Multi-octave, CEP-stable source for high-energy field synthesis

High-contrast front end based on Yb: YAG solid-state laser for PW-level Ti: sapphire laser

In this paper, we demonstrate a high-contrast front-end laser system based on Yb: YAG solid-state laser for Ti: sapphire terminal amplification. An ultrafast Yb: YAG solid-state laser is used to generate a broad-spectrum seed through white light generation (WLG), and then the signal light near 1600 nm is amplified by three-level colinear optical parametric chirped pulse amplification (OPCPA). Finally, a fs second harmonic generation (SHG) is used to obtain a laser output with a central wavelength of 795 nm, a pulse width of 40.2 fs, a contrast baseline of nearly 10-12 within 1 ns, and almost no ps pedestal. We used this laser system as a seed to amplify it to about the theoretical estimated 30 TW level, showing good dispersion properties and contrast preservation capabilities.

Gouy phase effects on photocurrents in plasmonic nanogaps driven by single-cycle pulses

The investigation of optical phenomena in the strong-field regime requires few-cycle laser pulses at field strengths exceeding gigavolts per meter (GV/m). Surprisingly, such conditions can be reached by tightly focusing pJ-level pulses with nearly octave spanning optical bandwidth onto plasmonic nanostructures, exploiting the field-enhancement effect. In this situation, the Gouy phase of the focused beam can deviate significantly from the monochromatic scenario. Here, we study the effect of the Gouy phase of a pulse exploited to drive coherent strong-field photocurrents within a plasmonic gap nanoantenna. While the influence of the specific Gouy phase profile in the experiment approaches the monochromatic case closely, this scheme may be utilized to identify more intricate phase profiles at sub-diffraction scale. Our results pave the way for Gouy phase engineering at picojoule (pJ) pulse energy levels, enabling the optimization of strong-field optical phenomena.

Role of the Porras factor in phase matching of high-order harmonic generation driven by focused few-cycle laser pulses

We investigate the role of the Porras factor (or laser focusing effect) on the macroscopic high-order harmonic generation (HHG) driven by a focused broadband few-cycle laser beam. By employing a non-adiabatic phase-matching analysis method, we reveal that phase mismatch due to the induced-dipole phase varies with the Porras factor, which is dominant in phase matching at low gas pressure. We also find that in a strongly ionized medium when gas pressure is high, the nonlinear propagation is dominated by a plasma effect such that the focusing effect is mitigated, resulting in similar poor phase matching of HHG regardless of the Porras factor. Our results are expected to assist experimentalists identifying optimal conditions for HHG using ultrashort laser pulses.

Parametric waveform synthesis: a scalable approach to generate sub-cycle optical transients

The availability of electromagnetic pulses with controllable field waveform and extremely short duration, even below a single optical cycle, is imperative to fully harness strong-field processes and to gain insight into ultrafast light-driven mechanisms occurring in the attosecond time-domain. The recently demonstrated parametric waveform synthesis (PWS) introduces an energy-, power- and spectrum-scalable method to generate non-sinusoidal sub-cycle optical waveforms by coherently combining different phase-stable pulses attained via optical parametric amplifiers. Significant technological developments have been made to overcome the stability issues related to PWS and to obtain an effective and reliable waveform control system. Here we present the main ingredients enabling PWS technology. The design choices concerning the optical, mechanical and electronic setups are justified by analytical/numerical modeling and benchmarked by experimental observations. In its present incarnation, PWS technology enables the generation of field-controllable mJ-level few-femtosecond pulses spanning the visible to infrared range.

Extension of the bright high-harmonic photon energy range via nonadiabatic critical phase matching

The concept of critical ionization fraction has been essential for high-harmonic generation, because it dictates the maximum driving laser intensity while preserving the phase matching of harmonics. In this work, we reveal a second, nonadiabatic critical ionization fraction, which substantially extends the phase-matched harmonic energy, arising because of the strong reshaping of the intense laser field in a gas plasma. We validate this understanding through a systematic comparison between experiment and theory for a wide range of laser conditions. In particular, the properties of the high-harmonic spectrum versus the laser intensity undergoes three distinctive scenarios: (i) coincidence with the single-atom cutoff, (ii) strong spectral extension, and (iii) spectral energy saturation. We present an analytical model that predicts the spectral extension and reveals the increasing importance of the nonadiabatic effects for mid-infrared lasers. These findings are important for the development of high-brightness soft x-ray sources for applications in spectroscopy and imaging.

Electro-optic characterization of synthesized infrared-visible light fields

The measurement and control of light field oscillations enable the study of ultrafast phenomena on sub-cycle time scales. Electro-optic sampling (EOS) is a powerful field characterization approach, in terms of both sensitivity and dynamic range, but it has not reached beyond infrared frequencies. Here, we show the synthesis of a sub-cycle infrared-visible pulse and subsequent complete electric field characterization using EOS. The sampled bandwidth spans from 700 nm to 2700 nm (428 to 110 THz). Tailored electric-field waveforms are generated with a two-channel field synthesizer in the infrared-visible range, with a full-width at half-maximum duration as short as 3.8 fs at a central wavelength of 1.7 µm (176 THz). EOS detection of the complete bandwidth of these waveforms extends it into the visible spectral range. To demonstrate the power of our approach, we use the sub-cycle transients to inject carriers in a thin quartz sample for nonlinear photoconductive field sampling with sub-femtosecond resolution.

Temporal characterization of two-octave infrared pulses by frequency resolved optical switching

We present the temporal characterization of infrared pulses with spectra extending from 0.55 to 2.5 μm by using the frequency resolved optical switching (FROSt) technique. The pulses are obtained by broadening femtosecond pulses at 1.75 μm central wavelength in a two-stage hollow core fiber setup. This work demonstrates the capability of the FROSt technique to temporally characterize pulses with ultra-broadband spectra. Being free of phase-matching constraints, it enables the characterization of pulses with very low energy at the limit of the detection threshold and with arbitrary long pulse duration. This strength of the FROSt technique is illustrated by the characterization of supercontinua pulses whose spectra span over two octaves and with only 150 nJ energy that is spread temporally over almost 40 ps. The FROSt capabilities provide a versatile tool for the characterization of sub-cycle pulses and to study nonlinear processes such as supercontinuum generation.

Simulating an ultra-broadband concept for Exawatt-class lasers

The rapid development of the optical-cycle-level ultra-fast laser technologies may break through the bottleneck of the traditional ultra-intense laser [i.e., Petawatt (PW, 10¹⁵ W) laser currently] and enable the generation of even higher peak-power/intensity lasers. Herein, we simulate an ultra-broadband concept for the realization of an Exawatt-class (EW, 10¹⁸ W) high peak-power laser, where the wide-angle non-collinear optical parametric chirped-pulse amplification (WNOPCPA) is combined with the thin-plate post-compression. A frequency-chirped carrier-envelope-phase stable super-continuum laser is amplified to high-energy in WNOPCPA by pumping with two pump-beamlets and injected into the thin-plate post-compression to generate a sub-optical-cycle high-energy laser pulse. The numerical simulation shows this hybrid concept significantly enhances the gain bandwidth in the high-energy amplifier and the spectral broadening in the post-compression. By using this concept, a study of a prototype design of a 0.5 EW system is presented, and several key challenges are also examined.