Generation and control of attosecond laser pulse

Abstract

Time-resolved measurement enables us to reveal the dynamical evolution of the system. The ultrashort laser and pump-probe method provide a powerful tool for achieving the time resolved measurement. With the development of laser technology, e.g., Q -switching, mode-locking, chirped pulse amplification and the application of the Ti:Sapphire crystal, the pulse width of laser is getting shorter and shorter, and the pulse energy is getting higher and higher. It has brought tremendous changes to the field of optics and photonics. For example, in the 1980s, femtosecond (1 fs =10–15 s) lasers have been used to study the evolution of molecular intermediate states during chemical reactions, which revealed the dynamics of chemical reactions at the molecular level. Nevertheless, the electron inside atoms and molecules moves even faster on a timescale of “attosecond” (1 as =10–18 s). At the dawn of the 21st century, attosecond pulses were produced from the high harmonics generated in the interaction between intense femtosecond laser and novel atoms. In 2001, an isolated attosecond pulse with a duration of 650 as was produced in the experiment via a 7 fs few-cycle driving laser. In the same year, an attosecond pulse train with a duration of 250 as for single pulse was produced by a multicycle driving laser. These results marked that the pulse width of the laser broke through the femtosecond timescale, and opened the door to attosecond science. In the next decades, lots of ultrafast processes inside atoms and molecules, e.g., tunneling ionization, Auger decay of atoms and electron localization and internal charge transfer of molecules, have been invested by pump-probe measurement combining the attosecond pulses and few-cycle near infrared laser pulses. In addition, the applications of attosecond pulses have been gradually extended from gas to solid materials. By using the attosecond pulse, it is possible to manipulate the electron motion in the attosecond time scale, which corresponds to an extremely high frequency rate of petahertz. Such manipulation is considered to be the basis for developing the photoelectric information processing at petahertz. Attosecond science has become one of the most important achievements in the field of ultrafast optics in the past two decades. The crucial issue is the generation and control of attosecond laser pulses. Many schemes have been demonstrated to produce the isolated attosecond pulses. The most popular scheme is few-cycle driving laser, which requires carrier-envelope phase stabilized ultrashort laser pulse. On the other hand, since the generation efficiency of the attosecond pulse depends on the driving laser ellipticity, a polarization gating scheme has also been developed, which was later generalized to double optical gating. Another scheme is the two-color field scheme, which synthesizes the laser pulses with different wavelengths. The advantage of this method is that multi-cycle driving laser pulse can be used to produce isolated attosecond pulse. Recently, this method has been extended to the three-color cases. In addition, ionization gating and lighthouse schemes are also demonstrated in experiment. With these methods, the pulse duration of the attosecond pulse decreases gradually. At present, the shortest pulse width has reached 43 as and high power isolated attosecond pulse with a peak power of 2.6 GW has been produced. But how to improve the attosecond pulse energy and generation of circularly polarized attosecond pulses are still the hot topics currently.