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The introduction of mid-IR optical parametric chirped pulse amplifiers (OPCPAs) has catalyzed interest in multi-millijoule, infrared femtosecond pulse-based filamentation. As tunneling ionization is a fundamental first stage in these high-intensity laser-matter interactions, characterizing the process is critical to understand derivative topical studies on femtosecond filamentation and self-focusing. Here, we report constructive-elastic microwave scattering-based measurements of total electron count, electron number densities, and photoionization rates generated by 3.9 micron femtosecond mid-infrared tunneling ionization of atmospheric air. Consequently, we determine photoionization rates in the range of 5.0x10$^{8}$-6.1x10$^{9}$ s$^{-1}$ for radiation intensities 1.3x10$^{13}$-1.9x10$^{14}$ W/cm$^{2}$, respectively. The proposed approach paves the wave to precisely tabulate photoionization rates in mid-IR for broad range of intensities and gas types and to study plasma dynamics at mid-IR filamentation.

Next-generation high-speed optical networks demand the development of ultrafast optical interconnects capable of Tbit s(-1) data rates. By utilizing colloidal CdSe/CdS core/shell quantum dots, gated by intense THz pulses, a proof of concept of an all-optical femtosecond electro-absorption switch is presented in this work. Without any additional enhancement of the THz electric field, an extinction contrast of more than 6 dB and transmission changes in the visible of more than 15% are achieved, with the latter setting a new record for solution-processed electro-absorption materials at room temperature. The absence of physical artifacts, originating from electrodes and field enhancing structures, allows to employ a simple and intuitive numerical model, which rationalizes the large field-induced electro-absorption response. Supported by theoretical calculations, the importance of the energy band alignment of heterostructure quantum dots are discussed for the first time and suggest that further improvement of the modulation depth and contrast may be achieved with Type-II quantum dots., Advanced Optical Materials, 10 (9), ISSN:2195-1071

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 experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems., Powerful terahertz pulses are generated during the nonlinear propagation of ultrashort laser pulses in gases. Here, the authors demonstrate efficient sub-cycle THz pulse generation by using two-color midinfrared femtosecond laser filaments in ambient air.

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

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.

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 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.

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.

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 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 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.

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.

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.

Optical Parametric Chirped-Pulse Amplification (OPCPA) is an emerging laser technology, providing a flexibility in operating wavelengths and power upscaling [1]. In this work, we show that mid-IR OPCPA technology could overcome major challenges in enhancing intense broadband THz generation from organic crystals. Strong THz fields are presently under extensive research for its applications in particle accelerations and fundamental understanding of structural dynamics [2]. Currently, the most efficient technique relies on optical rectification (OR) of femtosecond near-IR OPA laser pulses (1.3–1.5 pm) in highly nonlinear organic crystals [3]. However, the technique is still limited to a few mJ of pump energies and organic crystals suffer from a relatively low damage threshold at this pump wavelength. To improve THz generation, it is therefore crucial to use alternative laser pump technologies, such as Cr-Forsterite lasers [4]. However, the associated pump wavelength of 1.25 pm typically leads to an increase in nonlinear absorption, which decreases the crystal damage threshold and lowers the effectively emitted THz spectral contents. Here, we used a high-energy OPCPA system with 3.9 pm central wavelength (20 Hz repetition rate) [1] to generate THz from DSTMS organic crystal (Swiss Terahertz LLC) by OR. The emitted spectrum is broad and extends beyond 5 THz (compared to 2 THz central frequency in the case of Cr-Forsterite pump [4]). In terms of conversion efficiency, we did not observe saturation up to energy fluence of 180–200 mJ/cm2. This is remarkably high as compared to the typical saturation values of sub-10 mJ/cm2 from both Cr-Forsterite and near-IR OPA pump systems [4]. We evaluated the damage threshold of DSTMS at 3.9 pm beyond 250 mJ/cm2, more than an order of magnitude higher than for alternative near-IR pump technologies. These two advantages, due to the small pump photon energies, which are much below the crystal band gap, allow overcoming the major challenge of technology-limited crystal size and address the experimental needs to high spectral contents.

We report on THz generation driven by 3.9 µm pulses, via either optical rectification in organic crystals or in two-color plasma filaments. Outstanding THz conversion efficiency of more than 2% and bandwidth exceeding 15 THz are achieved.

We report results of experimental investigation on polarization evolution and losses in linearly and circularly polarized mid-IR filaments in air under different focusing conditions.

We report on, to the best of our knowledge, the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared (IR) laser pulses (λ=3.9 μm, 100 fs, 0.25 TW), which enable near- and above-critical density interactions with moderate-density gas jets. Relativistic electron acceleration up to ∼12 MeV occurs when the jet width exceeds the threshold scale length for relativistic self-focusing. We present scaling trends in the accelerated beam profiles, charge, and spectra, which are supported by particle-in-cell simulations and time-resolved images of the interaction. For similarly scaled conditions, we observe significant increases in the accelerated charge, compared to previous experiments with near-infrared (λ=800 nm) pulses.

ACS Nano, 9 (10), ISSN:1936-0851, ISSN:1936-086X

Filamentation of Mid-IR pulses in ambient air is significantly more complicated and challenging as compared to near-IR filaments, due the changing sign of dispersion in the vicinity of 3.6 μm, substantially lower ionization rates and hence plasma density [1], as well as because of the presence of molecular resonances around the atmospheric transparency windows. Experimental observations reveal that despite its low concentration in ambient air, which is only 400–600 ppm (0.04–0.06%), CO2 strongly affects filamentation of 3.9-μm pulses by influencing spectral and temporal dynamics, as well as spatial beam transformations and propagation losses. As a main loss mechanism during filamentation of loosely focused 3.9-μm pulses in ambient air, dynamic absorption by CO2 is identified. Since in the case of loose focusing plasma density in the filament is negligible, pulses during filamentation in both main constituents, O2 and N2, experience pronounced red-sided broadening (Fig.la), which is governed by stimulated Raman scattering. In ambient air, newly generated spectral components roll over CO2 resonant absorption band situated in the vicinity of 4.2-μm and are effectively absorbed, which results in up to 50% filamentation losses. To our surprise, an increase of CO2 concentration to ∼5% level (by exhaling air [3] into 2-m long open-ended tube) leads to complete disappearance of losses.

We present the first results from laser plasma wakefield acceleration driven by ultrashort mid-infrared laser pulses (100fs, λ=3.9µm). The onset of relativistic self-focusing and electron acceleration scale with critical laser power and target width.

Organic pigments such as indigos, quinacridones, and phthalocyanines are widely produced industrially as colorants for everyday products as various as cosmetics and printing inks. Herein we introduce a general procedure to transform commercially available insoluble microcrystalline pigment powders into colloidal solutions of variously sized and shaped semiconductor micro- and nanocrystals. The synthesis is based on the transformation of the pigments into soluble dyes by introducing transient protecting groups on the secondary amine moieties, followed by controlled deprotection in solution. Three deprotection methods are demonstrated: thermal cleavage, acid-catalyzed deprotection, and amine-induced deprotection. During these processes, ligands are introduced to afford colloidal stability and to provide dedicated surface functionality and for size and shape control. The resulting micro- and nanocrystals exhibit a wide range of optical absorption and photoluminescence over spectral regions from the visible to the near-infrared. Due to excellent colloidal solubility offered by the ligands, the achieved organic nanocrystals are suitable for solution processing of (opto)electronic devices. As examples, phthalocyanine nanowire transistors as well as quinacridone nanocrystal photodetectors, with photoresponsivity values by far outperforming those of vacuum deposited reference samples, are demonstrated. The high responsivity is enabled by photoinduced charge transfer between the nanocrystals and the directly attached electron-accepting vitamin B2 ligands. The semiconducting nanocrystals described here offer a cheap, nontoxic, and environmentally friendly alternative to inorganic nanocrystals as well as a new paradigm for obtaining organic semiconductor materials from commercial colorants.

Many atmospheric applications such as free-space communication or spectroscopy require targeted delivery of high-energy ultrashort pulses with a good beam quality. Often atmospheric turbulences complicate this task, causing random variations of the refractive index and resulting in fluctuations of light intensity on the target. Propagation of light in filamentation regime, when intensity is clamped, to a certain extent helps to overtake this problem, but brings ionization-related problems, such as energy loss, temporal pulse splitting, etc. Moreover, maximum peak power P peak , which can be delivered in a single filament is limited to about ten critical powers of self-focusing P crit , after which multiple filamentation and hence small-scale beam distortions take place. In the case of Ti:Sapphire drivers delivering 40-fs, 800-nm pulses, the energy in a single filament in air doesn't exceed 1 mJ, unless special conditions are applied [1]. However, since P crit ∼λ2 (λ is the driver wavelength), essentially more energy can be deposited in a single filament driven by 3.9-μm pulses [2]. Furthermore, mid-IR spectral range is beneficial in virtue of lower ionization rates and higher resistance to modulation instabilities and scattering by natural atmospheric obstacles, such as water droplets [3]. Finally, a unique combination of high atmospheric transparency and anomalous dispersion of air between 3.6–4.2 μm promote an opportunity for a lossless high-energy ultrashort pulse delivery and simultaneous solitonic self-compression [4]. However, multiple molecular resonances responsible for the anomalous dispersion also complicate filamentation dynamics by adding a new channel of energy loss, namely nonlinear enhanced absorption losses [2,5], when mainly stimulated rotational Raman scattering (SRRS) governs spectral dynamics dominated by an essential spectral redshift and immediate absorption of newly generated spectral components by CO 2 , having vibrational resonance in the vicinity of 4.2 μm. Due to this non-linearly enhanced absorption up to 50% of energy can be lost over several meters of propagation of 30 mJ 130-fs pulses [2]. However, since the intrapulse SRRS is sensitive to a temporal spacing between spectral components, pre-chirping of drivers pulses can help to reduce and control the energy loss. Moreover, we have demonstrated, that a proper choice of the chirp can also lead to ∼5-fold self-compression in time and postponed onset of filamentation in space, what fulfils a goal of targeted high-energy ultrashort pulse delivery.

We investigate THz generation by optical rectification (OR) in the organic crystal BNA with respect to the crystal thickness and central wavelength of the driving pulse in the near-IR spectral range. We report on high optical- to THz conversion efficiencies around 1 J.μm and broad spectra exceeding 5 THz.

Despite the rapid development of terahertz (THz) science during the last two decades, the majority of available THz sources remains rather weak, limiting the interactions of THz radiation with matter mostly in the realm of linear optics. Development of powerful THz sources will open the way to many exciting applications spanning from switching and controlling of magnetic domains to THz-enhanced attosecond pulse generation [1,2].

We demonstrate, strong THz emission from two-color mid-infrared (3.9 μm) femtosecond laser filaments. The conversion efficiency approaches the percent level, at least one order of magnitude higher than the previously reported for plasma-based THz sources.

We present a comparative study on efficient THz generation by optical rectification of 3.9 µm and 1.95 µm pulses in DAST, achieving up to 40 MV/cm field amplitudes. The ultrafast free-space THz signal is encoded onto an optical signal probing the absorption of CdSe/ CdS core/shell quantum dots.

Variation of the sign of dispersion in air in the vicinity of 3.6 pm [1] makes propagation and filamentation of mid-IR pulses extremely sensitive to the chirp, which allows to take control over the temporal pulse splitting, spectral broadening, energy losses and spatial profile. Here we report on filamentation of 30-mJ, 3.9-pm pulses produced by a hybrid OPA/OPCPA system [2]. Filamentation in ambient air is assisted by loose 7-m focusing and controlled by adjusting the chirp of the output of the OPCPA system.