Search

We present a novel approach for flexible adjustment of the media nonlinear response for efficient harmonics generation. Molecular resonances located in mid-IR have shown to change refractive index for pump radiation significantly and provide opportunity for effective phase matching. We use mixture containing 48 bar of xenon (Xe) and 1 bar of carbon dioxide (CO 2 ) to generate eleven harmonics with efficiency of 1% for third harmonic and 0.3% for higher ones (from 5th to 11th) in total. Theoretical calculations predict simultaneous phase matching of large number of harmonics in XUV region paving the way for seeding FELs and pump-probe XUV diagnostic.

We report the graphene-based passive mode-locking in Fe:ZnSe. The laser delivers 415 mW average power output radiation at a wavelength of 4.4 μm being pumped by 7-W fiber Er:ZBLAN laser. A pulse duration of 732 fs retrieved from the first-order autocorrelation function. Besides, we observed pulsed nanosecond oscillation under continuous-wave pumping and strong amplitude modulation caused by Kerr self-focusing. A numerical simulation of the soliton propagation in a laser cavity using the nonlinear Schrodinger equation supports obtained results. Such simple and effective ultrafast mid-IR high-repetition-rate coherent sources are designed to become a valuable tool in various fields of photonics.

The development of high power mid-IR laser applications requires a study on laser induced damage threshold (LIDT) in the mid-IR. In this paper we have measured the wavelength dependence of the plasma formation threshold (PFT) that is a LIDT precursor. In order to interpret the observed trends numerically, a model describing the laser induced electron dynamics, based on multiple rate equations, has been developed. We show both theoretically and experimentally that PFT at mid-IR wavelengths is controlled by a transition from weak- to strong-field regime of free carrier absorption. In the case of MgF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 this transition occurs around 3–4 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}μm corresponding to the region of the lowermost PFT. The region of the uppermost PFT is reached around 1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}μm and is governed by an interplay of photoionization and weak-field free carrier absorption which manifests itself in both MgF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 and SiO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2. The PFT observed in considered materials exhibits a universal dependence on the excitation wavelength in dielectrics. Thus, the presented results pave the route towards efficient and controllable laser-induced material modifications and should be of direct interest to laser researchers and application engineers for prevention of laser-induced damage of optical components in high-intensity mid-IR laser systems.

We report, for the first time, to the best of our knowledge, a femtosecond mode-locked Fe:ZnSe laser. Passive mode locking is implemented using graphene as a saturable absorber. The laser operates at 4.4 µm with a repetition frequency of 100 MHz and 415 mW output power pumped by a fiber 7 W Er:ZBLAN laser. The pulse duration of about 732 fs is retrieved from the first-order autocorrelation function. Additionally, we observe pulsed nanosecond oscillation under continuous-wave pumping and strong amplitude modulation caused by Kerr self-focusing. This Letter fills the gap in operating regimes of Fe:ZnSe lasers and paves the way for the development of powerful ultrafast high-repetition-rate mid-IR sources for the most advanced fields of science.

It has been found that the third harmonic generated under the tight focusing of a near-infrared (1.24-μm) femtosecond laser pulse indicates the energy deposition into the bulk transparent dielectric and can be used as a feedback system in the process of microstructuring. A third harmonic signal is sensitive to a change in both the size of a laser-induced microplasma and the free-electron density, which allows the detection of the microplasma with submicron longitudinal sizes. It has been shown that the method based on the third harmonic is universal and can be applied in both single-pulse and two-color microstructuring.

Pulses with a spectral width of 450 nm at a wavelength of 2.2 μm are generated in a parametric amplifier based on a type-I BBO crystal pumped by the second harmonic (620 nm) of a Cr:forsterite laser. Such a broadband radiation is obtained in a specific regime of the group velocity matching near the vertical region of the tuning curve. The generated pulses can be compressed to a duration of about 2.5 optical cycles (34 fs) in a medium with positive group-velocity dispersion.

Femtosecond microprocessing is of particular interest for various fields of science and technology ranging from precise cell surgery to industrial fabrication of opto-electronic devices [1,2]. The dependence of deposited energy density (DED) on laser parameters provides unique information about the breakdown initiation and dominant mechanisms. Detailed knowledge of the parameter dependence of DED is essential for controlled formation of micro- and nano-structures with predictable morphologies.

The nonlinear response of the matter irradiated by the strong light fields is of particular interest for both fundamental studies of laser microplasma properties and practical applications of generated coherent UV/VUV radiation. Gases provides unique properties for such strong field experiments since they are free from optical breakdown and their properties (nonlinearity, ionization cross section, dispersion) can be properly adjusted for a specific interaction regime. Nowadays the most attention is attracted to high harmonic generation (HHG) due to its applicability in attoscience [1]. However, efficient generation of low harmonics is also highly required for time-resolved spectroscopy [2], seeding free electron lasers [3] and other applications.

Mid-IR laser systems have a variety of applications ranging from generation of coherent x-ray and THz radiation to microstructuring of bulk dielectrics and semiconductors. To reach the mid-IR region, optical parametric amplifiers (OPAs) are employed for down-conversion of high energy near-IR pump lasers and require seed radiation at 1.5–2 μm. Several attempts have been made to improve the supercontinuum (SC) generated in bulk [1,2] and gas [3], but the spectral brightness and/or temporal quality in the region of 1.5–2 μm stays quite poor. We performed an extensive parametric analysis of SC generation in different dielectric materials and found a regime with well-resolved local maxima in the red wing that is highly appropriate as an OPA seed. These results provide strong ground for further development of SC-seeded ultrafast OPAs in the mid-IR spectral range using Cr:Forsterite and ytterbium based lasers as pump sources [4].

We report on entering a new era of mid-IR femtosecond lasers based on amplification in a relatively new gain chalcogenide medium, Fe:ZnSe. Our hybrid all-solid-state laser system is based on direct pulse amplification of femtosecond seed from three-stage AGS-based-optical parametric amplification (OPA) in a Fe:ZnSe laser crystal optically pumped by a Cr:Yb:Ho:YSGG Q-switched nanosecond laser. The development of the pump source with output energy up to 90 mJ operating at a 10 Hz repetition rate regime and highly efficient grating compressor (80%) provides 3.5-mJ 150-fs femtosecond pulses centered at 4.4 μm. Diode-pumped Er:YAG/Er:YLF lasers make it possible to increase the beam quality and repetition rate of the proposed laser system up to 100 Hz. Focusing such a laser radiation into the ∼3λ beam diameter allows us to reach a focus laser intensity up to 1016 W/cm2 which is only an order of magnitude lower than a relativistic intensity of 1017 W/cm2 and enough to drive strong nonlinear optics in mid-IR. We show as a proof-of-principle experiment the generation of four-octave spanning (from 350 nm up to 5.5 μm) supercontinuum in xenon.

The deposited energy density (DED) serves as a key parameter in the process of the femtosecond laser pulse energy delivery into the bulk of transparent dielectrics. The laser-induced micromodification can be created if the value of DED exceeds a certain threshold, which is specific for each material and does not depend on the laser wavelength. In this contribution, we present a comprehensive study of the DED evolution with the driving pulse energy and wavelength under femtosecond microstructuring of transparent dielectrics. To precisely determine the laser impact area we applied for the first time a real-time diagnostic of microplasma based on third harmonic generation. This technique gives submicron spatial resolution and is extremely sensitive to the free electron density (about 10-5 of the critical electron density). We found out that the threshold DED equals to approximately 2.5 kJ/cm3 for fused silica and roughly corresponds to excess of glass transition temperature. The highest DED is achieved for the shortest wavelength (620 nm) and equals to 16 kJ/cm3.

We demonstrate a novel approach of femtosecond laser pulse spectral-temporal control under filamentation in a wide range of pressures from 5 bar up to supercritical state. We showed that laser-induced processes such as supercontinuum generation and pulse self-compression could be tuned both by pressure and by energy adjustment due to the nonlinear refractive index alteration via pressure change. Thanks to the high pressure-controlled nonlinearity, a broadband (from 250 to 2500nm) supercontinuum with total efficiency more than 50% of energy can be generated in supercritical Xe and CO2. It was also demonstrated that the femtosecond laser pulse can be compressed in Xe in the femtosecond monofilamentation mode by 3.5 times with efficiency as high as 60%.

We report the first-of-its-kind compact and robust coherent source operating in mid-IR based on Fe:ZnSe chalcogenide gain medium optically pumped by Er:ZBLAN fiber laser. In the research, we study the CW operation of cryogenically cooled laser based on Fe:ZnSe single crystals with different doping level grown from the vapor phase on a single-crystal seed by using the concurrent-doping technology. The maximal output power achieved is 2.1 W with 59% slope efficiency with respect to absorbed pump power, which is close to the Stokes shift limit. Measured Fe:ZnSe output spectra indicate a significant influence of re-absorption on generation wavelength. For high doping levels and output powers, spectrum shifts to the red wing, which makes possible continuous tuning from 4012 to 4198 nm. As well, tunability of the laser in a wide range of temperature is investigated.

Dielectric properties of bionanocomposites resulting from regulated self-organization of chitosan and nanoparticles of synthetic saponite clay have been investigated by terahertz pulsed spectroscopy. Spectral characteristics of the composites considered in correlation with their structural features, which have been characterized by atomic force microscopy and scanning electron microscopy, depend on the concentration ratio of the components. The study of the effect of temperature on terahertz absorption spectra has led to the conclusion that hydrogen bonding may be involved in the formation of the bionanocomposites.

In this Letter, a multi-pass amplifier based on polycrystalline Fe:ZnSe element grown by the chemical vapor deposition (CVD) method and doped through diffusion technique is realized with output energy of more than 3 mJ at 4.5 μm. The crystal used in the experiment has external doping layers with peak Fe ions concentration of (1-2)х1019 сm−3 located on the surface and Fe ions diffusion depth of hundreds of microns. The crystal demonstrates small-signal gain per pass of 5.7 (corresponding to effective gain of 7.1 cm−1 taking into account 8 mm total length of the crystal) for short pump pulse (40-ns) under the optimal pump fluence of around 0.8 J/cm2 and upper-level lifetime of 670 ± 10 ns under moderate cooling to 7-8 °C, which makes such a crystal very attractive for mid-IR femtosecond multi-pass amplifiers and generators. Moreover, properties of the polycrystalline Fe:ZnSe elements grown by CVD and doped through diffusion and CVD methods with higher Fe ions concentration are also studied at different temperatures and compared with a single crystal element.

We present a novel approach to the characterization of ultrafast laser–matter interaction processes in solids and liquids under extreme conditions of microplasma generation. Through the combination of three-dimensional propagation imaging, absorption measurements, shadowgraphy and photoacoustic imaging we can restore plasma electron density distribution, laser pulse fluence profile and the value of deposited energy density inside the bulk of the material and characterize the regime of the laser pulse propagation. The developped concept is important for understanding the physics of ultrafast laser–matter interactions with strong implications for precision control of laser micromachining, bioprocessing and biotreatment.

We demonstrate that third-harmonic can be applied as a feedback during the process of femtosecond micromachining of solids with tightly focused laser beams. The third harmonic was used for 3D mapping of laser-induced microplasma and micromodification.

The first CW single-crystal Fe:ZnSe laser optically pumped by Er:ZBLAN fiber laser with an output power of 2.1 W is reported. Temperature-dependent spectral characteristics and tuning (3.8-5.1 μm) are investigated.

We present femtosecond parametric amplification of ultra-broadband (1.8 – 2.5 μm) seed radiation produced by near-IR BBO OPA in relatively novel nonlinear LiGaS 2 (LGS) crystal. The mixed BBO/LGS OPA shows 11% efficiency with 70 μJ output and 250-nm bandwidth near degeneracy (2.5 μm).

We have developed a robust optical parametric amplifier (OPA) based on three-AgGaS2-crystal pumping by a Cr:Forsterite GW femtosecond laser system, generating 150 fs pulses in dual bands of 1.6–2.0 μm (signal wave) and 3.5–5.5 μm (idler wave). By introducing a negative prechirp to the pump, a combined efficiency in two waves of greater than 10% was achieved, with signal energy up to 110 μJ and idler energy up to 43 μJ. Operation parameters of the system (intensity up to 90 TW/cm2) make OPA a promising tool for driving nonlinear optical phenomena, including generation of optical harmonics and laser-induced extreme states of matter in solids and liquids. As a proof of principle, we generated harmonics up to the sixth order and sum frequencies in 5 mm thick polycrystalline ZnSe.

Femtosecond mid-IR laser sources are of great demand due to its usefulness in a number of applications ranging from strong field phenomena to medical and environmental sensing. Nowadays devices based on OPA and OPCPA techniques had become a typical approach to produce mid-IR radiation. However, with Ti:Sapphire pump laser efficiency of OPAs in 4–5 μm range is rather low mainly due to the lack of suitable nonlinear crystals, whereas OPCPA schemes are incredibly complex [1, 2].

We demonstrate an efficient amplification of the broadband tunable (from 3.8 μm up to 4.8 μm) mid-IR femtosecond (∼200 fs) seed pulse in optically pumped Fe2+:ZnSe amplifier [1]. The experimental setup is sketched in Fig. 1a. Seed radiation was generated in two-stage AGS-based optical parametric amplifier pumped by Cr:forsterite laser [2]. Mid-IR output radiation with energy up to 1 μJ was then directed into a multipass amplifier. Iron-doped ZnSe crystal was pumped by Q-switched Cr:Yb:Ho:YSGG laser with 40 ns pulse duration, total energy of 20 mJ, operating at 2.85 μm wavelength [3]. In order to prevent carbon dioxide absorption, the last OPA stage and multi-pass amplifier was placed in the chamber, where atmospheric air was substituted by noble gas. Prior to the main experiments, gain evolution measurements were carried out. They have shown that the lifetime of population inversion of Fe2+:ZnSe strongly depends on the temperature. It increases from 350 ns (at room temperature) up to 1 μs (at 7 degrees Celsius) paving the way for multi-pass amplification. As a result, femtosecond seed radiation with energy 0.7 μJ centered at 4.3 μm was amplified in six-pass Fe2+:ZnSe amplifier up to the energy 70 μJ (Fig. 1b), which corresponds to 0.5 GW peak power. Decreasing of the input energy down to 40 nJ did not prevent to decrease in output energy. Therefore the total gain was about 2000. Inset in fig. 1 (b) presents far-field beam profile of amplified pulses. Substitution of air by argon removes spectral distortions in the laser pulse caused by the absorption by atmospheric CO 2 molecules. It does not radically improve the amplification process in Fe2+:ZnSe but stabilizes the operating regime (Fig. 1c).

We demonstrate a first-of-its-kind efficient chirped pulse amplification of broadband mid-IR (4-5 μm) femtosecond seed pulse (230 ps, 4μJ) generated in AgGaS2 based OPA driven by Cr:forsterite laser in multi-pass Fe2+:ZnSe amplifier optically pumped by solid-state Q-switched Cr:Yb:Ho:YSGG laser (2.85 μm, 30mJ, 5Hz, 0.6 J/cm2). The system delivers 1.2 mJ at pulse duration of 230 ps. Straightforward compression to 150 fs pulse is achievable with 70% efficiency using diffraction grating pair with peak power of about 6 GW. Further non-linear compression in a bulk CaF2 due to the SPM and anomalous GVD should provide the enhancement of peak power up to 20 GW. Possible routes to reach sub-TW and even TW power level in mid-IR are discussed.

We demonstrate novel approaches for femtosecond laser pulse spectral–temporal control under filamentation in a wide range of high pressures from 5 bar up to supercritical xenon. Laser-induced processes, such as supercontinuum (SC) generation and pulse self-compression, can be tuned both by pressure and by energy adjustment as a result of the nonlinear refractive index alteration via pressure change. Due to the high pressure-controlled nonlinearity of xenon, a broadband (from 250 to 2500 nm) SC with a total efficiency more than 50% of the energy can be generated in the supercritical state. We showed that the filament-induced SC in the visible wavelength range is accompanied by an amplified spontaneous emission observed in the forward direction. The maximum emission efficiency (about 0.5%) was achieved at a pressure of 30 bar for the 466 nm line. It was demonstrated that the femtosecond laser pulse can be compressed in Xe in the regime of femtosecond monofilamentation by 3.5 times with an efficiency as high as 60%.

We present a method of ultrafast laser-induced microplasma mapping based on the third harmonic generation at tightly focused laser beams in solids. The technique gives a submicron resolution and can be applied for two-dimensional imaging of laser-induced microplasma with the electron density as low as 10−5 ncr produced by a femtosecond laser driver at the plasma formation threshold. High sensitivity and contrast are dictated by nonlinear behavior of the third harmonic yield with the laser field. The demonstrated method gives an opportunity to increase the precision of plasma volume determination in the field of material microstructuring.

In this Letter, we report a compact and robust coherent source, operating in mid-infrared (IR) and based on the Fe:ZnSe chalcogenide gain medium, optically pumped by an Er:ZBLAN fiber laser. The output power of 2.1 W with a 59% slope efficiency with respect to the absorbed pump power at liquid nitrogen cooling is achieved. We show that strong re-absorption at a high pump power and iron ion doping concentrations leads to a continuous tuning of central wavelength from 4012 to 4198 nm. The robustness of a high-power Er:ZBLAN fiber laser combined with prominent spectroscopic properties of Fe:ZnSe media pave the way for the development of a reliable tunable continuous-wave mid-IR sources for scientific and industrial purposes.

We demonstrate a first-of-its-kind efficient amplification of a broadband tunable (from 3.8 to 4.8 µm) mid-IR femtosecond seed pulse generated from a AgGaS2-based optical parametric amplifier pumped by a Cr:forsterite laser in a multi-pass Fe2+:ZnSe amplifier optically pumped by a solid-state nanosecond Cr:Yb:Ho:YSGG laser. A total gain of 2000 for an input seed energy of 40 nJ has been obtained. The magnitude of the output energy reaches 80 µJ at a pulse duration of 200 fs.

We report an increase in conversion efficiency and significant broadening of signal and idler bandwidth in the collinear KTA optical parametric amplifier operating close to mid-IR transparency cutoff (5.2 μm) for a visible and near IR pump. Using a 1.24 μm pump we have produced 2 μJ idler pulses at 3.8 μm central wavelength with a bandwidth supporting 65 fs transform limited duration. Further tuning to mid-IR up to 5 μm could be achieved with a 620 nm pump.

For the first time, an experimentally measured seed pulse gain of about 2 cm−1 allows possibilities in the scaling power of such a femtosecond laser system in terawatts. The concept of a subterawatt power level hybrid femtosecond mid-IR (4–5 μm) laser system, based on a weak pulse from an optical parametric mid-IR seeder that is amplified in chalcogenide monocrystalline Fe2+:ZnSe, to gain medium has been proposed and designed. The method and approach for optimizing the choice of nonlinear medium, its length, and the required light intensity for the efficient non-linear self-compression of an ultrashort pulse has also been proposed and considered.