Physicists have just discovered the discontinuous nature of synchrotron radiation due to its coherence. This makes it possible to use coherent synchrotron radiation as one of the most efficient sources to produce a frequency comb in the terahertz range.
The metrology of time and frequency has been revolutionized by the use of radiation whose spectrum is composed of a series of very fine and regularly spaced lines similar to the graduations of a ruler. Initially developed for visible and near infrared radiation, the generation of frequency combs presently covers the mid-infrared to ultraviolet range. The challenge now is to expand this range to the terahertz radiation between infrared light and microwave radiation. Current sources, based either on the conversion of visible or infrared light, or on the use of quantum cascade lasers, produce very low powers or cover very limited spectral ranges. Physicists from the Laboratory of Physical Chemistry of the Atmosphere (University of the Littoral Opal Coast), the AILES infrared beamline on the SOLEIL synchrotron, the Orsay Institute for Molecular Sciences - ISMO (CNRS / University Paris South) and the Institute of Electronics, Microelectronics and Nanotechnology or IEMN (CNRS / Lille University) were able to show that coherent synchrotron emission consisted of a powerful frequency comb with high spectral density. The development of a heterodyne spectrometer with ultra-high resolution led them to discover that this radiation, hitherto regarded as a continuum of frequencies, was actually composed of a very dense discrete spectrum of regularly spaced frequencies. This study has been published in Nature Communications.
For the past fifteen years, researchers have been looking to increase synchrotron radiation power and have developed the "coherent" emission mode by reducing the electron bunch length that emits radiation when circulating round the instrument. This size reduction causes phase coherence between photon emissions with wavelengths comparable to the electron bunch size, creating radiation intensity nearly a hundred thousand times greater than standard radiation. In this case, the intensity is proportional to the square rather than simply to the number of stored electrons (typically one bunch is formed of several million electrons). To study the precise properties of this radiation, the researchers developed a heterodyne receiver that transposed the measured terahertz radiation to the microwave range. This frequency conversion allowed them to benefit from advanced spectral or temporal analysis tools and record the spectra around 0.2, 0.4 and 0.6 THz, with a resolution below 100 Hz. With this very high spectral resolution they had access to the purely discrete nature of coherent synchrotron emission. The frequency comb they discovered covered the 0.1-1 THz range and had a high mode density with spacing between modes of 846 kHz, corresponding to the revolution frequency of the electron bunches in the storage ring. The peak width was a few hundred hertz and the comb had no offset frequency. These remarkable properties of coherent synchrotron radiation offer very interesting prospects in the areas of ultra-high resolution spectroscopic studies and reaction kinetics.