Photographing a light helix
With their newly developed “nanoTIPTOE” technique, physicists from the Max Planck Institute of Quantum Optics and the Ludwig Maximilian University of Munich, in cooperation with Stanford University, have managed for the first time to record a helical light field on shortest time and length scales.
It has been known since the end of the 19th century that light is an electromagnetic wave, whereby its frequency determines its color. With around one quadrillion oscillations per second, light oscillates so quickly that it was not until the beginning of the 21st century that methods were developed to measure the temporal evolution of its field directly. Since then, more and more secrets of light have been revealed. Now, physicists from the Ultrafast Electronics and Nanophotonics group led by Dr. Boris Bergues and Prof. Matthias Kling from the attoworld team at the Ludwig-Maximilians-Universität (LMU) and the Max Planck Institute for Quantum Optics (MPQ) have developed a new technique, the so-called “nanoTIPTOE” technique, which allows measuring the electrical field of ultrashort laser pulses in time and space. This makes it possible to take “photographs” of light waves with a spatial and temporal resolution that has not been achieved before.
Light oscillations are incredibly fast. The frequency range of our current electronic technology does not even come close to that of light. For comparison: A modern computer works in the single-digit gigahertz range – around a million times slower than light oscillates. If we could operate our computers with light, they would be many times faster. A first step in this direction is to learn how to exactly steer and control light. Because the measurement has to take place inside a laser focus, within a volume much smaller than the focus size, this requires high precision temporal measurements in combination with high spatial resolution. This poses new challenges for physics because when we focus light onto a point (much like sunlight with a magnifying glass) the resolution is of the order of the focus size. Because light is diffracted, the theoretically achievable resolution is limited to about the size of the wavelength, that is, to a few hundred nanometers. In typical applications, however, this limit is difficult to achieve and the focus size is often in the range of a few micrometers. Therefore, effects on scales that are smaller than the focus size cannot be examined with focused light alone.
Physicists from the ultrafast electronics and nanophotonics group led by Dr. Boris Bergues and Prof. Matthias Kling of the attoworld team have now elegantly circumvented this problem. For their measurements, they used a tiny metallic nanotip that was much smaller than the focus of the light. This has the advantage that the field enhancement at the end of the tip, which is similar in principle to a lightning rod, allows to confine the measurement to the tiny end of the tip. The conductivity of the tip material in turn, enables the use of state-of-the-art electronic measurement methods, making the technique easy to handle and at the same time precise. The tip itself is only a few nanometers in size and serves to probe the field in one point in space. To obtain an overall image of the light field, the tip is scanned across the focus. Each tip position thus corresponds to one pixel of the image. In addition, the physicists can simultaneously measure the temporal evolution of the field in each pixel.
When light hits the needle tip, a short current pulse is generated. The latter flows through the tip in a few hundred attoseconds (an attosecond is one billionth of a billionth of a second). The laser field to be characterized modulates the induced current that is subsequently measured. With these current changes within the extremely short time interval, the physicists achieve the temporal resolution necessary to observe the light field.
With their technique, which they called “nanoTIPTOE”, the physicists have introduced a new approach to lightwave measurement. As a first application, scientists Johannes Blöchl and Dr. Johannes Schötz from the Ultrafast Electronics and Nanophotonics group, together with international collaborators, measured the field of an optical vortex beam, a specially structured type of laser field, which resembles a spiral of light. The light frequency of the beam is many orders of magnitude higher than what conventional electronics can detect. The achieved spatial resolution made it possible to reconstruct the spatial and temporal field distribution of the optical vortex in the focus of the laser beam and to observe the field amplitudes of the femtosecond vortex pulses rotating around the propagation axis (a femtosecond is one millionth of a billionth of a second).
“With our new methodology based on current measurements, we can achieve a new quality in spatially resolved spectroscopy and thus also drive applications in field-resolved scanning microscopy,” explains Johannes Blöchl.
Picture
A nanometric needle tip interacting with a few-cycle femtosecond laser pulse and a near-petahertz vortex field. The femtosecond pulse induces an ultrashort current of electrons that escape from the tip. The vortex field is probed by measuring the change in the electron current it induces. The localized field enhancement at the tip of the needle facilitates the spatial resolution of the helicoid wave front of the vortex field within the laser focus. Illustration: RMT.Bergues.
Origin publication
Spatiotemporal sampling of near-petahertz vortex fields
Authors: J. Blöchl, J. Schötz, A. Maliakkal, N. Šreibere, P. Rosenberger, P. Hommelhoff, A. Staudte, P. Corkum, B. Bergues, M. Kling
Journal: Optica 9, (2022)
Contact
Dr. Boris Bergues
Head of the Strong-Field Dynamics Group
Laboratory for Attosecond Physics
LMU Munich / Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching
Tel.: (+ 49 89) 32905 – 595
E-Mail: boris.bergues@mpq.mpg.de
http://attosecondimaging.de/strong-field-dynamics/
Prof. Dr. Matthias Kling
Ultrafast Electronics and Nanophotonics Group
SLAC, Stanford University, 2575 Sand Hill Rd, CA 94025, USA
LMU Munich / Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching, Germany
Tel.: +1-650-926-2745
E-Mail: kling@stanford.edu
https://uen.stanford.edu
Light-controlled reactions at the nanoscale
Controlling strong electromagnetic fields on nanoparticles is the key to triggering targeted molecular reactions on their surfaces. Such control over strong fields is achieved via laser light. Although laser-induced formation and breaking of molecular bonds on nanoparticle surfaces have been observed in the past, nanoscopic optical control of surface reactions has not yet been achieved. An international team led by Dr. Boris Bergues and Prof. Matthias Kling at Ludwig-Maximilians-Universität (LMU) and the Max Planck Institute of Quantum Optics (MPQ) in collaboration with Stanford University has now closed this gap. The physicists determined for the first time the location of light-induced molecular reactions on the surface of isolated silicon dioxide nanoparticles using ultrashort laser pulses.
Picture
A nanoparticle in the field of a femtosecond laser pulse with tailored waveform and polarization. The controlled enhancement of the field in specific nanoscopic regions of the nanoparticle (yellow spots) induces site-selective photochemical reactions of the molecules adsorbed on the surface. Imaging of the molecular fragments emitted from these regions enables all-optical control of the reaction sites with nanometer resolution. Illustration: RMT.Bergues
Origin publication
All-optical nanoscopic spatial control of molecular reaction yields on nanoparticles
Authors: W. Zhang, R. Dagar, P. Rosenberger, A. Sousa-Castillo, M. Neuhaus, W. Li, S. A. Khan, A. S. Alnaser, E. Cortes, S. A. Maier, C. Costa-Vera, M. F. Kling, B. Bergues
Journal: Optica 9, (2022)
Contact
Dr. Boris Bergues
Head of the Strong-Field Dynamics Group
Laboratory for Attosecond Physics
LMU Munich / Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching
Tel.: (+ 49 89) 32905 – 595
E-Mail: boris.bergues@mpq.mpg.de
http://attosecondimaging.de/strong-field-dynamics/
Sensing light
Future electronics will be fast. It could be driven at the frequencies of light waves. This implies that the switching speeds would be roughly 100,000 times faster than today. The development of electronics driven by light requires a detailed characterization of the light waves’s electromagnetic fields. Modern so-called field-sampling methods allow for probing the temporal evolution of a light field. While these techniques have been established, a complete and detailed understanding of their underlying mechanism has been lacking. Researchers from the Ultrafast Electronics and Nanophotonics group have now uncovered what exactly happens during the sampling of light fields and how their interaction with matter induces measurable currents in electronic circuits.
Light is a fascinating phenomenon of nature. It is incredibly fast and its electromagnetic field varies with extremely high frequencies. In the visible spectral range, light oscillates with nearly one petahertz (one thousand trillion oscillations per second). This property can be exploited in technology to drive future electronics with the help of the light fields, making it 100,000 times faster than today.
Sophisticated technologies, however, are required in the first step to precisely capture and control these ultrafast oscillations. This is enabled by so-called field-sampling methods. Short laser pulses ionize air near a metal electrode. Thereby, charge carriers are released, which induces currents in the electrode. The direction and strength of the electric current depend on the exact form of the light field, which enables its control. In previous studies, however, it has remained unclear how exactly the generated charge carriers determine the measured current. Moreover, there has been no possibility so far to predict the expected current quantitatively.
Johannes Schötz and coworkers have now uncovered this mechanism. With the help of experimental studies and numerical calculations, they were able to quantitatively model the process. As Johannes explains: “the study has shown that scattering and charge interaction of the generated charge carriers play an essential role in the formation of the macroscopic signal via ultrafast current generation in gases”.
In particular, the study has revealed how the strength of the generated signals can be increased by an order of magnitude. This will help further increase the sensitivity and accuracy of field sampling measurements and is an important step towards novel opto-electronic applications, paving the way to future light-field-controlled electronics.
Origin publication
The emergence of macroscopic currents in photoconductive sampling of optical fields
Authors: J. Schötz, A. Maliakkal, J. Blöchl, D. Zimin, Z. Wang, P. Rosenberger, M. Alharbi, A. Azzeer, M. Weidman, V. Yakovlev, B. Bergues, M. Kling
Journal: Nature Communications 13, (2022)
Picture
A strong few-cycle laser pulse leads to strong-field ionization of gas atoms and molecules. The motion of electrons (green) ejected from the atoms and molecules by the light induces a current in the nearby electrode. The process is influenced by the charge interaction between the charge carriers as well as by electron scattering. Illustration: RMT.Bergues
Exploration of new frequency frontiers
If physicists want to learn more about the dynamics of electrons in solids, they probe them with electromagnetic fields. A particularly powerful method is transient field-resolved spectroscopy. When a laser pulse excites electrons in a solid, its transmission or reflectivity changes. Transient field-resolved spectroscopy permits to monitor these changes after the excitation in the waveform of a sampling light field. Experiments so far, however, concentrated on frequency ranges below 50 terahertz.
Neuhaus and coworkers have now extended transient field-resolved reflectometry to frequency ranges of up to 100 terahertz, doubling the maximum frequency. The physicists could furthermore achieve repetition rates in the megahertz region in the measurements. They excited semiconductors with near-infrared laser pulses (at 800 nanometer wavelength), which lasted only for a few cycles in the femtosecond range. “The advantage of measurements in this extended frequency range is that it is free of other resonances. This permits to accurately monitor the response of free electrons in the materials”, explains Marcel Neuhaus, first author of the study. This way, the technique provided a time resolution of electron dynamics in solids below 10 femtoseconds.
By the extended frequency range the physicists studied dynamics of free charges in the semi-conductors germanium and gallium arsenide in a resonance-free range and could among other things observe, how electrons scatter between the different energy minima in the conduction band, the so called valleys. This allowed them to draw conclusions how the electrons influence each over in their scattering dynamics.
“The demonstrated field-resolved transient reflectometry at frequencies of 50-100 terahertz paves the way to studies of a broad range of intramolecular dynamics, including in molecular/organic electronics”, Prof. Kling explains. He adds: ”In the future, the new frequency range might enable measurements with high sensitivity to molecular vibrations in organic and novel 2D materials.”
Origin publication
Transient field-resolved reflectometry at 50–100 THz
Authors: M. Neuhaus, J. Schötz, M. Aulich, A. Srivastava, D. Kimbaras, V. Smejkal, V. Pervak, M. Alharbi, A. Azzeer, F. Libisch, C. Lemell, J. Burgdörfer, Z. Wang, M. Kling
Journal: Optica 9, (2022)
Picture
A near-infrared pulse (blue) excites a semiconductor. The unfolding ultrafast dynamics is interrogated by field-sampling spectroscopy, where changes to the waveform of a reflected mid-infrared light field (red) are recorded. Illustration: RMT.Bergues
Trihydrogen formation by intense irradiation
The Ultrafast Imaging and Nanophotonics team, led by Prof. Matthias Kling, in collaboration with the American University Sharjah, reports a new route to the formation of trihydrogen ions (H3+). The route involves a bimolecular reaction between water molecules adsorbed on nanoparticle surfaces, a situation that closely mimics irradiated ice and ice/dust particles in outer space. All routes identified so far involved organic molecules, which cannot exist under high-energy irradiation in space. The unprecedented generation of H3+ was enabled by “engineering” a suitable reaction environment comprising water-covered silica nanoparticles exposed to intense, femtosecond laser pulses.
Trihydrogen ions are arguably the most important triatomic species from the perspective of generating living matter in the universe. Such ions have hitherto not been possible to generate using conventional techniques of chemical synthesis. Their significance stems from their involvement in the production of many organic compounds in the universe and H3+ ions are regarded as an important precursor for the origin of life. The studies explore a new mechanism for the production of H3+ that mimics the conditions in outer space and, thus, provides new insights into the process that governs its creation and, consequently, the beginning of life in the universe. Furthermore, the new approach has the potential to be extended to the production of other new molecules that can have significant biological and environmental applications in terrestrial and non-terrestrial situations.
Origin publication
M. Said Alghabra, R. Ali, V. Kim, M. Iqbal, P. Rosenberger, S. Mitra, R. Dagar, P. Rupp, B. Bergues, D. Mathur, M. Kling, A. Alnaser
Anomalous formation of trihydrogen cations from water on nanoparticles
Picture
Physicists of the research group “field resolved nano spectroscopy”, led by Prof. Matthias Kling (left, together with Philipp Rosenberger) of the Ludwig-Maximilians-Universität, found a new route to the formation of trihydrogen ions (H3+) triggered by strong laserpulses.
Picture: Thorsten Naeser
Contact
Prof. Matthias Kling
Ultrafast Imaging and Nanophotonics, attoworld-team
Ludwig-Maximilians-Universität Munich
Am Coulombwall 1
85748 Garching
Germany
Phone: +49.89.298.54080
Email: matthias.kling@lmu.de
Poster award for Najd Altwaijry
At this year’s Max Planck School of Photonics Spring School Conference, Najd Altwaijry won a poster award. Her poster, titled: “Small, cheap method to sample electric fields up to 1PHz” included a 5-min video and explained the technique of “Nonlinear Photoconductive Sampling”. Using this technique, the team managed to sample an extremely short pulse (2,8fs) centred at 450 nm using a synthesised waveform from a three channel synthesiser.
Switching insulators to conductors at high rate
Scientists at the Wigner Research Centre for Physics of ELKH have reached an important milestone on the pathway toward ultrafast optoelectronic devices. Members of the Ultrafast Nanooptics Research group led by Péter Dombi, within an international collaboration involving attoworld-researchers around the Field-resolved Nano-spectroscopy group of Matthias Kling, made different insulating media conductive with a state-of-the-art compact laser source.
Researches at Wigner RCP have now made a huge step towards faster and more effective signal processing circuits. They exploited a recently discovered effect that allows to momentarily convert an insulating material (e.g. glass) into a conductor. This change of the state of matter lasts only for the time when the insulator is illuminated with an extremely short flash of laser light. But this time is long enough for the electrons to be transported by the action of the laser and to induce electric current in insulators. The team of scientists from the Ultrafast Nanooptics group led by Péter Dombi managed to trigger this effect in a special device with parameters adjusted to work with a simple laser oscillator. They succeeded to control the direction of the electric current at MHz repetition rates.
„It is fascinating to see in the lab, how an ultrashort laser pulse transforms insulators to a conductor within only a few femtoseconds. This effect has only been demonstrated with large, complex laser systems. Now, we managed to show this with a compact laser which is a very important step toward practical applications,” says Péter Dombi.
Origin publication
V. Hanus, V. Csajbók, Z. Pápa, J. Budai, Z. Márton, G. Kiss, P. Sándor, P. Paul, A. Szeghalmi, Z. Wang, B. Bergues, M. Kling, G. Molnár, J. Volk, P. Dombi
Light-field-driven current control in solids with pJ-level laser pulses at 80 MHz repetition rate
Picture
Sample under investigation, mounted in a sample holder, illuminated with laser and connected to acquisition electronics. (Source: Wigner Research Centre for Physics)
Metallization of nanoparticles on the cover
The article “Ionization-Induced Sub-cycle Metallization of Nanoparticles in Few-Cycle Pulses” by Qingcao Liu et al. was featured on the cover of ACS Photonics in the November 2020 issue. The cover was designed by Qingcao Liu with support by Lennart Seiffert and Matthias Kling, and shows the metallization of nanoparticles in few-cycle laser pulses. The hallmarks for the metallization are a phase change between inner and outer fields (depicted in false color), the screening of inner fields, and a characteristic increase in the cutoff energy of emitted electrons.
Prestigious fellowship for Wenbin Zhang
Dr. Wenbin Zhang, postdoc in the research group of Prof. Matthias Kling, has been awarded a very prestigious fellowship of the Alexander von Humboldt Foundation. With the Humboldt Research Fellowship, the Alexander von Humboldt Foundation sponsors above-average qualified researchers from all over the world.
During the fellowship, Dr. Zhang will work on directly probing and controlling molecular adsorbate dynamics on nanoparticles. To achieve this goal, he intends to develop new instruments. These include modification of a Nano-Target Recoil Ion Momentum Spectroscopy (NanoTRIMS) setup by introducing a time-of-flight (TOF) mass spectrometer (MS) on the electron side, and building a two-color laser setup for a novel 100 kHz, 2 µm laser system in the Kling lab.
Using the new instrumentation, Dr. Zhang’s research will concentrate on probing and controlling the effect of induced near-fields, such as plasmonic resonances in nanoparticles, on adsorbate dynamics. He will study the dissociation dynamics of organic molecules, the fundamental aspects of which are important in other scientific fields, for instance, in catalytic chemistry, and atmospheric photochemistry.
X-ray Light in Overdrive
An international collaboration, with physicists from the Ludwig-Maximilians-Universität Munich, the Max Planck Institute of Quantum Optics, and the King-Saud University in Riyad, has shown attosecond pulse generation under extreme conditions.
The generation of coherent x-ray light by high-harmonic generation (HHG) with strong laser pulses is one of the major building blocks of attosecond science. It allows for the generation of isolated attosecond pulses (IAPs), which provide unprecedented insight into the ultrafast dynamics of electrons in atoms, molecules and solids on a timescale of one billionth of a billionth second (1 as = 10-18 as). As the production of IAPs is pushed to higher photon energies, the interaction of the generating medium with the driving laser severely limits the HHG efficiency. A team of researchers from Ludwig-Maximilians-Universität (LMU) Munich and Max-Planck-Institute of Quantum Optics (MPQ), Germany and King-Saud University (KSU) in Riyadh, Saudi-Arabia, has now experimentally and theoretically investigated the IAP generation process in this overdriven regime, where the laser starts to get strongly affected by the medium.
HHG occurs when a strong laser field interacts with an atom or molecule leading to the emission of an electron through tunnel ionization (see figure). Subsequently, the freed electron is accelerated in the laser field and eventually recollides with the parent ion. The atom can be thought of as a tiny dipole emitting an HHG photon, whose phase is determined by the driving laser field and the motion of the electron outside the atom. For an efficient build-up of HHG, the radiation emitted along the laser propagation direction needs to add up constructively, referred to as phase-matching.
In the experiments, conducted at the Attosecond Science Laboratory (ASL) in Riyadh in Saudi-Arabia both isolated attosecond and driving laser pulses after the HHG process were characterized by the attosecond streaking technique. Surprisingly, it was found the IAPs could be generated at higher energies and under conditions, where it hadn’t been expected. The researchers found that changes to the driving laser are counteracted by both the decrease of the laser intensity and its blue shift in the gas medium, which permits perfect HHG phase-matching and boosts the efficiency. The blueshift was so far not considered in the description of experimental results but turned out to be especially important for few-cycle lasers.
The results improve the understanding of HHG phase-matching with few-cycle laser pulses and will help to push the current efforts of the generation of IAPs at higher energies and fluxes which, in turn, will open up a plethora of new applications.
Publication
Phase-Matching for Generation of Isolated Attosecond XUV and Soft-X-Ray Pulses with Few-Cycle Drivers
J. Schötz, B. Förg, W. Schweinberger, I. Liontos, H. A. Masood, A. M. Kamal, C. Jakubeit, N. G. Kling, T. Paasch-Colberg, S. Biswas, M. Högner, I. Pupeza, M. Alharbi, A. M. Azzeer, and M. F. Kling
Phys. Rev. X 10, 041011 (2020)
https://doi.org/10.1103/PhysRevX.10.041011
Contact
Johannes Schötz
Laboratory for Attosecond Physics
Department of Physics, LMU Munich and
Max Planck Institute of Quantum Optics
Am Coulombwall 1
85748 Garching
Phone: +49-89-289-54087
E-mail: johannes.schoetz@physik.uni-muenchen.de
Prof. Matthias Kling
Laboratory for Attosecond Physics
Department of Physics, LMU Munich and
Max Planck Institute of Quantum Optics
Am Coulombwall 1
85748 Garching
Phone: +49-89-289-54080
E-mail: matthias.kling@lmu.de
