Subfemtosecond

Subfemtosecond refers to time intervals shorter than one femtosecond (10^-15 seconds). The most developed portion of this regime is the attosecond scale, around 10^-18 seconds, which is used to observe and control electron dynamics in atoms, molecules, and solids. Subfemtosecond techniques aim to resolve processes that occur on the natural timescale of electronic motion.

The primary embodiment of subfemtosecond light is attosecond pulses generated in the extreme ultraviolet through high-harmonic generation in noble gases, driven by femtosecond infrared laser pulses. Isolated attosecond pulses can be produced by spectral gating or temporal gating, while attosecond pulse trains arise from the continued emission of high harmonics. In addition to gas-based HHG, advances in laser compression, phase control, and novel optical architectures continue to push pulse shaping and spectral reach. In the X-ray domain, certain free-electron laser schemes are pursued to approach subfemtosecond durations, broadening the possible applications.

Characterization of subfemtosecond pulses relies on attosecond metrology. Techniques such as attosecond streaking and the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) are used to time-resolve photoemission and to determine pulse duration, while methods like FROG-CRAB (frequency-resolved optical gating) aid spectral-temporal reconstruction. Typical attosecond pulses span tens to a few hundred attoseconds (about 10^-17 to 2×10^-16 s).

Applications of subfemtosecond science include tracking electron motion during chemical reactions, measuring time delays in photoionization, and enabling ultrafast investigations of matter at its most fundamental electronic level.

See also: attosecond science, high-harmonic generation, ultrafast spectroscopy.