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The modern era is, as you probably know, a pretty amazing time to be alive and working in science. It's not just the super-exotic stories likedetecting gravitational waves that move matter less than the width of a proton, either, but things that are fairly commonplace. It's now a fairly routine matter to make pictures of materials with a resolution comparable to the size of an atom-- the small college where I teach has a couple of different microscopes that can do it. If you have any sense of history of science, that's absolutely astounding-- only a little over a hundred years ago, there was still some debate as to whether atoms were real physical objects or just a calculational tool, and I recall reading (slightly outdated) science books as a kid that confidently asserted the impossibility of taking pictures of a single atom. And now, it's an undergrad lab.

Of course, these kinds of technologies don't just appear out of nowhere. They're not giant leaps, but an accumulation of tiny steps, putting together lots of other technologies that have built up over time. Last week, I got a press release about a new paper from the lab of Markus Raschke at the University of Colorado that makes a good jumping-off point to talk about this sort of process. It comes with the jargon-heavy title "Plasmonic nanofocused four-wave mixing for femtosecond near-field imaging" but the main result is very cool: they have demonstrated a laser-based "microscope" that makes images with nanometer-scale resolution in space, and a "frame rate" of a few femtoseconds for tracking the motion of electrons in time.

This is an image captured by CU-Boulder researchers using an ultrafast optical microscope shows clouds of electrons oscillating in gold material in space and time. The width of the image is 100 nanometers (about the size of a particle that will fit through a surgical mask), while the time between the top and bottom frame (10 fs, or femtoseconds) is less than 1 trillionth of a second. (Photo credit: University of Colorado)

This is a really cool result, but it doesn't come out of nowhere. Instead, it combines a bunch of different technologies that developed in other (sub)fields-- ultrafast lasers, scanning probe microscopy, near-field imaging, and non-linear optics-- to make something amazing. If you don't follow the field closely, it looks like a giant leap, but it's really a combination of little steps. Which I will now attempt to un-pack a little.

It's always good science to start with a question you'd like to answer, the bigger the better. This is more or less how we define subfields of physics-- by the questions they try to answer-- and the defining question of condensed matter physics is basically "How do the electrons and atoms inside a chunk of material behave?" This is really difficult to answer, because even a chunk of material whose dimensions are sensibly measured in nanometers will contain an astronomical number of particles, far too many to track them individually. And the electrons, at least, re-arrange themselves astonishingly rapidly-- the "Fermi velocities" for electrons in ordinary solids are close to 1% of the speed of light, or about one nanometer per femtosecond. So, if you want to achieve the pie-in-the-sky dream of a particular branch of condensed matter physics, you need to be able to make maps of where things are on a scale of nanometers, and look at how those change on a time scale of femtoseconds.

So, how can you do that? Well, there's a well-developed branch of laser optics built up for the femtosecond part. People have been working for years to make pulsed lasers that flash on for femtoseconds, and even less-- the leading edge of the field is down in the attosecond range. The trick here is to get a material for your laser that can amplify a wide range of frequencies, because a rapid change in time requires the addition of lots of frequencies. When you get down to femtosecond pulse durations, you need frequencies that span a full octave, and these "frequency combs" are now a tool used in lots of precision measurements, from comparing atomic clocks to searching for extrasolar planets.

So, you get a broad-band pulsed laser, do some clever tricks to manipulate the exact mix of frequencies included in the pulse, and you can make a laser flash on for a few femtoseconds at a time. Then you can use this to do a sort of stroboscopic movie: zap something with a bright pulse to set the start time, then send in a second pulse a few femtoseconds later (if you can make these pulses at all, you can split them up and delay one part by a tiny amount) to see how things have changed. Repeat that over and over, and you can make a "movie."