. The analysis revealed a significant reduction in the friction coefficient – up to 27% and 19% at room temperature and 500°C, respectively. The combination of these results demonstrates that the proposed method can be scaled up for the mass production of functionalized machining tools for dry machining.

Adaptive optics system for a short wavelength mid-IR laser based on a Shack–Hartmann wavefront sensor and analysis of thermal noise impacts

Huang Zhou, Jan Pilař, Martin Smrž, Liyuan Chen, Miroslav Čech, Tomáš Mocek

2022 — Applied Optics — DOI: —

In the article, the author team presents an adaptive optics (AO) system for a 1.94-µm laser source. The system consists of a home-made Shack–Hartmann wavefront sensor and silver-coated bimorph deformable mirror operating in a closed-loop control scheme. The wavefront sensor used an uncooled vapor phase deposition PbSe focal-plane array for the actual light sensing. An effect of thermal afterimage was found to be reducing the centroid detection precision significantly. The effect was analyzed in detail and finally has been dealt with by updating the background calibration. System stability was increased by reduction of control modes. The system functionality and stability were demonstrated by improved focal spot quality. By replacing some of the used optics, the range of the demonstrated mid-IR AOS could be extended to cover the spectral range of 1–5 µm. To the best of the authors’ knowledge, it is the first AO system built specifically for mid-IR laser wavefront correction.

Facile Manufacture of Oxide-Free Cu Particles Coated with Oleic Acid by Electrical Discharge Machining

Irshad Ahamad Khilji, Siti Nadiah Binti Mohd Safee, Sunil Pathak , Chaitanya Reddy Chilakamarry , Amiril Sahab Bin Abdul Sani, Venugopal Jayarama Reddy

2022 — Micromachines — DOI: —

Particle synthesis has seen significant advances in current trends. However, the synthesis of metal particles without oxidation is a challenge for researchers. The current study presents a straightforward, convenient, and convincing approach for manufacturing copper (Cu) particles free of surface oxide. The die-sink Electrical Discharge Machine (EDM) of copper alloys with oleic acid resulted in the formation of Cu particles with diameters between 10 to 20 µm. X-ray diffraction (XRD) was used for particle examination after cleaning and sonication with distilled water. Cu particles with oleic acid coating retained a Cu phase without oxidation after synthesis. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to determine the size and morphology of generated particles. Fourier transforms infrared (FT-IR) analysis revealed the oleic acid-coated Cu surface bonded with an oxygen atom. Also, the agglomeration and change of size involving Cu particles with increasing voltages in the pulse supply in EDM were reported.

Thermal-stress-induced birefringence management of complex laser systems by means of polarimetry

Ondřej Slezák, Magdalena Sawicka-Chyla, Martin Divoký, Jan Pilař, Martin Smrž, Tomáš Mocek

2022 — Scientific Reports — DOI: —

The novel method of the thermally-induced polarization changes driven power losses (TIPCL) analysis in the complex laser systems has been developed. The measurement has been tested on the amplifier chain of the 100 J / 10 Hz laser system ‘Bivoj’ operated at HiLASE Centre. By the usage of the measured non-uniform Mueller matrix of the amplifier chain, the optimization of the ideal input and output polarization state has been calculated numerically. The results of the optimization have been applied to the laser system, thus reducing the TIPCL from originally observed more than 33% to 7.9% for CW beam and to 9% for pulsed laser beam, respectively. To the best of our knowledge, this result represents the most efficient TIPCL suppression method for complex laser systems so far. The method also allows the definition of the ideal fully polarized non-uniform pre-compensation of input beam consequently suffering from zero TIPCL.

TOF Analysis of Ions Accelerated at High Repetition Rate from Laser-Induced Plasma

Evan Russell, Valeria Istokskaia, Lorenzo Giuffrida, Yoann Levy, Jaroslav Huynh, Martin Cimrman, Martin Srmž, Daniele Margarone

2022 — Applied Sciences — DOI: —

The generation, detection, and quantification of high-energy proton spectra that are produced from laser-target interaction methodologies is a field of increasingly growing popularity over the last 20 years. Generation methods such as target normal sheath acceleration or similar allow for collimated laminar ion beams to be produced in a compact environment through the use of short-burst terawatt lasers and are a growing field of investment. This project details the development and refinement of a python-based code to analyze time-of-flight ion spectroscopy data, with the intent to pinpoint the maximum proton energy within the incident beam to as reliable and accurate a value as possible within a feasible processing time. TOF data for 2.2 × 1016 W/cm2 intensity laser shots incident on a 2 mm Cu target that were gathered from the PERLA 1 kHz laser at the HiLASE center were used as training and testing data with the implementation of basic machine learning techniques to train these methods to the data being used. These datasets were used to ensure more widely applicable functionality, and accurate calculation to within 1% accuracy of an assumed correct value was seen to be consistently achievable for these datasets. This wider functionality indicates a high level of accuracy for previously unseen TOF datasets, regardless of signal/noise levels or dataset size, allowing for free use of the code in the wider field.

Post-processing of selective laser melting manufactured SS-304L by laser shock peening

Sunil Pathak, Sanin Zulić, Jan Kaufman, Jaromír Kopeček, Ondřej Stránský, Marek Böhm, Jan Brajer, Libor Beránek, Ashish Shukla, Michal Ackermann, Filip Véle, Tomáš Mocek

2022 — Journal of Materials Research and Technology — DOI: —

This paper investigates the post-processing of additively manufactured stainless steel (SS) 304 L samples through laser shock peening (LSP). Four samples, differently processed, (i) as-built (AB); (ii) as-built + annealed; (iii) as-built + polished; and (iv) as-built + annealed + polished, were used to study the effect of LSP at pre-identified parameters to understand the mechanism and behavior of processing different conditions. In addition, compressive residual stresses and surface morphology were considered as a response to measure the effects of LSP on processed samples. The results have shown significant improvements in compressive residual stresses where the values have changed from initial tensile to compressive (−1170 MPa) and remain compressive up to the depth of 1 mm in most cases. The results of this research help better understand the LSP mechanism in the post-processing of AM Parts.

Role of Van Hove singularities and effective mass anisotropy in polarization-resolved high harmonic spectroscopy of silicon

Pawan Suthar, František Trojánek, Petr Malý, Thibault J.-Y. Derrien, Martin Kozák

2022 — Communications Physics — DOI: —

Nonlinear optical response of materials exposed to strong nonresonant light fields leads to production of high energy photons whose spectra contain fingerprints of the coherent electron dynamics in the material. In this paper we investigate how the high harmonic spectra generated in crystalline silicon are linked to specific properties of its band structure. By comparing the polarization anisotropy of high harmonic spectra for two distinct frequencies of the driving pulses we show that the anisotropy has two sources. When driven by mid-infrared light, the signal at specific photon energies is enhanced by the presence of Van Hove singularities in the joint density of states of silicon. With near-infrared driving pulses, in contrast, the high harmonic yield is mainly influenced by the anisotropy of the reduced mass of electron-hole pair, which is related to the nonresonant excitation probability. The experimental results are compared with numerical calculations using time-dependent density functional theory.

Improving functional performance characteristics of spur gears through flank modifications by non-contact advanced finishing process

Vivek Rana, Neelesh Kumar Jain, Sunil Pathak

2022 — The International Journal of Advanced Manufacturing Technology — DOI: —

This paper describes reduction in noise, vibrations, total and tooth-to-tooth, and longwave transmission errors, total and tooth-to-tooth composite errors, and radial runout of spur gears by imparting them flank modifications and their combinations by a non-contact advanced finishing process referred to as pulsed electrolytic dissolution. Eight spur gears were modified using innovatively developed 5 cathode gears and apparatus by imparting them 5 flank modifications (i.e. tip relief, root relief, end relief, profile crowning, lead crowning) individually and their 4 selected combinations. Vibrations at all rotational speeds and noise at 900 and 1200 rpm are reduced for all modified spur gears at all the values of applied loads. End relieved spur gear showed maximum reductions in noise and vibrations by 5 dBA and 3.77 m/s2 respectively for 1200 rpm speed. Reduction amount in noise and vibrations of modified gears increase with rotary speed. Lead crowned gear showed maximum reductions of 146 µm in total transmission error, 109 µm in total composite error, and 102 µm in radial runout. End relieved gear showed maximum reductions of 37 µm in tooth-to-tooth and 139 µm longwave transmission errors. Tip relieved, and tip and root relieved gear showed maximum reduction of 121 µm in tooth-to-tooth composite error. This work proves that the developed non-contact process can very effectively impart different flank modifications individually and their combinations to spur gears without any twist error. Lead crowning, end relieving, tip relieving, and tip and root relieving are main flank modifications that significantly improve functional performance characteristics of spur gears. It will result in their enhanced operating performance and service life, which will help their manufacturers and end-users.

Morphology of Meteorite Surfaces Ablated by High-Power Lasers: Review and Applications

Anna Křivková, Vojtěcj Laitl, Elias Chatzitheodoridis, Lukáš Petera, Petr Kubelík, Antonín Knížek, Homa Saeidfirozeh, Barbora Drtinová, Václav Cuba, Dan Páclík, Tomáš Mocek, Jan Brajer, Jan Kaufman, Martin Divoký, Jakub Koukal, Roman Dudžák, Nikola Schmidt, Petr Boháček, Svatopluk Civiš , Libor Lenža, Miroslav Krůs, Martin Ferus

2022 — Applied Sciences — DOI: —

Under controlled laboratory conditions, lasers represent a source of energy with well-defined parameters suitable for mimicking phenomena such as ablation, disintegration, and plasma formation processes that take place during the hypervelocity atmospheric entry of meteoroids. Furthermore, lasers have also been proposed for employment in future space exploration and planetary defense in a wide range of potential applications. This highlights the importance of an experimental investigation of lasers’ interaction with real samples of interplanetary matter: meteorite specimens. We summarize the results of numerous meteorite laser ablation experiments performed by several laser sources—a femtosecond Ti:Sapphire laser, the multislab ceramic Yb:YAG Bivoj laser, and the iodine laser known as PALS (Prague Asterix Laser System). The differences in the ablation spots’ morphology and their dependence on the laser parameters are examined via optical microscopy, scanning electron microscopy, and profilometry in the context of the meteorite properties and the physical characteristics of laser-induced plasma.

Harmonically mode-locked laser pulse accumulation in a self-resonating optical cavity

Yuya Koshiba, Seiya Otsuka, Koki Yamashita, Chikara Fukushima, Sakae Araki, Alexander Aryshev, Tsunehiko Omori, Konstantin Popov, Tohru Takahashi, Nobuhiro Terunuma, Yuuki Uesugi, Junji Urakawa, Masakazu Washio

2022 — Optics Express — DOI: —

Optical enhancement cavities enabling laser pulses to be coherently stacked in free space are used in several applications to enhance accessible optical power. In this study, we develop an optical cavity that accumulates harmonically mode-locked laser pulses with a self-resonating mechanism for X-ray sources based on laser-Compton scattering. In particular, a Fabry-Perot cavity composed of 99% reflectance mirrors maintained the optical resonance in a feedback-free fashion for more than half an hour and automatically resumed the accumulation even if the laser oscillation was suspended. In contrast to conventional optical enhancement cavity systems with a dedicated feedback controller, this characteristic is highly beneficial in practical applications, such as for laser-Compton scattering X-ray sources. Lastly, upscaling and adoption of the proposed system might improve the operability and equipment use of laser Compton-scattering X-ray sources.

Modeling thermal response of Mo thin films upon single femtosecond laser irradiation: dynamics of film melting and substrate softening

Kryštof Hlinomaz, Yoann Levy, Thibault J.-Y.Derrien, Nadezhda M.Bulgakova

2022 — International Journal of Heat and Mass Transfer — DOI: —

Energy relaxation in a thin molybdenum film exposed to a single laser pulse (wavelength 400 nm, pulse duration 200 fs at FWHM) has been investigated numerically. For this purpose, a two-temperature model was used accounting for the optical and thermodynamical properties of molybdenum deposited on a selection of substrates. The effect of the substrate on laser-induced film heating has been studied for the cases of fused silica, silicon, soda-lime glass as well as for a free-standing film. For a fused silica substrate, the calculated melting threshold fluence is in a good agreement with thickness-dependent experimental data available in literature. It has been found that, for Mo films with the thickness 107 nm, the softening point of the fused silica substrate is exceeded already at the Mo melting threshold fluence. This suggests the possibility of substrate damage related to glass deformation. The melting dynamics of the Mo film and the effects of energy transfer from the irradiated film to the substrate are discussed based on the modeling results.

Experimental study on the fluctuation–magnification effect of chirped volume Bragg grating compressor in high-energy, ultrafast, thin-disk chirped pulse amplification system

Huang Zhou, Ondřej Novák, Martin Smrž, Liyuan Chen, Lukáš Roškot, Bianka Csanaková, Michal Chyla, Miroslav Čech, Tomáš Mocek

2022 — Optical Engineering — DOI: —

The fluctuation–magnification effect on the peak intensity of a laser pulse caused by the nonlinear Kerr effect in the chirped volume Bragg grating (CVBG) compressor is investigated experimentally for a high-energy, thin-disk, chirped pulse amplification system. The nonlinear Kerr effect occurs at the blue end, and the accumulated nonlinear phase shift (B-integral) in the CVBG rises with the increase of laser pulse energy. Experiments show that small fluctuations in peak power of uncompressed pulses cause increasing of the temporal fluctuation and spatial fluctuation due to high Kerr-nonlinearity in the CVBG when B-integral is larger than π. Thus the initial fluctuation would be magnified by the CVBG compressor.

Fatigue life enhancement of additive manufactured 316l stainless steel by LSP using a DPSS laser system

Sanin Zulić, Danijela Rostohar, Jan Kaufman, Sunil Pathak, Jaromír Kopeček, Marek Böhm, Jan Brajer, Tomáš Mocek

2022 — Surface Engineering — DOI: —

Present work aims to study the fatigue life behaviour of additively manufactured (AM) stainless steel 316L in a laser shock peening (LSP) process. The fatigue behaviour of as-built samples and LSPeened sample were studied on the AM samples. A new diode-pumped laser system was utilized as an instrument to perform LSP operations. LSP operation was performed under two different sacrificial layers of different materials i.e. vinyl tape and aluminium tape. The other essential LSP parameters, namely energy, pulse width, spot size, overlap, and number of layers were kept constant based on the previous experimental findings. The characterization was done for analyzing the residual stresses using a hole drilling process, and a 4-point bending machine. A considerable improvement in compressive residual stress and fatigue life was observed in the LSPeened samples. Such improvements in residual stresses and fatigue life may pave a road to establishing diode-pumped lasers for LSP applications.

Investigation of the lasing performance of a crystalline-coated Yb:YAG thin-disk directly bonded onto a silicon carbide heatsink

Jan Cvrček, Martin Cimrman, David Vojna, Denisa Štěpánková, Ondřej Foršt, Martin Smrž, Ondřej Novák, Ondřej Slezák, Michal Chyla, Michal Jelínek, Tomáš Mocek

2022 — Optics Express — DOI: —

We investigated the use of crystalline coatings as the highly reflective coating of an Yb:YAG thin disk directly bonded onto a silicon carbide heatsink. Compared to commonly used ion-beam-sputtered coatings, it possesses lower optical losses and higher thermal conductivity, resulting in better heat management and laser outputs. We pumped the disk up to 1.15 kW at 969 nm and reached 665 W of average output power, and disk temperature of 107 °C with a highly multi-modal V-cavity. These promising results were reached with this novel design despite the adoption of a cheap silicon carbide substrate having more than 3 times lower thermal conductivity compared to frequently used CVD diamond.

Surface Morphology of Nimonic Alloy 263™ in Nanosecond Pulsed Laser Ablation

Sunil Pathak, Zhehao Jiang, S. Subramani, J. Radhakrishnan, Sundar Marimuthu

2022 — Springer Cham — ISBN: 978-3-031-03830-3 — DOI: —

This book covers the fundamentals of different laser-based manufacturing and processing, namely laser shock peening, laser micromachining, laser cleaning, cladding, remelting, laser honing, and other several aspects of lasers. The book discusses the general laser interaction with different materials. The application of laser-based post-processing of additive manufacturing and repair engineering is reported. It also provides the reader with mechanism of lasers in manufacturing and recent developments in tools, technologies, controls, and operations.

Laser induced damage threshold (LIDT) of β-barium borate (BBO) and cesium lithium borate (CLBO) – Overview

Hana Turčičová, Ondřej Novák, Jiří Mužík, Denisa Štěpánková, Martin Smrž, Tomáš Mocek

2022 — Optics & Laser Technology — DOI: —

Published data of laser induced damage thresholds (LIDT) of nonlinear BBO and CLBO crystals, which are used for the generation of deep-ultraviolet radiation, are collected and plotted against laser pulse duration. The LIDT measurement techniques are briefly described and their relevance relative to the intended application of the crystal is discussed. The intrinsic and extrinsic factors affecting the evolution of the LIDT are reported. Our own experience with the LIDT of BBO and CLBO crystals in the fifth harmonic generation (FiHG) process is presented. Lowering of the LIDT for crystals with antireflection coatings is treated.

Titanium Alloy Particles Formation in Electrical Discharge Machining and Fractal Analysis

Irshad Ahamad Khilji, Siti Nadiah Binte Mohd Saffe, Sunil Pathak, Ştefan Ţălu, Slawomir Kulesza, Miroslaw Bramowicz, Venugopal Jayarama Reddy

2022 — Journal of The Minerals — DOI: —

A large amount of material is machined by electrical discharge machining (EDM) and the machined debris is flushed away as waste. The present study aims to establish EDM as a top-down method which can effectively machine hard-to-cut materials while the by-product, i.e., metal debris, can be utilized as micro/nano particles. In the present work, titanium-alloy grade 5 was machined using a die-sink EDM and the debris was collected from the dielectric fluid with a multistage filtration system. The collected debris was cleaned and the particles were characterized. Particles were examined by scanning electron microscopy, x-ray diffraction, and energy-dispersive x-rays to determine size, shape, and chemical crystal structure. The average size of the formed particles was 5.14 µm and they were spherical. A fractal analysis was performed on the SEM images to deepen the study of the formed particles.

10-µJ few-cycle 12-µm source based on difference-frequency generation driven by a 1-kHz mid-wave infrared OPCPA

Martin Duda, Lorenz von Grafenstein, Martin Bock, Dennis Ueberschaer, Pia Fuertjes, Lukáš Roškot, Martin Smrž, Ondřej Novák, Uwe Griebner

2022 — Optics Letters — DOI: —

We report on high-energy, few-cycle pulse generation in the long-wave infrared spectral region via difference-frequency generation (DFG) in GaSe and AgGaSe2 nonlinear crystals. The DFG is driven by the signal at 3.5 µm and idler at 5 µm of a high-power mid-wave infrared optical parametric chirped pulse amplification (OPCPA) system operating at a 1-kHz repetition rate. The DFG pulses contain up to 17 µJ of energy and cover a spectrum from 8.5 µm to 14.5 µm. They are generated with a conversion efficiency of 2.1 %. Compression results in 10.2-𝜇 J pulses with sub-150-fs duration, corresponding to less than four optical cycles.

Extreme laser pulse-energy measurements by means of photon momentum

Paul A. Williams, Kyle A. Rogers, Martin Divoky, Geiland Porrovecchio, Jiří Tesař, Marek Smid, John H. Lehman

2022 — Optics Express — DOI: —

Extreme lasers capable of short, high-energy pulses are probing the frontiers of science and advancing practical technology. The utility of such lasers increases with their average power delivery, which enables faster data acquisition, higher flux of laser-driven particle and radiation sources and more efficient material processing. However, the same extreme energies and electric field strengths of these lasers are currently preventing their direct and high accuracy measurement for these experimental applications. To overcome this limitation, we use the momentum of the laser pulses as a measurement proxy for their energy. When light reflects from an ideal mirror, its momentum is transferred to the mirror, but its energy is reflected. We demonstrate here a force-sensing mirror configuration to measure laser pulse energies up to 100 J/pulse (10 ns duration, 10 Hz repetition rate) from a kilowatt-level average power multi-slab laser operated at the HiLASE facility of the Czech Academy of Sciences. We combine a radiation-pressure power meter with a charge integrator photodiode to form what we refer to as a Radiation Pressure Energy Meter. To our knowledge, this is the first demonstration of a high-accuracy, non-absorbing, SI traceable primary standard measurement of both single and average pulse energies of a 1-kW-average-power pulsed laser source. With this, we demonstrate a practical method for in-situ calibration of the traditional thermal instruments (pyroelectric detectors) currently used for indirect measurements of energy and power of such extreme lasers.

Silicon surface patterning by regular stripes of laser-induced periodic surface structures

J. Sládek, Y. Levy, T.J.-Y. Derrien, Z. Bryknar, N.M. Bulgakova

2022 — Applied Surface Science — DOI: —

Laser-induced periodic surface structures (LIPSS) formation on the surface of mono-crystalline silicon is reported for a specific fabrication regime of large-area structuring. Using a femtosecond laser at 1030 nm wavelength, a particular range of fluence and beam overlap in two-dimensional scanning scheme has been identified where LIPSS-covered areas organize into stripes perpendicular to the scanning direction. The stripes, where the LIPSS appear, are regularly spaced and located in between the centers of two subsequent Gaussian pulses while the regions receiving the peak pulse fluence are free of LIPSS. The formation of the LIPSS stripes is examined by analyzing local integrated fluence, N-on-1 pulse damage geometry and thresholds of modifications. Processes at play in the generation of such stripes of LIPSS are discussed and an explanation based on interpulse feedback from amorphized areas is proposed.

Photoionization and transient Wannier-Stark ladder in silicon: First-principles simulations versus Keldysh theory

Thibault J.-Y. Derrien, Nicolas Tancogne-Dejean, Vladimir P. Zhukov, Heiko Appel, Angel Rubio, Nadezhda M. Bulgakova

2021 — Physical Review B — DOI: —

Nonlinear photoionization of dielectrics and semiconductors is widely treated in the framework of the Keldysh theory whose validity is limited to photon energies that are small compared to the band gap and relatively low laser intensities. The time-dependent density functional theory (TDDFT) simulations, which are free of these limitations, enable one to gain insight into nonequilibrium dynamics of the electronic structure. Here we apply TDDFT to investigate the photoionization of silicon crystal by ultrashort laser pulses in a wide range of laser wavelengths and intensities and compare the results with predictions of the Keldysh theory. Photoionization rates derived from the simulations considerably exceed the data obtained with the Keldysh theory within the validity range of the latter. Possible reasons for the discrepancy are discussed and we provide fundamental data on the photoionization rates beyond the limits of the Keldysh theory. By investigating the features of the Stark shift as a function of photon energy and laser field strength, a manifestation of the transient Wannier-Stark ladder states is revealed, which become blurred with increasing laser field strength. Finally, it is shown that the TDDFT simulations can potentially provide reliable data on the electron damping time that is of high importance for large-scale modeling.

Wide spectral range optical characterization of yttrium aluminum garnet (YAG) single crystal by the universal dispersion model

Daniel Franta, Mihai-George Mureșan

2021 — Optical Materials Express — DOI: —

Pure YAG single crystal was optically characterized in the wide spectral range (from far IR to vacuum UV) by applying the universal dispersion model. Data obtained from a broad range of characterization instruments and methods was simultaneously processed using least-square method and the result were compared with literature findings. The universal dispersion model describes individual elementary electron and phonon excitations in materials as separate contributions. For the first time recorded, an asymmetric Voigt peak approximation was used for modeling the contribution of one-phonon absorption in crystalline material. The optical constants are presented both graphically and in detailed dispersion parameters sets.

Stereoselective Cyclopropanation of Boron Dipyrromethene (BODIPY) Derivatives by an Organocascade Reaction

Vojtěch Dočekal, Tereza Koberová, Jan Hrabovský, Andrea Vopálenská, Róbert Gyepes, Ivana Císařová, Ramon Rios, Jan Veselý

2021 — Advanced Synthesis & Catalysis — DOI: DOI: 10.1002/adsc.202101286 —

The synthesis of enantiopure chiral boron dipyrromethenes (BODIPYs) is of importance due the intrinsic properties of BODIPYs as fluorophores that could be used as probes for molecular sensing. The present study reports an asymmetric organocatalytic cascade reaction of meso-chloromethyl BODIPY derivatives with α,β-unsaturated aldehydes catalyzed by a chiral secondary amine. The corresponding BODIPY-derived cyclopropanes were produced in isolated yields 66–98%, and with diastereomeric ratios 3/2–>20/1, and 92–99% ee for major diastereomer. The synthetic utility of the protocol was exemplified on a set of additional transformations of the corresponding optically pure compounds. In addition, a study explaining the reaction mechanism (DFT computations) and photophysical characterization of all enantioenriched products were accomplished.