Invisible laser pulses push electrons in a quantum swing

Unlike natural light sources such as the sun, lasers have special coherence properties, resulting e.g. in the intriguing observation of interference, where the overlap of two lasers can lead to darkness. The coherence of lasers can also be used to push electrons in a quantum swing inside an atom, meaning that electrons oscillate between two quantum states that lie on different energy levels. Such quantum swings are known as Rabi oscillations and have been observed with low-frequency lasers up to the ultraviolet spectral range. In this paper, we show that Rabi oscillations can be driven in an argon atom by an intense laser pulse in the extreme-ultraviolet spectral range, which is invisible for the human eye. By using a second laser pulse, the ultrafast oscillation of electrons between the ground state (the lowest energy level) and an excited state (at a higher energy level) is traced directly in the time domain, showing that it takes place within tens of femtoseconds (corresponding to 10^-14 seconds). Our results pave the way towards multi-photon coherent control techniques in the extreme-ultraviolet range, allowing extension to situations where interactions between electrons play a role, or more complex systems including molecules.

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Fast electrons at long wavelengths

Efficient electron acceleration was observed in clusters induced by a laser field at 1.8 μm that consisted of only two optical cycles. In this regime that is dominated by electronic rather than nuclear dynamics, clear signatures of direct electron emission were observed as well as rescattering of electrons that gain additional kinetic energy during laser-driven collisions with ions and with the cluster potential. The results, which were obtained at the Imperial College London, promise efficient particle acceleration in clusters at mid-infrared and terahertz wavelengths.

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Invisible light flash ignites nano-fireworks

We have demonstrated a new way to turn initially transparent nanoparticles suddenly into strong absorbers for intense laser light and let them explode.

Intense laser pulses can transform transparent material into a plasma that captures energy of the incoming light very efficiently. At the MBI we have discovered a trick to start and control this process in a way that is so efficient that it could advance methods in nanofabrication and medicine. We studied the interaction of intense near-infrared (NIR) laser pulses with atomic nanoclusters. The visible NIR light pulse alone can only generate a plasma if its electromagnetic waves are so strong that they rip individual atoms apart into electrons and ions. We could outsmart this so-called ignition threshold by illuminating the clusters with an additional weak extreme-ultraviolet (XUV) laser pulse that is invisible to the human eye and lasts only a few femtoseconds (a femtosecond is a millionth of a billionth of a second). With this trick we could “switch on” the energy transfer from the near-infrared light to the particle at unexpectedly low NIR intensities and created nano-fireworks, during which electrons, ions and colourful fluorescence light are sent out from the clusters in different directions (Figure 1). The results open unprecedented opportunities for both fundamental laser-matter research and applications and was published in the latest issue of Physical Review Letters.
The experiments were carried out at the Max Born Institute at a 12 meter long high-harmonic generation (HHG) beamline. Our collaborators Mathias Arbeiter and Thomas Fennel from the University of Rostock modelled the light-matter processes with numerical simulations and uncovered the origin of the observed synergy of the two laser pulses. They found that only a few seed electrons created by the ionizing radiation of the XUV pulse are sufficient to start a process similar to a snow avalanche in the mountains. The seed electrons are heated in the NIR laser light and kick out even more electrons.
The novel concept of starting ionization avalanching with XUV light makes it possible to spatially and temporally control the strong-field ionization of nanoparticles and solids. Using HHG pulses paves the way for monitoring and controlling the ionization of nanoparticles on attosecond time scales, which is incredibly fast. One attosecond compares to a second as one second to the age of the universe. Moreover, the ignition method is expected to be applicable also to dielectric solids. This makes the concept very interesting for applications, in which intense laser pulses are used for the fabrication of nanostructures. By applying XUV pulses, a smaller focus size and therefore a higher precision could be achieved. At the same time, the overall efficiency can be improved, as NIR pulses with a much lower intensity compared to current methods could be used. In this way, novel nanolithography and nanosurgery applications may become possible in the future.

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Energy exchange in highly ionized nanoparticles

Excited atoms often decay via the emission of radiation, a process that is known as fluorescence. A different scenario can emerge when an excited atom is surrounded by other excited atoms, ions and electrons. Such a situation is achieved when an intense laser pulse interacts with a nanoscale object. In this case, an excited atom can decay by transferring its excess energy to another particle in the environment. Researchers from the Max-Born-Institut in Berlin, the University of Rostock, and the University of Heidelberg found evidence for such an energy exchange involving electrons that are trapped within a nanocluster. They observed a so far unidentified peak in the electron spectrum following the ionization of a nanocluster by a near-infrared (NIR) laser pulse. The researchers attributed this signal to the relaxation of one electron from an excited Rydberg atom and the simultaneous transfer of the excess energy to a second electron that can escape from the cluster. The obtained results, which were published in Nature Communications, are of universal nature and expected to play an important role in other nanoscale systems including biomolecules.

Interatomic Coulombic decay (ICD) describes the relaxation of an excited atom by transferring its excess energy to a neighboring atom that gets ionized. This effect has received significant attention in recent years, as it may be a source of radiation damage in biological systems. At the same time, it was proposed to exploit ICD for novel cancer therapies. So far, ICD has been observed following the ionization or excitation of clusters by high-energy photons in the extreme-ultraviolet (XUV) and X-ray range. In contrast, it had not been expected that ICD could be induced by low-energy photons in the NIR regime.

The ionizaton of a cluster by an intense NIR laser pulse triggers highly complex dynamics. A so called nanoplasma is formed that consists of a large number of ions and electrons interacting with each other. Recombination of electrons and ions has been found to result in the generation of Rydberg atoms and ions, which can decay via fluorescence. However, in a strongly ionized cluster, Rydberg atoms may also relax via correlated electronic decay (CED) processes similar to ICD, i.e. without the emission of radiation. In CED, one electron can relax from a Rydberg state to the ground state and transfer its excess energy to a second electron, which is either located in the same atom, in the nanoplasma, or which is in a Rydberg state of a nearby atom (see Figure 1). Using this additional energy, the second electron can escape from the cluster. “Even though CED may be expected in nanoplasmas, the effect had neither been observed in experiments nor had it been predicted by theoretical models.”, explains Dr. Bernd Schütte from the Max-Born-Institut. “The major challenge in the experiment was to find suitable conditions that allow a direct observation of correlated electronic decay.”

Just recently, the researchers were rewarded for their search and found evidence of CED in the electron spectrum from argon clusters ionized by an intense NIR laser pulse. Their results have now been published in Nature Communications. The emergence of a peak in the energy spectrum of emitted electrons that is close in energy to the atomic ionization potential (see Figure 2) was found to be the signature of an electronic decay process involving bound atomic states. Surprisingly, the scientists found that the energy exchange between electrons takes place almost 100 picoseconds after the cluster is ionized. This is much slower than for typical ICD processes that proceed on 100 femtoseconds timescales.

Support for this explanation was obtained by modeling the complex dynamics taking place in the expanding clusters by the group of Prof. Thomas Fennel from the University of Rostock. “The tricky aspect of the experiment is that the charged and expanding cluster disturbs the electrons emitted via CED. Electrons that have been emitted in early expansion stages will have lost their specific bound-state signatures.”, explains Fennel. The ICD expert Dr. Alexander Kuleff from the University of Heidelberg adds “Our calculations show that ICD between lowly excited argon atoms takes place on a timescale of 200 femtoseconds, but the process significantly slows down, when higher Rydberg states are involved. This is in good agreement with the experiment, which suggests that the observed electrons are emitted from higher Rydberg orbitals.”

Although the first experiments on clusters with intense NIR laser pulses were already performed in the 1990s, it took a long time to observe correlated electronic decay in expanding nanoplasmas for the first time. One reason why this effect could not be revealed in previous experiments is that it can only be directly observed in a very small range of laser intensities and cluster sizes. However, after having understood the involved dynamics, the researchers could show that CED has a universal nature. The process was observed in all the investigated clusters, which include atomic krypton and xenon clusters as well as molecular oxygen clusters. “CED takes place as soon as a nanoplasma is born within the cluster and excited states are populated by recombination”, explains Dr. Arnaud Rouzée from the Max-Born-Institut, adding “CED is therefore expected to be important also for experiments, in which intense XUV and X-ray laser pulses that interact with nanoscale objects, including biomolecules.” Further experiments are under way in order to elucidate the overall significance of correlated electronic decay in highly excited complex systems.

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Classical or not? Physics of nanoplasmas

The interaction of an intense laser pulse with a nanometer-scale particle results in the generation of an expanding nanoplasma. In the past, nanoplasma dynamics were typically described by classical phenomena, like the thermal emission of electrons. In contrast, a new study on the interaction of intense near-infrared (NIR) laser pulses with molecular oxygen clusters now demonstrates that phenomena, which can only be described quantummechanically, play an important role. For the first time, evidence of efficient formation of autoionizing states in nanoplasmas is found. Autoionization of so called superexcited states of atomic oxygen is directly observed on a nanosecond time scale, whereas indirect signatures are visible for decay processes occurring on shorter time scales. Autoionization is found to take place in various systems and is expected to be important also in the interaction of finite systems with intense extreme-ultraviolet (XUV) and X-ray pulses from novel free-electron laser sources.

Following the interaction of intense NIR laser pulses with clusters, the recorded electron spectra typically show a smooth distribution. In the past, the absence of discrete state signatures in these spectra led to the conclusion that the dynamics of charged particles during the cluster expansion can be well described by fully classical behavior. As a consequence, simulations that model the interaction of intense lasers with clusters, nanoparticles or large molecules, often make use of quasiclassical approaches. With the advent of novel laser sources and time-resolved techniques during the last year, this picture began to falter. Recently, extensive formation of excited atoms in nanoplasmas driven by electron-ion recombination processes was reported. When an atom with 2 electrons in excited states is formed, it may decay via an electron correlation effect, where one electron is released into the continuum, while the second electron relaxes to a lower bound state. However, since the electrons emitted via such autoionization processes exchange kinetic energy with the cluster environment, they had not been observed in experiments so far.

In a collaboration led by scientists from the Max-Born-Institut, the first evidence of autoionization following intense NIR laser-cluster interactions is now reported. In the current issues of Physical Review Letters [114, 123002 (2015)] Bernd Schütte, Marc Vrakking and Arnaud Rouzée, and their colleagues Jan Lahl, Tim Oelze and Maria Krikunova from the TU Berlin present results obtained from oxygen clusters. This system was chosen, because oxygen atoms have previously been shown to exhibit long-lived autoionizing states. In the present study, clear peaks were observed in the electron spectrum from oxygen clusters ionized by intense NIR pulses (Fig. 1). These peaks could be assigned to well-known autoionizing states, and it was shown that they decay on a nanosecond time scale, when the cluster has already significantly expanded. Therefore, the influence of the environment on the electrons emitted via autoionization was negligible. The observed autoionization contributions were found to be very sensitive on the intensity of the NIR laser pulse. At higher intensities, the autoionization peaks were blurred out, but still visible. These results indicate that autoionization plays an important role in many experiments that study the interaction of intense laser pulses with nanometer-scale systems, even when these processes cannot be directly observed in the electron spectrum. Previously, it was demonstrated that the observed nanoplasma dynamics following intense XUV and NIR ionization of clusters are similar, and therefore, the current results are expected to be highly relevant as well for experiments at novel free-electron lasers. The experimental findings of autoionization are also important for improving theoretical models of nanoplasmas in the future in order to gain a better understanding of the underlying microscopic processes.

The presented results demonstrate that a description of nanoplasma dynamics by classical approaches is insufficient. Quantum phenomena like autoionization play an important role during the expansion of clusters following the interaction with intense light pulses.

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Freedom of electrons is short-lived

During the interaction of an intense extreme-ultraviolet (XUV) laser pulse with a cluster, many ions and free electrons are created, leading to the formation of a nanoscale plasma. In experiments using XUV/X-ray free electron lasers (FELs) it was previously demonstrated that only a small fraction of these electrons can leave the cluster, while the majority of the electrons remain trapped within the cluster and may therefore recombine with ions. In a novel approach using a laboratory-scale XUV source, we have now measured the time scale of these electron-ion recombination processes leading to a strong formation of excited atoms, which is in the picosecond range. The results show that it is even possible to follow the laser-induced cluster expansion process up to nanosecond times.

The formation of a large number of charges in a cluster by an intense laser pulse can lead to the generation of a transient nanoplasma consisting of free electrons and ions. In the past, fascinating processes could already be observed in nanoplasmas, including nuclear fusion or the creation of neutral atoms with very high kinetic energies. While nanoplasmas are routinely generated during the interaction of clusters with intense XUV pulses from free-electron lasers, a detailed understanding of the processes inside the plasma is challenging. Theoretical models have predicted that the majority of electrons remains trapped in the cluster and may eventually recombine with ions such that both transient species cannot be observed in usual experiments. However, an experimental investigation of these dynamics is crucial, since processes in clusters are complex and manifold, and their detailed prediction is difficult. A promising route towards a better understanding of the different mechanisms in nanoplasmas is the development of time-resolved experiments. In this context, intense high-order harmonic generation (HHG) sources that can deliver light pulses down to the attosecond regime are very interesting. This laboratory-scale XUV sources provide a straightforward way to carry out pump-probe experiments on clusters and can significantly improve the possibilities for the understanding of cluster dynamics.

In an international collaboration led by researchers from the Max-Born-Institut, the first pump-probe experiment on clusters using an intense HHG source was now performed. In the current issue of Physical Review Letters [112, 253401 (2014)] Bernd Schütte, Marc Vrakking and Arnaud Rouzée and their colleagues Filippo Campi from the University of Lund and Mathias Arbeiter and Thomas Fennel from the University of Rostock present the results of these investigations. The development of a technique allowing the Reionization of Excited Atoms from Recombination (REAR) makes it possible for the first time to infer information on ion charge states prior to recombination. By using near-infrared (NIR) probe pulses, a surprisingly extensive formation of excited atoms was observed and could be shown to originate from recombination between electrons and ions. It was demonstrated that in the nanoplasma electrons released by means of photo-ionization only remain quasi-free for a short time up to 10 picoseconds before they undergo a recombination process with the surrounding ions. More information about these processes was obtained by generating special clusters that consist of a xenon core and an argon shell. These investigations showed that recombination preferentially takes place in the xenon core of the cluster. The wavelength of the ionizing pulse interacting with the cluster was shown not to be important: excited atom formation attributed to recombination processes was also observed when using NIR or blue pump pulses instead of XUV pulses. This demonstrates the general implications of the current findings for the explanation of previous experiments carried out in different wavelength regimes. Moreover, the cluster expansion dynamics could be traced up to the nanosecond range by using the REAR technique.

Our results show the remarkable versatility of intense HHG pulses for the study of dynamic processes in clusters. In the future, the investigation of other extended systems like biomolecules will benefit from the availability of these laboratory-scale XUV light sources.

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