In the past 20 years laser technology has blossomed to the point where table-top plasma accelerators are now a common sight. In these machines, electrons surf on electric fields 1,000 times stronger than those offered by conventional accelerators. The fields are created inside plasmas travelling close to the speed of light, in the wake of an intense laser pulse. Until now, the features of these laser wakefields were invisible. But a clever camera has revealed details not seen before.

Picturing the plasma behind the laser beam is important because its structure tells us about the quality — the divergence and energy spread — of the electron beam produced by the accelerator. Existing methods used to image wakefields are slow and unwieldy; they only photograph the plasma at individual points in time, therefore many snapshots have to be combined together to get a complete dynamic image. In contrast, the approach adopted by researchers from Texas and Michigan simplifies things by illuminating the entire plasma at once (Nature Phys. 2, 749–753; 2006).The result is precise information about the temporal and spatial behaviour of the wake.

Their technique, frequency-domain holography, is a twist on conventional holography geared towards imaging structures travelling at luminal velocities. An intense 800-nm, 30-fs pump laser is focused into a jet of helium gas, creating a plasma and laser wakefield. Meanwhile two chirped 400-nm, 1-ps pulses — a 'reference' beam in front and a 'probe' beam trailing 3 ps behind — ride along with the pump beam. Because the probe waves overlap with the pump pulse, they become imprinted with phase shifts that reveal information about the plasma disturbance. A two-dimensional holographic image is obtained by detecting interference between the probe and reference beams with a spectrometer and a CCD. The wake structure is recovered by Fourier-transforming this data.

The resulting images can distinguish features smaller than the plasma frequency. They show sharp ionization fronts, followed by a wake containing a He2+ core and an outer He+ 'corona'. A 30-TW laser pulse generates a wake with curved 'horse-shoe' wavefronts, arising from nonlinear laser–plasma interactions. But this new camera does more than just offer us a window into the world of relativistic laser–plasmas; it takes us a step closer to using plasma accelerators to their full potential.