A unit of time equal to one quadrillionth of a second, used to measure ultrafast processes.A femtosecond is a unit of time equal to one quadrillionth of a second (10^-15 s). To grasp this scale: light travels about 300 nanometers in a femtosecond, roughly the size of a small molecule. This timescale is where the fastest processes in nature unfold—the vibrations of atoms in a crystal, the breaking and forming of chemical bonds, and the initial quantum events in photosynthesis.
The first femtosecond laser pulses were achieved in the 1980s, building on earlier work in picosecond pulse generation. The technique of pump-probe spectroscopy was developed in the late 20th century, enabling the study of chemical reactions in real time.
Femtosecond physics became possible with the development of mode-locked lasers, which produce pulses of light lasting only a few femtoseconds. By splitting such a pulse into a 'pump' (to trigger a process) and a 'probe' (to observe it at a precise delay), scientists can take snapshots of ultrafast events. Varying the delay between pump and probe reconstructs a movie of the process with femtosecond time resolution.
Femtosecond lasers are used in eye surgery (LASIK), precision micromachining, and as the basis for optical frequency combs that measure time with extraordinary accuracy.
In one femtosecond, light travels only about the width of a single water molecule.

