Atoms in Strong Laser Fields
When a laser field becomes comparable in strength to the fields binding electrons in an atom, perturbation theory breaks down and non-perturbative processes such as above-threshold ionization and high-harmonic generation appear.
Definition
Atoms in strong laser fields is the study of atomic ionization and emission when the oscillating electric field of the laser is intense enough that the atom's response is non-perturbative, so that the field substantially distorts or suppresses the binding Coulomb potential within an optical cycle.
Scope
This topic covers the behaviour of atoms in intense laser fields: the transition from multiphoton to tunnelling ionization characterized by the Keldysh parameter, above-threshold ionization in which an electron absorbs more photons than needed to ionize, the three-step recollision model, and high-harmonic generation that produces coherent extreme-ultraviolet and attosecond pulses. It treats the regime where the laser field rivals the internal Coulomb field.
Core questions
- When does the laser–atom interaction stop being describable by perturbation theory?
- What distinguishes multiphoton ionization from tunnelling ionization?
- How does an electron returning to its parent ion generate high harmonics?
- How do strong-field processes produce attosecond pulses of light?
Key concepts
- Keldysh parameter
- Multiphoton ionization
- Tunnelling ionization
- Above-threshold ionization
- Three-step recollision model
- High-harmonic and attosecond pulse generation
Key theories
- Keldysh theory of strong-field ionization
- Keldysh introduced a parameter comparing the laser frequency to a tunnelling rate, separating the multiphoton regime, where ionization proceeds by absorbing many photons, from the tunnelling regime, where the field bends the potential barrier enough for the electron to tunnel out.
- Three-step recollision model
- Corkum's model describes strong-field emission as tunnelling ionization, acceleration of the freed electron in the laser field, and recollision with the parent ion, which can recombine to emit a high-energy photon and so generate high harmonics.
Clinical relevance
Strong-field processes are the foundation of attosecond science: high-harmonic generation provides coherent extreme-ultraviolet and attosecond light sources used to film electron motion in matter, and strong-field ionization underlies laser filamentation and intense-laser machining and diagnostics.
History
Keldysh's 1965 theory framed strong-field ionization before intense lasers existed to test it. Multiphoton and above-threshold ionization were observed as lasers grew more powerful through the 1970s and 1980s; high-harmonic generation, explained by Corkum's 1993 recollision model, then opened attosecond science, recognized by the 2023 Nobel Prize in Physics.
Key figures
- Leonid Keldysh
- Paul Corkum
- Anne L'Huillier
- Ferenc Krausz
Related topics
Seminal works
- keldysh1965
- corkum1993
- krausz2009
Frequently asked questions
- What does the Keldysh parameter tell you?
- The Keldysh parameter compares the time an electron would take to tunnel through the suppressed barrier with the laser's optical period. A value much greater than one indicates the multiphoton regime, while a value much less than one indicates the tunnelling regime.
- How does strong-field physics make attosecond pulses?
- In high-harmonic generation, electrons recollide with their parent ions once per optical half-cycle, emitting bursts of extreme-ultraviolet light. Combining many harmonics produces pulses lasting only attoseconds, short enough to resolve electron dynamics.