Waldemar Schneider and Lauryna Lötscher generate in the LMU laser laboratory under the guidance of Matthias Kling light pulses in the extreme ultraviolet spectral range at a repetition rate of 50 megahertz. Such rates are needed to record high-resolution images of electron motion on nanostructured surfaces. So far, the pulses repeat very often but are a few hundred femtoseconds long. The goal of the scientists is now to generate light pulses with the laser system, which last only a few femtoseconds or even attoseconds. These pulses can then be used to also record temporally sharp images of the electron motion on nanostructured surfaces.

The ATLAS-Laser in its final state at MPQ. At the end of 2011 the system produced 60 terawatt laser pulses. Courtesy of Thorsten Naeser.

A light field synthesizer divides incident coherent white light into three color channels and modifies it afterwards. The composition creates laser pulses with a complex, however fine adjustable waveform. Using this synthesizer, the team of Dr. Eleftherios Goulielmakis, leader of the ERC-research group “Attoelectronics, generated for the first time “white” laser pulses and they are able to sculpt their field on time scales shorter than an optical cycle. These new tools hold promise for unprecedented control of the motion of electrons in the microcosm. Courtesy of Thorsten Naeser.

What is the fine structure of bones, teeth or seashells like? Sergiu Amarie knows that. He examines the structure and chemical composition of the surfaces of mineralised biological samples. The physicist is able to see tiny, 20 nanometer large structures (one nanometer corresponds to one billionth of a meter). Since a usual light microscope is only able to resolve structures which approximately correspond to the size of the wavelength of light, e. g. for infrared radiation several micrometers, the physicist uses a new kind of microscopy, namely the Scanning Nearfield Optical Microscopy (SNOM). This new technique uses an AFM (Atomic Force Microscope) to place a tiny tip with a diameter of 20 nanometres with a precision of a few nanometres above the sample surface. A broad-band laser pulse build in our labs containing frequencies in the infrared spectral range is being focused on the sample at the tip. The AFM tip creates a further nanofocus through its antenna properties and modulate the near-field signal to suppress unwanted background thus shrinking the measured sampled volume down to the tip size. Specific nano-crystals present in our body, like calcium-phosphate or those present in seashells like aragonite or calcite interact differently with the applied broad-band laser and only specific frequencies which mach the lattice vibrations of those crystals are enhanced. Compared to a usual light microscope the resolution of the SNOM is around 500 fold better. The resolution is solely determined by the diameter of the tip. The analysis of the light scattered by the tip with a FTIR spectrometer (Fourier Transformation Infrared spectrometer) then provides the vibration spectrum - the fingerprint of the sample material at the location of the tip. By scanning the surface the structure the chemical composition of the sample can hence be mapped with nanometre resolution. In the future this technique is intended to be used in biomedical studies such as the investigation of osteoporosis. Courtesy of Thorsten Naeser.

Producing isolated attosecond extreme-UV pulses via high harmonic generation requires high-energy fewcycle pulses preferably with kilohertz repetition rates for exploring hyperfast electronic phenomena. Ultrashort pulses in the range of a few femtoseconds at approximately kilohertz repetition rates have been demonstrated using a complex design consisting of an oscillator, a multipass chirped pulse amplifier, and an additional nonlinear compression stage. However, it has been difficult to extend these systems to the multimillijoule level. Optical parametric chirped pulse amplifiers (OPCPAs) have emerged as a powerful alternative for creating broadband fewcycle pulses and are the only method by which highenergy multimillijoule few-cycle coherent light pulses have been generated. The use of shorter pump pulses in the range of a few picoseconds eliminate the need for a large stretching and compression ratio and allow stretching of the seed pulses by passing through a few-centimeters-long dispersive optical material and recompression by a highly efficient compressor made up of a few chirped multilayer mirrors. Furthermore, the threshold intensity for optical damage of transparent materials increases for laser pulse durations decreasing below 20 ps. As a consequence, exposing the nonlinear crystal to higher intensities allows the same optical parametric amplification (OPA) gain to be attained with a shorter crystal and hence over a broader bandwidth. Courtesy of Thorsten Naeser.

Electrons (depicted as green particles) are released by the laser field (red wave). These electrons are first accelerated away from the particle surface and then driven back to it by the laser field. After an elastic collision with the surface, they are accelerated away again and reach very high kinetic energies. The figure shows three snapshots of the acceleration (from left to right): 1) the electrons are stopped and forced to return to the surface , 2) when reaching the surface, they elastically bounce right back 3) the electrons are accelerated away from the surface of the particle reaching high kinetic energies. Courtesy of Christian Hackenberger.

The local field on the polar axis is plotted as function of time, where time within the few-cycle wave runs from the lower right to the upper left. The fields show a pronounced asymmetry along the polarization axis of the laser (i.e. along the rims and valleys of the wave). This asymmetry leads to higher energies gained by electrons on one side of the nanoparticle as compared to the other side. For the given example the most energetic electrons are emitted from the backside, where the highest peak field is reached. The energies of the electrons and their emission directions are determined from the experiment. Courtesy of Christian Hackenberger.

Development of new light sources. LAP-Laser physicists aim at developing a brilliant, coherent source of ultrashort-pulsed XUV and SXR pulses at MHz repetition rates. This new source will open up unprecedented applications including highly temporal and spatially resolved spectroscopy on water-solvated proteins, time-resolved coincidence studies, biomedical imaging, nanoplasmonics as well as XUV holography and lithography. Courtesy of Thorsten Naeser

An artists view on attosecond physics: An attosecond light flash (blue) and a few-cycle femtosecond light pulse (red) coming from the right side are focused on a tungsten crystal. The attosecond light flash kicks electrons from the crystal into the vacuum. Subsequently, the energy of the now free electrons is modified by the electric field of the femtosecond light pulse. The system reveals how subtle differences between the electrons starting conditions determine their behaviour in the microcosm. Photo: Thorsten Naeser; image editing: Christian Hackenberger

Attosecond pulses of extreme ultraviolet light (depicted as a blue beam) are focused by a mirror (right) on a jet of neon atoms effusing from a thin valve. At the same time an infrared beam is striking the atoms. Both beams in combination allow real-time observation of the motion of electrons in the neon atoms and measurement of the duration of the attosecond pulse. Photo: Thorsten Naeser; image editing: Christian Hackenberger

Artificial view on chemical reactions, triggered by the dynamics of valence electrons in molecular orbitals. A proof-of-principle demonstration reported in Nature (Vol.466, 2010) shows how attosecond spectroscopy can be adapted to follow the hyperfast (subfemtosecond) motion of electron wavepackets in the valence shell — the bond-forming electrons — of krypton ions. Attosecond transient absorption spectroscopy of this type has the potential to reveal the elementary electron motions in molecules and solid-state materials that determine physical, chemical and biological properties. The cover picture of the magazine depicts a sequence of snapshots of the oscillatory motion of a valence electron inside an atomic ion, reconstructed from attosecond pump—probe measurements. Credit: Christian Hackenberger

Insight into an attosecond laboratory at the MPQ. The scientists Frederik Süßmann and James Kapaldo are working on the attosecond beamline AS5, an interferometric attosecond beamline for UHV experiments. Courtesy of Thorsten Naeser.

In elementary chemical reactions of electronically excited molecules the temporal evolution of the electronic and nuclear degrees of freedom are strongly coupled. One can therefore speak of ultrafast chemistry, if the coupled motion is temporally resolved. While the change of the molecular geometry occurs within several tens to hundreds of femtoseconds, the electrons respond to nuclear changes in a few femtoseconds or even attoseconds. The research teams of Reinhard Kienberger at the Technische Universität München (TUM) and Eberhard Riedle at the Ludwig-Maximilians-Universität (LMU) investigate these complex dynamics of molecules. Selected systems are excited with few femtosecond UV pulses tuned to the characteristic absorption wavelength of each molecule. A second temporally delayed and wavelength optimised pulse ionizes the molecules. The light thus knocks one ore more electrons out of the molecules. The physicists measure the time these electrons need to reach the detector. The time of flight yields the electron kinetic energy, which is related to the energy of the state, the electrons come from prior to ionisation. The evolution of the electron kinetic energy with the change in temporal delay of the two laser pulses sheds light on the very first events occurring in the molecule after it has been excited. In the laser laboratory at the TUM, Peter Lang is operating this spectroscopic experiment in a molecular beam apparatus. A gas beam seeded with benzene molecules is expanded through a small nozzle into a differentially pumped vacuum chamber and passed through a skimmer before reaching the laser interaction region. The laser pulses ionise the cold molecules and make the electronic relaxation visible. In future experiments, electron transfer processes and the bond cleavage at specific molecular positions will be investigated. The new insights will deepen the microscopic understanding of the fundamental processes in chemical reactions. The ultimate goal is to use this knowledge to control chemical reactions. Courtesy of Thorsten Naeser.

LAP physicists at the LMU target fabrication are able to produce diamond like carbon foils with a thickness as small as three nanometers. Being irradiated by focused highly intensive laser pulses, the few atomic layers eventually act like a light sails and positively charged atoms (ions) are emitted. This ion-acceleration by the pressure of light has proved to be very efficient and offers high potential in future biomedical applications. Courtesy of Thorsten Naeser.

The acceleration of ions with intense light fields requires targets which are opaque for optical radiation and adjustment with accuracies of some micrometers. Hit by the laser, the enourmous light pressure of the laser can accelerate a microscopic particle bunch with almost solid density to velocities of a few 10 percent of the speed of light. Courtesy of Thorsten Naeser.

Artificial view on Attosecond photoemission of electrons: Attosecond photoemission of electrons from two different atomic orbitals has been captured by a few-cycle light wave (yellow line). The resultant attosecond streak images (represented by the red surface plot) reveal an unexpected delay in photoemission. Courtesy of Christian Hackenberger.

Producing ultrashort high-energy laser pulses with repetition rates of over a million per second present laser engineers with severe challenges. For this purpose, LAP physicists of the Ultrafast Optics (UFO) group with the chair of Experimental Physics (Laser Physics) of LMU focus long infrared laser pulses into a crystal to produce so called white light. This can be done with various types of crystals, such as the Yttrium aluminium garnet crystal. This white light is then increased in energy with long infrared laser pulses and then compressed in time with special mirrors to flashes of light, now lasting just very few femtoseconds. Courtesy of Thorsten Naeser.

Producing ultrashort high-energy laser pulses with repetition rates of over a million per second present laser engineers with severe challenges. For this purpose, LAP physicists of the Ultrafast Optics (UFO) group with the chair of Experimental Physics (Laser Physics) of LMU focus long infrared laser pulses into a crystal to produce so called white light. This can be done with various types of crystals, such as the blue-gleaming Yttrium vanadate crystal. This white light is then increased in energy with long infrared laser pulses and then compressed in time with special mirrors to flashes of light, now lasting just very few femtoseconds. Courtesy of Thorsten Naeser.

For the first time ever, a European research team has managed to use attosecond laser pulses to observe the motion electrons in molecules. This report is published in the journal Nature, issue June 10th. Courtesy of Christian Hackenberger.

The photoemission of electrons by an attosecond light pulse (blue beam) is time resolved by controlling the electron motion with an ultrashort visible laser pulse (shown as red beam). This attosecond streaking uncovers that electrons from different atomic orbitals are released with a delay comparable to the atomic unit of time. Courtesy of Thorsten Naeser and Christian Hackenberger.

In future physicists at LAP are to produce particle beams in black chambers like that of Rainer Hörlein. For this purpose they use ultrashort high-intensity laser pulses lasting about 50 femtoseconds. The laser pulses are directed at diamond –like carbon foils just nanometres thick. The photons of the laser pulses exert pressure on the foil and eject ions, i.e. charged atoms. The pressure of light can attain up to ten gigabar (one billion bar). For comparison: On earth the prevailing pressure is one bar. The ions ejected are thus accelerated to ten per cent o the velocity of light. These ions are then applied to samples of tissue cells. The scientists now want to determine how the laser-accelerated ions have to be dosed in order to destroy cancer cells in tissue. This would afford a possibility of combating tumors by means of laser light. The scientists are thus on the threshold to developing efficient and, primarily, controlled acceleration of ions with laser light. The aim is to use lasers in future to provide ion radiotherapy with facilities that are compact and hence available to smaller hospitals. Courtesy of Thorsten Naeser.

Researchers from LMU and MPQ (group of Prof. Kleineberg) are investigating the ultrafast dynamics of collective electron motion (commonly referred to as “localized surface plasmons”) and induced localized optical fields in various kinds of metallic nanostructures, which take place on a nanometer spatial scale as well as on a sub-fs temporal scale. The results may help to understand and develop ultrafast electronics in the future based on localized and propagating surface plasmons rather than on drifting electrons. Courtesy of Thorsten Naeser.

View into the experimental chamber of the attosecond beamline AS2: Atomic gas streams from a nozzle into the interaction volume where attosecond XUV pulses and few-cycle laser pulses are employed for pump probe experiments. Photoelectrons are detected by a time of flight detector (TOF) mounted from the top, the ionized atoms are investigated by a mass spectrometer (reflectron) oriented horizontally. Courtesy of Thorsten Naeser.

A nozzle emits a rare gas that converts laser pulses, coming from the right, into attosecond light flashes. Courtesy of Thorsten Naeser.

The scientists Matthias Kling (l) and Sergey Zherebtsov are discussing the setup of the beamline AS5. Courtesy of Thorsten Naeser.

Ultraviolet photons are required for photoemission of femtosecond electron pulses. These have a 50,000-fold shorter wavelength than the blue light in this picture, and are our tool to visualize atoms and their motion in all four dimensions of space and time. See www.ultrafast-electron-imaging.com for details. Courtesy of Thorsten Naeser.

The slender stainless-stell nozzles in a vacuum chamber of the AS4b attosecond beamline extend like stalactites into the centre of the experimental setup. From the right-hand edge of the photo the physicists focus invisible infrared light pulses lasting about 3.5 femtoseconds exactly on the centre of the left-hand gas nozzle, which emits neon gas whose atoms are exited by the infrared light pulses, thus giving them a reddish fluorescence. The right-hand gas nozzle, on the other hand, emits the argon, which emits a blue fluorescence, likewise due to the excitation caused by the infrared light. In this excitation process the rare-gas atoms produce light pulses both in the ultraviolet (UV wavelength about 250 nanometers) and in the extreme ultraviolet spectrum of the light (XUV, wavelength about 8 nanometers). Courtesy of Thorsten Naeser.

Mirror production at the clean room of the Munich Centre for Advanced Photonics (MAP). Courtesy of Thorsten Naeser.

Image of the field oscillations in a few-cycle pulse of red laser light sampled with 250-attosecond x-ray pulses. Courtesy of Dr. Eleftherios Goulielmakis.