I wrote up the ultra-fast, ultra-intense laser part of this year’s Nobel prize a couple of days ago, but of course, there’s another half I didn’t cover. This was awarded to Arthur Ashkin, for the development of “optical tweezers,” which let you manipulate small objects with laser light, a very different subject that’s prizeworthy in its own right.
This is a prize that many would’ve guessed had been foreclosed by the 1997 Nobel Prize in Physics to Steve Chu, Claude Cohen-Tannoudji, and Bill Phillips (full disclosure: Bill was my Ph.D. thesis advisor). Chu had worked with Ashkin at Bell Labs, and his citation included mention of optical tweezers. The Nobel folks don’t like to repeat themselves, so a lot of people in the field expected Ashkin would never win. It’s nice to be wrong sometimes.
So, what are “optical tweezers,” and how do they work? The key idea, as with the 1997 prize for laser cooling of atoms, is that light carries momentum. In laser cooling, the momentum of a single photon is used to slow the motion of a single atom, which when repeated billions of times a second can slow an atoms from the speed of sound to speeds of a few centimeters per second. In optical tweezers, the momentum of a beam of light containing huge numbers of photons is used to push around a solid object.
The easiest way to understand this is by considering a small transparent object, which being a physicist I will represent as a sphere in the figure above. This will act like a tiny lens, bending the path of a light beam coming in, which leads to a force pulling the sphere into the center of a focused laser beam.
The force is basically a reaction force based on the change in momentum that comes from redirecting the light beam. In the case of the transverse force (perpendicular to the laser direction), shown in the left half of the figure above, the light starts out headed straight down, with its momentum directed the same way, but is bent to the left or right. After passing through the sphere, the light has acquired some leftward or rightward momentum, but the total momentum of the universe as a whole isn’t allowed to change, so the sphere must have gotten a push in the opposite direction. If the light bends right, the sphere is pushed left, and vice versa.
If the sphere is centered in the beam, these forces cancel out, but if the intensity varies, the force will be larger on the side where the light is more intense. This leads to a force that pulls the sphere toward the brightest point available.
A similar process will work along in the longitudinal direction, if the beam is brought to a tight focus, as shown in the right half of the above figure. If the bead is centered in the beam near but slightly below the focus, the bending leads to forces that go up-and-right and up-and-left as drawn in the figure. The and-right and and-left portions cancel each other out, leading to a vertical force. The net effect is to pull the object into the focus of the light beam.
The cartoons above draw the object as a refractive sphere, but the result is much more general than that. We usually demonstrate this with small beads, but it can work with basically anything refractive. I’ve had a series of undergraduate research students work on optical tweezers at Union (a photo of the apparatus is above), and a staple activity when a given student gets the trap up and working is to go out to the creek and get some water with algae in it, and hunt for microscopic critters to trap in the tweezers. It’s great fun.
You can also extend this basic idea to things that aren’t floating in water, and even objects that are opaque. In some of these cases, you need to use two counter-propagating beams to balance out the push that comes from the light that reflects off the object, but the basic idea is similar.
What’s this good for? Well, it’s a system for pushing around microscopic objects using forces from light. Since the force depends on the intensity of the light, this offers an excellent method for measuring the tiny forces between biological objects– people have attached motor proteins to little glass beads, and measured the force exerted as the proteins drag the bead along. The Block lab at Stanford pioneered a lot of these biophysics applications, and they’ve got some cool pictures and explanations.
The force also depends on the scattering of light, which depends on the position of the trapped object relative to the focus, so by measuring the beam after the trap, you can get information about how the trapped object is moving. The Raizen group at Texas has used this to do some really cool work on Brownian motion of little trapped beads, which is a much more physics-y angle on the use of optical tweezers.
Like the chirped-pulse amplification process described in the previous post, this prize is mostly for the development of a tool that’s become widely used. There are dozens of biophysics labs around the world making use of optical tweezers to manipulate organisms, genes, and even single molecules, and all of that research traces back to Ashkin’s original ideas about how to take advantage of the momentum of light.
