Gas-filled waveguides enable few-cycle, spatio-temporally coupled (STC) pulses with programmable structure, opening new routes to control charged particles with optical fields. This review maps the landscape of optical-field-driven photoemission, then surveys gas-based nonlinear drivers, photonic crystal fibers (PCFs) for low-energy, high-repetition operation and hollow-core capillaries (HCCs) for high-power, few-cycle synthesis. We highlight mechanisms for deterministic pulse shaping, including four-wave-mixing-based spectral-phase transfer in HCCs, and show how tailored STC waveforms steer emission dynamics from the multiphoton to tunneling regimes, enabling sub-cycle gating, momentum control, and brightness scaling. We conclude with open challenges: phase stability, mid-IR scalability, coupling to nanophotonic emitters, metrology of vectorial fields, and outline a path toward compact, ultrafast, phase-coherent electron sources and emerging quantum applications powered by nonlinear photonics.

Strong-field tunneling and multiphoton excitation in materials provide a sub-cycle temporal gate that enables access to not only the temporal profile of the laser-pulse envelope but also the rapid oscillations associated with the carrier phase. This approach extends waveform metrology from the mid-infrared to the near-infrared spectral range and can be implemented using all-optical techniques and integrated on-chip devices, including operation in a single-shot detection geometry.

We demonstrate nonlinear compression of mid-infrared pulses from a Cr:ZnSe chirped-pulse amplifier using a gas-filled stretched hollow-core fiber followed by bulk-material compression. Starting from 90 fs, 2.45 µm pulses with 5.3 mJ energy, spectral broadening in the gas-filled capillary combined with optimized dispersion management enables compression to 15 fs, less than two optical cycles at 2.45 µm, with 3.3 mJ pulse energy, corresponding to a peak power of approximately 0.12 TW. The simplicity of the approach, based on a single hollow-core fiber stage and bulk dispersion compensation, makes it scalable to higher energies and establishes a robust route to mid-infrared drivers for high harmonic generation and attosecond applications.

Next-generation attosecond spectroscopies require high-repetition-rate driving lasers with few-cycle pulse duration, stable carrier-envelope phase (CEP), and ease of operation. Industrial-grade Yb-doped solid-state regenerative amplifiers perform very well at high repetition rates, but their relatively long pulse durations necessitate post-compression techniques with large compression factor to reach the few-cycle limit. Here, we demonstrate the two-stage hybrid nonlinear compression of 280 fs pulses from a Yb:KGW regenerative amplifier to 5.6 fs, 142 mu J, with an overall transmission efficiency of >70%. In the first stage, a gas filled multi-pass cell is used to compress the pulses down to 38 fs, while a second stage gas filled hollow-core fiber further compresses the pulses to below two optical cycles. We also demonstrate the stabilization of the CEP in the compressed pulse, with a measured root-mean-squared error of 423 mrad on a shot-to-shot basis.

Field-resolved measurements of few-cycle laser waveforms allow access to ultrafast electron dynamics in light–matter interactions and are key to future lightwave electronics. Recently, sub-cycle gating based on nonlinear excitation in active pixel sensors has allowed the first single-shot measurements of mid-infrared optical fields. Extending the techniques to shorter wavelengths, however, is not feasible using silicon-based detectors with bandgaps in the near-infrared. Here, we demonstrate an all-optical sampling technique for near-infrared laser fields, wherein an intense fundamental field generates a sub-cycle gate through nonlinear excitation of a wide-bandgap crystal, in this case, ZnO, which can sample the electric field of a weak perturbing pulse. By using a crossed-beam geometry, the temporal evolution of the perturbing field is mapped onto a transverse spatial axis of the nonlinear medium, and the waveform is captured in a single measurement of the spatially resolved fluorescence emission from the crystal. The technique is demonstrated through field-resolved measurements of the field reshaping during nonlinear propagation in the ZnO detection crystal.

We present an in-depth study on the impact of spatiotemporal Raman enhancement in molecular gas-filled hollow-core fibers (HCFs), demonstrating the efficient generation and post-compression of multidimensional solitary states (MDSS). Through different experimental scenarios—employing large-core HCFs filled with molecular gases (N2 and N2O) and driven by high energy, sub-picosecond and picosecond Fourier transform-limited ytterbium laser pulses—this work leverages multimode propagation and enhanced spatiotemporal interactions to achieve significant spectral broadening and asymmetric redshift, contrasting sharply with self-phase modulation. Our findings reveal that, beyond the regime of maximum nonadiabatic molecular alignment, spatiotemporal nonlinear enhancement primarily governs spectral broadening for input pulse durations up to 1 ps. The process shows limited sensitivity to input pulse duration and the two investigated molecular gases (N2 and N2O), with only subtle differences in broadening arising from their distinct Raman spectroscopic properties. Furthermore, post-compression of MDSS was achieved in various cases. Notably, using 7 mJ, 1 ps laser pulses, we generated 22 fs pulses with a 47% energy conversion efficiency of the input pulse energy. These results position MDSS as a powerful platform for generating high-energy, ultrashort pulses with tunable wavelengths, offering a robust solution for applications such as high harmonic generation.

We investigate, by solving the time-dependent Schrödinger equation in the single-active-electron approximation in helium, a two-color scheme for tabletop high-order harmonic generation (HHG) that combines a mid-infrared (MIR) driving field with an ultrashort ultraviolet (UV) pulse that could be generated via resonant dispersive wave emission in gas-filled hollow-core fibers. This configuration enables the generation of bright, isolated, and tunable attosecond X-ray pulses. In contrast to single-color driving schemes, which suffer from low conversion efficiency, unfavorable wavelength scaling, and limited spectral control, the MIR+UV approach provides a practical and controllable route for advancing tabletop ultrafast spectroscopy and real-time molecular imaging within current experimental capabilities.

We perform field-resolved measurements of vibrational nonlinearity in fused silica using sub-10 fs pulses at 1 micron, generated via a two-stage hybrid compression system, and sampled with the TIPTOE technique.

We demonstrate compression of few-cycle ultraviolet (UV) resonant dispersive waves (RDWs) generated in a cascaded hollow capillary fiber setup using a Yb laser system. Temporal characterization is performed using both tunneling ionization with a perturbation for the time-domain observation of an electric field (TIPTOE) and self-diffraction frequency-resolved optical gating (SD-FROG), which show good agreement. Through careful dispersion management, we compress the RDW pulse to 6.9 fs at a ∼390-nm central wavelength. This is the first, to our knowledge, measurement of an RDW using the TIPTOE method and demonstrates the viability of this technique to reliably characterize few-cycle UV pulses with μJ pulse energies.

Yb-doped laser amplifiers have, in recent years, found applications in attosecond science, driven largely by the opportunities associated with high-average-power and high-repetition-rate attosecond sources. In this Perspective article, we outline the recent advances and upcoming opportunities in attosecond pulse generation and attosecond measurements based on Yb-doped laser amplifiers.

Using 7 mJ, 1 ps laser pulses, we generated 22 fs pulses with 47% energy conversion efficiency, demonstrating MDSS as a robust platform for high-energy, ultrashort pulses with tunable wavelengths, ideal for applications like HHG.

Yb-doped lasers are an attractive platform for attosecond science at high repetition rate. Here, we demonstrate carrier-envelope phase-dependent high-order harmonic generation using polarization gating of few-cycle pulses resulting from two-stage-fiber compression.

A 250fs 4.07μm laser driven filamentation in polycrystalline ZnS is imaged by a synchronous orthogonal femtosecond 520nm probe. Multiple filaments are visible for each power, where larger filaments appear at the highest intensity.

In this chapter, we review the topic of high-order harmonic generation in solid-state thin films. High-order harmonic generation was first observed in bulk ZnO crystals in 2011, and has since attracted significant interest both for compact attosecond source development and spectroscopic studies of condensed matter systems. However, as the harmonic spectrum generated from bulk materials is highly susceptible to the effects of nonlinear propagation on the driving laser field, it is often difficult to extract the relevant microscopic physics from the harmonic spectra. Thin film samples do not suffer from the same issues, and moreover offer access to structural phases not present in bulk, as well as novel physics associated with quantum confinement and topological effects. In the past several years, high-order harmonic generation has been demonstrated in a wide array of thin-film samples, including materials which can be cleaved from bulk crystals, grown epitaxially on dielectric substrates, and exfoliated down to the 2D limit, leading to important insights into macroscopic build-up of harmonic emission and nanoscale physics. We review these advances, starting from a basic perspective on the theoretical underpinnings and experimental achievements of harmonic generation in bulk solids, and highlighting the recent extensions of the technique to thin film and monolayer materials. Finally, we give our perspective on the upcoming physics frontiers which may be addressed by harmonic generation in thin film systems.

In this paper, we analyze the effects of electron-electron correlations in the high-order harmonic (HH) spectrum of perovskite BaTiO3 excited by an intense, few-cycle midinfrared laser pulse by using time-dependent density-functional theory with the exchange-correlation (XC) kernel obtained from the dynamical mean-field theory solution of the Hubbard model. As we show, the correlation effects considerably alter the HH spectrum, shifting the spectral weight to higher frequencies and leading to the generation of "superharmonics"-an energyperiodic enhancement and suppression of certain harmonic orders-when the correlation strength exceeds a critical value. The results are stable for variation of the peak field strength, duration, and central frequency of the driving laser pulse. Using a correlated two-level model, we find that the superharmonic emission results from transitions between correlation-induced Hubbard subband states and has frequency proportional to the effective electron-electron correlation energy.

We observed the effect of stain on higher harmonics generation (HHG) in Copper Indium Thiophosphate (CIPS) film. HHG in strain sample enhanced by 1 order of magnitude compared to pure CIPS.

Copper indium thiophosphate, CuInP 2 S 6 (CIPS), has unique properties including van der Waals layered structure and strong room temperature ferroelectricity. Here, we study high-order harmonic generation driven by mid-infrared pulses in CIPS, observing intense emission up to the 9th harmonic, which lies above the band gap.

Multiphoton excitation in AlGaN provides a sub-cycle gate capable of sampling few-cycle waveforms in the near-infrared. We demonstrate near-infrared laser waveform sampling using an AlGaN photodiode in a scanning TIPTOE geometry.