Ultraintense Ultrashort Laser Pulses

• Ultraintense ultrashort laser pulses are electromagnetic bursts exceeding 10^18 W/cm² with femtosecond to sub-femtosecond durations, crucial for exploring strong-field physics.
• Generation techniques such as chirped-pulse amplification, self-phase modulation, and OPCPA yield few-cycle pulses with excellent CEP stabilization and broad spectral ranges.
• These pulses drive nonlinear phenomena including electron acceleration, high-harmonic generation, and relativistic light–matter interactions, enabling advanced experimental regimes.

An ultraintense ultrashort laser pulse is a temporally brief electromagnetic field with peak intensity in excess of 10¹⁸ W/cm² and duration ranging from a few femtoseconds down to the sub-femtosecond regime, comprising only a few optical cycles at its central wavelength. These pulses are both extreme in power and spectral bandwidth, displaying unique physical interactions with matter, nonlinear light–matter coupling, and enabling phenomena inaccessible to longer or less intense pulses. They are central to advancements in strong-field physics, laser–plasma acceleration, attosecond science, relativistic optics, and various high-energy-density applications.

1. Fundamental Parameters and Spectral Properties

Ultraintense ultrashort laser pulses are defined by peak intensity $I_0\gtrsim10^{18}\,$W/cm$^2$ and durations τ shorter than a few optical cycles. The carrier frequency $\omega_0$ alone is insufficient to characterize such pulses due to broad spectral content ($\Delta\omega/\omega_0\sim1$ or greater). The principal frequency $\omega_P$, computed as

$$\omega_P = \frac{\int \omega^2 |E(\omega)|^2 d\omega}{\int \omega |E(\omega)|^2 d\omega},$$

provides a more accurate clock for many strong-field and ultrafast phenomena, particularly for describing extrema in the electric field relevant for sub-cycle dynamics, high-order harmonic generation (HHG) cutoff scaling, and timing of ionization events (Neyra et al., 2021). As $\Delta\omega$ increases, $\omega_P$ shifts above $\omega_0$, making it the relevant parameter for photonic and electron dynamics in sub-2-cycle regimes.

2. Generation Techniques and State-of-the-Art Systems

Ultraintense ultrashort pulses are generated via chirped-pulse amplification (CPA) and further compressed through self-phase modulation, nonlinear spectral broadening, parametric amplification, or hollow-core fiber systems:

• Hollow-core fiber systems with spectral broadening (SPM/Raman) and compression yield few-cycle pulses with mJ energy (e.g., 15 fs, 10–12 mJ) tunable from IR to UV (Popmintchev et al., 2023).
• Gain-managed nonlinear fiber amplification combined with resonant dispersive wave emission in antiresonant hollow-core fiber enables sub-20 fs pulses, $E=10$–40 nJ, $P_{\text{peak}}>2$ MW, at $\sim 5$ MHz, continuously tunable from 400–700 nm (Sabbah et al., 3 Feb 2025).
• OPCPA systems with difference-frequency generation and BBO/BiBO stages deliver 15–21 fs, 2 µm pulses with energies up to 30 µJ, 200 kHz rep-rate, $P_{\text{peak}}=1.5$ GW, and $I_0>10^{14}$ W/cm² (Meier et al., 14 Oct 2024).

Table: Representative Sources

Method/Platform	Duration	Energy/Peak Power	Spectral Range
HCF+SPM+SHG/Raman–populated	8–30 fs	2–3 mJ / 3×10¹⁵ W/cm²	460–580 nm (VIS), 230–290 nm (UV-C) (Popmintchev et al., 2023)
GMNA + RDW in antiresonant fiber	13–20 fs	10–40 nJ / >2 MW	400–700 nm
OPCPA (DFG + NOPA, BBO/BiBO)	<20 fs	20–30 µJ / 1.5 GW	2 µm

Pulses from these systems are often passively stabilized in carrier-envelope phase (CEP), with jitter $<0.1$ rad, enabling reproducible strong-field and attosecond experiments (Meier et al., 14 Oct 2024, Sabbah et al., 3 Feb 2025).

3. Pulse–Matter Interaction Regimes

At intensities $I_0\gtrsim 10^{18}\,\mathrm{W/cm}^2$ and durations $\lesssim$100 fs:

• In solids, collisional (inverse-bremsstrahlung) absorption is dominant for circular polarization, leading to rapid ($<500$ fs) bulk electron heating to 2.5–3.5 keV at solid density, producing Maxwellian distributions with minimal nonthermal tails. The absorbed energy scales as $\Delta U\propto a_0^{1.48}\tau^{1.13}$ for normalized amplitude $a_0$ and duration $\tau$ (Sundström et al., 2019).
• In near-critical and underdense plasmas, strong self-focusing (when $P_L\P_c$) collapses the pulse into a micrometer or sub-micrometer volume, depositing a significant fraction (25–35%) of energy within $\lesssim$100 fs, generating MeV electrons, and sustaining multi-megagauss ($\sim$10 MG) magnetic dipoles and relativistic ionization fronts expanding at $v_{\text{exp}}\sim c/3$ (Sylla et al., 2012).
• In structured targets (e.g., converging wedges), subpicosecond pulses are concentrated by geometric focusing and relativistic self-focusing into near-wavelength spots. With optimal geometry ($\theta=17^\circ$), $\sim 10 \times$ intensity amplification is achieved, with $I_{\text{peak}}/I_0\approx 8.8$, sustained for 200–300 fs. The focusing is described by an analytical model wherein the focal peak recedes at velocity $u_f$ given by

$$u_f/c=(1/\sin\theta)\left(m_e/M_i\right)^{1/2}~\text{for}~a\gg1,$$

enabling design scaling to $I_0>10^{20}$ W/cm$^2$ for future high-power systems (Levy et al., 2011).

4. Relativistic and Nonlinear Phenomena

Ultraintense ultrashort pulses access highly nonlinear, relativistic regimes:

• In laser-plasma interaction, the "relativistic electronic spring" (RES) model quantitatively describes how a $p$-polarized, %%%%33$P_{\text{peak}}>2$34%%%% W/cm$^2$ pulse at oblique incidence drives nanometer-thick electron bunches on the plasma surface, converting energy into isolated attosecond bursts with intensities up to $\sim 10^{26}$ W/cm². The duration and amplitude scale as $\tau_{\text{att}}\propto I_0^{-3/4}$ and $E_{\text{att}}\propto I_0^{3/4}$, much more favorably than scaling in the oscillating mirror regime (Gonoskov et al., 2011).
• For ultraintense Laguerre–Gaussian (LG$_{10}$) pulses in underdense plasma, the unique spatial profile creates electron pillars and focusing fields, enabling stable multi-GeV proton acceleration while minimizing witness scattering, e.g., 1 GeV $\rightarrow$ 7 GeV in 35 fs at $2.14\times 10^{22}$ W/cm² (Zhang et al., 2014).
• Sub-cycle waveforms (duration $<2\pi/\omega_P$) can be generated by exploiting plasma wakefields: an intense driver excites a nonlinear spike, which acts as a moving mirror/amplifier for a seed, yielding CEP-tunable, broadband, relativistically strong, isolated sub-cycle pulses with efficiency $\sim$1% and durations of a few femtoseconds (Siminos et al., 2019).

5. Quantum and Ultrafast Light–Matter Effects

Ultraintense ultrashort pulses enable and require fully quantum treatments of light–matter processes:

• Nonlinear Compton scattering with sub-cycle pulses necessitates Volkov-state QED calculations accounting for the temporally finite envelope and high $a_0$. The differential photon spectrum displays broadened harmonic structure with intensity-dependent overlap and quantum recoil effects, even when the classical recoil parameter $y_\ell\ll1$. A universal scaling law maps the quantum result to the classical (Thomson) result, but in regions of overlapping harmonics, quantum modifications persist and are quantitatively significant (Seipt et al., 2010).
• The correct description of photon/electron emission timings in ultrashort pulses (e.g., HHG cutoff, attosecond bursts) relies on the principal frequency $\omega_P$ rather than the carrier, due to the breakdown of the slowly-varying envelope approximation for few-cycle fields (Neyra et al., 2021).

6. Applications and Outlook

Ultraintense ultrashort laser pulses underpin:

• High harmonic generation (HHG) to XUV and soft-X-ray frequencies with sub-100-attosecond bandwidth-limited pulse trains for time-resolved imaging, with intensity enhancement disciplined by nonlinearly assisted compression and spectral broading methods (Popmintchev et al., 2023).
• Electron and ion acceleration for compact, high-repetition-rate sources (e.g., gas-jet-driven MeV–GeV beams, quasi-monoenergetic proton and electron beams), essential for secondary gamma, neutron, and positron production (Sylla et al., 2012, Zhang et al., 2014).
• Warm/Hot Dense Matter studies via rapid, uniform solid-density heating, facilitating precision equation-of-state, opacity, and transport measurements free from suprathermal-electron contamination—optimized by using CP at high intensity (Sundström et al., 2019).
• Exploration of extreme-field quantum electrodynamics (QED) phenomena (vacuum birefringence, radiation reaction, nonperturbative pair production) utilizing isolated attosecond spikes exceeding $10^{26}$ W/cm$^2$ (Gonoskov et al., 2011).

Ultraintense ultrashort laser pulses continue to drive advances in both applied and fundamental high-field science, with innovations in source engineering, CEP control, pulse shaping, and target design expanding their capacity for next-generation probing and manipulation of matter on attosecond timescales and at relativistic energies.