femtoJoules

Femttojoule (symbol fJ) is a unit of energy in the International System of Units (SI). It represents 10^-15 joules. The name combines the prefix femto- (10^-15) with the joule. In SI terms, 1 joule equals 10^15 femtojoules.

Femttojoules are used to describe very small energy quantities encountered in physics and engineering, such as the energy contained in tiny light pulses, nanoscale electronic signals, or energy budgets of ultra-low-power devices. In optics, the energy of a single visible-light photon is around a few 10^-19 J, so a femtojoule corresponds to on the order of thousands of photons, depending on wavelength.

Typical contexts include ultrafast laser research, single-photon detection, and measurements of nanoscale energy transfer. The fJ scale is common when discussing pulse energies in femtosecond laser systems and the sensitivity of photodetectors.

Note that femtojoule is an energy unit and is distinct from femtosecond, a time unit; they share the same prefix but apply to different physical quantities.