Materials respond to light in different ways based on their electronic structure; this is true even in the extreme state of matter: plasma. Plasmas transmit or block light waves based on the number of electrons available to respond collectively to the light and the light’s colour/oscillation frequency. This is the reason why ionosphere that surrounds the Earth transmits visible light but reflects radio waves.

This simple description of light-matter interaction changes significantly when the light becomes very intense. Plasma electrons are accelerated to near-light speeds, making them “heavier” due to Einstein’s mass-energy equivalence principle. These heavier electrons cannot respond to light fast enough, thereby making the plasma instantly transparent.

A team of researchers from the University of Strathclyde, led by Prof. Paul McKenna and working in collaboration with researchers from the CLF and Queens University Belfast, have used this relativistic induced transparency effect to create an instantaneous “hair-sized pin-hole” in the otherwise opaque plasma, through which part of the laser can pass through. This creates patterns in the transmission by diffraction, the phenomenon that makes edges of your shadows blurred.

By changing this diffraction pattern through variation of the laser parameters, the researchers were able to control the shape of the accelerated electron beam. By changing the polarisation of the laser, for example, the electron beam was also made to twist, creating a spiral shape beam of electrons travelling close to the speed of light. Thus diffraction of ultra-intense laser light passing through a thin foil is used to control charged particle motion, which can be used in the development of laser-driven particle accelerators - with far-reaching impact not just in science but also in medicine, industry and security. Helical magnetic fields induced using this concept could potentially be used in laboratory investigations of similar field structures in astrophysical jets.

This work, published in the high impact journal Nature Physics, was performed using the Gemini laser at the CLF and the simulations were carried out using the ARCHIE-WeSt (University of Strathclyde) and ARCHER (Edinburgh) high performance computers. The research is supported by EPSRC funding.