Attosecond, xuv, supercontinuum
Dynamics occurring on microscopic scales, such as electronic motion inside atoms and molecules, are governed by quantum mechanics. However, the Schroedinger equation is usually too complicated to solve analytically for systems other than the hydrogen atom. Even for some simple atoms such as helium, it still takes months to do a full numerical analysis. Therefore, practical problems are often solved only after simplification. The results are then compared with the experimental outcome in both the spectral and temporal domain. For accurate experimental comparison, temporal resolution on the attosecond scale is required. This had not been achieved until the first demonstration of the single attosecond pulse in 2001. After this breakthrough, "attophysics" immediately became a hot field in the physics and optics community. While the attosecond pulse has served as an irreplaceable tool in many fundamental research studies of ultrafast dynamics, the pulse generation process itself is an interesting topic in the ultrafast field. When an intense femtosecond laser is tightly focused on a gaseous target, electrons inside the neutral atoms are ripped away through tunneling ionization. Under certain circumstances, the electrons are able to reunite with the parent ions and release photon bursts lasting only tens to hundreds of attoseconds. This process repeats itself every half cycle of the driving pulse, generating a train of single attosecond pulses which lasts longer than one femtosecond. To achieve true temporal resolution on the attosecond time scale, single isolated attosecond pulses are required, meaning only one attosecond pulse can be produced per driving pulse. Up to now, there are only a few methods which have been demonstrated experimentally to generate isolated attosecond pulses. Pioneering work generated single attosecond pulse using a carrier-envelope phase-stabilized 3.3 fs laser pulse, which is out of reach for most research groups. An alternative method termed as polarization gating generated single attosecond pulses with 5 fs driving pulses, which is still difficult to achieve experimentally. Most recently, a new technique termed as Double Optical Gating (DOG) was developed in our group to allow the generation of single attosecond pulse with longer driving pulse durations. For example, isolated 150 as pulses were demonstrated with a 25 fs driving laser directly from a commercially-available Ti:Sapphire amplifier. Isolated attosecond pulses as short as 107 as have been demonstrated with the DOG scheme before this work. Here, we employ this method to shorten the pulse duration even further, demonstrating world-record isolated 67 as pulses. Optical pulses with attosecond duration are the shortest controllable process up to now and are much faster than the electron response times in any electronic devices. In consequence, it is also a challenge to characterize attosecond pulses experimentally, especially when they feature a broadband spectrum. Similar challenges have previously been met in characterizing femtosecond laser pulses, with many schemes already proposed and well-demonstrated experimentally. Similar schemes can be applied in characterizing attosecond pulses with narrow bandwidth. The limitation of these techniques is presented here, and a method recently developed to overcome those limitations is discussed. At last, several experimental advances toward the characterization of the isolated 25 as pulses, which is one atomic unit time, are discussed briefly.
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Doctor of Philosophy (Ph.D.)
College of Optics and Photonics
Optics and Photonics
Optics and Photonics
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Optics and Photonics; Optics and Photonics -- Dissertations, Academic
Zhang, Qi, "Generation and characterization of sub-70 isolated attosecond pulses" (2014). Electronic Theses and Dissertations. 4562.