Have you heard of an x-ray laser before? Did you know that there is a huge x-ray laser in the heart of Silicon Valley in California that can help us discover a myriad of information about the Universe we live in. This x-ray laser lives in the heart of the SLAC National Accelerator Laboratory which is also home to the world’s largest linear particle accelerator.
Recently, researchers at SLAC have invented a way to produce bursts of x-rays that can see the movement of electrons on timescales of the order of billionths of a billionth of a second (attoseconds). This process is called X-ray laser-enhanced attosecond pulse generation or XLEAP for short. With this process, chemical and crucial biological reactions can be probed on incredibly short timescales, allowing scientists to find out even more about them.
So, how do they do this? Let me start with the x-rays. SLAC has its own x-ray laser known as the Linac Coherent Light Source (LCLS) which spits out x-ray pulses that last a few millionths of a billionth of a second (femtoseconds). In an x-ray laser like LCLS, the x-rays are produced by first accelerating the electrons to speeds close to the speed of light and then wiggling them with magnets in an object called an undulator. This is where some of the electrons’ energy is converted into the production of x-rays. The team had to find a way of converting these femtosecond x-ray pulses into attosecond pulses. This is where the magic happens.
They did this by adding two sets of magnets in front of the undulator. If you go back to your high school physics days, you might remember that magnetic fields influence the direction of moving charges. In this case, the electron packets that are being wiggled in the undulator are further influenced by these two magnets, which allows the scientists to manipulate the electron bunches into a desired shape. This allows them to, in turn, control the x-rays bursts.
But how did they know that these x-ray pulses had attosecond pulse durations? The scientists used a special device that is similar to how LCLS scientists characterise their x-ray pulses. The method entails shooting the x-rays through gas, which ionizes the gas (liberating electrons) and creating an electron cloud. An infrared laser is then shone at the electron cloud, which gives the electrons a little boost. Some of the electrons are boosted more than others because the infrared laser is circularly polarised. Polarisation describes the direction of the electric field of the light, which means that some electrons move in different ways according to the direction of the electric field of the laser light. Circularly polarised light is light that has an electric field that essentially varies in such a way that it traces a circle. By measuring the speed and direction of the electrons after they are influenced by the laser, the researchers can work out the x-ray pulse length.
But the XLEAP researchers don’t want to stop here, they want to make their x-rays even brighter and possibly even shorter. This could be possible after LCLS gets upgraded to LCLS-II which is happening next year and will mean that the x-ray pulses will be spat out 8000 times faster. Experiments that you can look out for from the team will involve looking at processes in details unobtainable before. These include looking at individual molecules moving on incredibly fast timescales in vital processes like photosynthesis. So, watch this space. You can find out more information about this work published in Nature Photonics today.