HIJ (Jena, Germany)

Few-Cycle-Laser

• Carrier-envelope phase measurement
  • Stereographic photoelectron spectroscopy provides a high-precision, single-shot, and real-time carrier-envelope phase (CEP) measurement at 1.8 μm laser wavelength based on stereographic photoelectron spectroscopy [Optics Letters 42, 5150 (2017)].
• Controlling laser-driven processes
  • Experiments in the NLO lab demonstrated how a nanoscale metal tip can be used as a phase sensitive probe to map out the phase distribution throughout the laser focus [Nature Phys. 13, 947 (2017)].
• Imaging
  • The first demonstration of coherence tomography, i.e., non-invasive cross-sectional imaging, using a laser-driven high-harmonic source provides a technique for achieving nanoscale axial resolution and very good material contrast [Optica 4, 903 (2017)] and a full reconstruction of the reflectivities of internal surfaces and the dispersion of internal layers [Optica 8, 230 (2021).
• Molecular Physics
  • The first experimental investigation of the simplest asymmetric molecule, the helium hydride ion (HeH⁺), in strong laser fields was performed, showing evidence of control of the molecular bond length by direct vibrational excitation [Phys. Rev. Lett. 121, 073203 (2018)].

POLARIS

POLARIS [1] [2] has been used in various experiments on laser-driven particle acceleration. Using 20µm-diameter, solid-hydrogen jets as the target we could show that the efficiency of laser-to-proton-beam energy could be enhanced to the few-percent level [3].

POLARIS has also been used in proton-acceleration experiments using ultra-thin foils, which were possible due to its extreme temporal intensity contrast. An investigation of the proton beam profile (using a gated CCD camera to record the energy-resolved proton beam profile) revealed important details about the acceleration mechanism [4].

Recently, POLARIS has been equipped with a frequency-broadened synchronized probe pulse system [5], which has enabled pump-probe experiments with a temporal resolution below the main-pulse duration. Recently, we could monitor the formation of the laser-induced plasma prior to the high-intensity interaction and identify the dominating ionization processes in a space- and time-resolved measurement [6].

The research focus of the Few-Cycle-Laser laboratory at the Friedrich Schiller University Jena is on strong field and attosecond laser physics, including nanoscale XUV imaging. For this purpose, lasers are operated, which are capable of generating ultrashort laser pulses with a FWHM of less than two optical cycles over a large spectral range and at high laser repetition rates.  The laboratory is particularly known for its unique technology to measure the so-called absolute (or carrier-envelope) phase, i.e. the position of the maxima of the carrier wave relative to the pulse envelope, has high significance. The dependence of the laser field on the CEP enables the investigation of strong field phenomena with sub-cycle and thus attosecond resolution. Tagging the CEP on a single-shot basis and in real-time in experiments with a Time-of-flight-spectrometer or coincidence spectroscopy allows the time-resolved study of ionization and dissociation of atoms and molecules.

The second major topic in this context is the generation of attosecond XUV pulses and their application. One of them is coherence tomography in the extremely ultraviolet and soft X-ray spectral range (XCT). The XUV radiation is produced via high harmonic generation. XCT makes efficient use of almost the entire harmonic spectrum and allows cross-sectional imaging, i.e. resolving depth structures on the nanometer scale and to achieve a very good material contrast. A full reconstruction of the field (!) reflectivities of internal interfaced and the dispersion of internal layers is achieved, i.e. the attosecond structure of the reflected field is determined. 

HI-Jena and FSU Jena have a long-standing tradition in the development and realization of schemes in laser-driven particle acceleration and the generation of secondary radiation and in providing access to external users e.g. through Laserlab. Furthermore, Jena has a year-long expertise in x-ray spectroscopy, which has also been used in a variety of such experiments. These research activities are carried out on the different high-power laser systems in Jena with POLARIS being one of them.

In addition to the application of high-power laser systems, their development and optimization is also one of Jena’s major research themes. This allowed for the development and commissioning of POLARIS in the past – being a worldwide unique system – but also for the constant optimization of the systems. In addition to an extended research activity to improve the temporal contrast of such systems (extending the more classical approach using double-CPA architecture and the application of plasma mirrors into passive solutions, which are included in the laser system itself), we have equipped all our high-power laser systems with synchronized, few-cycle optical probe pulses which facilitate very complex pump-probe scenarios. In these investigations, we could either exploit the extreme temporal resolution of the probe pulses down to few fs to visualize ultra-fast transient phenomena such as the laser-driven plasma wave. On the other hand, a high flexibility in the probe pulse’s wavelength allows for almost background-free probing scenarios, e.g. from solid or liquid targets.

Our institute offers access to standard laboratory equipment and laser diagnostics.

The POLARIS system delivers its laser pulses into a target area, which is fully equipped with high-resolution diagnostics for the generated particle and radiation pulses. Different focusing geometries and target types (solid, liquid, cryogenically cooled and gaseous targets) can be accommodated in this target area giving the experimental user a high flexibility in the realization of various experimental ideas. In addition to these passive diagnostics, a synchronized, frequency-broadened probe pulse systems is available allowing for a large variety of complex pump-probe experiments. Furthermore, we can rely on the world-known expertise in x-ray spectroscopy available at our institute.