A new way of producing coherent light in the ultra-violet spectral region, which points the way to developing brilliant table-top x-ray sources, has been produced in research led at the University of Strathclyde. The scientists have developed a type of ultra-short wavelength coherent light source that does not require laser action to produce coherence. Common electron-beam based light sources, known as fourth-generation light sources, are based on the free-electron laser (FEL), which uses an undulator to convert electron beam energy into X-rays.
However, using a free-electron laser to produce coherent radiation is not the only way; a "pre-bunched" beam or ultra-short electron bunch can also be used to achieve exactly the same coherence in a very short undulator that is less than a metre in length. As long as the electron bunch is shorter than the wavelength of the light produced by the undulator, it will automatically produce coherent light—all the light waves will add up or interfere constructively, which leads to very brilliant light with exactly the same properties of light from a laser.
The researchers have demonstrated theoretically that this can be achieved using a laser-plasma wakefield accelerator, which produces electron bunches that can have a length of a few tens of nanometres. They show that if these ultra-short bunches of high energy electrons pass through a short undulator, they can produce as may photons as a very expensive FEL can produce. Moreover, they have also shown that by producing an electron bunch that has an energy "chirp", they can ballistically compress the bunch to a very short duration inside the undulator, which provides a unique way of going to even shorter electron bunches and therefore produce even shorter wavelength light.
This new way of producing coherent radiation could revolutionise light sources, as it would make them highly compact, essentially table-top size, and capable of producing ultra-short duration pulses of light, much shorter than can be produced easily by any other means.
The group is now planning a proof-of-principle experiment in the ultraviolet spectral range to demonstrate this new way of producing coherent light. If successful, it should dramatically accelerate the development of even shorter wavelength coherent sources based on the same principle.
Photo credit: University of Strathclyde Glasgow