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High‐energy, few‐cycle laser pulses are essential for numerous applications in the fields of ultrafast optics and strong‐field physics, due to their ultrafast temporal resolution and high peak intensity. In this work, different from the traditional hollow‐core fibers and multiple thin solid plates, the generation of an octave‐spanning supercontinuum broadening is demonstrated by utilizing multiple ultrathin liquid films (MTLFs) as the nonlinear media. The continuum covers a range from 380 to 1050 nm, corresponding to a Fourier transform limit pulse width of 2.5 fs when 35 fs Ti: sapphire laser pulse is applied on the MTLFs. The output pulses are compressed to 3.9 fs by employing chirped mirrors. Furthermore, a continuous high‐order harmonic spectrum up to the 33rd order is realized by subjecting the compressed laser pulses to interact with Kr gas. The utilization of flowing liquid films eliminates permanent optical damage and enables wider and stronger spectrum broadening. Therefore, this MTLFs scheme provides new solutions for the generation of highly efficient supercontinuum and nonlinear pulse compression, with potential applications in the fields of strong‐field physics and attosecond science. [ABSTRACT FROM AUTHOR]

In this study, we employ strong field approximation (SFA) to investigate the influence of the number of pulse cycles on above-threshold ionization within the framework of nondipole theory. The SFA enables the analysis of the ionization process under the dominance of the electric field, compared to other factors such as the binding potential of an atom. Nondipole effects, including higher-order multipole fields, can significantly impact ionization dynamics. However, the interaction between nondipole effects and pulse cycles remains unclear. Therefore, we investigate the pulse cycle dependence of ionization and examine peak shifts in Kr and Ar atoms. Our findings have implications for comprehensively understanding the effects of electromagnetic fields on electron behavior. The insights gained from this study provide valuable guidance for future research in strong field ionization. [ABSTRACT FROM AUTHOR]

Gas-filled hollow-core fibers are a convenient tool for laser pulse compression down to a few-cycle duration. The development of compact, efficient and high quality compression schemes for laser pulses of relatively low μJ-level energies is of particular interest. In this work, temporal pulse compression based on nonlinear spectral broadening in a xenon-filled revolver fiber followed by a chirped mirror system is investigated. A 250 fs pulse at a central wavelength of 1.03 μm is compressed to 13.3 fs when the xenon pressure was tuned to provide zero group velocity dispersion near 1.03 μm. The energies of input and compressed pulses are 3.8 and 2.7 μJ, respectively. The compression quality factor of 1.8 is achieved. [ABSTRACT FROM AUTHOR]

In this study, we employ strong field approximation (SFA) to investigate the influence of the number of pulse cycles on above-threshold ionization within the framework of nondipole theory. The SFA enables the analysis of the ionization process under the dominance of the electric field, compared to other factors such as the binding potential of an atom. Nondipole effects, including higher-order multipole fields, can significantly impact ionization dynamics. However, the interaction between nondipole effects and pulse cycles remains unclear. Therefore, we investigate the pulse cycle dependence of ionization and examine peak shifts in Kr and Ar atoms. Our findings have implications for comprehensively understanding the effects of electromagnetic fields on electron behavior. The insights gained from this study provide valuable guidance for future research in strong field ionization.

• The damage behavior of two UBCMs with different designs is studied under 7.3-fs pulse irradiation for the first time. • A time-domain electric field model is proposed to explain the origin of the damage and the discrepancy in LIDTs of the two UBCMs. • Incorporating the time-domain electric field model as a merit function contributes to designing high-threshold UBCMs. The laser-induced damage resistance performance of ultra-broadband chirped mirrors (UBCM) is a critical factor influencing the generation of petawatt or exawatt laser pulses in thin film compression techniques. In this study, a few-cycle pulse damage testing platform was set up, and two UBCMs with different designs were fabricated for a comparative study of their laser-induced damage. Because light of varying wavelengths forms an electric field at different times within UBCMs under few-cycle pulse irradiation, a time-domain electric field model was developed to obtain transient electric field variations and explain the damage behavior. The model successfully elucidated the origin of the damage and discrepancy between the two UBCMs in terms of damage thresholds, showcasing its potential applications in designing high-threshold UBCMs for high-power laser systems. [ABSTRACT FROM AUTHOR]

Gas-filled hollow-core fibers are a convenient tool for laser pulse compression down to a few-cycle duration. The development of compact, efficient and high quality compression schemes for laser pulses of relatively low μJ-level energies is of particular interest. In this work, temporal pulse compression based on nonlinear spectral broadening in a xenon-filled revolver fiber followed by a chirped mirror system is investigated. A 250 fs pulse at a central wavelength of 1.03 μm is compressed to 13.3 fs when the xenon pressure was tuned to provide zero group velocity dispersion near 1.03 μm. The energies of input and compressed pulses are 3.8 and 2.7 μJ, respectively. The compression quality factor of 1.8 is achieved.

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided.

Generation of few-cycle optical pulses in free-electron laser (FEL) oscillators has been experimentally demonstrated in FEL facilities based on normal-conducting and superconducting linear accelerators. Analytical and numerical studies have revealed that the few-cycle FEL lasing can be explained in the frame of superradiance, cooperative emission from self-bunched systems. In the present paper, we review historical remarks of superradiance FEL experiments in short-pulse FEL oscillators with emphasis on the few-cycle pulse generation and discuss the application of the few-cycle FEL pulses to the scheme of FEL-HHG, utilization of infrared FEL pulses to drive high-harmonic generation (HHG) from gas and solid targets. The FEL-HHG enables one to explore ultrafast science with attosecond ultraviolet and X-ray pulses with a MHz repetition rate, which is difficult with HHG driven by solid-state lasers. A research program has been launched to develop technologies for the FEL-HHG and to conduct a proof-of-concept experiment of FEL-HHG.

Without using the approximation of slowly varying envelopes, we investigate spectral transformations of vector few-cycle and quasi-monochromatic solitons under propagation in the birefringent medium of asymmetric molecules possessing permanent dipole moments. We show that when an ordinary pulse enters the medium, the self-modulation effect arises. As a result, the spectrum of the ordinary component undergoes significant changes from the violet shift and the generation of odd harmonics with close frequency satellites to the supercontinuum including zero frequency. The spectrum of the extra-ordinary few-cycle pulse also has the properties of a supercontinuum. When a quasi-monochromatic ordinary pulse enters the medium in the spectrum of an extra-ordinary component, the effect of optical rectification with simultaneous generation of the second harmonics is clearly manifested. [ABSTRACT FROM AUTHOR]

We have demonstrated experimentally that few-cycle pulses can be achieved from an Er-doped fiber laser via single-stage soliton compression. Adopting negative prechirp amplification is to avoid the high-energy pulse break-up and increase the spectral width. Through optimizing the fiber length and pulse energy in different parts of the fiber laser system, the generated pulse can be as short as 22.7 fs, which is about four cycle duration at 1550 nm. The average power is up to 120 mW with the corresponding pulse energy of 2.8 nJ. Numerical simulation has been carried out to illustrate the dynamic of pulse prechirp, amplification, and compression. [ABSTRACT FROM PUBLISHER]

The self-compression of sub-TW mid-IR ( λ =4–5 µm) femtosecond laser pulse in a non-linear dielectric medium was numerically modeled. Combination of the Kerr non-linearity and negative group velocity dispersion in CaF 2 crystal was found to result in compression of the incident laser pulse to a 1–2-cycle pulse width. The possibility for suppression of small-scale self-focusing in conditions when the pulse power was several orders of magnitude above the critical self-focusing power was analyzed. Intrinsic concentration of free carriers was found to play important role in pulse evolution during its propagation. [ABSTRACT FROM AUTHOR]

Abstract: We propose a theoretical scheme to coherently control the transport of a single electron in an asymmetric double-quantum-dot system. The single-electron transport originates from the intrinsic interplay between the externally applied few-cycle pulse and the inter-dot tunneling. Solving the equations of motion for dot-density matrix, we reveal numerically that the current exhibits a significant dependence on the carrier-envelope phase (CEP) of the few-cycle pulse, which is similar to the magnetic flux controlled coherent transport in an Aharnov–Bohm (AB) interferometer. As a result, by varying the CEP of the pulse one can suppress or enhance the current either instantaneously or periodically. Our results illustrate the potential to utilize few-cycle pulses for excitation in quantum dot systems through the CEP control, as well as a guidance in the design for possible experimental implementations. [Copyright &y& Elsevier]

The scattering of a single relativistic electron with few-cycle plane wave laser pulse with intensity of about $$I=1.38\times 10^{14}\,\text{ W/cm }^{2}$$ is theoretically and numerically analyzed in the linear regime, and the radiated energy spectra of electron shows that zeptosecond X-ray pulses can be supported. The influences of the initial carrier-envelope phase offset $$\varphi _0$$ of the incident few-cycle laser pulses are studied, and the results demonstrate that a single zeptosecond pulse can be produced from scattering by using a single-cycle laser pulse with fixed initial carrier-envelope phase offset $$\varphi _0 =\pi /2$$. It is discovered that the influence of the initial carrier-envelope phase $$\varphi _0$$ on the spectrum of the radiation is apparent for low and high frequency of the spectrum, but there is no influence of the central part of the spectrum. [ABSTRACT FROM AUTHOR]

Abstract: Based on the classical radiation theory, the spatial and spectral characteristics of linear scattering with linearly polarized few-cycle pulses have been obtained. Most of the radiation is sharply centered in the backward direction. The research results show that the shorter the duration of the few-cycle pulse is, the wider the backward scattering spectrum, the bigger backward scattered central frequency of the spectrum and the weaker intensity of the spectrum are. It is discovered that the influence of the initial phase on the spectrum of the radiation is apparent for low and high frequency of the spectrum, but there is no influence of the central part of the spectrum. [Copyright &y& Elsevier]

A numerical study is reported of the transparent propagation of few-cycle laser pulses with arbitrary intensity. The phenomenon is based on the preparation of steady maximum coherent superposition state of the lower two levels in double-lambda atomic systems. The transparency of the few-cycle pulse is sensitive to its parameters such as the arriving time and relative phase. The advantages of the method are discussed. [ABSTRACT FROM AUTHOR]

The propagation equation for a single- and a few-cycle pulses was derived in a cubic nonlinear medium including the Raman response. Using this equation, the propagation characteristics of a single- and a 4-cycle pulse, at 0.8 µm wavelength, were studied numerically in one spatial dimension. It was shown that Raman term does influence the propagation characteristics of a single-as well as a few-cycle pulses by counteracting the self-steepening effect. [ABSTRACT FROM AUTHOR]

We present the results of theoretical studies of the generation process of difference frequency radiation arising via interaction of mutually orthogonal linearly polarized few-cycle laser pulses propagating in an isotropic nonlinear medium. Numerical time-integration by the finite-difference method of nonlinear Maxwell-equation systems has been performed. We consider the interaction of pulses having the central wavelengths of 1.98 and 1.55 µm, duration of 30 fs with the corresponding electric field amplitudes of 295 × 106 and 463 × 106 V m-1, propagating along the normal to the 〈110〉 plane in 854 µm thickness of GaAs crystal. The process of difference frequency pulse formation arising via spectral filtration of a supercontinuum formed in the spectra of pump pulses at the output of a nonlinear crystal is studied. [ABSTRACT FROM AUTHOR]

Abstract: A non-variational Time-Dependent Multi-Configuration Self-Consistent Field (TDMCSCF) scheme has been developed to describe, by an ab-initio approach, the time-resolved electron dynamics of a laser-driven many-electron atomic or molecular system. Its implementation within a typical quantum chemical package is discussed and its use demonstrated on the laser-driven electron dynamics of LiH. As an method, this approach faces severe challenges when ionization and large-amplitude electronic motions are addressed. We present here an extension of this TDMCSCF scheme to include multiple ionization processes, using a Feschbach state-partitioning formalism, allowing the bound electrons’ dynamics to be treated by the -TDMCSCF method, while the ionized electrons can be treated separately by an alternative approach. We present results of proof-of-principle calculations on the strong field ionization of and discuss the effect of the carrier-envelope phase (CEP) on electron localization and ionization processes in this model system. [Copyright &y& Elsevier]

Impulsive stimulated resonant Raman scattering was induced by a 5.7 fs visible pulse from a non-collinear optical parametric amplifier. The pulse duration corresponded to 2.8 cycles of the central wavelength of the laser spectrum at 610 nm. The induced spectral change was time-resolved with a 0.8 fs delay time step and the effective absorbance change ΔA(λ, t) was calculated from the normalized transmittance change ΔT(λ, t) /T. The observed optical and vibrational 2-D spectrum was analyzed to separate the electronic and vibrational contributions to the transient difference absorption spectrum. The probe wavelength dependence of the vibrational amplitude was explained in terms of the coupling between the laser field, Stokes field, and the vibrational coordinates. Simultaneous measurement of both the time and wavelength dependence of the vibrational amplitudes clearly showed that the broadband spectra of the components corresponding to the laser and Stokes lights are oscillating in π out of phase with the corresponding vibrational frequency. The probe delay time and probe wavelength dependence of the oscillation in the absorbance change induced by the femtosecond laser were fully analyzed. The results were explained by the imaginary and real part of the third-order susceptibility, respectively. [ABSTRACT FROM AUTHOR]

Propagation of a few-cycle laser pulse in a dense V-type three-level atomic medium is investigated by the numerical solution of the full Maxwell-Bloch equations without the rotating wave and slowly varying envelope approximations, and the numerical solution is obtained by using the predictor-corrector method and the finite-difference time-domain method. It is shown that, due to the strength of the electric field induced by the macroscopic polarization in a dense medium being stronger than that in a dilute medium and the influence of the near dipole-dipole (NDD) interaction, the time evolution of a few-cycle pulse in the dense medium is remarkably different from that in the corresponding dilute medium. In the dilute medium, oscillation arises at the trailing edge of the pulse; while in the dense medium, it appears at both the leading and trailing edges of the pulse; moreover, the oscillation at the leading edge is more obvious with the pulse area decreasing. The carrier-envelope phase has an obvious difference in the two cases with and without NDD interaction. The ratio, γ, of the transition dipole moments has strong influence on the time evolution and split of the pulse. In the dense medium, when γ = 1, NDD interaction delays propagation and split of the pulse; while when γ > 1, NDD interaction accelerates propagation and split of the pulse, moreover, the phenomenon is more obvious with the input pulse area decreasing. In the dilute medium, the larger area pulse doesn't split when γ = 1 while it splits when γ > 1. [ABSTRACT FROM AUTHOR]

Abstract: By solving numerically the full Maxwell–Bloch equations without the slowly varying envelope approximation and the rotating-wave approximation, we investigate the effects of Lorentz local field correction (LFC) on the propagation properties of few-cycle laser pulse in a dense Λ-type three-level atomic medium. We find that: when the area of the input pulse is larger, split of pulse occurs and the number of the sub-pulses with LFC is larger than that without LFC; at the same distance, the time interval between the first sub-pulse and the second sub-pulse in the case without LFC is longer than that with LFC, the time of pulse appearing in the case without LFC is later than that in the case with LFC, and the two phenomena are more obvious with propagation distance increasing; time evolution rules of the populations of levels |1>, |2> and |3> in the two cases with and without LFC are much different. When the area of the input pulse is smaller, effects of LFC on time evolutions of the pulse and populations are remarkably smaller than those in the case of larger area pulse. [Copyright &y& Elsevier]

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided. [ABSTRACT FROM AUTHOR]

Generation of few-cycle optical pulses in free-electron laser (FEL) oscillators has been experimentally demonstrated in FEL facilities based on normal-conducting and superconducting linear accelerators. Analytical and numerical studies have revealed that the few-cycle FEL lasing can be explained in the frame of superradiance, cooperative emission from self-bunched systems. In the present paper, we review historical remarks of superradiance FEL experiments in short-pulse FEL oscillators with emphasis on the few-cycle pulse generation and discuss the application of the few-cycle FEL pulses to the scheme of FEL-HHG, utilization of infrared FEL pulses to drive high-harmonic generation (HHG) from gas and solid targets. The FEL-HHG enables one to explore ultrafast science with attosecond ultraviolet and X-ray pulses with a MHz repetition rate, which is difficult with HHG driven by solid-state lasers. A research program has been launched to develop technologies for the FEL-HHG and to conduct a proof-of-concept experiment of FEL-HHG. [ABSTRACT FROM AUTHOR]

Propagation of ultra-short optical pulses in an environment with quadratic nonlinearity and modulated dispersion is studied. It is shown that periodic modulation of the velocity-mismatch sign and of the third-order dispersion coefficient allows one to suppress pulse scattering and decompression, which noticeably increases their interaction efficiency. [ABSTRACT FROM AUTHOR]

• The laser presented here is the first 50 fs pulse demonstration generated directly from a NALM fiber oscillator with long term stability. • Our laser cavity is constructed by only four all-PM fiber components. Thus it enjoys the advantages including easy self-starting, long-term reliable, concise in structure and cost-effective. • We numerically proved that shorter cavity can produce pulses with broader spectral bandwidth and shorter transform-limited pulse width. We report on a long-term stable 50 fs pulse generation directly from a nonlinear amplified loop mirror based fiber oscillator. The laser cavity with all-polarization-maintaining fiber components can generate pulses with 0.16 nJ pulse energy and 50 fs pulse width under 85 MHz repetition rate. The 0–45 °C high-low temperature test, together with the 15 h output power test, shows the excellent stability of the few-cycle pulsed laser. [ABSTRACT FROM AUTHOR]

A model estimate of the energy eigenvalues of the E2 g phonon mode of the ion vibrations in the plane of boron atoms is obtained. It is shown that the anharmonic contribution to the energy of E2 g monotonously increases with the increase in z = A4/ A, where A4 is the fourth-order anharmonic coefficient and A2 is the harmonic constant. The results agree with the estimate in the Hartree-Fock approach. However, the discrepancy between these results and Raman spectroscopy data indicates the necessity to account for the influence of the electron-phonon interaction on the energy of E2 g in more detail. [ABSTRACT FROM AUTHOR]