[Paper Review] Quantum regime of laser-matter interactions at extreme intensities

A. M. Fedotov|arXiv (Cornell University)|Dec 6, 2016

Laser Design and Applications2 references3 citations

TL;DR

This paper investigates the quantum regime of laser-matter interactions at extreme intensities, focusing on non-perturbative quantum electrodynamics (QED) effects. It derives scaling laws for pair production, explores self-sustained electron-positron-photon cascades at a₀ ≳ 10², and identifies a breakdown of the Intense Field QED (IFQED) approach when αχ²/³ ≳ 1, signaling a transition to truly non-perturbative dynamics.

ABSTRACT

A survey of physical parameters and of a ladder of various regimes of laser-matter interactions at extreme intensities is given. Special emphases is made on three selected topics: (i) qualitative derivation of the scalings for probability rates of the basic processes; (ii) self-sustained cascades (which may dominate at the intensity levels attainable with next generation laser facilities); and (iii) possibility of breaking down the Intense Field QED approach for ultrarelativistic electrons and high-energy photons at certain intensity level.

Motivation & Objective

• To analyze the transition from classical to quantum regimes in laser-matter interactions at intensities approaching 10²⁴–10²⁶ W/cm².
• To derive scaling laws for key quantum processes such as pair production and radiation emission in strong-field QED.
• To investigate the emergence and sustainability of self-sustained electron-positron-photon cascades at high laser intensities.
• To assess the breakdown of the Intense Field QED (IFQED) approach when quantum corrections become non-perturbative.
• To identify the critical intensity threshold where standard perturbative QED fails due to large effective coupling αχ²/³ ≳ 1.

Proposed method

• Uses the dimensionless laser strength parameter a₀ = eE/mωc to characterize the non-perturbative regime, with a₀ ≳ 1 indicating relativistic electron motion.
• Applies the QED effective action and proper-time method to sum all-order corrections in the external field, particularly for the electron self-energy and polarization operators.
• Employs the optical theorem to relate higher-order vacuum polarization and radiation corrections to observable cascade processes.
• Derives scaling laws via dimensional and uncertainty-principle arguments, estimating characteristic lengths and timescales such as l∥,P ≃ lCκ⁻²/³ and tC ≃ ℏ/mc.
• Analyzes the expansion parameter of IFQED perturbation theory as αχ²/³, derived from comparing 6th-order to 4th-order contributions in the mass operator.
• Considers the role of Bose enhancement in effective coupling, showing that √Nγ ≈ a₀, leading to an effective coupling α → a₀α in the presence of many background photons.

Experimental results

Research questions

• RQ1 What are the scaling laws for the probability rates of key quantum processes such as pair production and radiation emission in extreme laser fields?
• RQ2 Under what conditions can self-sustained electron-positron-photon cascades dominate laser-matter interactions at intensities accessible to next-generation lasers?
• RQ3 At what intensity level does the Intense Field QED (IFQED) approach break down due to non-perturbative quantum corrections?
• RQ4 How does the effective coupling in QED evolve in strong laser fields, and when does it exceed the perturbative regime?
• RQ5 Can the breakdown of IFQED be linked to the onset of macroscopic cascade multiplicity and field depletion?

Key findings

• The scaling of pair production rates follows ∝ α²χ²/³ for high-intensity laser fields, indicating a strong dependence on the quantum parameter χ.
• Self-sustained cascades become macroscopically significant when the pair density exceeds the relativistic critical plasma density, leading to laser field depletion.
• The Intense Field QED (IFQED) approach breaks down when αχ²/³ ≳ 1, which corresponds to a critical field strength where higher-order corrections become non-perturbative.
• The effective expansion parameter of IFQED perturbation theory is identified as αχ²/³, derived from the ratio of 6th- to 4th-order contributions in the electron self-energy.
• For χ ≳ α⁻³/² ≈ 1.6 × 10³, radiation corrections to the electron mass (M⁽²⁾ ≈ αmχ²/³) become comparable to the bare mass, invalidating perturbation theory.
• The characteristic timescale for radiation processes, tγ ≈ tC ≈ ℏ/mc, implies that radiation-free motion is unphysical at the Compton scale, undermining the foundation of IFQED.