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.

(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?

This is not my field– I’m a CW laser guy– but looking at it from the outside, there seem to be two basic categories of work done in this field:

Pushing the wavelength limit: One major area of research is working to extend the HHG process to higher and higher energies, basically making tabletop coherent x-ray sources (for a somewhat large table, anyway). This is somewhat tricky because the x-rays emitted come out in phase with the original infrared field, and as the x-rays and the IR see different indices of refraction in the ionized gas of the HHG medium, they quickly get out of phase. This means, essentially, that x-rays produced early in the cloud interfere destructively with x-rays from further in, and prevent you from building up a substantial x-ray intensity.

To get around this, there are a bunch of clever tricks you can play to do “phase-matching,” using nanostructured optical fibers and the like. The undisputed champions of this field appear to be Henry Kapteyn and Margaret Murnane at JILA, and they have pushed the photon energy they can produced up to several hundred eV:

(The plot shows the x-ray intensity (red indicates higher intensity) as a function of the photon energy (horizontal) and the pressure of the gas in the HHG medium (vertical). Figure from this PRL, which again is available free from the KM group.)

Once you can make coherent x-ray beams, you can look at using them to study all kinds of things. Kapteyn and Murnane talk about things like imaging of single cells, and studying nanostructures with less overhead than needed to do SEM imaging of the same systems. These aren’t the sort of x-ray lasers that will fry spaceships– the peak power is in the microwatt sort of range– but it’s not hard to come up with things you might do with even a weak x-ray laser.

Ultrafast Dynamics: The other big category of stuff people do with these ultrashort pulses is to measure the behavior of atoms and molecules on a femtosecond or attosecond time scale. The idea here is generally a sort of pump-probe arrangement: you do HHG to get attosecond pulses of UV light, then shine both the UV and the original IR pulse onto a sample of something you’d like to study. If you delay one pulse with respect to the other– which can be as simple as shifting a mirror by a few nanometers (light travels 0.3 nm in 1 as)– you can use one wavelength to watch what happens after excitation by the other wavelength. You can monitor the process by looking at absorption of light by the excited target (such as this paper on the motion of a valence electron, which sadly is paywalled), light scattered from the target (as in this paper using a clever interferometric technique, also paywalled), or electrons knocked out by the second pulse (as in this measurement of a delay in photoemission from different electronic states, also paywalled).

Here’s a pretty picture to give you the idea of the sort of things they can do:

This shows an oscillation in the absorbance of ultraviolet light by a sample of krypton gas excited by an infrared pulse. The particular wavelength they’re looking at here is absorbed by singly ionized krypton, and the oscillation happens because the “hole” left behind in the outer shell of electrons is moving around. When the wavefunction is aligned along the axis of the laser polarization, light is absorbed more strongly; when it’s perpendicular to the laser, light is not absorbed as strongly. The absorbance thus lets them reconstruct the motion of the electrons inside the ion on a femtosecond time scale.

There’s a third category of ultrafast laser research at DAMOP, namely optical frequency combs, but that’s much more of a precision measurement application, so I’ll talk about those later. There are also numerous groups working with free electron lasers, which can make coherent VUV or x-ray beams by a different process, and groups working on using ultrafast lasers to accelerate electron beams– the Diocles group at Nebraska is a good example, accelerating electrons to energies of 100 MeV over a few mm– but that stuff feels less like AMO physics to me, so I’m going to skip it for attention conservation purposes.

Anyway, subject to those constraints, that’s my outsider’s take on what’s exciting in the subfield of ultrafast and ultraintense lasers.

Names to Conjure With: If you want to know what’s going on in this subfield, but can only keep track of a few names or groups, or just want to be able to sound vaguely informed, big names to drop are the aforementioned Henry Kapteyn and Margaret Murnane; Phil Bucksbaum, formerly at Michigan and now at Stanford; and the Max Planck Institute attosecond laser group. That only scratches the surface, of course, but those groups have some involvement in most of the hot topics in the field.

As a bonus, the Kapteyn-Murnane group being at NIST means none of their papers are copyrightable, so they’re all freely available via the group’s website. You could do a lot worse than to troll through their publication list for fun things to read.