Petahertz

Petahertz (symbol PHz) is a unit of frequency equal to 10^15 hertz, or one quadrillion cycles per second. The prefix peta denotes 10^15 in the International System of Units.

One cycle period is 1 femtosecond; thus petahertz-scale oscillations correspond to optical frequencies. In practice, many visible- and near-infrared light fields lie in the hundreds of terahertz, with the visible range roughly 4×10^14 to 8×10^14 hertz, i.e., about 0.4 to 0.8 PHz. Therefore, petahertz is the natural scale for describing optical carrier frequencies and ultrafast phenomena.

Generation and measurement: Generating light in the petahertz range and beyond uses ultrafast laser systems and high-order harmonic generation, which can push frequencies into the extreme ultraviolet and soft X-ray regions, corresponding to tens or hundreds of petahertz. Frequency combs provide precise, phase-coherent references for optical frequencies and enable metrology and spectroscopy at these scales; mode-locked lasers operate in the hundreds of terahertz, bridging into the petahertz regime.

Applications and significance: The petahertz scale is central to ultrafast science, including femtosecond and attosecond experiments that probe electron dynamics in atoms, molecules, and solids, as well as high-resolution metrology and optical clock technologies. It serves as a reference point for discussions of optical frequencies and the limits of ultrafast measurement and control.