Attosecond chirp compensation over broadband high-order harmonics to generate near transform- limited 63 as pulses

By generating broadband high-harmonic pulses from neon and compensating for attosecond chirp by the material dispersion of argon, the generation of near transform-limited 63 as pulses was achieved. The spectral phase analysis showed that, without proper compensation, the attosecond chirp of the broadband harmonics caused splitting of attosecond high-harmonic pulses in addition to pulse broadening. Although it was attained only within a limited spectral range, the attosecond chirp compensation was successful in bringing out pulse compression over broad harmonics, which signifies the effectiveness of the attosecond chirp compensation by material dispersion. High harmonics emitted from atoms driven by an intense femtosecond laser pulse is a new kind of light source with attosecond duration [1]. As the spectral bandwidth of high harmonics can be very broad with an equal spacing between adjacent harmonics, an attosecond pulse train can be generated. The high-harmonic pulses, however, contain inherent chirp originating from the harmonic generation processes [2], causing pulse broadening. In order to obtain transformlimited attosecond pulses, the attosecond chirp (called ‘atto-chirp’) has to be compensated. As the attosecond chirp of harmonics contributed from short trajectory components is positive, its compensation in a material with negative group-delay dispersion (GDD) was proposed [2] and realized using metallic [3, 4] and gaseous media [5]. As negative GDD can be found only in 3 Author to whom any correspondence should be addressed. New Journal of Physics 12 (2010) 063008 1367-2630/10/063008+07$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft