The paradox of a free electron laser without a laser

A new way of producing coherent light in the ultraviolet spectral region, which paves the way for the development of bright tabletop x-ray sources, has been produced as part of research conducted at the University of Strathclyde.

Scientists have developed a type of ultra-short wavelength coherent light source that does not require laser action to produce coherence. Common electron beam light sources, called fourth generation light sources, are based on the free electron laser (FEL), which uses an inverter to convert the energy of the electron beam into x-rays.

Coherent light sources are powerful tools that enable research in many fields of medicine, biology, materials science, chemistry and physics.

This new way of producing coherent radiation could revolutionize light sources, as it would make them very compact, essentially the size of a table, and capable of producing light pulses of ultra-short duration, much shorter than those that can be used. easily produced by any other means.

Making ultraviolet and X-ray coherent light sources more widely available would transform the way science is done; a university could have one of the devices in one room, on a table, for a reasonable price.

The group is now planning a proof-of-principle experiment in the ultraviolet spectral domain to demonstrate this new way of producing coherent light. If successful, it should significantly accelerate the development of coherent sources of even shorter wavelengths based on the same principle. The Strathclyde Group has set up a facility to study these types of sources: the Scottish Center for the Application of Plasma-based Accelerators (SCAPA), which houses one of the most powerful lasers in the UK.

The new research was published in Scientific reports, one of the Nature family of journals.

Professor Dino Jaroszynski, of the physics department at Strathclyde, led the research. He says that “this work considerably advances the state of the art of synchrotron sources by proposing a new method of producing coherent radiation of short wavelength, using a short inverter and electron packets of attosecond duration. “.

“This is more compact and less demanding on the quality of the electron beam than free electron lasers and could provide a paradigm shift in light sources, which would stimulate a new direction of research. He proposes to use packet compression, as in modulated pulse amplification lasers. —In the inverter to significantly improve the brightness of the radiation. “

“The new method presented would be of great interest to a diverse community developing and using light sources.”

In FELs, as in all lasers, the intensity of the light is amplified by a feedback mechanism that locks the phases of individual radiators, which in this case are “free” electrons. In the FEL, this is achieved by passing a high energy electron beam through the inverter, which is an array of alternating polarity magnets.

The light emitted by electrons as they wriggle through the inverter creates a force called a ponderomotive force which groups electrons together – some are slowed down, others are accelerated, causing grouping, similar to traffic on a freeway that periodically slows down and accelerates.

Electrons passing through the inverter emit inconsistent light if they are evenly distributed. For every electron that emits light, there is another electron that partially cancels out the light because they radiate out of phase. An analogy of this partial cancellation is rain on the sea: it produces many small ripples that partially cancel each other out, effectively smothering the waves, reducing their amplitude. On the other hand, a constant or pulsating wind will cause an amplification of the waves by the mutual interaction of the wind with the sea.

In FEL, the grouping of electrons causes the light to amplify and increase its coherence, which usually takes a long time – so very long inverters are required. In an X-ray FEL, inverters can be over a hundred meters long. The accelerators that power these X-ray FELs are miles long, making these devices very expensive and some of the largest instruments in the world.

However, using a free electron laser to produce coherent radiation is not the only way; a “pre-bundled” beam or an ultra-short electron bundle can also be used to achieve exactly the same coherence in a very short inverter less than a meter long. As long as the electron bundle is shorter than the wavelength of light produced by the inverter, it will automatically produce coherent light – all light waves will constructively add or interfere with each other, leading to a very bright light with exactly the same light properties of a laser.

The researchers theoretically demonstrated that this could be done using a laser-plasma wake-field accelerator, which produces packets of electrons that can be a few tens of nanometers long. They show that if these ultra-short packets of high energy electrons pass through a short inverter, they can produce as many photons as a very expensive FEL can produce. In addition, they have also shown that by producing a bundle of electrons which has a ‘chirp’ of energy, they can ballistically compress the bundle to a very short time inside the inverter, which offers a unique way to go to even shorter electron packets and thus produce light of even shorter wavelength.