Particle physics experiments

Terahertz Light Experiments Herald Small Particle Accelerators | Research & Technology | Jul 2022

OAK RIDGE, Tenn., July 25, 2022 – Oak Ridge National Laboratory (ORNL) scientists studying how to produce and use terahertz (THz) light to enable particle accelerators have developed an electro-optical sampling technique which measures THz wavelengths while at the same time maintaining the correlation between position and time in THz pulses. The technology could pave the way for reducing the immense size of particle accelerator facilities.

The Spallation Neutron Source proton accelerator, a Department of Energy user facility at ORNL, is the length of three football fields.

With longer wavelength THz light, the particles could reach the same energy in less than the length of an end zone. Such miniaturization could help particle accelerators reach higher energy levels to better support scientific discoveries.

The sampling technique measures the shape of a THz pulse and how its shape changes when directed at a target. The researchers used the technique to measure THz light pulses made with intense lasers. Each pulse contained concentrated THz energy strong enough to create high acceleration fields, and each pulse comprised several different THz frequencies.

When the researchers directed a pulse at a target, the frequencies separated from each other, much like the frequencies of white light can separate into the colors of a rainbow.

A pulse of terahertz light is focused (green) onto a miniaturized particle accelerator to energize particles (blue spheres). The technology developed by a team at ORNL measures how the shape of the terahertz pulse (inserts) changes when focused on its target. Courtesy of ORNL.

If not taken into account, the separation of THz frequencies can lead to imperfections in the shape of the THz pulses. The imperfections, in turn, can make the light from the pulses less focused, which could result in weaker particle acceleration and potentially affect particle accelerator performance.

To measure the imperfections of the THz light pulses, the researchers combined the sampling technique with modeling tools and used the resulting measurements to develop an optical design to correct for the shape and imperfections of the pulses. The researchers believe that by developing an optimal optical design, they may be able to improve the shape of the THz lightballs enough to use the lightballs to improve particle acceleration.

The researchers analyzed the spatial chirp in the subcycle THz light pulses. They showed that free-space propagation directly drives lateral spatial chirp, with the on-axis spectrum being significantly bluer than the overall energy spectrum. The team measured the dimensional profile of THz subcycle light pulses from organic crystals and compared the measurements with observed spatiospectral correlations consistent with the model.

The research has been published in Physical examination A (