Scientists at Oak Ridge National Laboratory (ORNL) investigating how to produce and use terahertz (THz) light to enable particle accelerators developed an electro-optical sampling technique that measures THz wavelengths while at the same time maintaining the correlation between position and time in the THz pulses. The technology could provide a path to reducing the immense size of particle accelerator facilities. The proton accelerator at the Spallation Neutron Source, a Department of Energy user facility located at ORNL, is the length of three football fields. With longer-wavelength THz light, particles could reach the same energy in less than the length of an end zone. Such miniaturization could help particle accelerators achieve higher energy levels to better support scientific discovery. The sampling technique measures the shape of a THz pulse and how its shape changes when it is directed at a target. The researchers used the technique to measure THz light pulses made with intense lasers. Each pulse held concentrated THz energy that is strong enough to create high acceleration fields, and each pulse comprised several different THz frequencies. When the researchers aimed a pulse at a target, the frequencies separated from each other, similar to the way the frequencies in white light can separate into the colors of a rainbow. A pulse of terahertz light is focused (green) on a miniaturized particle accelerator to give energy to particles (blue spheres). Technology developed by a team at ORNL measures how the shape of the terahertz pulse (insets) changes as it is focused on its target. Courtesy of ORNL. If not accounted for, the separation of THz frequencies can lead to imperfections in the shape of the THz pulses. The imperfections, in turn, can cause the light from the pulses to become less concentrated, which could result in weaker particle acceleration and potentially affect particle accelerator performance. To measure imperfections in 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 the shape and imperfections in the pulses. The researchers believe that by developing an optimal optical design, they may be able to improve the shape of THz light bullets enough to use the light bullets to enhance particle acceleration. The researchers analyzed the spatial chirp in the subcycle THz light pulses. They showed that free-space propagation led directly to lateral spatial chirp, with the spectrum on-axis substantially bluer than the overall energy spectrum. The team measured the dimensional profile of the THz subcycle light pulses from organic crystals and compared the measurements with observed spatiospectral correlations consistent with the model. The research was published in Physical Review A (www.doi.org/10.1103/PhysRevA.104.032229).