femtoseconds

Femtoseconds (fs) are a unit of time equal to 10^-15 seconds, or one quadrillionth of a second. The prefix femto denotes 10^-15, and thus 1 fs is 0.001 picoseconds (ps). This ultrafast timescale is used to describe processes in physics, chemistry, and biology, where events such as molecular vibrations and electron dynamics unfold on femtosecond timescales. In this regime, light travels about 0.3 micrometers in vacuum during a single femtosecond, providing intuition for the rapidity of these events.

Femtosecond pulses are generated primarily by mode-locked lasers; titanium-sapphire (Ti:sapphire) systems are the dominant source in research. To reach the shortest durations, pulses are often amplified using chirped pulse amplification (CPA). Contemporary systems produce pulses from a few femtoseconds up to hundreds of femtoseconds. Pulse duration and phase are characterized by techniques such as autocorrelation, frequency-resolved optical gating (FROG), or SPIDER (spectral phase interferometry for direct electric-field reconstruction).

Applications include time-resolved spectroscopy and pump-probe experiments that watch chemical reactions, energy transfer, and phase transitions as they occur. Femtosecond lasers enable precise materials processing, medical imaging, and nonlinear optical studies. The study of femtosecond dynamics informs understanding of fundamental processes in molecules and solids and complements longer timescale research in picoseconds and beyond. The field continues to push toward shorter durations and higher peak powers while maintaining pulse quality and stability.