An international team of researchers has demonstrated a new concept for the generation of intense extreme-ultraviolet (XUV) radiation by high-harmonic generation (HHG). Its advantage lies in the fact that its footprint is much smaller than currently existing intense XUV lasers. The new scheme is straightforward and could be implemented in many laboratories worldwide, which may boost the research field of ultrafast XUV science. The detailed experimental and theoretical results have been published in Optica.

The invention of the laser has opened the era of nonlinear optics, which today plays an important role in many scientific, industrial and medical applications. These applications all benefit from the availability of compact lasers in the visible range of the electromagnetic spectrum. The situation is different at XUV wavelengths, where very large facilities (so called free-electron lasers) have been built to generate intense XUV pulses. One example of these is FLASH in Hamburg that extends over several hundred meters. Smaller intense XUV sources based on HHG have also been developed. However, these sources still have a footprint of tens of meters, and have so far only been demonstrated at a few universities and research institutes worldwide.

A team of researchers from the Max Born Institute (Berlin, Germany), ELI-ALPS (Szeged, Hungary) and INCDTIM (Cluj-Napoca, Romania) has recently developed a new scheme for the generation of intense XUV pulses. Their concept is based on HHG, which relies on focusing a near-infrared (NIR) laser pulse into a gas target. As a result, very short light bursts with frequencies that are harmonics of the NIR driving laser are emitted, which thereby are typically in the XUV region. To be able to obtain intense XUV pulses, it is important to generate as much XUV light as possible. This is typically achieved by generating a very large focus of the NIR driving laser, which requires a large laboratory.

Scientists from the Max Born Institute have demonstrated that it is possible to shrink an intense XUV laser by using a setup which extends over a length of only two meters. To be able to do so, they used the following trick: Instead of generating XUV light at the focus of the NIR driving laser, they placed a very dense jet of atoms relatively far away from the NIR laser focus, as shown in Fig. 1. This has two important advantages: (1) Since the NIR beam at the position of the jet is large, many XUV photons are generated. (2) The generated XUV beam is large and has a large divergence, and can therefore be focused to a small spot size. The large number of XUV photons in combination with the small XUV spot size makes it possible to generate intense XUV laser pulses. These results were confirmed by computer simulations that were carried out by a team of researchers from ELI-ALPS and INCDTIM.

To demonstrate that the generated XUV pulses are very intense, the scientists studied multi-photon ionization of argon atoms. They were able to multiply ionize these atoms, leading to ion charge states of Ar²⁺ and Ar³⁺. This requires the absorption of at least two and four XUV photons, respectively. In spite of the small footprint of this intense XUV source, the obtained XUV intensity of 2 x 10¹⁴ W/cm² exceeds that of many already existing intense XUV sources.

The new concept can be implemented in many laboratories worldwide, and various areas of research may benefit. This includes attosecond-pump attosecond-probe spectroscopy, which has so far been extremely difficult to do. The new compact intense XUV laser could overcome the stability limitations that exist within this technique, and could be used to observe electron dynamics on extremely short timescales. Another area that is expected to benefit is the imaging of nanoscale objects such as bio-molecules. This could improve the possibilities for making movies in the nano-cosmos on femtosecond or even attosecond timescales.

Compact intense extreme-ultraviolet source

B. Major, O. Ghafur, K. Kovács, K. Varjú, V. Tosa, M. J. J. Vrakking and B. Schütte

Optica 8, 960 (2021).

Scientists from the Max Born Institute (MBI) have developed the first refractive lens that focuses extreme ultraviolet beams. Instead of using a glass lens, which is non-transparent in the extreme-ultraviolet region, the researchers have demonstrated a lens that is formed by a jet of atoms. The results, which provide novel opportunities for the imaging of biological samples on the shortest timescales, were published in Nature.

A tree trunk partly submerged in water appears to be bent. Since hundreds of years people know that this is caused by refraction, i.e. the light changes its direction when traveling from one medium (water) to another (air) at an angle. Refraction is also the underlying physical principle behind lenses which play an indispensable role in everyday life: They are a part of the human eye, they are used as glasses, contact lenses, as camera objectives and for controlling laser beams.

Following the discovery of new regions of the electromagnetic spectrum such as ultraviolet (UV) and X-ray radiation, refractive lenses were developed that are specifically adapted to these spectral regions. Electromagnetic radiation in the extreme-ultraviolet (XUV) region is, however, somewhat special. It occupies the wavelength range between the UV and X-ray domains, but unlike the two latter types of radiation, it can only travel in vacuum or strongly rarefied gases. Nowadays XUV beams are widely used in semiconductor lithography as well as in fundamental research to understand and control the structure and dynamics of matter. They enable the generation of the shortest human made light pulses with attosecond durations (an attosecond is one billionth of a billionth of a second). However, in spite of the large number of XUV sources and applications, no XUV lenses have existed up to now. The reason is that XUV radiation is strongly absorbed by any solid or liquid material and simply cannot pass through conventional lenses.

In order to focus XUV beams, a team of MBI researchers have taken a different approach: They replaced a glass lens with that formed by a jet of atoms of a noble gas, helium (see Fig. 1). This lens benefits from the high transmission of helium in the XUV spectral range and at the same time can be precisely controlled by changing the density of the gas in the jet. This is important in order to tune the focal length and minimize the spot sizes of the focused XUV beams.

In comparison to curved mirrors that are often used to focus XUV radiation, these gaseous refractive lenses have a number of advantages: A ‘new’ lens is constantly generated through the flow of atoms in the jet, meaning that problems with damages are avoided. Furthermore, a gas lens results in virtually no loss of XUV radiation compared to a typical mirror. “This is a major improvement, because the generation of XUV beams is complex and often very expensive,” Dr. Bernd Schütte, MBI scientist and corresponding author of the publication, explains.

In the work the researchers have further demonstrated that an atomic jet can act as a prism breaking the XUV radiation into its constituent spectral components (see Fig. 2). This can be compared to the observation of a rainbow, resulting from the breaking of the Sun light into its spectral colors by water droplets, except that the ‘colors’ of the XUV light are not visible to a human eye.

The development of the gas-phase lenses and prisms in the XUV region makes it possible to transfer optical techniques that are based on refraction and that are widely used in the visible and infrared part of the electromagnetic spectrum, to the XUV domain. Gas lenses could e.g. be exploited to develop an XUV microscope or to focus XUV beams to nanometer spot sizes. This may be applied in the future, for instance, to observe structural changes of biomolecules on the shortest timescales.

Original publication:
“Extreme-ultraviolet refractive optics”
Lorenz Drescher, Oleg Kornilov, Tobias Witting, Geert Reitsma, Nils Monserud, Arnaud Rouzée, Jochen Mikosch, Marc Vrakking & Bernd Schütte
Nature doi.org/10.1038/s41586-018-0737-3

 

When a nanoscale particle is exposed to an intense laser pulse, it transforms into a nanoplasma that expands extremely fast, and several phenomena occur that are both fascinating and important for applications. Examples are the generation of energetic electrons, ions and neutral atoms, the efficient production of X-ray radiation as well as nuclear fusion. While these observations are comparably well understood, another observation, namely the generation of highly charged ions, has so far posed a riddle to researchers. The reason is that models predicted very efficient recombination of electrons and ions in the nanoplasma, thereby drastically reducing the charges of the ions.

In a paper that was published in the current issue of Physical Review Letters, a team of researchers from the Imperial College London, the University of Rostock, the Max-Born-Institute, the University of Heidelberg and ELI-ALPS have now helped to solve this riddle. Tiny clusters consisting of a few thousand atoms were exposed to ultrashort, intense laser pulses. The researchers found that the vast majority of the emitted electrons were very slow (see Fig. 1). Moreover, it turned out that these low-energy electrons were emitted with a delay compared to the energetic electrons.

Lead scientist Dr. Bernd Schütte, who performed the experiments at Imperial College in the framework of a research fellowship and who now works at the Max-Born-Institute, says: “Many factors including the Earth’s magnetic field influence the movement of slow electrons, making their detection very difficult and explaining why they have not been observed earlier. Our observations were independent from the specific cluster and laser parameters used, and they help us to understand the complex processes evolving on the nanoscale.”

In order to understand the experimental observations, researchers around Professor Thomas Fennel from the University of Rostock and the Max-Born-Institute simulated the interaction of the intense laser pulse with the cluster. “Our atomistic simulations showed that the slow electrons result from a two-step process, where the second step relies on a final kick that has so far escaped the researchers’ attention”, explains Fennel. First, the intense laser pulse detaches electrons from individual atoms. These electrons remain trapped in the cluster as they are strongly attracted by the ions. When this attraction diminishes as the particles move farther away from each other during cluster expansion, the scene is set for the important second step. Therein, weakly bound electrons collide with a highly excited ion and thus get a final kick that allows them to escape from the cluster. As such correlated processes are quite difficult to model, the computing resources from the North-German Supercomputing Alliance (HLRN) were essential to solve the puzzle.

The researchers found the emission of slow electrons to be a very efficient process, enabling a large number of slow electrons to escape from the cluster. As a consequence, it becomes much harder for highly charged ions to find partner electrons that they can recombine with, and many of them indeed remain in high charge states. The discovery of the so-called low-energy electron structure can thus help to explain the observation of highly charged ions from intense laser cluster interactions. These findings might be important as low-energy electrons are implicated as playing a major role in radiation damage of biomolecules – of which the clusters are a model.

Senior author Professor Jon Marangos, from the Department of Physics at Imperial, says: “Since the mid-1990’s we have worked on the energetic emission of particles (electrons and highly charged ions) from laser-irradiated atomic clusters. What is surprising is that until now the much lower energy delayed electron emission has been overlooked. It turns out that this is a very strong feature, accounting for the majority of emitted electrons. As such, it may play a big role when condensed matter or large molecules of any kind interact with a high intensity laser pulse.”

Fig. 1: The electron kinetic energy spectrum from argon clusters interacting with intense laser pulses is dominated by slow electrons (orange area). The inset shows the same spectrum on a logarithmic scale, indicating the slow electrons (indicated by the red curve) and the fast electrons (indicated by the green curve).

 

 

 

Fig. 2: Atomistic simulation of the laser-induced cluster explosion. Credit: Thomas Fennel

 

 

 

 

Original publication:

Physical Review Letters 121, 063202 (2018), doi: https://doi.org/10.1103/PhysRevLett.121.063202

“Low-energy electron emission in the strong-field ionization of rare gas clusters”

Bernd Schütte, Christian Peltz, Dane R. Austin, Christian Strüber, Peng Ye, Arnaud Rouzée, Marc J. J. Vrakking, Nikolay Golubev, Alexander I. Kuleff, Thomas Fennel and Jon P. Marangos

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.

Link to publication

Bernd Schütte has received the 7th ISUILS Award for Young Researchers. This prize is sponsored by the Japan Intense Light Field Science Society and was awarded at the 15th International Symposium on Ultrafast Intense Laser Science, currently taking place in Cassis in the South of France. Bernd Schütte has received this award for his work on ultrafast cluster dynamics during the past few years.

The second workshop on “Ultrafast Cluster Dynamics” took place from August 23-24 at the Max-Born-Institut, and was organized together with the TU Berlin. Our 40 guests presented 14 exciting posters and 9 talks, which resulted in many lively discussions. A lot of new fascinating results have been obtained since the last workshop that took place 2 years ago in Rostock. We are already looking forward to the next edition of this workshop taking place in Freiburg in 2 years!

 

The second summer workshop on “Ultrafast Cluster Dynamics” will take place on August 23-24 at the Max-Born-Institut. Following the first edition organized by Thomas Fennel in Rostock in 2014, this workshop will be organized by Bernd Schütte in collaboration with Daniela Rupp and Maria Krikunova from the TU Berlin. We expect about 45 participants from Germany, but also from abroad. In addition to 9 talks about nonlinear cluster dynamics, we will have 14 interesting poster presentations. Please contact the organizers for more information.

Programm_UltrafastClusterDynamics

After our recent observation of autoionization following NIR strong-field ionization of molecular oxygen clusters (Link), we have now discovered autoionization in a number of different systems including atomic krypton and xenon clusters.

Link to publication

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.

Link to publication

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.

Link to publication