Two-color circularly polarized laser fields-- A platform for observing the ultrafast and the ultrasmall?

8:52:00 AM

Amanda Grennell, 5th year PhD candidate in Chemistry


Imagine you are an electron. You aren’t very heavy, so gravity doesn’t really affect you. Instead, your world is all about charge. The electric field surrounding you dictates the motion of charged particles; since you have a negative charge you move away from the direction the field points (yeah, you’re a rebel). Boringly, most of your time is spent hanging out close to a positively charged nucleus. But, if you find yourself in the lab with Chris Mancuso and Dan Hickstein, you might be in for the ride of your life.

Motivated by the possibility of imaging atoms and molecules using their own electrons, these CU Boulder researchers, led by Profs Henry Kapteyn and Margaret Murnane, have figured out how to make electrons do some incredible acrobatics. Chris, a graduate student, Dan, a post-doc, and their colleagues use a combination of two laser pulses to create a unique electric field that can pull an electron far away from its atom, and then shoot it back, much like a boomerang.

Why is this significant? Using electrons to image very small samples is a well-established technique: bombard the sample with a beam of electrons and the momenta of scattered electrons are transformed into an image. But this technique uses electrons from outside the sample, and just can’t capture processes in the sample that happen really fast.

To use a sample’s own electrons to image it, Chris explains, “You need an electron to get accelerated away from the atom, and then come back at a very high velocity. If it doesn’t come back, it can never collide with the atom and record the structural information. And since this process [of an electron leaving and returning to its atom] happens extremely quickly, it allows for the possibility of measuring ultrafast dynamics.”

In order to boomerang an electron back to re-sample the atom in a controlled way, Chris and Dan take advantage of a property of light known as polarization.  

You’ve probably seen the word “polarized” when shopping for sunglasses – but what does it really mean? All forms of light are partly composed of an ever-changing electric field. Most light sources create unpolarized light, which means that the electric field oscillates back and forth but keeps changing direction chaotically. After passing through polarized sunglasses (or special filters in the lab), the electric field will instead oscillate with a very regular pattern. If the electric field points up and down, tracing out a wave as it moves, the light is linearly polarized. But if direction of the field rotates in a circle, tracing out a spiral, the light is circularly polarized (Figure 1). 

Figure 1. Animation of linearly (left) and circularly (center) polarized light. Arrows represent the magnitude and direction of the electric field. Chris & Dan combine two circularly polarized laser pulses to make the “counter-rotating circular” field (right), which resembles a three leaf clover.  

Linearly polarized light has already been used by other groups to boomerang an electron, but Chris points out a big drawback: “With linearly polarized light, you can drive an electron out, but it returns in the same direction. This gives information about the sample in one dimension (1D), but there are two more spatial dimensions to be accessed.”

So, instead of using linearly polarized light, Chris and Dan make use of laser pulses that are circularly polarized. Then, by adding together two laser pulses of different colors, they can make the electric field trace out much more complicated patterns. In their recent paper, they create a pattern that looks like a three leaf clover when viewed end-on (Figure 2). 



Figure 2. Animation of the electric field
of the two laser pulses (blue and red) and 
their combined electric field (pink). A head-on 
view of the combined electric field is shown on 
the bottom. As the blue laser pulse is delayed relative to 
the red laser pulse, the combined electric field and 
photoelectron distribution rotate. 
This clover pattern actually loops the electron back to the atom three times. Now that’s a fancy boomerang.

Remarkably, experiments using their new technique showed that electrons can be pulled off an atom and directed in a two dimensional plane, rather than just along a line. This allows Chris and Dan to collect a 2D image of the electron’s momentum, known as a photoelectron distribution.

Actually, this technique is so fancy that Chris and Dan can produce a 3D image by changing how the two laser pulses overlap in time. This causes the 2D image to rotate (Figure 2), allowing them to extract information from all three dimensions.

Dan says this is like a CAT scan in the hospital – several 2D “slices” are taken to build up the final 3D image. But Chris furthers the analogy. “In a 3D scan, the x-ray camera moves around the patient, but we can’t move our detector. What we can do is just move the object. It would be like if you are sitting in the doctor’s office and they were like ‘move two degrees’ as they kept the camera still.”

This research is the very beginnings of a new, powerful imaging technique, so the possibilities are wide open. Dan is brimming with so many ideas he can barely say them fast enough.

“If you want to design the next generation computer chip, you need to understand how the heat travels on the nanometer scale, or it will melt. If you want to design the next drug, you may need to find the structure of a reaction intermediate that may only exist for a small fraction of a second. Name a technology that’s being developed, and there is a strong probability that this technique can help on a fundamental level.”

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Chris and Dan’s paper, published in Physical Review A, can be found here.

Special thanks to Dan Hickstein for making the awesome animations in this post.

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