femtosecond

Femtosecond (fs) is a unit of time equal to 10^-15 seconds. It is used to describe events that unfold on ultrafast timescales, many orders of magnitude shorter than typical chemical reaction times. Light travels about 0.3 micrometers in one femtosecond, so a femtosecond pulse may occupy a fraction of a micron in space.

Femtosecond pulses are typically produced by mode-locked lasers and can be stretched and amplified using chirped pulse amplification (CPA) to achieve very high peak powers while keeping average energy manageable. Common femtosecond lasers operate in the near-infrared and emit pulses from a few tens to a few hundred femtoseconds in duration; shorter durations require broad spectral bandwidth due to the time-bandwidth limit. The field strength of such pulses enables nonlinear optical effects and precise control of light–matter interactions.

Applications span fundamental science and technology. In chemistry and physics, femtosecond spectroscopy and pump–probe experiments reveal ultrafast dynamics of electrons and nuclei, a field often referred to as femtochemistry. In biology and materials science, it enables time-resolved measurements of structural changes and energy flow. In industry and medicine, femtosecond lasers are used for micromachining, precise material processing, and certain ophthalmic procedures.

Characterization of femtosecond pulses employs autocorrelation, frequency-resolved optical gating (FROG), spectral phase interferometry (SPIDER), and related techniques to retrieve pulse duration and phase. The term remains central to the broader family of ultrafast science, which continues to push toward ever shorter light pulses, including attoseconds.