VOLUME 80, NUMBER 3 PHYSICAL REVIEW LETTERS 19 JANUARY 1998 Above-Threshold Ionization by an Elliptically Polarized Field: Quantum Tunneling Interferences and Classical Dodging G. G. Paulus, F. Zacher, and H. Walther* Max-Planck-Institut f ür Quantenoptik, 85748 Garching, Germany A. Lohr, W. Becker, † and M. Kleber Physik-Department T30, Technische Universität München, 85747 Garching, Germany (Received 11 April 1997) Measurements of above-threshold ionization electron spectra in an elliptically polarized field as a function of the ellipticity are presented. In the rescattering regime, electron yields quickly drop with increasing ellipticity. The yields of lower-energy electrons rise again when circular polarization is approached. A classical explanation for these effects is provided. Additional local maxima in the yields of lower-energy electrons can be interpreted as being due to interferences of electron trajectories that tunnel out at different times within one cycle of the field. [S0031-9007(97)05059-X] PACS numbers: 32.80.Rm, 03.65.Sq The interaction of strong laser fields with matter has become a rapidly evolving discipline in recent years. One typical phenomenon is above-threshold ionization (ATI) of atoms, i.e., the absorption of more photons than necessary for ionization, leading to a series of peaks in the electron energy spectra; for a review see Ref. [1]. Above- threshold ionization spectra display various characteristic properties, such as the plateau [2] and side lobes in the angular distributions [3]. Remarkably, these and other features are qualitatively similar for different atomic systems, in particular for the rare gases. A lot of insight was gained from a classical model that treats the atom as just a source that provides electrons via tunneling [4]. Subsequently, the atomic binding potential is ignored and the electrons are described by their classical trajectories merely in the laser field. This simple picture has explained [5] both the plateau and the side lobes. Thus many strong-field effects are “universal” since they do not qualitatively depend on any particular property of the individual atom, as much as they are “classical” to the extent that they can be qualitatively explained by the just mentioned classical model. In this Letter we report novel features in the ATI spectra generated by an elliptically polarized laser field. All of them are universal, i.e., have a similar appearance for all rare gas atoms studied, but only two of them are classical in terms of the above classification. The third effect owes its existence to a genuine quantum phenomenon: interference of electrons that reaches the continuum via tunneling at different times. The quantum interferences reported in this Letter are observed and discussed for elliptically polarized laser fields. They are, however, present for other polarizations as well, although more difficult to detect in experiments. In addition to establishing the relevance of quantum interference to intense-field physics, this Letter presents an interesting link to other tunneling phenomena: In three-dimensional tunneling more than one most probable escape path may exist [6] and the contributions from these paths may interfere. In fact, the interference effect measured here in intense-field ionization with elliptically polarized laser light is exactly such as interference of two tunneling paths, whose experimental observation is, to our knowledge, described here for the first time. Futhermore, there is a close connection to the problem of tunneling times, in our case for a dynamic tunneling phenomenon. The theory outlined below to explain the interference effects in the ATI spectra also provides means to extract from the data complex times with an imaginary part that sets the scale for the tunneling time. ATI by an elliptically polarized field has been investigated before [7], but with different emphasis. The experimental setup consists of a femtosecond dye laser whose pulses are brought to an energy of about 15 mJ in a two-stage optical amplifier pumped by a copper-vapor laser with a repetition rate of 6.2 kHz. The amplified pulses have a FWHM dura- tion of 50 fs at a wavelength of 630 nm and can be focused down to 12 mm giving rise to intensities in excess of 10 14 Wcm 2 . For the analysis of the kinetic energy of the photoelectrons we use a high-resolution time-of-flight spectrometer capable of recording several electrons per laser shot with a collecting angle of 5 ± . The ellipticity of the laser polarization is controlled by a quarter wave plate mounted on a stepper-driven rotary stage. The big axis of the polarization ellipse always points in the direction of the electron detector. In a measurement of an ellipticity distribution, the ellipticity is scanned several times in order to minimize artifacts from possible long-term drifts in the laser intensity. The respective ATI spectra are recorded with a computer that mimics 73 different multichannel analyzers, one for each position of the quarter-wave plate. Data have been taken for all the rare gas atoms. The effects to 484 0031-9007 98 80(3) 484(4)$15.00 © 1998 The American Physical Society