Overview of Undulator Concepts for Attosecond Single-Cycle Light

The production of intense attosecondlight pulses is an active area in accelerator research, motivated by the stringent demands of attosecondscience: (i) short pulse durationfor resolving the fast dynamics of electrons in atomsand molecules; (ii) high photon flux for probing and controlling such dynamics with high precision. While the free-electron laser(FEL) can deliver the highest brilliance amongst laboratory x-ray sources today, the pulse durationis typically 10–100 femtoseconds. A major obstacle to attaining attosecondduration is that the number of optical cycles increases with every undulator period. Hence, an FEL pulse typically contains tens or hundreds of cycles. In recent years, several novel concepts have been proposed to shift this paradigm, providing the basis for single-cycle pulses and paving the way towards high-brilliance attosecondlight sources. This article gives an overview of these concepts.