3.5-mJ 150-fs Fe:ZnSe hybrid mid-IR femtosecond laser at 4.4 μm for driving extreme nonlinear optics

Cryogenically cooled Fe:ZnSe-based chirped pulse amplifier at 4.07 µm

A femtosecond chirped pulse amplifier based on cryogenically cooled Fe:ZnSe was demonstrated at 333 Hz-33 times higher than previous results achieved at near-room-temperature. The long upper-state lifetime allows free-running, diode-pumped Er:YAG lasers to be used as pump lasers. 250-fs, 4.59-mJ pulses are produced with a center wavelength of 4.07 µm, which avoids strong atmospheric CO₂ absorption that cuts on around 4.2 µm. It is therefore possible to operate the laser in ambient air with good beam quality. By focusing the 18-GW beam in air, harmonics up to the ninth order were observed indicating its potential for use in strong-field experimentation.

Long-wave-infrared pulse production at 11 µm via difference-frequency generation driven by femtosecond mid-infrared all-fluoride fiber laser

We present an ultrafast long-wave infrared (LWIR) source driven by a mid-infrared fluoride fiber laser. It is based on a mode-locked Er:ZBLAN fiber oscillator and a nonlinear amplifier operating at 48 MHz. The amplified soliton pulses at ∼2.9 µm are shifted to ∼4 µm via the soliton self-frequency shifting process in an InF₃ fiber. LWIR pulses with an average power of 1.25-mW centered at 11 µm with a spectral bandwidth of ∼1.3 µm are produced through difference-frequency generation (DFG) of the amplified soliton and its frequency-shifted replica in a ZnGeP₂ crystal. Soliton-effect fluoride fiber sources operating in the mid-infrared for driving DFG conversion to LWIR enable higher pulse energies than with near-infrared sources, while maintaining relative simplicity and compactness, relevant for spectroscopy and other applications in LWIR.

High-gain broadband laser amplification of mid-IR pulses in Fe:CdSe crystal at 5 μm with millijoule output energy and multigigawatt peak power

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.

Femtosecond tunable solitons up to 4.8 µm using soliton self-frequency shift in an InF3 fiber

A tunable ultrashort soliton pulse source reaching up to 4.8 µm is demonstrated based on a 2.8 µm femtosecond fiber laser coupled to a zirconium fluoride fiber amplifier followed by a small core indium fluoride fiber. This demonstration is extending by 300 nm the long wavelength limit previously reported with soliton self-frequency shift (SSFS) sources based on fluoride fibers. Our experimental and numerical investigation highlighted the spectral dynamics associated with the generation of highly redshifted pulses in the mid-infrared using SSFS enhanced by soliton fission. This study is intended at providing a better understanding of the potential and limitations of SSFS based tunable femtosecond fiber sources in the 3-5  µm spectral range.

Mid-Infrared Laser Generation of Zn1-xMnxSe and Zn1-xMgxSe (x ≈ 0.3) Single Crystals Co-Doped by Cr2 and Fe2 Ions—Comparison of Different Excitation Wavelengths++

Two different mid-infrared (mid-IR) solid-state crystalline laser active media of Cr2+, Fe2+:Zn1−xMnxSe and Cr2+, Fe2+:Zn1−xMgxSe with similar amounts of manganese or magnesium ions of x ≈ 0.3 were investigated at cryogenic temperatures for three different excitation wavelengths: Q-switched Er:YLF laser at the wavelength of 1.73 μm, Q-switched Er:YAG laser at 2.94 μm, and the gain-switched Fe:ZnSe laser operated at a liquid nitrogen temperature of 78 K at ∼4.05 μm. The temperature dependence of spectral and laser characteristics was measured. Depending on the excitation wavelength and the selected output coupler, both laser systems were able to generate radiation by Cr2+ or by Fe2+ ions under direct excitation or indirectly by the Cr2+→ Fe2+ energy transfer mechanism. Laser generation of Fe2+ ions in Cr2+, Fe2+:Zn1−xMnxSe and Cr2+, Fe2+:Zn1−xMgxSe (x ≈ 0.3) crystals at the wavelengths of ∼4.4 and ∼4.8 μm at a temperature of 78 K was achieved, respectively. The excitation of Fe2+ ions in both samples by direct 2.94 μm as well as ∼4.05 μm radiation or indirectly via the Cr2+→ Fe2+ ions’ energy transfer-based mechanism by 1.73 μm radiation was demonstrated. Based on the obtained results, the possibility of developing novel coherent laser systems in mid-IR regions (∼2.3–2.5 and ∼4.4–4.9 μm) based on AIIBVI matrices was presented.

Kinetics of excitation transfer from Cr2+ to Fe2+ ions in co-doped ZnSe

The transfer of electronic excitations from Cr²⁺ to Fe²⁺ ions in co-doped epitaxially grown ZnSe is studied by time-resolved photoluminescence (PL) spectroscopy with unprecedented sub-10 ns time resolution. Upon excitation of Cr²⁺ ions by a picosecond pulse at 2.05 µm wavelength, PL from Fe²⁺ ions displays a delayed onset and a retarded decay in comparison to Fe²⁺ PL directly excited at 3.24 µm. We measure an extremely rapid 60 ns buildup of the Fe²⁺ luminescence, which is followed by a slower relaxation on the few micrometer scale. The experimental results are analyzed in the framework of Förster radiationless resonant energy transfer. Directly connecting to the work of Fedorov et al. [Opt. Mater. Express9, 2340 (2019)10.1364/OME.9.002340], the 60-ns buildup time of energy transfer is found to correspond to a Cr²⁺-Fe²⁺ distance of 0.95 nm, close to the length of the space diagonal of the ZnSe unit cell. This result demonstrates a significant density of spatially correlated Cr²⁺-Fe²⁺ ion pairs at short distance, in parallel to ions with a random distribution at a larger mutual separation.

Control of spectral shift, broadening, and pulse compression during mid-IR self-guiding in high-pressure gases and their mixtures

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.

Cr:ZnS-based soliton self-frequency shifted signal generation for a tunable sub-100 fs MWIR OPCPA

We present a tunable, high-energy optical parametric chirped pulse amplification system with a front-end based on a femtosecond Cr:ZnS laser. By taking advantage of the broad emission spectrum of the femtosecond Cr:ZnS master oscillator, we are able to directly seed the holmium-based pump around 2 µm. At the same time, the signal pulses for the parametric process are generated via Raman self-frequency shifting of the red end of the spectrum centered at 2.4 µm. The solitons, generated in a fluoride fiber, are tunable over the wavelength range between 2.8 and 3.2 µm. The optical parametric amplifier operates at a 1 kHz repetition rate, and consists of two stages equipped with ZGP as nonlinear crystal. The generated idler pulses are tunable between 5.4 and 6.8 µm with a pulse energy of up to 400 µJ. Dispersion management using bulk material stretching and compression in combination with precise phase shaping prior to amplification enables idler pulses of a sub-100 fs duration, translating into a peak power as high as 4 GW.

High-Power Solid-State Near- and Mid-IR Ultrafast Laser Sources for Strong-Field Science

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.

Diode-side-pumped watt-level high-energy Q-switched mid-IR Er:YLF laser

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.

Octave-spanning mid-infrared femtosecond OPA in a ZnGeP2 pumped by a 2.4 μm Cr:ZnSe chirped-pulse amplifier

We report on the highly efficient, octave-spanning mid-infrared (mid-IR) optical parametric amplification (OPA) in a ZnGeP₂ (ZGP) crystal, pumped by a 1 kHz, 2.4 μm, 250 fs Cr:ZnSe chirped-pulse amplifier. The full spectral coverage of 3-10 μm with the amplified signal and idler beams is demonstrated. The signal beam in the range of ∼3 - 5 μm is produced by either white light generation (WLG) in YAG or optical parametric generation (OPG) in ZGP using the common 2.4 μm pump laser. We demonstrate the pump to signal and idler combined conversion efficiency of 23% and the pulse energy of up to 130 μJ with ∼2 μJ OPG seeding, while we obtain the efficiency of 10% and the pulse energy of 55 μJ with ∼0.2 μJ WLG seeding. The OPA output energy is limited by the available pump pulse energy (0.55 mJ at ZGP crystal) and therefore further energy scaling is feasible with multi-stage OPA and higher pump pulse energy. The autocorrelation measurements based on random quasi-phase matching show that the signal pulse durations are ∼318 fs and ∼330 fs with WLG and OPG seeding, respectively. In addition, we show the spectrally filtered 30 μJ OPA output at 4.15 μm suitable for seeding a Fe:ZnSe amplifier. Our ultrabroadband femtosecond mid-IR source is attractive for various applications, such as strong-field interactions, dielectric laser electron acceleration, molecular spectroscopy, and medical surgery.

Role of wavelength in photocarrier absorption and plasma formation threshold under excitation of dielectrics by high-intensity laser field tunable from visible to mid-IR

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[Formula: see text] this transition occurs around 3-4 [Formula: see text]m corresponding to the region of the lowermost PFT. The region of the uppermost PFT is reached around 1 [Formula: see text]m and is governed by an interplay of photoionization and weak-field free carrier absorption which manifests itself in both MgF[Formula: see text] and SiO[Formula: see text]. 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.

Attosecond science based on high harmonic generation from gases and solids

Recent progress in high power ultrafast short-wave and mid-wave infrared lasers has enabled gas-phase high harmonic generation (HHG) in the water window and beyond, as well as the demonstration of HHG in condensed matter. In this Perspective, we discuss the recent advancements and future trends in generating and characterizing soft X-ray pulses from gas-phase HHG and extreme ultraviolet (XUV) pulses from solid-state HHG. Then, we discuss their current and potential usage in time-resolved study of electron and nuclear dynamics in atomic, molecular and condensed matters.

Different methods are demonstrated in recent years to produce attosecond pulses. Here, the authors discuss recent development and future prospects of the generation of such pulses from gases and solids and their potential applications in spectroscopy and ultrafast dynamics in atoms, molecules and other complex systems.

Femtosecond graphene mode-locked Fe:ZnSe laser at 4.4 µm

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.

Powerful 3-μm lasers acousto-optically Q-switched with KYW and KGW crystals

We report, to the best of our knowledge, the first use of KY(WO₄)₂ and KG(WO₄) acousto-optical Q-switches in 3-μm powerful lasers. Q-switches of two different designs (normal incidence with antireflection coatings, and Brewster-angle cut crystals) operate in lasers based on Er:YAG, Cr:Yb:Ho:YSGG, and Cr:Er:YSGG laser crystals. Gain and lifetime of laser crystals significantly influence the regime of Q-switched generation. Er:YAG and Cr:Yb:Ho:YSGG lasers deliver pulses with 10.8 mJ and 17.5 mJ energy, respectively. Pulses with energy of 29.6 mJ and duration of 75 ns in the TEM₀₀ mode are obtained in a Cr:Er:YSGG laser. The energy is scaled up to 85.7 mJ in the two-stage master oscillator power amplifier system. Powerful laser systems of this kind are in the region of interest for pumping other mid-IR laser media (e.g., Fe:ZnSe and Fe:CdSe), OPOs, CO₂ lasers, and amplifiers. Preliminary experiments on microstructuring of transparent materials by the laser-induced backside wet etching method demonstrate the potential of such lasers to build the foundation for dye-free tissue and cell engineering concepts.