Femtosekunder

Femtosekunder, or femtoseconds, are a unit of time equal to 10^-15 seconds. The prefix femto- denotes one quadrillionth of a second. Because of their extreme brevity, femtosecond-scale events reveal ultrafast dynamics in physics, chemistry, and biology.

Femtosecond pulses are produced primarily by mode-locked lasers, notably titanium-doped sapphire (Ti:Sapphire) systems, which can emit pulses from tens to a few hundred femtoseconds. The shortest pulses are obtained through chirped-pulse amplification, which stretches pulses for amplification and then recompresses them to durations near the transform limit.

Characterization methods include interferometric autocorrelation, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric-field reconstruction (SPADE). These techniques reconstruct the electric-field amplitude and phase of the pulse.

Applications span time-resolved spectroscopy and pump-probe experiments that track molecular dynamics, electron motion, and vibrational processes. In materials science, femtosecond lasers enable precision micromachining and microfabrication with reduced thermal damage. In medicine, femtosecond lasers are used in ophthalmology for corneal surgery and in other surgical and imaging contexts.

Challenges include managing dispersion, nonlinear propagation, and optical damage thresholds in optical components. Generating shorter pulses often requires advanced optics, carefully designed gain media, and precise pulse shaping.