How fast is ultrafast, really?

When I assemble the December issue each year, I like to look over the technology advances we have already covered and anticipate what might be coming down the pike, so to speak. Ultrafast laser technology, for example, has been making headlines. In the five or so years since Ti:sapphire lasers burst into the market, these femtosecond lasers have been welcomed by the research community. Synchronized ultrafast systems are useful sources for pump-probe spectroscopy, and various amplifier systems can produce tunable ultrashort pulses with more energy over a wider spectral range than ever before possible. New materials have allowed frequency-doubling and nonlinear-conversion schemes (optical parametric oscillators, for example) to extend the wavelength coverage of ultrafast lasers from the ultraviolet to the infrared.

But how fast is ultrafast, really? Research groups worldwide are pushing the limits of Ti:sapphire lasers to measure the shortest possible pulses. Fifteen years ago, modelocked lasers that produced picosecond pulses (pulse durations of 10^-12 seconds) were state of the art; "simple" nonlinear-optics-based autocorrelators could characterize picosecond pulses. But take a few orders of magnitude from the pulse width and you arrive in the femtosecond world (pulse durations of 10^-15 seconds). New technology was needed to completely characterize these brief pulses, resulting in the development of more-sophisticated correlation techniques, such as frequency-resolved optical gating. The cover photo and the article on p. 55 illustrate one approach to measurement at the femtosecond limit.

Still on the fast track

Recently I read predictions that future ultrashort performance would break the attosecond barrier (pulse durations of 10^-18 seconds). These shorter pulse durations will likely be possible only at x-ray wavelengths. As with femtosecond pulses, attosecond technology will require new tools to characterize it and to solve new problems. Laser Focus World will be watching and ready to describe those tools.

Scale back a few more orders of magnitude, and there will be zemptoseconds (pulse durations of 10^-21 seconds). No one has predicted pulses this short yet, but if they can be produced, in future years Laser Focus World will be sure to explain the ultrafast world according to zemptoseconds!