FERMI | LDM The interaction of light with matter provides indispensable insight into the quantum mechanical world of atoms and molecules on their intrinsic time and length scale. Compared to the macroscopic world, these scales are extreme: The motion of nuclei happens on time scales of about 10 fs (1 fs = 10-15 sec), the electron motion on even shorter time scales in the order of 10 as (1 as = 10-18 sec), while typical length scales are given by the bond lengths between individual atoms, that are in the order of just 0.2 nm. A long-standing aim in science is to resolve these extreme scales with the help of laser light, which remains a challenge. An even greater goal is to go beyond the role of a passive observer of the quantum world and instead actively control and steer the processes happening in atoms and molecules. Based on the concepts of quantum mechanics, specially tailored light fields can be used to address this problem. Here, the electromagnetic wavecharacter of light is exploited. By shaping the amplitude, phase and polarization of the electromagnetic waves, fields can be sculpted that enhance certain quantum processes while suppressing others, resulting in a net control of the system. The prerequisite is the ability to shape the electromagnetic field of ultrashort laser pulses with durations of just a few femtoseconds. Such pulses enabled scientists for the first time to trigger and control the atomic and molecular processes on their natural time scale. In the visible range of the electromagnetic spectrum, the spectrotemporal shaping of ultrashort laser pulses is a mature technique. Potential applications can be found in physics, chemistry and material science, for instance in the control of chemical reactions, efficient qubit manipulation and the exploration of complex reaction pathways. The short-wavelength domain, that is the extreme ultraviolet (XUV) and x-ray spectral regime, provides a new perspective for the control of matter. In this spectral region, extremely short laser pulses with attosecond duration are available with which the fastest processes can be potentially controlled. Moreover, the energetic radiation can be used to selectively address the highly localized inner-shell electrons of atoms and molecules. This opens up a perspective to control quantum processes in a very localized way, basically on the spot. Thus, extending spectro-temporal pulse shaping to the short-wavelength regime promises the control of matter on extremely short time scales and with unprecedented spatial resolution. However, to date, corresponding concepts for the spectrotemporal shaping of XUV and x-ray pulses are hardly explored. In our experiment, a key step towards this development is achieved. We established for the first time the control of quantum dynamics based on the spectro-temporal shaping of XUV pulses. For this purpose, a new technique for the manipulation of XUV pulses was implemented at the free electron laser FERMI. As a test case, the novel concept was applied to a fundamental problem, that is the control of the quantum dynamics in a strongly driven fewelectron system. To this end, helium atoms were irradiated with highly intense XUV pulses produced by FERMI reaching an intensity in the order of 100 TW per cm2. Under these extreme conditions, the electrons in the atoms feel an extraordinary strong force imposed by the electro-magnetic radiation of the XUV pulses. As a consequence, the probability of the electrons to be in one of the atomic orbitals starts to rapidly oscillate between the 1s and the 2p orbital. This back-and-forth oscillation of the orbital occupation probability is called Rabi oscillation (Fig. 1 a). The oscillation period is directly proportional to the electromagnetic field strength of the penetrating XUV pulse, at the peak of the pulse, as 2 fs. To describe this situation theoretically, the electrons in the atom and the photons in the XUV pulse cannot be treated separately anymore. Instead, they are described as hybrid electronphoton states, so called “dressed states”. A consequence is that the energy of these hybrid states can take two values. To visualize this effect, we ionize the atoms during the short time window while the intense XUV pulse drives the Rabi oscillations (Fig. 1 a). In the kinetic energy distribution of the emitted electrons, we observe a distinct splitting, called the Autler-Townes doublet, which reflects the two energy values of the dressed states of the atom. By shaping the phase of the XUV pulses we achieved the control of these ultrafast Rabi dynamics. Specifically, we imprint a frequency chirp in the electromagnetic field. Quantum theory predicts, that with an up-chirp (frequency increases with time) the hybrid electronphoton state occupancy is steered towards the lower energy state. Whereas for a down-chirp the inverse process happens. In the experiment, we could successfully validate this prediction and showed the precise control of the occupancy of the hybrid quantum states during the duration of the XUV pulse (Fig. 1 b). These results are a fundamental demonstration that quantum control using shaped laser fields can be indeed extended to the short-wavelength domain. This opens an exciting route towards the exploration and control of matter with unprecedented capabilities. Controlling quantum dynamics with shaped extreme ultraviolet pulses Elettra Highlights 2024–2025 50
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