Lawrence Berkeley National Laboratory

The terahertz (THz) and sub-THz region of the electromagnetic spectrum bridges the infrared and the microwave. This boundary region is beyond the normal reach of optical and electronic measurement techniques normally associated with these better-known neighbors. Only over the past decade has this THz region become scientifically accessible withbroadband sources of moderate intensity being produced by ultra-fast laser pulses incident on biased semiconductors or non-linear crystals [1,2]. Very recently, a much higher power source of THz radiation was demonstrated: coherent synchrotron radiation (CSR) from short, relativistic electron bunches [3-5]. Coherent synchrotron radiation will open up new territory in the THz frequency range with intensities many orders of magnitude higher than previous sources. The energy range between microwave and the far infrared, 3 33 cm-1 (0.1 1 THz), has proven to be challenging to accesses and is therefore referred to as the THz gap . However, with the new CSR source at BESSY [3,5] we have been able to extend traditional infrared measurements down into this sub-terahertz frequency range. This source is broadband and is made up of longitudinally coherent single-cycle sub-picosecond pulses with a high repetition rate (100 s of MHz). With the combination of high intensity and short pulse duration new opportunities for scientific research and applications are enabled across a diverse array of disciplines from condensed matter physics, to medical, technological, manufacturing, space and defense industries. Imaging, spectroscopy, femtosecond dynamics, and driving novel non-linear processes are all among the potential applications. The high average power of the CSR source allows one to extend experimental conditions to lower frequencies than have been possible with thermal and conventional synchrotron sources (Figure 1). In this paper, we make use of the stable CSR THz source at BESSY for threeinitial scientific demonstration experiments.