Zenghu Chang

The science of light-matter interactions at extremely short times is an emerging research field at the scientific and technological frontier. Our research is focused mainly on the dynamics of electrons at a time scale similar to the time needed for electrons to change energies in an atom. This time is extremely short -- an attosecond or 10-18 seconds. In 2009, the highlight of the research in our group has been on the attosecond light pulse generation, characterization and applications.

Single isolated attosecond pulses are powerful tools for studying electron dynamics in atoms and molecules. Such pulses, as short as 80 as, have been generated. The duration of the driving laser pulses is 3.5 fs, corresponding to one and half optical cycles. Also, the carrier-envelope (CE) phase of the laser pulses must be stabilized. It is a technical challenge to reproduce daily such pulses. Since the bandwidth of such pulses is so broad, special optics are required to transport them and reliable measurement of the pulse duration is very difficult. Since the first demonstration of generating such attosecond pulses in 2001, still very few laboratories have access to such unique light sources. It is thus highly desirable to find a technique that can generate single attosecond pulses using longer driving pulses.

We demonstrated a technique called double optical gating for generating isolated attosecond pulses with lasers pulses as long as 28 fs that was directly from a chirped pulse amplifier. This new gating scheme, with a relaxed requirement on laser pulse duration, makes attosecond experiments more accessible to many laboratories that are capable of producing such multi-cycle laser pulses. These XUV pulses, generated from noble gas, are characterized by reconstructing the spectrogram of the photoelectrons produced by the XUV pulse in a laser field. We have used isolated 140 attosecond pulses in several applications, which include a direct characterization of laser fields and a demonstration that the two-electron dynamics in helium could be observed and controlled.