observing attosecond phenomena in real time: the attosecond transient recorder
Modern electronics can visualize fast electrical signals, by means of instruments called a transient recorder, with a temporal resolution of picoseconds. Streak cameras extend this capability into the femtosecond regime for short flashes of light, see Fig 1. Even the most advanced of these measuring instruments had a resolution not better than 100 femtoseconds, thousand times poorer than required for resolving attosecond phenomena. So how can we possibly overcome this enormous gap? Measuring brief time intervals or events relies on a physical quantity that varies on the relevant time scale in a controlled fashion. This quantity is the electric field deflecting the electron beam in both the electronic transient recorders and the optical streak cameras. The controlled variation of these microwave deflecting fields within pico- to femtoseconds permits measurements on these time scales.
Having recognized this, the solution looks, in principle, simple: let us replace the microwave field by a controlled optical field varying a thousand- to ten-thousand-times faster
. This can deflect electrons correspondingly faster, resolving their emission with a correspondingly higher temporal resolution. Fig. 2 shows the first implementation of this concept: the optical-field-driven streak camera. It is capable of recording the emission of electrons from atoms, molecules, or solids with a resolution of 100 attoseconds or better. The electrons may be knocked free directly by the exciting pulse (photoelectrons) whilst some others (secondary electrons) may be released by the remaining bound electrons as they relax toward states of lower energy. The emission of photoelectrons and secondary electrons reflects the history of the incident excitation (e.g. attosecond xuv or x-ray pulse) and subsequent relaxation processes, respectively. Recording the history of this emission, see Fig. 3, thus provides access to the duration of attosecond pulses and a look into the motion of electrons in the interior of atoms. Controlled light fields constitute an enabling technology for this attosecond chronometry.
. This can deflect electrons correspondingly faster, resolving their emission with a correspondingly higher temporal resolution. Fig. 2 shows the first implementation of this concept: the optical-field-driven streak camera. It is capable of recording the emission of electrons from atoms, molecules, or solids with a resolution of 100 attoseconds or better. The electrons may be knocked free directly by the exciting pulse (photoelectrons) whilst some others (secondary electrons) may be released by the remaining bound electrons as they relax toward states of lower energy. The emission of photoelectrons and secondary electrons reflects the history of the incident excitation (e.g. attosecond xuv or x-ray pulse) and subsequent relaxation processes, respectively. Recording the history of this emission, see Fig. 3, thus provides access to the duration of attosecond pulses and a look into the motion of electrons in the interior of atoms. Controlled light fields constitute an enabling technology for this attosecond chronometry.More information:
The first measurement with an optical-field-driven streak camera revealed the existence of precisely reproducible isolated sub-femtosecond pulses with a duration of 250 attosecond:
Atomic transient recorder, R. Kienberger et al. Nature 427, 817 (2004)
Single-Cycle Nonlinear Optics E. Goulielmakis et al. Science, 320, 1614 (2008).
The resolution of the atomic transient recorder was improved to the atomic unit of time, 24 as, and used to measure 80 as XUV pulses.
You may also be interested in the appreciation of William Safire
Attoboy, attosecond! William Safire, The New York Times , Section 6 (3/7/2004)
or the cover story of Hasel Muir:
Welcome to attoworld: where a second lasts the age of the universe, Hazel Muir, New Scientist , 33-36 (11/6/2004)Single-Cycle Nonlinear Optics E. Goulielmakis et al. Science, 320, 1614 (2008).
The resolution of the atomic transient recorder was improved to the atomic unit of time, 24 as, and used to measure 80 as XUV pulses.
Fig. 1. A short flash of light knocks electrons off from a metal plate (physicists call it a photocathode), which are deflected by transverse electric field rapidly (within about a nanosecond) growing from zero to maximum on their way to a screen. Upon their arrival, their resultant spatial distribution on a screen (streak image) reflects the temporal shape of the electron bunch (i.e. that of the light flash) with its width Δx measuring the pulse duration Δt.
Fig. 2. To record streak images of an attosecond emission process, streaking must be speeded up. To this end, the electrons released from atoms by an attosecond x-ray pulse are accelerated or braked - depending on their instant of release - by the controlled electric field of a few-cycle laser pulse that previously generated the attosecond pulse and is beamed in simultaneously. Hence the electrons' final energy distribution is broadened, see Fig. 3.
Fig. 3. The hyperfast-varying optical field maps the electron’s release time on a final velocity (or energy) distribution. This broadened electron energy distribution can be regarded as a streak image (of width ΔW), permitting determination of the duration Δt and temporal variation of electron emission. Imaging photo- or secondary electrons with this attosecond-resolution transient recorder allows measurement of the attosecond pulse as well as real-time observation of electron relaxation deep inside atoms and atomic-scale electron transport in solids.