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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.

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.

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.

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.

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, 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

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.

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.

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.