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Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise, and accurate measurement mode with easy registration of the combs beat note in the radio frequency domain. Extensions of this technique already found application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump–probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of 2 free-running oscillators. We confirm the versatility of this delay generation method by measuring χ (2) cross-correlation and χ (3) multicomponent population recovery kinetics.

All-optical switching of 75 fs pulses centered at 1560 nm, driven by 270 fs, 1030 nm pulses in a dual-core optical fiber exhibiting high index contrast is presented. The fiber is made of a thermally matched pair of lead silicate and borosilicate glasses used as core and cladding material, respectively. The novel switching approach is based on nonlinear balancing of dual-core asymmetry, by control pulse intensity induced group velocity reduction of the fast fiber channel. Due to the fiber core made of soft glass with high nonlinearity high switching contrast exceeding 20 dB is reached by using control pulses of only few nanojoule energy. The optimum fiber length is 14 mm, which closely match the calculated coupling length at the signal wavelength. The results express significant progress in comparison to similar studies based on self-switching of solitonic pulses in dual-core fibers and represent high application potential.

We demonstrate bright, narrow-bandwidth, high order harmonics driven by VIS lasers, continuously tunable towards higher and lower EUV – X-ray photon energies, ideal for resonant, ultrafast, coherent diffractive imaging at the absorption edges of ferromagnets.

We report on direct ultrafast electro-absorption switching in colloidal CdSe/CdS core/shell quantum dots driven by intense THz pulses without a field enhancing structure, and suggest a route to optimize electro-absorption modulators with respect to the energy band alignment of the heterostructure material.

We investigate experimentally and theoretically effects of the inter-core propagation mismatch on nonlinear switching in dual-core high-index-contrast soft-glass optical fibers. Incident femtosecond pulses of various energy are fed into a single ("straight") core, to identify transitions between different dynamical regimes, viz., inter-core oscillations, self-trapping in the cross core, and retaining the pulse in the straight core. The transfer between channels, which has solitonic character, is controlled by the pulse's energy. A model based on the system of coupled nonlinear Schr\"{o}dinger equations reveals the effect of the mismatch parameter and pulse duration on the diagram of the various energy dependent dynamical regimes. Optimal values of the mismatch and pulse width, which ensure stable performance of the nonlinear switching, are identified. The theoretical predictions are in agreement with experimental findings., Comment: 25 pages, 9 figures. To be published in "Chaos, Solitons & Fractals"

We present a complex study of pulse-energy-controlled solitonic self-switching of femtosecond pulses at wavelengths of 1700 and 1560 nm in two nonlinear high-index contrast dual-core fibers having different levels of slight asymmetry. In the case of the fiber with higher dual-core asymmetry excited by 1700 nm pulses, the highest switching contrast of 20.8 dB at 40 mm fiber length was demonstrated. It was accompanied by multiple exchanges of the dominant core at the fiber output, which is a strong signature of the soliton-based switching process. In the case of the fiber with lower dual-core asymmetry, excited by 1560 nm pulses, the highest switching contrast of 21.4 dB at 35 mm fiber length was achieved with a broadband character of the switching in the spectral range of 1450–1650 nm. Both demonstrations represent progress in all-optical switching studies at these particular wavelengths thanks to a comparison between their results, which reveals the requirement of a higher level of dual-core symmetry for applicable C-band operation.

Multidimensional solitary states generated by intermodal stimulated Raman scattering in a gas-filled hollow-core fiber form red-shifted, few-cycle pulses after propagation thought glass. These pulses, when used to drive the high harmonic generation process, allow for tailoring the extreme ultraviolet source to specific photon-demanding applications.

High harmonic generation (HHG) in gasses has become a method of choice among table-top extreme ultraviolet (XUV) sources. In order to push the harmonic cut-off towards higher photon energies, many strategies can be implemented, including red-shifting and compressing the driver pulses. Here, we propose a new approach for tuning the red-shift of sub-picosecond driver pulses and compressing them to few-cycle durations in a single stage. This method uses newly discovered multidimensional solitary states (MDSS) pulses as a spatio-temporal engineered source based on the nonlinear Raman process in gas-filled hollow-core fibres (HCF) [1] . It has a few key advantages: 1) The MDSS pulses exhibit a negative quadratic spectral phase and can be compressed by simple propagation through glass. 2) In contrast with commonly-used optical parametric amplifiers, a small redshift can be obtained, allowing to reach a target XUV photon energy while minimizing the detrimental effects of long driver wavelengths on HHG efficiency. We apply our method to X-ray Resonant Magnetic Scattering (XRMS) measurements on a cobalt/platinum ferromagnet, an application that requires a high photon flux at the M 2,3 edge of cobalt (60 eV).

Nonlinear self-channeling (filamentation) of intense, ultrashort laser pulses in air has been extensively studied with ultrafast, near-infrared and visible laser sources [1] . Recently these investigations advanced to longer wavelengths, as well as to the regime of few-cycle pulses, where the effects associated with the carrier-envelope phase (CEP) of the laser pulse become important. It has been shown that spectral interference in the white light generated on filamentation of an ultrafast, short-wave infrared laser source carries CEP information [2] . Here, we report using this spectral interference for single-shot characterization of the CEP of an ultrafast, mid-infrared (MIR) laser source at 3.9 µm wavelength.

Due to numerous unique properties, such as the capability of resonantly exciting crystal lattice vibrations, molecular rotations and other low energy transitions in solids, liquids and gases, terahertz radiation belongs to one of the most interesting but yet less explored spectral regions. Recently, a considerable progress has been demonstrated in THz generation by optical rectification (OR) in organic crystals, conventionally driven by near-IR fs-pulses centred at the telecommunication wavelength of 1.5 µm. Although high optical- to THz conversion efficiencies of 1-3% with pulse energies of up to 0.9 mJ [1] can be achieved, the conversion efficiency is nevertheless deteriorated by multiphoton absorption and is fundamentally limited by the damage threshold of the crystal. This restricts the applicable pump fluence to nothing more than 20 mJ/cm 2 .

Within the last decades, electro-optic (EO) modulators based on the quantum confined Stark effect (QCSE) in quantum well structures came to prominence for high-speed optical networks. When an external electric field is applied to such a quantum structure, it bends the energy levels and shifts the absorption edge to smaller energies. In addition, the overlap between electron and hole wave functions decreases, which diminishes the amount of absorption [1] . This technique is ultimately limited to the time scale necessary to change the electric field on the attached electrodes. Moreover, largest modulation of the energy levels can be achieved for structures with a strong confinement and large excition binding energies. Hence, low dimensional materials like quantum dots (QDs) in combination with intense THz pulses which provide the electric field driver, are promising candidates for sub-picosecond all-optical switching of optical absorption. Pioneering work has been performed by Pein and co-workers [2] wherein they report on a THz-field induced QCSE in CdSe/CdS colloidal QDs. However, due to difficulties to generate THz radiation with the electric field in the MV/cm range, necessary to alter the optical transmission, THz field enhancing structures have been employed.

We demonstrate a time-domain method to quantitatively retrieve the angular dependence of laser-induced electron rescattering from the measured rotational half-revival signals of molecules with a reaction microscope. The method is based on the measured channel-resolved ion yields of single ionization and electron-rescattering-induced double ionization and the in situ measured alignment distribution of molecules. From the measured data for ${\mathrm{CO}}_{2}$, we retrieve the angular dependence of electron rescattering in nonsequential double ionization for both nondissociative and dissociative cases. The results imply that the angular dependence of electron rescattering is affected by the state of the parent ion populated during the first ionization step. The method demonstrated here opens up the possibility to quantitatively retrieve from measured data which molecular orbitals are involved in the interaction of strong laser fields with molecules.

We report on the observation of quantum confined stark effect (QCSE) in CdSe/CdS core/shell quantum dots (QDs) directly driven by an intense THz field, generated in the organic crystal DAST, which is pumped with multi-millijoule mid-IR pulses.

The generation of ultrashort laser pulse bursts has already contributed to many applications such as materials processing [1] , pulsed laser deposition [2] , laser induced breakdown spectroscopy (LIBS) [3] and seeding of free electron lasers [4] . Currently, there is a rising demand on burst sources providing terahertz (THz) intraburst repetition rates or higher corresponding to pulse spacings in the order of a few picoseconds, given by applications such as resonant driving of optical nonlinearities [5] , time-resolved nonlinear spectroscopies [6] , coherent control of quantum systems [7] or as driver for secondary radiation sources, e.g. generation of THz bursts [8] . However, the amplification of bursts to multi-millijoule energies at THz intraburst repetition rates is highly problematic. This is given by the appearance of spectral modes that are mapped into the time-domain, formed by spectral interference of closely spaced pulses which are overlapping when applying Chirped Pulse Amplification (CPA). This leads to insufficient energy extraction in an amplifier and, if pulse spacing is not stabilized on an interferometric scale, to temporally unstable burst generation. Recently, we have successfully demonstrated energy-safe, stabilized CPA of ultrashort pulse bursts with THz intraburst frequencies up to multi-millijoules by applying Vernier-enabled phase-modulation techniques, breaking the mentioned energy-limiting barrier [8] . In this contribution, we demonstrate a new version of our burst-generation technique, which drastically lowers system complexity compared to our previous approach, allowing its application to any master-oscillator regenerative-amplifier combination setup with only minor modifications.

A pair of sub-mJ femtosecond pulses separated by 110 ns, generated in a single cw-pumped Yb regenerative amplifier, is employed as a frontend for mid-IR OPCPA operating in the atmospheric transparency window.

By exploiting stimulated Raman scattering in long nitrogen-filled capillary fibers, we demonstrate continuous tunability of Yb laser systems over the 1.0-1.7 μm range, with conversion efficiency up to 82%, and an up to 10-fold pulse compression.

Generating intense ultrashort pulses with high-quality spatial modes is crucial for ultrafast and strong-field science and can be achieved by nonlinear supercontinuum generation (SCG) and pulse compression. In this work, we propose that the generation of quasi-stationary solitons in periodic layered Kerr media can greatly enhance the nonlinear light-matter interaction and fundamentally improve the performance of SCG and pulse compression in condensed media. With both experimental and theoretical studies, we successfully identify these solitary modes and reveal their unified condition for stability. Space-time coupling is shown to strongly influence the stability of solitons, leading to variations in the spectral, spatial and temporal profiles of femtosecond pulses. Taking advantage of the unique characteristics of these solitary modes, we first demonstrate single-stage SCG and the compression of femtosecond pulses from 170 to 22 fs with an efficiency >85%. The high spatiotemporal quality of the compressed pulses is further confirmed by high-harmonic generation. We also provide evidence of efficient mode self-cleaning, which suggests rich spatiotemporal self-organization of the laser beams in a nonlinear resonator. This work offers a route towards highly efficient, simple, stable and highly flexible SCG and pulse compression solutions for state-of-the-art ytterbium laser technology.

Optical rectification in lithium niobate and semiconductor nonlinear optical materials, utilizing the tilted-pulse-front technique, has become a versatile source of intense single- and multicycle THz pulses. Novel approaches recently proposed, and some of them demonstrated, are promising to scale the technology to unprecedented field strengths by enabling uniform interaction length for THz generation across a large beam. The generation of continuously-tunable, narrowband THz pulses from phase-locked multi-millijoule femtosecond pulse bursts by optical rectification is demonstrated.

We experimentally demonstrate that temporally confined spatial solitons can be realized by space-time coupled propagation of strong femtosecond pulses in a nonlinear optical resonator, consisting of periodic layered Kerr media (PLKM). A universal relationship between the characteristic beam size and the critical nonlinear phase of the solitary modes is revealed, defining different regions of soliton stability. Taking advantage of the unique characters of these solitary modes, we demonstrate supercontinuum generation and pulse compression of initially 260 μJ, 170 fs pulses down to 22 fs in a single-stage PLKM resonator with an efficiency >90%.

We demonstrate a pulse energy-governed nonlinear self-switching of femtosecond pulses at the wavelengths of 1700 nm and 1560 nm in a highly nonlinear high index contrast dual-core fiber. The improved technology process allowed the first-time registration of the switching effect in the communication C-band.

We establish a generalized picture of the phase sensitivity of laser-induced directional bond breaking using the H_{2} molecule as the example. We show that the well-known proton ejection anisotropy measured with few-cycle pulses as a function of their carrier-envelope phases arises as an amplitude modulation of an intrinsic anisotropy that is sensitive to the laser phase at the ionization time and determined by the molecule's electronic structure. Our work furthermore reveals a strong electron-proton correlation that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in larger molecules.

All-optical switching of 77 fs pulses centered at 1550 nm, driven by 270 fs, 1030 nm pulses in dual-core optical fibre exhibiting high index contrast is presented. The switching is achieved by nonlinear balancing of dual-core asymmetry. High switching contrast exceeding 20 dB is attained with driving pulses of only few nanojoule energy, which is due to the fibre core made of soft glass possessing high nonlinearity.

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here we demonstrate both theoretically and experimentally the realization of a novel THz source with high peak power performance based on two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm. Our theory predicts that under this scheme sub-cycle THz pulses with multi-millijoule energies and record conversion efficiencies can be produced. Besides, we elucidate the origin of this high efficiency, which is made up of several factors, including a novel mechanism where the harmonics produced by the mid-infrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation. In our experiments we verify the theoretical predictions by demonstrating ultrashort sub-cycle THz pulses with sub-millijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm-1. Moreover, we show that these intense THz fields can drive nonlinear effects in bulk semiconductors (ZnSe and ZnTe) in free space and at room temperature. Our numerical simulations indicate that the observed THz yield can be significantly upscaled by further optimizing the experimental setup leading to even higher field strengths. Such intense THz pulses enable extreme field science, including into other, relativistic phenomena.

We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.

The performance of regenerative amplifiers at high repetition rates is often limited by the occurrence of bifurcations induced by a destabilization of the pulse-to-pulse dynamics. While bifurcations can be suppressed by increasing the seed energy using dedicated pre-amplifiers, the availability of adjustable filters and control electronics in modern pulse amplifiers allows to exploit feedback strategies to cope with these instabilities. In this paper, we present a theoretical and experimental analysis of active feedback methods to stabilize otherwise unstable operational regimes of regenerative amplifiers. To this end, the dynamics of regenerative amplifiers are investigated starting from a general space-dependent description to obtain a generalization of existing models from the literature. Suitable feedback strategies are then developed utilizing measurements of the output pulse energies or the transmitted pump light, respectively. The effectiveness of the proposed approach is highlighted by experimental results for a Yb:CaF2-based regenerative amplifier.

A conventional hollow core fiber (HCF) scheme is implemented to investigate spectral broadening of Titanium:Sapphire (Ti-Sa) femtosecond laser pulses in saturated hydrocarbon molecules compared to unsaturated ones. While the saturated molecules exhibit a spectral broadening similar to noble gases, for the unsaturated ones with π bonds, broadening towards blue is restrained. Numerical simulations underpin that it is a combination of group velocity dispersion (GVD) and Raman scattering which limits the spectral broadening for the unsaturated molecules. Compression of low energy ∼40fs pulses to ∼8fs using saturated hydrocarbons is demonstrated, suggesting the feasibility of this media for high repetition rate laser pulse compression.

This works shows a new approach to study the electron recapture process during double and multiple ionization of atoms. This scheme is applicable to non-dissociative processes in molecules, which are not accessible with existing methods based on the measurement of kinetic energy released from fragmentation processes. The experiments show a transition of frustrated double ionization in argon from a non-sequential scenario to a sequential scenario.

We report on unexpected efficient THz generation in organic crystals (DAST and DSTMS) by optical rectification (OR) of intense mid-infrared pulses. The crystals experience superior damage threshold and an ultra-broadband THz spectrum reaching 10 THz.

We investigate the femtosecond supercontinuum generation and compression in layered Kerr media. The condition for extending the setup in a sustainable way is revealed, with characteristic spectral and temporal features. We demonstrate tenfold pulse compression in a single-stage multiplate setup by including 14 layers of the thin medium.

We report on the retrieval of the angular dependence of laser-induced electron rescattering in CO2 with a novel method from the measured rotational half revival signals of filed-free aligned molecules using a reaction microscope.

We report on efficient THz generation in DAST, driven by mid-infrared pump sources. Due to the suppression of multi-photon absorption, high pump fluences can be applied and outstanding optical to THz conversion efficiencies of more than 4% can be reached.

We present an energy-efficient and scalable technique to realize energetic sub-100-fs pulses, continuously tunable in the 1030÷1730 nm range, by exploiting stimulated Raman scattering in long N2-filled hollow-core fibers.

A conventional hollow-core fiber (HCF) scheme is implemented to investigate the effect of group velocity dispersion (GVD) and Raman scattering in spectral broadening in molecular gases for low energy pulse compression application.

In this paper, we demonstrate a pulse energy-governed nonlinear self-switching of femtosecond pulses at wavelength of 1700 nm in a highly nonlinear high index contrast dual-core fibre. The fibre structure is composed of two microdimension cores and a solid cladding made of two thermally matched soft glasses developed in-house. A double switching behaviour under monotonic increase of the input pulse energy from 1 nJ to 3 nJ was observed at 35 mm fibre length in its anomalous dispersion region. A spatial intensity distribution measurement identified a switching contrast of 16.7 dB, and the registered spectra of the two output channels revealed a broadband switching behaviour within 150 nm. The obtained results indicate a novel type of solitonic switching and represent a significant progress in comparison to the previous experimental works and has a high application potential.

We present strong field autocorrelation measurements of molecular ionization and dissociation with a reaction microscope. Time-frequency analysis of the time-domain signals was performed to obtain electron and nuclear dynamics during molecular ionization and dissociation.

A high harmonics source is used to perform x-ray resonant magnetic scattering on magnetic samples. The dynamics of laser-induced ultrafast demagnetization are probed and longer pump wavelengths are found to increase the initial quenching levels.

In this paper, we present an iterative learning algorithm for pulse-shape control applications of optical chirped pulse amplifiers for ultra-short, high-energy light pulses. For this, we first introduce a general nonlinear and infinite-dimensional mathematical model of chirped pulse amplifiers. By reducing the complexity of this detailed model and reformulating the control task, we are subsequently able to apply inversion-based iterative learning control to track desired output pulses. Using the reduced model to estimate both internal states and unknown parameters yields a fast and simple way of consistently estimating the input–output behavior without relying on a calibrated system model. The effectiveness of the resulting adaptive algorithm is finally illustrated with simulation scenarios on an experimentally validated mathematical model.

Zusammenfassung Die Beschreibung optischer Pulsverstärker für ultra-kurze Laserpulse resultiert in mathematisch aufwendigen, verteilt-parametrischen Modellen, deren detaillierte Modellierung und adaptive Regelung unlängst präsentiert wurde. In diesem Beitrag werden nun Aspekte der Identifikation geeigneter Parameter betrachtet, um ein gemessenes Eingangs-Ausgangs-Verhalten möglichst gut nachbilden zu können. Dazu werden effiziente Simulationsmodelle präsentiert, die den numerischen Aufwand – besonders bei hohen Wiederholraten und der dadurch auftretenden starken Verkopplung aufeinanderfolgender Pulse – in handhabbaren Grenzen halten.

We report on the results of a joint experimental and numerical study on the sub-cycle laser field-driven electron dynamics that underlie the population of highly excited electronic states in multiply ionized argon dimers by electron recapture processes. Our experiments using few-cycle laser pulses with a known carrier-envelope phase (CEP) in combination with reaction microscopy reveal a distinct CEP-dependence of the electron emission and recapture process and, furthermore, a small but significant CEP-offset to the scenario in which no excited argon dimers are produced. With the help of classical ensemble trajectory simulations we trace down these different CEP-dependencies to subtle differences in the laser-driven sub-cycle electron trajectory dynamics that involve in both cases the transfer of an electron from one argon ion across the system boundary to the neighboring ion and its transient capture on this ion.

We report on an experimental and theoretical study of the ionization-fragmentation dynamics of argon dimers in intense few-cycle laser pulses with a tagged carrier-envelope phase. We find that a field-driven electron transfer process from one argon atom across the system boundary to the other argon atom triggers sub-cycle electron-electron interaction dynamics in the neighboring atom. This attosecond electron-transfer process between distant entities and its implications manifest themselves as a distinct phase-shift between the measured asymmetry of electron emission curves of the $\text{Ar}^{+}+\text{Ar}^{2+}$ and $\text{Ar}^{2+}+\text{Ar}^{2+}$ fragmentation channels. Our work discloses a strong-field route to controlling the dynamics in molecular compounds through the excitation of electronic dynamics on a distant molecule by driving inter-molecular electron-transfer processes.

We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.

Nonlinear (NL) interactions of strong THz electric and magnetic fields with matter are the next frontier in nonlinear optics. NL THz spectroscopy requires ultrashort/broadband THz pulses with high electric field strengths. Here, we report on THz generation from two-color laser filaments, driven by intense midIR pulses centered at $3.9 {\mu } \mathrm {m}$. We demonstrate generation of THz pulses with a spectral bandwidth of $\gt 15$ THz, extraordinary conversion efficiency of 2.34% and remarkably high energy of 0.185 mJ.

Optical Parametric Chirped-Pulse Amplification (OPCPA) is an emerging laser technology, providing a flexibility in operating wavelengths and power upscaling. Here, we investigate THz generation in organic crystals (DAST) by optical rectification (OR) of intense Mid-Infrared (MIR) pulses. The broadband spectrum exceeds several octaves when pumped by 100 fs, 3.9 μm OPCPA pulses. We report on an extraordinarily high THz energy density of 3.1 mJ/cm $^{\mathbf {2}}$ and crystal damage threshold $ {>}150$ mJ/cm $^{\mathbf {2}}$.

We demonstrate that intense free space THz fields can nonlinearly induce excitonic photoluminescence in room temperature semiconductors as well as cross-phase modulation in an electro-optic crystal.

A high harmonics source is used to perform x-ray resonant magnetic scattering on a magnetic sample. The dynamics of laser-induced ultrafast demagnetization are probed with respect to the wavelength of the pump beam. Longer pump wavelengths are found to increase the initial quenching levels.

Ultrafast laser sources operating in the mid-infrared spectral region, where many molecules and their functional groups have vibrational and rotational resonances, are very important tools for studying novel regimes of non-linear optics and strong field physics. Next to optical parametric amplifiers (OPAs) based on ZnGeP 2 (ZGP) crystals which, on the one hand, allow generation of energetic ultrashort mid-IR pulses at a mJ energy level, but, on the other, rely on complex and immature holmium pump lasers [1], a difference frequency generation (DFG) in AGS or GaSe crystals starting from a single broadband near-infrared (NIR) pulse [2] or via cascaded two-stage parametric down-conversion [3] is an alternative way to generate mid-IR pulses above 4-μm wavelength. In the later case more conventional Ytterbium, Neodymium or Ti:Saphire lasers are used to pump the frequency conversion stages. The use of a longer-wavelength 1.03 μm Ytterbium laser as compared to Ti:Sapphire pump sources yields an improved phase matching bandwidth, lower group velocity miss-match, as well as a lack of three-photon absorption, which reduces detrimental photo-darkening in AGS.

Nonlinear self-channeling (filamentation) of intense, ultrashort laser pulses in air has numerous potential applications ranging from remote sensing in the atmosphere to lightning control [1]. Recent progress in optical parametric chirped-pulse amplification (OPCPA) technology pushes the investigations of filamentation into short-wave infrared [2] and mid-wave infrared [3] regimes. At longer wavelengths, the propagation dynamics is affected by the presence of windows of anomalous dispersion due to the trace constituents of air [4,5]. Multiple odd harmonics of the infrared driver pulse can be easily detected and analyzed.

A generation of optical harmonics by circularly polarized light, which is in general forbidden in an isotropic medium, was recently observed in molecular gases where molecules were aligned and oriented by another linearly polarized laser pulse [1] or by pulses with twisted polarization [2]. In order to generate nth-harmonic with circularly polarized light, a molecule has to "absorb" n photons, having the same handedness, and to emit a single nth harmonic photon while fulfilling the energy and the momentum conservation laws, meaning a necessity to excite the molecule to appropriate rotational state. One can do this by means of intrapulse rotational Raman scattering, which prepares the molecule in the excited state through a subsequent absorption of (n-1) same-handed photons [1]. All previously reported experimental studies on circular harmonic generation in molecular gases were performed with visible/near-IR drivers, in either gas jets or in standing cells and with pre-alignment of molecules by an additional laser pulses. Here we report on the studies of self-action of elliptically polarized mid-IR pulses on the generation of low-order harmonics in filaments ignited in air and in argon, representing a molecular and an atomic gas respectively. The harmonics were driven by 20-mJ, sub-100 fs mid-IR pulses, centred at 3.9 pm wavelength. In the experiments filamentation was assisted by focusing light with a spherical mirror of ROC=-2500 mm into an open-end tube, through which Ar gas was flown at slight overpressure. Polarization of the pulses was controlled by a broadband quarter wave plate (QWP). Spectra of 5th and 7th harmonics were recorded after the filaments with a grating spectrometer (HR4000, OceanOptics). Polarization state of the generated harmonics was analysed with a Glan-Taylor polarizer.