Phase-dependent ionization and CE-phase measurement at long wavelengths

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The development towards shorter and shorter laser pulses has reached the point, where further progress is hardly possible: So-called few-cycle pulses consist of virtually only one optical cycle. Few-cycle pulses have the remarkable property that their waveform can be asymmetric, i.e. the field strength in the opposing directions of the laser polarization axis is different. A quantitative characterization of the waveform of few-cycle pulses is possible with the so-called carrier-envelope (CE) phase, which is of outstanding significance also in, e.g., frequency metrology.An optical period lasts one to a few femtoseconds. Electronic dynamics in atoms, molecules and solids, however, proceeds on the attosecond time scale. In order to learn something about nature on this time scale with the tools of laser physics, it is therefore necessary to focus on processes within an optical cycle. The respective approach of the QUTIF priority program is to manipulate the optical waveform in specific ways and to observe the subsequent reactions of the quantum dynamics. When few-cycle pulses are used, this can obviously be achieved by varying the CE phase. Few-cycle pulses have the additional advantage of confining the specific perturbation to a well-defined optical cycle.The measurement of the CE phase is consequently of huge importance. We have chosen the intuitive approach that builds on the conjecture that asymmetric laser pulses will induce asymmetric photoelectron emission which, in turn, can be used to infer the CE phase. The details of the underlying mechanisms are intricate. However, the concept has proven very fruitful. Unfortunately, rendering the method for the infrared spectral region, which is of particular interest because of molecular resonances, encounters serious difficulties: The very effects that are exploited for CE phase measurement decrease with the forth power of the wavelength.Nevertheless, we succeeded to expand the measurement range to wavelengths up to 1800nm. In the next years, the region up to 3500nm shall be opened up. To this end, it will be necessary to investigate CE phase-dependent photoionization of entirely different atomic and molecular systems with infrared few-cycle pulses. Since there are no suitable methods for measuring the CE phase in the infrared regime at present, we are confronted with a typical chicken-egg dilemma which we have to resolve.