SC4 / HIGH-INTENSITY LASER SYSTEMS
Light-matter interactions drive countless physical, chemical and biological processes. The perhaps best-known example is photosynthesis – the process by which plants store the energy of sunlight in the form of carbohydrates – which was pivotal in the development of life on earth.
However, manipulating matter with light is also of fundamental importance in science and technology. Without a doubt, light is one of the most powerful tools for investigating and processing matter. High-intensity laser systems allow inducing drastic changes in the state of matter, which otherwise only exist in the very core of stars. It is also possible to accelerate particles to extreme velocities approaching the speed of light .
Laser systems with high peak powers and short pulse durations, therefore, allow progress in the following areas :
- Fundamental research: High-intensity laser systems enable the analysis of extreme states of matter without the need for massive particle accelerator facilities. These capabilities will drive laboratory-scale research in nuclear, high-energy and vacuum physics as well as cosmology.
- Medicine / healthcare: Radiation sources driven by high-intensity lasers are versatile tools for biomedical imaging as well as novel therapy approaches. Along these lines, substantial progress can be expected in the early diagnosis as well as the treatment of cancer.
- Energy: Laser systems are important building blocks for the development of sustainable strategies for the global energy supply. Laser-induced nuclear fusion has the potential to replace conventional sources of energy. At the same time, laser systems will be instrumental in understanding and harnessing the dynamics of light-harvesting complexes to convert photons to chemical energy.
- Laser materials processing / Industry 4.0: Ultrashort pulsed laser systems offer unique capabilities for the rapid, precise and energy efficient processing of materials. In many cases, they constitute the only method for non-destructive structuring of complex materials. Consequently, exploring light-matter interactions is closely linked with industrial and technological applications that will drive the increased versatility and digitalization of the industrial production.
Tackling these scientific challenges requires higher peak powers and shorter pulse durations. A crucial role on this road will be played by innovative laser sources, nonlinear light converters as well as robust and durable integrated-optical systems.
ENABLER: EFFICIENT ULTRA-PRECISE PULSE COMPRESSION GRATINGS
Pulse compression gratings must be extremely efficient and operate for a broad bandwidth on very large areas. They must conform to highest requirements regarding wave front accuracy, scattering light suppression, robustness and power handling capability.
Depending on the use-case the gratings can operate in transmission or reflection. Typically, transmission gratings require higher aspect ratios and smaller parameter tolerances as well as very thin substrates. Reflection gratings are commonly manufactured on torsion-resistant substrates with a thickness of several centimeters. Accordingly, they have a large mass.
By far the greatest limitation of current diffraction gratings is the rather low power resistance per grating area, the so-called damage threshold. Consequently, gratings and laser beam areas are extraordinarily large (up to 500 mm) to allow for an operation with low power densities. Higher damage thresholds would allow
- more compact laser systems with smaller and cheaper components while keeping the peak power,
- increasing the peak power without increasing the size or price of the laser system,
- increasing efficiency of the laser system by reducing process times due to higher average power and more durable components.
A recent overview of typical performance parameters of compression gratings like they will be used in the ELI beamline , is shown in the following table.
Gratings for high-power laser systems can be manufactured on the surface or in the volume of a substrate. However, high efficiency and large bandwidth cannot be combined with volume gratings . The advantage of surface gratings lies in a superior control of the grating nanostructure and, thus, a significantly improved flexibility in the choice of the optical parameters. Depending on the manufacturing process the gratings are differentiated in:
- ruled gratings, which are directly ruled into the substrate with a defined tip,
- holographic gratings, which are written by interfering laser beams in photoresist and transferred into the substrate via etching processes,
- lithographic gratings, where the grating pattern is realized with direct writing or diffraction lithography in the photoresist and afterwards transferred into the substrates by means of etching processes .
The fabrication of ruled as well as holographic gratings does not scale well to large areas. Furthermore, the control over the nanostructure of the grating is limited when using holographic methods.
In contrast, direct writing processes can be scaled to very large areas while ensuring a high uniformity and accuracy over the whole grating area. Additionally, these techniques allow for a high degree of control over the nanostructure in the unit cell and flexibility with respect to the used materials and coatings. These superior properties render lithography the most promising candidate to achieve the parameters from Tab. 4. For many applications the combination of electron beam lithography and optical diffraction lithography can represent a complementary technology with even better scalability to large areas [7, 8].
Gratings with diameters of about 500 mm require substrates with a weight of over 30 kg. To handle such substrates, coat them with photoresist, clean them, etc. special procedures must be developed to ensure an excellent quality of the final nanostructures. For ultra-precise positioning of such substrates special stage systems for lithography facilities will be developed.
The approach chosen by NPL combines outstanding structuring technology with innovative material systems and novel coating methods to create gratings with highest efficiency and large bandwidth. Novel materials require new etching processes. The developed technologies will increase the degrees of freedom in the optical design and, thus, will lead to improved optical properties.
Only a combined approach that applies the entire scientific-technological bandwidth of NPL can create the necessary innovations to address the abovementioned challenges of the high-intensity laser community. The core scientific challenges which will be tackled and to which technological solutions will be developed are
- electron beam lithography, scanning beam interference lithography, diffraction lithography as well as etching and coating techniques will be scaled to substrate sizes of >500mm and weights >30kg,
- for high-power resistant optical material systems coating procedures with high homogeneity (e.g. ALD) will be established,
- high-resolution characterization technologies for sub-micrometer structures (e.g. scanning electron microscopy or atomic force microscopy) will be applied to large substrates.
The scientific partner has excellent prerequisites to develop the required technological chain. The long standing expertise is illustrated by the successful work on numerous projects from the envisioned topic. Optical high-end components as well as worldwide recognition for excellence in characterizing large nanostructured components were achieved [9-14]. The projects were always tightly connected to R&D activities of equipment manufacturers. Essential processing facilities, technologies as well as metrology for the planned scaling of the technology chain will be further developed in intense cooperation with the users of high-power laser systems. Close scientific connections exist to the important research facilities in the field of high-power lasers. Those stand primarily on the development and application of:
- highly innovative laser systems, especially based on fiber lasers for ultrashort pulses,
- fiber based systems for nonlinear pulse compression,
- systems for measuring the absolute phase of optical pulses,
- systems for increasing the power of laser systems based on spatial and temporal coherent combination and enhancement cavities,
- as well as integrated, nonlinear high-power light converters.
To meet the abovementioned requirements the following investments for the core technologies are necessary.
IMPORTANT PROJECTS OF THE NPL-PARTNERS
- Pulse Compressor Grating (reflective) for CFEL at Deutsches Elektronensynchrotron (DESY) link
- Beam Splitter Grating for ELI at Extreme Light Infrastrcuture (ELI) link
- Pulse Compressor Grating (reflective) for ELI at Dausinger & Giesen.
- WDM-Grating for High Power Laser System at Trumpf Lasertechnik link
- Pulse Compressor Grating (transmissive) for High Power Laser System at Trumpf Lasertechnik link
- Pulse Compressor Grating (transmissive) for High Power Laser System at SpectraPhysics link
- WDM-Grating for High Power Laser System at RWM link
- Covered Pulse Compressor Grating (transmissive) for High Power Laser System at LightConversion link
- Pulse Compressor Grating (reflective) for High Power Laser System at Institute of Photonic Sciences (ICFO) link
 W.Leemans. White paper of the ICfa-ICuil joint task force - high power laser technology for accelerators. Technical report (2011).
 G.A. Mourou, G.Korn, W.Sandner etal., eds. Extreme Light Infrastructure Whitebook - Science and Technology with Ultra-Intense Lasers. THOSS Media GmbH (2011).
 J.König, S.Nolte, and D.Sutter. Ultrakurzpulslaser für die industrielle massenfertigung - produzieren mit lichtblitzen (2013).
 J.-K. Rhee, J.A. Arns, W.S. Colburn etal. Chirped-pulse amplification of 85-fs pulses at 250 khz with third-order dispersion compensation by use of holographic transmission gratings. Opt. Lett., 19:1550 (1994). doi: 10.1364/OL.19.001550.
 N.George and J.W. Matthews. Holographic diffraction gratings. Appl. Phys. Lett., 9:212 (1966). doi: http://dx.doi.org/10.1063/1.1754714.
 U.Zeitner, M.Oliva, F.Fuchs etal. High performance diffraction gratings made by e-beam lithography. Appl. Phys. A, 109:789 (2012). doi: 10.1007/s00339-012-7346-z.
 L.Stuerzebecher, F.Fuchs, T.Harzendorf etal. Pulse compression grating fabrication by diffractive proximity photolithography. Opt. Lett., 39:1042 (2014). doi: 10.1364/OL.39.001042.
 L.Stuerzebecher, F.Fuchs, U.D. Zeitner etal. High-resolution proximity lithography for nano-optical components. Microelectron. Eng., 132:120 (2015). doi: 10.1016/j.mee.2014.10.010.
 L.Coriand, M.Mitterhuber, A.Duparré etal. Definition of roughness structures for superhydrophobic and hydrophilic optical coatings on glass. Appl. Opt., 50:C257 (2011). doi: 10.1364/AO.50.00C257.
 A.Duparré. Handbuch zur Industriellen Bildverarbeitung : Qualitätssicherung in der Praxis, chapter Charakterisierung von Mikro- und Nanostrukturen für funktionale Oberflächen und Schichten. Fraunhofer IRB Verlag (2007).
 A.Duparré and L.Coriand. Advances in Contact Angle, Wettability and Adhesion, chapter Assessment Criteria for Superhydrophobic Surfaces with Stochastic Roughness. Wiley InterScience (2013).
 A.Duparré, J.Ferre-Borrull, S.Gliech etal. Surface characterization techniques for determining the root-mean-square roughness and power spectral densities of optical components. Appl. Opt., 41:154 (2002). doi: 10.1364/AO.41.000154.
 A.Duparré, M.Flemming, G.Notni etal. Nanorauheit statt lotusstruktur: Chancen für ultrahydrophobe optische Oberflächen. Photonik, 2 (2005).
 S.Schröder, T.Herffurth, H.Blaschke etal. Angle-resolved scattering: an effective method for characterizing thin-film coatings. Appl. Opt., 50:C164 (2011). doi: 10.1364/AO.50.00C164.
- High intensity laser labors
- Laboratories for laser-based material processing
- Nuclear physics
- Laboratory cosmology
- Basic physics
- Elementary Particle Physics
- Biology / Medicine
- Laser material processing
- Large-area, high-precision, high-efficiency, high-efficiency compressors