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Single Shot Stereo-ATI (above threshold ionization) Phase Meter
Ultraschnelle Nanophotonik (Prof. Kling)
The electric field of a laser pulse, which can be considered as a sine-like electromagnetic wave
modulated by an envelope function E0 (t ) , can be written as E  t   E0  t  cos t  ce  , where  is
the frequency of the carrier wave and ce is the carrier–envelope phase (CEP). The significance of the
CEP ce , or rather the rate of its variation dce / dt , was first recognized and exploited in frequency
metrology. Known as the offset frequency, dce / dt is the offset of the frequency modes of a shortpulse laser from integer multiples of the pulse repetition rate. Stabilization of this offset frequency led to
a revolution in frequency metrology and its pioneers, T. W. Hänsch and J. L. Hall, were awarded for their
contributions with the Nobel Prize in 2005.
Fig. 1. Illustration of the electric field for a few-cycle pulse. The electric field oscillations for a given
pulse envelope f(t) are plotted for two different carrier envelope phases ce  0 and  2 , as indicated.
For few-cycle laser pulses, the pulse envelope changes appreciably within one optical cycle;
therefore, the CEP effectively determines the temporal evolution of the underlying electric field (see Fig.
1). This is of great significance, as most strong-field laser–matter interactions depend on the electric field
rather than the intensity envelope of the pulse. Changes in the CEP shift the position of the cycle
maximum within the pulse envelope, which strongly affects the temporal shape of the electric field.
Extension of waveform control to multi-terawatt few-cycle lasers is expected to open the door to a
plethora of new phenomena. One of the most intriguing applications is the generation of isolated
attosecond bursts with keV photon energies. Obviously, these applications are limited by our ability to
control the carrier envelope phase.
Several techniques exploiting various phase-sensitive phenomena were shown to be able to
retrieve the CEP in proof-of-principle experiments. All of these methods need a large number of laser
shots for a single phase measurement, thus fluctuations of the CEP are averaged out. It also implies the
need to stabilize the CEP of the laser pulses, which, as a rule, is rather complex. Typically, the rate of
change of the CEP is detected with a so-called f-to-2f interferometer, and the CEP drift is actively
corrected for with a control loop. This is a way to stabilize the phase of intense few-cycle pulses. As an
alternative, recent efforts using quantum interference in photocurrents or linear optical interferometry
have been developed and can potentially be used to derive the evolution of the CEP. All of these
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techniques cannot detect the actual value of the CEP but only its relative change, and furthermore, they
all rely on active stabilization of the CEP, which is difficult to achieve for long periods of time.
Recently the ability to measure, in real time, the CEP for every single-shot using the strong-field
phenomena of above-threshold ionization (ATI) of xenon gas has been demonstrated. The stereo-ATI
carrier–envelope phase meter (CEPM) implementing this technique facilitates CEP data tagging, i.e.
measuring the CEP for each laser shot in parallel to another event-mode CEP-dependent measurement.
This allows one to probe CEP-dependent processes without CEP locking, facilitating the study of low
probability CEP-dependent phenomena. Moreover, the carrier–envelope phase meter (CEPM) represents
a powerful, real-time diagnostic tool with sub-cycle sensitivity, which is crucial for easy and reliable
tuning and monitoring of intense, ultrashort, Ti:sapphire sources, for instance, when trying to generate
isolated attosecond pulses in the XUV.
The goal of the present experiment is to characterize the CEP and duration of light pulses
generated by a state-of-the-art few cycle laser system with the use of a stereo-ATI phase meter. The
experiment provides many insights into various aspects of ultrafast laser science and the atomic physics
in strong laser fields.
Literature
1.
Kübel, M. et al. Carrier-envelope-phase tagging in measurements with long acquisition times.
New J. Phys. 14, 093027 (2012).
2.
Rathje, T. et al. Review of attosecond resolved measurement and control via carrier–envelope
phase tagging with above-threshold ionization. J. Phys. B At. Mol. Opt. Phys. 45, 074003 (2012).
3.
Wittmann, T. et al. Single-shot carrier–envelope phase measurement of few-cycle laser pulses.
Nat. Phys. 5, 357–362 (2009).
4.
Sayler, a M. et al. Carrier-Envelope Phase Measurement of Ultrashort Laser Pulses. Opt. Lett. 36, 1
(2011).
5.
Agostini, P., Fabre, F., Mainfray, G., Petite, G. & Rahman, N. K. Free-free transitions following sixphoton ionization of xenon atoms. Phys. Rev. Lett. 42, 1127–1130 (1979).
6.
Corkum, P. B. Plasma Perspective on Strong-Field Multiphoton Ionization. Phys. Rev. Lett. 71,
1994–1997 (1993).
7.
Paulus, G. G., Nicklich, W., Xu, H., Lambropoulos, P. & Walther, H. Plateau in above threshold
ionization spectra. Phys. Rev. Lett. 72, 2851–2854 (1994).
8.
B. Bergues, The circular-polarization phase-meter, Optics Express 20, 25317 (2012)