ATTOSECOND LIGHT SOURCES. Villeneuve, D. & Villeneuve, D.
ATTOSECOND LIGHT SOURCES [pdf]Paper  abstract   bibtex   
ince the invention of photography in the 19th cen-tury, scientists have been trying to freeze the motion in fast-moving events. Motion pictures of galloping horses were an early application of high speed photography, see Fig. 1. Shutter speeds of cameras are limited to about 1 millisecond, and scientists then SUMMARY The shortest-duration laser pulses now have durations below 100 attoseconds. This is less than the time it takes an electron to go around a hydrogen atom. This article describes how these new sources work. turned to brief light flashes to illuminate the subject for microseconds. The invention of the laser in the 1960s then reduced the duration of the light flashes to nanoseconds. F i g u r e 2 shows the progress that has been made in reducing the pulse dura-tion of laser sources over the past 40 years. For the first 20 years, the shortest pulse d u r a t i o n d e c r e a s e d exponential-ly, much like Moore's law in the semi-c o n d u c t o r industry. Then a floor of about 5 femtoseconds was reached. The reason for this floor is illustrated in Fig. 3. A pulse of light from a laser source is composed of two components – the carrier frequency corresponding to the optical wavelength of the laser, and the envelope that defines the pulse duration. Figure 3(a) shows a 50 fsec pulse containing many optical cycles, and Fig. 3(b) shows the corresponding Fourier transform. The spectrum is centered at the optical frequency of 375 THz correspon-ding to the 800 nm laser source. The spectral width and the pulse duration go hand-in-hand. A pulse of a certain duration requires at least the spectral bandwidth of its Fourier transform. The converse is not true – a broad spectrum does not guarantee that you have a short pulse. The solar spectrum is very broad, yet it is not composed of femtosecond pulses because the dif-ferent frequencies are not coherent. To work as a laser, the spectral bandwidth must be supported by the gain band-width of the laser source. This is the reason that titanium-doped sapphire (Ti:Sapphire) is the laser medium of choice for femtosecond lasers. It has a gain spectrum that covers the range of 700-1000 nm. This is short enough to support a 5 fsec pulse, as shown in Fig. 3(c,d).

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