Grey_Area

DHeitz31

phoenix

aarchiba

Cantonkid

If electrons behaved like ping-pong balls, you wouldn't see the "rings", all you'd see would be a spot moving up and down. Those rings are weird quantum behaviour.

aarchiba

lucky_you

FrankenPC

92BuickLeSabre

Start with an atom of argon. This is a positive nucleus with eighteen negative electrons zipping around it. Now zap it with an ultrashort pulse of ultraviolet light. This pulse knocks an electron out of its orbit. Now fire an infrared laser in.

Light is an "electromagnetic wave". What this means is, to make a beam of light, you set up an electric field, let's say pointing vertically. Now try to turn off this electric field. Nature "opposes" this change by creating a (horizontal) magnetic field. But if you stop fooling with the electric field, this magnetic field soon collapses. Nature opposes this collapse too, creating another vertical electric field - but a little further along. When this collapses, you get another horizontal magnetic field, which collapses, giving you another vertical electric field, which... you get the picture. This is an electromagnetic wave. It "ripples" forward at the speed of light. The colour depends on how fast you collapse the original electric field.

The point of this is if you have a powerful infrared laser going past an electron, there's a strong, oscillating, electric field. An electric field pushes charged particles (like the electron). When you bring your finger near a doorknob after shuffling on the carpet, there's a strong electric field which drives electrons to leap across the gap, forming a spark. So, in this case, the electric field gives the electron a shove.

If you look at a single frame of the video, what they've done is repeated this process many, many times. Each time, they measured the speed and direction of the outgoing electron. The image is a plot: they've coloured each pixel based on how many electrons came flying out with that speed and direction. Dark blue means "not many"; yellow and red mean "many".

In an ideal world, you might expect all the electrons to come out the same way, so you'd see a single bright spot. In reality, the world is more complicated than that. The infrared light that is doing the pushing is not just an electromagnetic wave, it's also made up of discrete particles called "photons". (If you wonder how it can possibly be both a wave and a particle, you're in good company; quantum mechanics is *weird*.) So you get a pattern of possible electron velocities in each frame. The pattern has a dark spot in the center because if the electron doesn't get a push at all it sticks itself back to the argon atom and we don't see it. The pattern is mostly vertical - not many electrons come out to the sides - because the electric field is pointed up or down, not to the sides. The pattern has rings because of the discreteness of the photons in the infrared light: if it gets hit by one, the electron winds up in the first ring, if it gets hit by two the electron winds up in the second ring, and so on.

Now, what's the *movie* about? Well, I said the infrared laser produced an *oscillating* electric field, remember. If you kick an electron off when it's pointing up, you get a different pattern than if you kick an electron off when it's pointing down. Each frame in the movie is picking a different time to kick the electron off.

In fact, that's the hard part of this experiment: making sure you always kick the electron at the same part of the IR laser's cycle. They do this in a very clever way by generating the pulses using the IR laser itself, using "higher harmonic generation".

"Higher harmonic generation" operates by putting (a split-off part of) the IR laser beam through a nonlinear medium. Like your speakers when you turn the volume too high, this distorts the signal. The distortion takes a very specific form: if you start with a nice oscillating signal at some frequency f, once you've mangled it, you have a signal at frequency f, but also a signal at frequency 2f, 3f, 4f, ..., which are "harmonics". The relative proportions and phases of these higher frequencies depends on the details of the nonlinear process. What these researchers did was find a nonlinear process that gave them a whole bunch of harmonics with roughly the same amplitude and phase. It turns out that if you screen out all the others, and just add up a whole bunch of harmonics with the same amplitude and phase, what you get is a string of very narrow pulses. (Explaining why this is requires Fourier analysis.) What's important here is that not only does this produce a string of nice narrow pulses, it produces them all in phase with the original laser - when the electric field is pointing up, there's a pulse (say). You can now point this split-off string of pulses and the original laser at a little vial of argon gas, and get the experiment these folks did. If you want to adjust the delay between the laser and the pulses, you just move a mirror a bit so that the pulses have to travel a little further before getting to the argon.

aarchiba

MaxTwice

Simon Piper

"See that dark spot in the middle, that is...."
"See each of those blue rings? Each of those is the approximate place the electron could be as it rides the light wave (that's the closest I've come to understanding it so far.)"

I'm fascinated by science, and I'm a perfectly bright guy, but it's been so long since I've had to think or study it, that even descriptions still throw me off a little.

Anyone up for it?

92BuickLeSabre

kurwinsky

aarchiba

corpore-metal

Yatta!

Suppose that you had an argon atom and you zapped it with a short pulse of ultraviolet light. The electron would (might, but let's assume it does) be knocked off the atom ("ionization").

Now let's suppose you bathed the argon atom in a steady beam of infrared light. The electron is charged, and the infrared light includes an oscillating electric field, so the infrared light would give the electron a push. The electron, being very light, would then go flying off; you could then collect it and measure its velocity and direction. You'd get a variety of velocities, since the electric field is oscillating all over the place. In particular, the infrared beam is polarized, so the electric field oscillates between up and down. If they kick the electron off the atom when the electric field is just starting to go up, the electron will be pushed up and take off before the infrared beam goes back down to zero. If they kick the electron when the electric field is starting to go down, the electron will go down. Under normal circumstances you get some electrons going up and some going down, more or less at random.

What these guys did was make sure that when the electron got kicked off, the electric field of the infrared beam was always at *exactly* the same phase. A strobe light that fires exactly when your spark plug does, makes your crankshaft look like it's not turning at all, since every time the light fires, your crankshaft is in just the same place. Here it's almost the same: by carefully synchronizing the ultrashort pulses and the infrared field, they made sure to always release the electron when the field was going up (say). So the electrons always went up.

But, you say, what are these weird pictures with all the fringes? Well, welcome to the quantum world. Even if you do everything exactly the same way, there are a range of possible ways - speeds and directions - the electron can come off. You plot them and you get a weird pattern of rings. Go figure.

What's new here is that you can change that pattern by changing the IR phase at which you release the electrons. You release at one phase, and more electrons go down. You release at a different phase, and more electrons go up. The fact that the pattern changes is proof that you're always hitting the atom at the same phase of the IR laser. So that's what this video is.

Each phase of the video is a particular phase between the IR laser and the short pulses. You can see than in some frames the image shifts down; in others it shifts up.

Why is this interesting? Well, suppose you could bash an electron off an atom, sweep it away a short distance, then bounce it off the now-charged atom. The way it bounces off would tell you something about the shape of the ion. You can actually do this with a powerful enough IR laser: As the field oscillates, the electron is pulled away, then back and past the ion. But to make any sense of the results, you need to know exactly how fast the electron was going when it bounced off the atom. With this technique, you can do that: you rip it off at a known phase of the IR laser, so you know just how fast it was going when you get it back to the ion.

These guys aren't quite ready to do that yet; they need to upgrade their IR laser. And understanding the results will be tricky. But the stroboscopic technique *works*.

aarchiba

Good for the guys who did it. Yay! But for Cap'n Layperson, ummmmmm.

Capn_Marrrrk

Munkles

braak

xeijix

Simpsons-Movie-ruled

I, too, have no idea what I'm seeing. Could be an old C-64 graphic demo for all I would know.

TommySez

Amiash

But I'd like to.

92BuickLeSabre