We present the approaches for improving the properties of mid-IR ultrafast lasers based on iron-doped chalcogenide crystals, including energy scaling due to multipass chirped pulse amplification in Fe:ZnSe, spectral synthesis in combination of Fe:ZnSe and Fe:CdSe gain media, as well as spectral broadening in chalcogenide fibers and studying of nonlinear properties of carbon nanotubes.

The generation of terahertz radiation in a BNA crystal pumped by 1.24-µm femtosecond laser radiation from a Cr:forsterite laser system with a pulse duration of 100 and 35 fs and a pump density of 10 mJ/cm 2 has been realized. The achieved generation efficiency is 0.1%. It is found that a decrease in the pump pulse duration from 100 to 35 fs leads to the generation of high-frequency components in the ranges of 2.5–6.5 THz and 9‒10.5 THz in the generated radiation spectrum. Simulation of the terahertz radiation generation based on the solution of Maxwell’s equations by the finite-difference time-domain method has made it possible to adequately describe the measured spectra. The generation of broadband high-frequency terahertz radiation in the BNA crystal pumped by the Cr:forsterite laser system allows one to consider this schematic as an alternative to sources based on the BNA crystal pumped by a Ti:sapphire laser system.

The femtosecond generation in the Cr:Forsterite laser synchronously pumped by an ytterbium ultrafast laser is implemented. It was found that synchronous pumping is a stable regime with respect to small changes in cavity length and allows mode-locked regime to be maintained in the temperature range 4 degrees Celsius higher. The energy and spectral parameters of pulses in various generation regimes are characterized. The dependence of the position of the spectrum on the pulse repetition rate is investigated. Difference frequency generation is realized with output radiation at a wavelength near 6 μm at a high repetition rate. Such a seeding source is promising for development powerful chirped pulse amplification systems in mid-IR range based on iron doped chalcogenide crystals.

Laser-Induced Forward Transfer (LIFT) technology has now found its wide application in the field of bioprinting. Laser bioprinting allows the spatial transfer of living micro-objects (cells, microorganisms) in a gel medium using the energy of a laser pulse. Typically, to effectively absorb laser pulse energy, a thin metal layer is deposited on the donor substrate. The disadvantages of this approach are the appearance of cytotoxic metal nanoparticles during laser bioprinting and the need for continuous movement of the donor substrate. The article proposes a method that allows moving to absorption film-free bioprinting using Q-switched Er:YAG nanosecond laser operating at a wavelength of 2.94 mu m. This wavelength is effectively absorbed in a top ten-micron hydrogel layer, allowing for a cleaner (without nanoparticles from absorbing layer) and simpler (without the need to constantly move the donor substrate from pulse to pulse) laser bioprinting process. Using strain Escherichia coli as an example, the suitability of the proposed method for successful laser bioprinting is demonstrated. A model describing in detail the processes occurring in the hydrogel layer is presented.

The effect of the length and pressure of an argon gas jet on low-order harmonics (5th, 7th, 9th, and 11th) generation by 4.5-μm femtosecond laser radiation of a Fe:ZnSe laser system has been studied experimentally. It has been shown that an increase in the length of the generation medium up to the waist length allows one to increase the generation efficiency by a factor of 12. It has also been demonstrated that a change in the length of the gas medium changes the pressure dependence of the energy of the generated radiation because of change in the phase matching conditions, whose correct simulation requires the inclusion of nonlinear propagation effects for pump pulse and generated harmonics.

It has been shown that the spectrum of intense few-cycle terahertz radiation generated in a DAST organic crystal can be controlled by chirping 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system. It has been found that an increase in the linear chirp of generating radiation results in the narrowing of the spectrum of terahertz radiation and its redshift. The simulation of the generation of terahertz radiation within the model of three-wave mixing has shown that this effect is due to a change in the phase matching width of the degenerate generation of the difference frequency of terahertz range. In addition, the comparative analysis of terahertz radiation spectra generated in DAST, DSTMS, OH1, and BNA organic crystals indicates that the spectral–temporal properties of terahertz radiation can be more roughly controlled by choosing an appropriate crystal. The proposed approach to control the terahertz radiation spectrum by chirping the pump pulse provides the foundation for spectroscopic studies using intense terahertz radiation with controlled spectral–temporal properties.

The chirped pulse amplification (CPA) systems based on transition-metal-ion-doped chalcogenide crystals are promising powerful ultrafast laser sources providing access to sub-TW laser pulses in the mid-IR region, which are highly relevant for essential scientific and technological tasks, including high-field physics and attosecond science. The only way to obtain high-peak power few-cycle pulses is through efficient laser amplification, maintaining the gain bandwidth ultrabroad. In this paper, we report on the approaches for mid-IR broadband laser pulse energy scaling and the broadening of the gain bandwidth of iron-doped chalcogenide crystals. The multi-pass chirped pulse amplification in the Fe:ZnSe crystal with 100 mJ level nanosecond optical pumping provided more than 10 mJ of output energy at 4.6 μm. The broadband amplification in the Fe:ZnS crystal in the vicinity of 3.7 μm supports a gain band of more than 300 nm (FWHM). Spectral synthesis combining Fe:ZnSe and Fe:CdSe gain media allows the increase in the gain band (~500 nm (FWHM)) compared to using a single active element, thus opening the route to direct few-cycle laser pulse generation in the prospective mid-IR spectral range. The features of the nonlinear response of carbon nanotubes in the mid-IR range are investigated, including photoinduced absorption under 4.6 μm excitation. The study intends to expand the capabilities and improve the output characteristics of high-power mid-IR laser systems.

В работе показана возможность управления спектром мощного малопериодного терагерцового излучения, генерируемого в органическом кристалле DAST, путем чирпирования излучения накачки, в качестве которого использовалось фемтосекундное лазерное излучение системы на кристалле хромфорстерита с длиной волны 1.24 мкм. Установлено, что увеличение линейного чирпа генерирующего излучения приводит к обужению спектра терагерцового излучения и его смещению в низкочастотную область. Проведенное моделирование процесса генерации терагерцового излучения в рамках модели трехволнового смешения показывает, что в основе данного эффекта лежит изменение ширины фазового синхронизма вырожденного процесса генерации разностной частоты терагерцового диапазона. Также проведен сравнительный анализ спектров терагерцового излучения, генерируемого в органических кристаллах DAST, DSTMS, OH1 и BNA, свидетельствующий о возможности более грубого управления спектрально-временными свойствами терагерцового излучения путем выбора требуемого кристалла. Продемонстрированный подход к управлению спектром терагерцового излучения путем чирпирования импульса накачки закладывает основу для проведения спектроскопических исследований с использованием мощного терагерцового излучения с управляемыми спектрально-временными свойствами.

High (15–25) harmonic generation in the vacuum ultraviolet spectral range (83–50 nm) has been realized by focused (NA = 0.033) near-infrared femtosecond laser radiation (wavelength λ = 1.24 μm) with a vacuum intensity of ~7.5 × 10 14 W/cm 2 irradiating a dense gas jet. It has been shown experimentally that the use of such a high-numerical aperture focusing requires high (up to 10 bar) gas jet pressures to optimize phase matching. The use of the dense gas jet results in a noticeable manifestation of nonlinear propagation effects for generating radiation, which affect the generation process through the change in the phase matching conditions. Furthermore, it has been shown that the prechirping of the generating pulse makes it possible to compensate a chirp appearing due to self-phase modulation and to increase the harmonic generation efficiency because of the nonlinear compression of the generating pulse. This approach has allowed 17th (73 nm) harmonic generation with an energy of 2 pJ in a pulse and a generation efficiency of 5.4 × 10 –9 . The estimates obtained have shown that this radiation can be used for single-pulse maskless photolithography in the extreme ultraviolet range.

It has been shown that the spectrum of intense few-cycle terahertz radiation generated in a DAST organic crystal can be controlled by chirping 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system. It has been found that an increase in the linear chirp of generating radiation results in the narrowing of the spectrum of terahertz radiation and its redshift. The simulation of the generation of terahertz radiation within the model of three-wave mixing has shown that this effect is due to a change in the phase matching width of the degenerate generation of the difference frequency of terahertz range. In addition, the comparative analysis of terahertz radiation spectra generated in DAST, DSTMS, OH1, and BNA organic crystals indicates that the spectral–temporal properties of terahertz radiation can be more roughly controlled by choosing an appropriate crystal. The proposed approach to control the terahertz radiation spectrum by chirping the pump pulse provides the foundation for spectroscopic studies using intense terahertz radiation with controlled spectral–temporal properties.

We report on a first of its kind, to our knowledge broadband amplification in a Fe:CdSe single crystal in the mid-IR beyond 5 µm. The experimentally measured gain properties demonstrate saturation fluence close to 13 mJ/cm2 and support the bandwidth up to 320 nm (full width at half maximum). Such properties allow the energy of the seeding mid-IR laser pulse, generated by an optical parametric amplifier, to be pushed up to more than 1 mJ. Dispersion management with bulk stretcher and prism compressor enables 5-µm laser pulses of 134-fs duration, providing access to multigigawatt peak power. Ultrafast laser amplifiers based on a family of Fe-doped chalcogenides open the route for wavelength tuning together with energy scaling of mid-IR laser pulses that are strongly demanded for the areas of spectroscopy, laser-matter interaction, and attoscience.

Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense (I ~ 1014 W/cm2) femtosecond mid-infrared radiation of a laser system based on a Fe:ZnSe crystal (wavelength is 4.55 μm, pulse duration by the FWHM level of intensity is 160 fs, and the pulse energy is up to 3.5 mJ) with an argon jet (pressure is up to 10 bar) in the tunneling ionization regime (Keldysh parameter is $$\gamma = 0.2$$ ). The maximum energy efficiencies of the 5th, 7th, and 9th harmonic generation are 2 × 10–7, 6 × 10‒9, and 3 × 10–10, respectively. It has been established that nonlinear effects of propagation of generating radiation under an increase in the pressure of the gas jet begin to significantly affect the process of generation.

В данной работе экспериментально исследовано влияние длины и давления газовой струи аргона на процесс генерации гармоник низкого порядка (5, 7, 9, 11) фемтосекундным излучением лазерной системы на кристалле Fe:ZnSe c длиной волны 4.5 мкм. Экспериментально установлено, что увеличение длины среды генерации вплоть до длины перетяжки позволяет более чем на порядок (в 12 раз) увеличить эффективность генерации. Кроме того, показано, что изменение длины газовой среды также приводит к изменению зависимости энергии генерируемого излучения от давления, что является следствием изменения условий фазового согласования, для корректного моделирования которых необходимо учитывать нелинейно-оптические эффекты распространения генерирующего излучения.

Precise control of the nonlinear optical phenomena is the limiting factor for the spectral broadening and pulse compression techniques for high-power laser systems. Here we demonstrate that generation of the blue and red components under filamentation of 4.55-μm mid-IR pulses can be easily adjusted independently through the use of inert and molecular gases, while uniform broadening up to 1-μm bandwidth at the 1/e2 level relies on the proper choice of gas mixture and its compounds partial pressure. Such synthesized media provide a feasible route for the free of damage control of pulse spectral broadening and compression for gigawatt peak power laser systems operating in the mid-IR. Additional management of a generated spectrum can be realized through the adjustment of focusing conditions. The resulted pulse is compressed by a factor of 2.6 down to 62 fs pulse duration (4.1 optical cycles) with additional dispersion compensation. Controllable nonlinear compression down to four optical cycles keeping the millijoule energy level of a mid-IR laser pulse provides direct access to extreme nonlinear optics.

The amplification and generation properties of erbium-doped laser crystals (Er:YAG, Er:YSGG, Cr:Er:YSGG) with a high (up to 50 ^4I_9/2 and ^4I_11/2 ) is decisive for the population inversion in Er:YAG, whereas this is less important for other erbium-doped media. Pulses with an energy of 62 mJ at 10 Hz have been obtained in the ТЕМ00 mode in the developed electro-optically Q-switched Er:YAG oscillator and two single-pass amplifiers. The optomechanical Q-switching based on a rotating mirror opens access to a higher output energy due to the absence of losses in optical elements and depolarization. Single pulses with an energy of 75 mJ and a duration of 123 ns at a repletion rate of 10 Hz have been obtained in the Er:YAG laser, whereas a high gain of Cr:ErYSGG limits obtaining single nanosecond pulses with this Q-switching method. The development of such sources with high peak and average powers is of interest for terawatt chirped-pulse amplification laser systems based on iron-ion-doped chalcogenides in the mid-infrared range (3–5 μm).

We present the single-shot optoacoustic method of the TEMoo mode laser pulse parameters retrieval (pulse duration and beam radius). The experimental proof of the concept is demonstrated with the use of Er:YAG mid-IR nanosecond laser radiation. The proposed method can be applied for any laser wavelength provided that the appropriate highly-absorptive medium was chosen. The presented method can be a subject of interest in the area of laser metrology and applications.

This review highlights the development of ultrafast sources in the near- and middle-IR range, developed in the laboratory of Nonlinear Optics and Superstrong Laser Fields at Lomonosov Moscow State University. The design of laser systems is based on a powerful ultrafast Cr:Forsterite system as a front-end and the subsequent nonlinear conversion of radiation into the mid-IR, THz, and UV spectral range. Various schemes of optical parametric amplifiers based on oxide and non-oxide crystals pumped with Cr:Forsterite laser can receive pulses in the range of 4–6 µm with gigawatt peak power. Alternative sources of mid-IR ultrashort laser pulses at a relatively high (MHz) repetition rate are also proposed as difference frequency generators and as a femtosecond mode-locked oscillator based on an Fe:ZnSe crystal. Iron ion-doped chalcogenides (Fe:ZnSe and Fe:CdSe) are shown to be effective gain media for broadband high-peak power mid-IR pulses in this spectral range. The developed sources pave the way for advanced research in strong-field science.

Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense ( I 10 14 W/cm 2 ) femtosecond mid-infrared radiation of a laser system based on a Fe:ZnSe crystal (wavelength is 4.55 μm, pulse duration by the FWHM level of intensity is 160 fs, and the pulse energy is up to 3.5 mJ) with an argon jet (pressure is up to 10 bar) in the tunneling ionization regime (Keldysh parameter is γ = 0.2 ). The maximum energy efficiencies of the 5 th , 7 th , and 9 th harmonic generation are 2 × 10 –7 , 6 × 10 ‒9 , and 3 × 10 –10 , respectively. It has been established that nonlinear effects of propagation of generating radiation under an increase in the pressure of the gas jet begin to significantly affect the process of generation.

Being the second most abundant element on earth after oxygen, silicon remains the working horse for key technologies for the years. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. Mid-IR (beyond 4 µm) ultrafast lasers provide the required control for all these parameters not only on the surface (as in the case of the lithographic techniques), but also inside the bulk of the semiconductor, paving the way to an unprecedented variety of properties that can be encoded via such an excitation. We estimated the deposited energy density as 6 kJ cm −3 inside silicon under tight focusing of mid-IR femtosecond laser radiation, which exceeds the threshold value determined by the specific heat of fusion (~ 4 kJ cm −3 ). In such a regime, we successfully performed single-pulse silicon microstructuring. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a “ring” with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 × 10 21 cm −3 . These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon.

We report on a powerful mid-IR diode-side-pumped tunable Er:LiYF4 (Er:YLF) laser electro-optically Q-switched with the help of a KTiOPO4 crystal. At a 20 Hz repetition rate, the laser pulses with output energy of 82 mJ and 13 ns duration at the wavelength of 2.67 µm are obtained. At higher repetition rates (up to 50 Hz), one can extract up to 20 mJ from the laser cavity. The developed mid-IR laser source demonstrates high peak (up to 6.3 MW) and average (up to 1.7 W) power. Realized wavelength tuning provides access for megawatt-peak power-level nanosecond laser pulses over the 2667-2851 nm wavelength region, which are highly demanded for mid-IR laser systems development and light-matter interaction study in the view of extreme-state creation in liquids and solids, paving the way to novel microprocessing techniques.