A method for measuring the group refractive index and dispersion of dielectrics using a pump-probe scheme demonstrated with a ZnSe crystal. In this approach, an intense, ultrashort pump pulse causes ionization of the crystal, leading to scattering and absorption effects on the subsequent broadband probe pulse, which experiences dispersive broadening as it propagates through the crystal. By adjusting the time delay between the pump and probe pulses, we can effectively control the spectrum transmitted through the crystal, enabling the retrieval of both the group refractive index and dispersion characteristics.

The generation of spectral components sensitive to the carrier-envelope phase of a laser pulse in a thin zinc selenide film has been experimentally demonstrated and confirmed by a numerical simulation. A pump–probe scheme has been implemented so that a pump pulse with a duration of about 1.5 field cycles, a central wavelength of 1.7 μm, and a stabilized carrier-envelope phase induces photoionization in a thin zinc selenide film. The probe pulse is scattered by the plasma, generating new phase-sensitive spectral components at the edges of its spectrum. The theoretical analysis has confirmed plasma nonlinearity as a mechanism for generating these components. The observed effect can be used to characterize the carrier-envelope phase of ultrashort pulses during the generation of high-order harmonics and sequences of attosecond pulses.

A novel, 2.6-to-10-μm wavelength-tunable laser source of sub-70-fs pulses is combined with mid-IR heterodyne detection to provide a versatile laser platform for broadband two-dimensional Fourier-transform infrared spectrochronography. The mid-infrared laser output serves as both a short-pulse driver and a broadband probe for time-resolved studies of ultrafast molecular coherence, population dynamics, and multimodal energy transfer in a vast class of complex molecular systems.

We present a particle-in-cell (PIC) analysis of terahertz (THz) radiation by ultrafast plasma currents driven by relativistic-intensity laser pulses. We show that, while the I0 [Formula: see text] product of the laser intensity I0 and the laser wavelength λ0 plays the key role in the energy scaling of strong-field laser-plasma THz generation, the THz output energy, WTHz, does not follow the I0 [Formula: see text] scaling. Its behavior as a function of I0 and λ0 is instead much more complex. Our two- and three-dimensional PIC analysis shows that, for moderate, subrelativistic and weakly relativistic fields, WTHz(I0 [Formula: see text]) can be approximated as (I0λ02)α, with a suitable exponent α, as a clear signature of vacuum electron acceleration as a predominant physical mechanism whereby the energy of the laser driver is transferred to THz radiation. For strongly relativistic laser fields, on the other hand, WTHz(I0 [Formula: see text]) closely follows the scaling dictated by the relativistic electron laser ponderomotive potential [Formula: see text], converging to WTHz ∝ [Formula: see text] for very high I0, thus indicating the decisive role of relativistic ponderomotive charge acceleration as a mechanism behind laser-to-THz energy conversion. Analysis of the electron distribution function shows that the temperature Te of hot laser-driven electrons bouncing back and forth between the plasma boundaries displays the same behavior as a function of I0 and λ0, altering its scaling from (I0λ02)α to that of [Formula: see text], converging to WTHz ∝ [Formula: see text] for very high I0. These findings provide a clear physical picture of THz generation in relativistic and subrelativistic laser plasmas, suggesting the THz yield WTHz resolved as a function of I0 and λ0 as a meaningful measurable that can serve as a probe for the temperature Te of hot electrons in a vast class of laser-plasma interactions. Specifically, the α exponent of the best (I0λ02)α fit of the THz yield suggests a meaningful probe that can help identify the dominant physical mechanisms whereby the energy of the laser field is converted to the energy of plasma electrons.

A high-intensity ultrashort laser pulse interacting with a thin plasma target is shown to couple to plasma electrons, driving electron oscillations within the plasma and making these electrons bounce back and forth between plasma boundaries. Each time these recirculating electrons traverse the plasma boundary, they emit bright subcycle terahertz (THz) field waveforms via laser-driven coherent transition radiation. As a concurrent process, laser-driven electrons near the front surface of the plasma target are accelerated to relativistic velocities to emit high-order harmonics (HHs), giving rise to attosecond pulses of vacuum-ultraviolet radiation. These attosecond pulses are shown to provide a high-precision clock for subcycle THz field waveforms. We demonstrate that the delay time between HH pulses and THz waveforms can be tuned with an attosecond precision by varying the thickness of the plasma target, thus opening an avenue toward HH-pump–THz-probe studies of ultrafast processes on the attosecond time scale with table-top laser sources.

Ultrafast nonlinear dynamics driven by high-peak-power ultrashort mid-infrared (mid-IR) pulses gives rise to ultrabroadband radiation whose spectrum spans over multiple decades, stretching from the terahertz (THz) to the microwave range. We show that, despite its enormous, multidecade bandwidth, an accurate spectral and spatial-mode characterization of this radiation is possible via a suitable combination of mutually complementary signal-analysis methods borrowed from ultrafast optics, radioelectronics, THz photonics, and microwave engineering. This analysis reveals intense subgigahertz radiation emitted as a part of mid-IR-driven supercontinuum generation and shows that microwave-to-THz field waveforms can almost reach the diffraction limit in their beam focusability, yielding field strengths above ≈3 MV/cm.

The smallness of the velocity $$v$$ of laser-field-induced motion of electrons compared to the speed of light c is one of the fundamental physical factors limiting the efficiency of nonlinear optical processes in plasma media. It has been shown in this work that the use of intense ultrashort mid-infrared pulses makes it possible to significantly enhance a wide class of $$v{\text{/}}c$$ -weak plasma nonlinearities primarily related to plasma currents induced by the laser field. This allows implementing laser plasma schemes of the efficient generation of coherent broadband terahertz and microwave radiation, i.e., terahertz–microwave supercontinuum.

Ultrafast laser-plasma interactions driven by ultrashort terawatt laser pulses are shown to give rise to a bright multioctave microwave radiation, whose polarization and spatial mode structure provides a sensitive probe for laser-driven plasma electrodynamics, helping detect the symmetries of plasma currents and signatures of multiple ionization. Polarization mode structure of this radiation is dominated, as polarization-resolved measurements show, by a radially polarized mode, indicating the significance of ponderomotively driven plasma currents as sources of microwave emission. Angle-resolved analysis of microwave supercontinua reveals regimes in which the microwave emission is drastically enhanced, via coherence buildup, manifested in a well-resolved Cherenkov-emission cone.

1 Physics Department, International Laser Center, M.V. Lomonosov Moscow State University, Russia 2 Russian Quantum Center, Skolkovo, Moscow Region, Russia 3 Department of Physics and Astronomy, Texas A&M University, College Station, TX, United States of America 4 National University of Science and Technology ‘MISiS’, Moscow, Russia 5 Institute on Laser and Information Technologies, Branch of FSRC ‘Crystallography and Photonics’, Russian Academy of Sciences, Shatura, Russia

Experiments on the generation of high optical harmonics demonstrating that relativistic regimes of interaction of radiation with matter can be implemented in the field of mid-infrared laser pulses with a peak power of 0.3 TW have been reported. The observation of relativistic phenomena at such extraordinary low peak powers of the laser field becomes possible because of the formation of a high-quality space-time mode of the laser field with an exactly specified polarization state. Such a field structure ensures a high intensity of radiation in the focus of the beam and the effective acceleration of electrons by a low-frequency electromagnetic field of a high-contrast laser pulse with an exactly specified polarization at the extremely sharp vacuum-solid interface.

Coherent-wake plasma emission induced by ultrashort mid-infrared laser pulses on a solid target is shown to give rise to high-brightness, high-order harmonic radiation, offering a promising source of attosecond pulses and a probe for ultrafast subrelativistic plasma dynamics. With 80-fs, 0.2-TW pulses of 3.9-μm radiation used as a driver, optical harmonics up to the 34th order are detected, with their spectra stretching from the mid-infrared region to the extreme ultraviolet region. The harmonic spectrum is found to be highly sensitive to the chirp of the driver. Particle-in-cell analysis of this effect suggests, in agreement with the generic scenario of coherent-wake emission, that optical harmonics are radiated as trains of extremely short, attosecond ultraviolet pulses with a pulse-to-pulse interval varying over the pulse train. A positive chirp of the driver pulse can partially compensate for this variation in the interpulse separation, allowing harmonics of the highest orders to be generated in the plasma emission spectrum.

Fiber-optic multioctave supercontinuum generation is a unique resource for ultrafast optical science, enabling ultrabroadband frequency-comb technologies and paving the way for the photonics of subcycle field wave forms. Extension of these methods to the midinfrared spectral range encounters numerous challenges, calling for radically new approaches in fiber optics and short-pulse generation technologies, as well as for closing the gaps in our understanding of optical nonlinearities in the midinfrared range. Here, we confront these challenges by showing that multioctave supercontinua spanning from the ultraviolet to the midinfrared range can be generated by shock-wave-coupled soliton self-compression of ultrashort midinfrared pulses in a gas-filled antiresonance-guiding hollow-core photonic-crystal fiber. Analysis of the fiber output spectra, measured within a broad range of gas pressures and input driver energies, shows that multioctave supercontinuum generation in this setting becomes possible due to soliton self-compression coupled to shock-wave pulse self-steepening, yielding extraordinarily short, sub-half-cycle field transients.

Fiber-optic multioctave supercontinuum generation is a unique resource for ultrafast optical science' enabling ultrabroadband frequency-comb technologies and paving the way for the photonics of subcycle field waveforms. Extension of these methods to the mid-infrared spectral range is challenging as it calls for new fiber solutions that would allow a high optical nonlinearity in the mid-IR to be combined with broadband transmission and suitably tailored dispersion. Kagome-cladding and antiresonance-guiding single-ring (SR) hollow-core (HC) photonic-crystal fibers stand out as examples of fiber designs that can provide this unique combination of properties [1-4].

High-order harmonic generation (HHG) is one of the central effects in strong-field nonlinear-optical physics [1]. Within the past two decades, HHG has been at the heart of a paradigm-shifting breakthrough to attosecond physics [2]. Much less limelight has been given to the ability of HHG to serve as a highly sensitive and in many ways unique analytical tool. When used in this modality, HHG can help detect and understand ultrafast electron dynamics in complex gas-phase, solid-state, and laser-plasma systems [3]. Here, we extend this concept by using HHG as a probe for relativistic laser-matter interactions driven by ultrashort pulses in the mid-infrared.