  The generation of the shortest isolated attosecond pulses requires both broad
spectral bandwidth and control of the spectral phase. Rapid progress has been
made in both aspects, leading to the generation of the world-record-shortest 67
as light pulses in 2012, and broadband attosecond continua covering a wide
range of extreme ultraviolet and soft x-ray wavelengths. Such pulses have been
successfully applied in photoelectron/photoion spectroscopy and the recently
developed attosecond transient absorption spectroscopy to study electron
dynamics in matter. In this Review, we discuss the significant recent
advancement in the generation, characterization, and application of
ultrabroadband isolated attosecond pulses with spectral bandwidth comparable to
the central frequency, which can in principle be compressed to a single optical
cycle.
