Development of complex X-ray optical elements
Prof. U. Kleineberg, Experimental Physics, LMU
Research profile for IMPRS:
Short wavelength radiation in the soft x-ray regime is recently gaining a lot of interest for experiments and applications in nanolithography, nanomicroscopy and ultrafast electron spectroscopy. Those highly fascinating fields of physics require the development of ultraprecise optical elements for steering, focusing and spectrally selecting short wavelenght (1-10 nm) as an important prerequisite. Recently, coherent soft X-ray sources like soft X-ray lasers, High Harmonic Generation sources or Free Electron Lasers are being developed and offer the potential for an entirely new class of experiments in the future, especially for studying electron dynamics in atoms, molecules, clusters or surfaces on an ultrashort time scale or to investigate single molecules or clusters microscopically.
The development of those complex X-ray optical elements (ultraprecise mirrors, gratings, zone plates, phase shifters and many more) based on multilayer-coated elements as well as the development of new experiments utilizing coherent soft X-ray radiation are the major research activities of my group.
Doctorate thesises are announced in the following fields :
1. Generation, characterization and use multilayer optics for attosecond pulses in the 200 eV – 1 keV photon energy range :
While almost all scientific experiments with attosecond pulses have been performed so far at photon energies below 100 eV, the further develoment of improved laser schemes as well as new conversion processes with higher conversion efficiencies will lead to the generation of attosecond pulses from HH sources with photon energies up to 1 keV or even beyond. The characterization and utilization of this radiation for time-resolved experiments requires the develoment of dedicated multilayer-based soft x-ray optical elements. This includes the development and fabrication of multilayer-coated chirped mirrors, beamsplitters, polarizers, diffractive optics, active (adaptive) optics and many more. This task is getting increasingly more difficult for higher photon energies due to the very small layer thicknesses (~ 1 nm) and extraordinary interface precision requirements (~0.1 nm) which are required. This work will be accompanied by extensive theoretical calculations and simulations (based on evolutionary algorithm codes) of aperiodic multilayer structures providing control of the spectral phase and thus control over the time response of the multilayer optics.
These new soft x-ray optical elements will be setup and used in time resolved pump-probe experiments like the setup of a new attosecond streak camera for photon energies exceeding 200 eV for the measurement of soft x-ray pulse durations on a 100 asec time scale (or even below) as well as the characterization or controlled generation/compensation of spectrally chirped soft X-ray pulses (in collaboration with Prof. Krausz, MPQ Garching).
2. Diffractive imaging of nanostructures using coherent High Harmonic radiation
Besides the timing properties High Harmonic radiation provides an almost perfectly coherent source of “laser-like” soft X-ray radiation. This radiation property together with its small wavelength is especially useful for performing holographic or diffraction experiments on nanostructures. Diffractive coherent imaging is a rather new technique for the imaging of noncrystalline nanostructures and macromolecules by recording diffraction patterns using coherent soft X-ray radiation and recalculating the diffracting structure (in 2D or 3D). While those experiments have been performed recently (by J. Miao, D. Sayre, J. Kirz and others) by using spatially filtered undulator radiation, I propose to utilize inherently coherent HH radiation to perform diffractive imaging. The diffracting test structures are fabricated on ultrathin silicon membranes by means of e-beam lithography or STM lithography. The diffraction patterns are planned to be recorded with very high resolution by using a Transmission Photoelectron Microscope with a CsI szintillator cathode as two-dimensional soft X-ray detector. First test experiments using such a Transmission PEEM for characterizing the spatial properties of focused High Harmonic radiation have already been performed (in my group) recently. The diffraction patterns will be recalculated by a phase retrieval algorithm making use of an oversampling technique based on the Fienup algorithm.
Future work will be extended towards “true” macromolecular imaging as well as the combination of diffractive imaging with time resolution to aquire a four-dimensional set of imaging data when single-shot experiments become available. The use of ultrashort radiation pulses in the sub-10 fs regime will then also provide insight in the mechanisms of radiation damage processes in macromolecules e.g. due to Coulomb repulsion effects.
3. Maskless interference nanolithography with coherent EUV radiation
The fabrication of ever smaller nanostructures is one of the key problems in modern nanoscience. While direct write techniques (e-beam, ion beam, STM/AFM) are intrinsically slow and limited to small field sizes, mask projection lithography (optical litho, EUV litho, X-ray proximity printing) suffers from severe problems with the accuracy and defectivity of the mask patterns.
A maskless approach to nanolithography is based on the interference pattern of multiple overlapping coherent EUV/SXR beams. The resulting interference pattern is recorded in ultrathin high resolution photoresists, e.g. inducing chemical modifications in Self Assembled Monolayers followed by guideed self-asssembly of block copolymers, colloids, quantum dots or many more.
The utilization of coherent EUV radiation from table-top HHG sources for interference lithography will thus be investigated in the near future.
Contribution to IMPRS curriculum:
Lecturer: Ulf Kleineberg (3+1 hours/week)
This lecture will present an introduction to the physics of short wavelength radiation with special emphesis of the soft x-ray regime. After presenting the basics the second part of the lexture will deal with sources, optics and detectors for short wavelenght radiation. The third part will cover technology and applications in the soft x-ray range like Extreme Ultraviolet lithography, Soft X-ray Microscopy and Spectroscopy.
+ Laboratory excursion “Working with Synchrotron Radiation”
This excursion offers the possibility to visit the third generation Synchrotron Radiation source BESSY II in Berlin and to join a team of researchers for a couple of days performing experiments utilizing highly brillant soft x-ray radiation. This part will give a unique insight in the daily work of a world renowned research spot.