Broadband laser sources based on supercontinuum generation by femtosecond laser filamentation have enabled applications from stand-off sensing and spectroscopy to the generation and self-compression of high-energy few-cycle pulses. Filamentation reli
es on the dynamic balance between self-focusing and plasma defocusing, mediated by the Kerr nonlinearity and multiphoton ionization respectively. The filament properties, including the supercontinuum generation, are therefore highly sensitive to the properties of both the laser source and the propagation medium. Here, we report the anomalous spectral broadening of the supercontinuum for filamentation in molecular gases, which is observed for specific elliptical polarization states of the input laser pulse. The resulting spectrum is accompanied by a modification of the supercontinuum polarization state and a lengthening of the filament plasma column. Numerical simulations confirm that the anomalous behavior originates from the delayed rotational response of the medium, demonstrating a new parameter to control supercontinuum generation.
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 pulse
s 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.
