Attoscience for non-specialists
Attosecond Science: A new area of research that’s moving FAST!
Designer chemistry, faster computers, better drug development, medical imaging, and movies of nature's fastest processes are just a few of exciting applications of attosecond science, a new field set to transform science and technology.
In the Imperial College Laser Consortium, we are at the forefront of this research. We use attosecond lasers to investigate and get to know how electrons and nuclei work, act and move within atoms and molecules - the very building blocks of everything you see around you.
This research will lead to a better understanding of how chemical reactions work and even the power to control them.
Back to basics:
In school we learn about atoms and how they are made up of electrons and nuclei forming elements like hydrogen, oxygen and nitrogen. We learn about how they make bonds to form molecules and the materials we see around us every day. Often we were taught by drawing diagrams or making models but….. have you ever considered how accurate those diagrams really are? Do they really look like that and how do those bonds really work? The truth is that little is known about how electrons and nuclei ‘move’ around in these systems. The reason we know so little is because they are on scales so small and happen over time durations so short they prove difficult to catch in action.
However, catching them in action is what we do! Processes like molecular motion and the time it takes for an electron to transfer between atoms happens over timescales of a few hundred attoseconds. These are the time scales the Imperial College Laser Consortium works on every day.
To see what is happening on these time scales you need something that is as fast, or faster. It’s like trying to produce an ever faster shutter on a camera. We, however, do it by producing ever shorter pulses of light, which are then used to look at what dynamics are going on.
What can we catch in action?
There are lots of fast things going on that now we have the chance to “film”:when a solar cell first absorbs sunlight, an electron is excited. How is essentially still a mystery - it happens too fast to measure without attosecond pulses. What influences the efficiency, and can we increase it? Maybe attoscience has the answers. Even more interesting is photosynthesis in plants. There's evidence that quantum processes are important in the world's plants, and with attosecond pulses we could unravel them, and maybe even copy them for our own solar cells. There's also the dream of designer chemistry, using light to control chemical reactions. We might be able to 'grab' and 'guide' electrons around a molecule with light, passing them from one pulse to another until we guide them to where we want them.
How do we do it?
In our group we already have some of the best, most uptodate lasers in the world, which produce femtosecond pulses of light (10-15 of a second). Femtoseconds pulses are already on time scales short enough to find out more about nuclear and molecular dynamics. This is a big part of our research here in the department. Also, these femtosecond pulses of light are being used to produce even shorter pulses of light that are of attosecond duration, another big area of development in our group.
Both the information we gain about molecules and the generation of attosecond pulses come as a result of a process called High Harmonic Generation (HHG). HHG is a process where laser light is sent into either a group of atoms or molecules and causes the atoms or molecules to emit light.
In the case of HHG where molecules are used, features of the emitted HHG light can be studied in order to determine the nuclear dynamics going on in the molecule and its electron structure. For example, the frequency spectrum of the HHG light produced can tell you whether the molecule changed while the HHG light was being emitted. Specifically looking at the HHG frequency spectrum to improve molecular understanding is also known as PACER: Probing Attosecond Dynamics by Chirp-Encoded Recollisions.
When HHG is carried out with atoms and the femtosecond pulses are so short, thatthere are only a few cycles of the laser field,a single attosecond pulse is formed by the process! The shortest pulses we have produced here are of a 260 attosecond duration. We have pulse measurement systems, one being Attosecond Streaking to confirm how short these pulses are. Once the attosecond pulses have been produced, they are usedto probe atoms/moleculeslooking at how the electrons are moving and behaving. A laser is used to excite the electrons in the atoms/molecules and then the attosecond pulses (produced by the HHG process) are sent in to check what’s happening. These types of experiment are known as Pump Probe Experiments.
So what is High Harmonic Generation (HHG)?
HHG can be broken down into a three step process that can be thought of as an electron’s attempt at “The Great Escape.”
Step one: an opportunity arises and an escape is made…
A pulse of light is focused onto molecules or atoms. The molecule/atom have bound electrons. They’re trapped in potential well, which like a prison wall keeps them from escaping. The forces caused by the laser pulse effectively bends the electron’s prison walls. This gives the electron an opportunity break free. It tunnel ionises out of the potential well and becomes “delocalised” - FREEDOM.
Step two: an electron on the run...
The electron is swept up by the force of the laser pulse. First it is accelerated away from the molecule/atom and then driven back towards it. During this time the electron gains kinetic energy.
Step three: the recapture...
When the electron gets propelled back towards its parent molecule/atom, it can be recaptured. It recombines, releasing its gained kinetic energy. The electron’s brief taste of freedom is over. But all is not lost, as the energy released is in the form of light. The very light we use to understand these atoms and molecule and the light we use to make attosecond pulses.
Interested in finding out more?
This is just a brief overview of what we do at Imperial College Laser Consortium - we hope you found it interesting. Why not find out a bit more? Have a detailed read of all the areas of development and pioneering projects we are heading and are involved in: Attosecond Electron Dynamics in Molecular and Condensed Phase Systems.