The generation of extremely short laser pulses with cutting edge parameters has always attracted significant attention, extending far beyond laser physics. For the MBI as a research institute devoted to short pulse spectroscopy and nonlinear optics the development of novel laser sources is of paramount importance. Laser sources developed in-house can offer parameters that are unavailable from com-mercial lasers. It is this availability of unique sources that enables unique experiments.

Consequently, one of the focal points of the research strategy of the MBI is the generation of ex-tremely short pulses in a broad wavelength region. Often, applications in spectroscopy demand a spe-cific wavelength range, and it is far from trivial to produce few-cycle pulses outside the visible/near-infrared range accessible by Ti:sapphire lasers. Therefore, we pursue Raman pulse compression as one favorable method to generate pulses outside this range, in particular in the vacuum ultraviolet range. Further efforts concentrate on the identification of novel materials and methods, both for build-ing mode-locked laser oscillators and for frequency conversion by harmonic and parametric nonlinear processes.

One of the key missions of the MBI is the amplification of ultra-short pulses to extremely high intensi-ties of the order of 10²⁰W/cm², or to high average powers for dedicated applications. The synchronisa-tion of two separate ultra-high intensity lasers with pulse energies in the Joule (Ti:Sa) and 10 Joule range (Nd:Glass) at MBI opens a new and exclusive route towards particle acceleration and proton imaging experiments in plasma physics. The development of novel high-average-power ps-lasers over the last decade has made MBI an indispensible partner for cooperation with the national and interna-tional high-energy physics and Free-Electron-Laser community.

The overall goal of the laser research at MBI is to generate of light pulses with record-breaking pa-rameters over a wide range of wavelengths and energies, and to directly enable their use for applica-tions in spectroscopy and related studies of ultrafast and nonlinear phenomena in the key fields of interest at the MBI (see focal areas 2 and 3). Laser research at MBI is strongly interconnected, both, among the different research themes within the laser research as well as to direct applications in the other focal areas. Many of these activities are embedded into international collaborations and are made accessible to external users, most notably through the Transnational Access Activity within the EU laser infrastructure network LASERLAB-EUROPE.

Ultrafast nonlinear optics and few-cycle pulses

The efforts in generating few-cycle pulses encompass the relatively widespread method of pulse com-pression in hollow gas-filled fibers where pulses with as few as 1.6 optical cycles have already been generated – corresponding to a duration of 4.3 fs. Similarly, we explore white-light continuum genera-tion processes in microstructured fibers. Our efforts include generation of very short pulses in the vac-uum ultraviolet, where Raman based compression methods, also in a hollow-fiber geometry, are in-vestigated for pulses of a few femtoseconds pulse duration. These activities are backed up by theo-retical work, paving the way for novel methods for the generation of extremely short pulses over the entire laser-accessible wavelength region, ranging from the deep ultraviolet, the visible and near-infrared up to mid-infrared pulse generation.

As a prerequisite for further pulse shortening in most of these spectral ranges, the ultrashort pulses have to be up- or downconverted from the near infrared, where the majority of the femtosecond laser systems operate. To improve such nonlinear optical conversion with respect to efficiency and control the bandwidth of the pulses novel materials and conversion schemes are examined.

It is very challenging to characterize and actively shape such extremely short laser pulses. The efforts of generating short pulses therefore have to be augmented by means to measure their complex spa-tio-temporal structure and, one step further, also to control spatial or temporal parameters of the few-cycle wave packets.

For details see research project 1-01.

Short pulse laser systems

Different concepts for advanced short-pulse lasers based on Ti:sapphire, rare-earth doped crystals and microstructure fibers for femtosecond and picosecond oscillator and amplifier systems are under investigation.

The potential of novel ytterbium and neodymium doped active materials are studied in the 1-µm spec-tral range. In particular, Yb-doped laser crystals are well-suited for building conceptually simple and highly efficient diode-pumped femtosecond lasers with high output power. Among those materials, the monoclinic double tungstates Yb:KY(WO₄)₂ and Yb:KGd(WO₄)₂ stand out because of their large ab-sorption and emission cross sections, which was demonstrated using diode-pumped oscillators with 100 fs pulse duration. The isotropic sesquioxides Sc₂O₃ , Y₂O₃ and Lu₂O₃ are, however, more attractive for high-power applications because of their excellent thermo-mechanical properties. We have proven the excellent potential of these materials by demonstrating a 54% efficiency for Yb:Sc₂O₃ in mode-locked operation, which is the highest optical efficiency ever reported from a mode-locked laser.

Compared to conventional fiber designs, microstructure fibers have considerably enhanced the possi-bilities of tailoring linear and nonlinear fiber properties. For mode-locked fiber lasers, dispersion engi-neering is of particular interest, as it enables intrinsic dispersion compensation or soliton propagation at virtually arbitrary wavelengths. Recently, we demonstrated mode-locking of Nd-doped microstruc-ture fiber laser, which is the first demonstration of mode-locked microstructure fiber lasers in the 1-mm region.

Besides these conceptual investigations this project also contains two main research efforts aiming at oscillator-amplifier systems with either high peak power or high average power. The MBI ultra-high intensity Ti:Sa laser, providing intensities in excess of 10¹⁹ W/cm², is with 35fs pulse duration among the shortest-pulse multi-10-TW systems world-wide. New research developments focus on contrast and beam quality improvement within the European SHARP collaboration. A series of novel concepts for high-average-power lasers has rendered MBI as one of the leading laboratories for burst-mode (with up to 5kW average power during ms-bursts) and quasi-cw ps-lasers. The most recent develop-ments employ fully diode-pumped and OPCPA systems, but also high-average power Ti:Sa fs-lasers. Applications include driver lasers for incoherent x-ray sources and, to a large extent, unique photo-cathode drivers and pump-probe user endstations for newly established Free Electron Lasers at col-laborating laboratories.

For details see research project 1-02.