What’s So Interesting About Extreme Lasers?

The second in the DAMOP research categories I talked about is “Extreme Lasers,” a name I was somewhat hesitant to use, as every time I see “Extreme [noun],” I get a flash of Stephen Colbert doing air guitar. It is, however, the appropriate term, because these laser systems push the limits of what’s possible both in terms of the pulse duration (attosecond pulses are common, with 1as = 0.000000000000000001 s) and the pulse intensity (1014 W/cm2 is a typical order-of-magnitude, and some systems get much higher than that).

One of the main tricks for generating these ultra-short pulses is to do high-harmonic-generation (HHG) by blasting a femtosecond duration, very intense infrared laser pulse into a sample of gas (typically noble gases: He, Ne, Ar, Kr, Xe). These pulses are sufficiently intense that they can be thought of as basically a huge classical electric field– the number of photons in the pulse is large enough that it’s not worth trying to keep track. When the field get big, it strips electrons off of the gas atoms, and accelerates them away. A short time later, though, the field reverses direction, and accelerates the electrons back toward the atoms they came from. When they get back, they have acquired a great deal of kinetic energy, which is carried off in the form of a high-energy photon when the electron recombines with the ionized atom.

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(Figure from this Nature Photonics article, available for free from the Kapteyn-Murnane Group page at JILA.)

When you work out all the details of the process, you find that you get pulses of high-energy photons (ultraviolet and even x-rays) that last a few attoseconds. What’s more, the pulses are generated at multiples of the original laser frequency (hence the “harmonic” in HHG), and come out in coherent beams along the original direction of the exciting laser.

So, what’s exciting to do with this sort of system?

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