Experimental Light Sources

Powis Group

Synchrotron Radiation

Synchrotron radiation is generated when a stream of bunches of relativistic electrons, travelling at nearly the speed of light, are continuously accelerated around an evacuated circular track, many tens of metres in diameter. A very broad band of usable radiation, extending from VUV into the X-ray region, is emitted and can be collected in further pipes, or beamlines, that are arranged tangentially to the ring. Usually, a specific wavelenth (energy) of the broad synchrotron radiation is selected by a monochromator and the beam of light is further conditioned by optics as it passes up the beamline before reaching an experimental station at the end. A major advantage of synchrotron radiation is the convenient, continuous, and rapid tunability of the photon energy delivered by a beamline.

Planar Undulators

schematic of planar undulator

Modern, 3rd generation, synchrotrons further extend their capabilities by the use of magnetic insertion devices. As it travels around the ring the beam of electrons can be caused to undergo rapid tranverse oscillations by an alternating magnetic field. Light emitted in the forward direction at each tight turn of the electron beam in such an undulator constructively interferes with that produced at adjacent turns, resulting in a much intensified, collimated light beam. This undulator outut has a much narrowed spectrum and is linearly polarized in the plane of the induced electron oscillations.

Elliptical Undulators

elliptical undulator

The latest undulator designs combine two magnetic field components in the vertical and horizontal planes — thus are capable of producing intense radiation with both horizontal and vertical polarization components. By controlling these components' amplitude and relative phase (effected by a translation along the electron beam direction of one of the magnetic arrays) arbitrary linear, elliptical, and circular light polarizations may be produced. Try the light polarization tutorial to see how!

Rare Gas Discharges

Rare gas discharge lamps are a traditional laboratory source of VUV radiation in the 10 – 40 eV range. Compared to other modern VUV sources discharges lack intensity, tunability (radiation is usually emitted by one of a small number of fixed frequency atomic resonance lines that are excited in an electrical discharge) and are un-polarized. Usually they provide genuine cw light with no temporal structure; while pulsed operation is possible the timing characteristics are poor compared to laser or even synchrotron sources. However, these lamps are economical and reliable, and can be fitted with partially polarizing optics.

Apparatus with He Lamp

Which Synchrotron?

We've used
  • Soleil, near Paris, France
  • BESSY, Berlin, Germany
  • SRS, Daresbury (UK)
  • Elettra, Trieste, Italy

Laser UV and VUV Sources

The multiple attractive features of lasers are well known. Unfortunately, laser action cannot be achieved in the VUV/SXR regions using laboratory scale systems (these spectral regions are being addressed by development of Free Electron Lasers that make use of large-scale undulator technology – see above).

The photon energy range of off-the-shelf lasers can, however, be extended by techniques that effectively combine several photons into one in some non-linear optical medium. This technique is offered commercially to extend near UV/Vis lasers into the far UV. In Nottingham we have developed (in collaboration with Prof. K. Reid) a VUV laser source that uses rare gases as the non-linear medium for mixing UV/Vis laser beams into a single VUV wavelength.