Blazed X-ray Optics
The diffractive zone plate optics developed and fabricated at the LMN enable ultra-high resolution for X-ray microscopy applications. However, the efficiency of conventional (binary) zone plates is limited. Therefore the design, fabrication and application of so called blazed diffractive X-ray optics is one of the research topics of the X-ray optics group. These special optics are designed to induce a continuous or a multi-level phase change, thus overcoming the fundamental limitations of conventional designs. Two main approaches under development are a tilted geometry of two specially designed 1D-lenses and the stacking of two Fresnel zone plates on top of each other.
The theoretical efficiency limits of different types of zone plate structures.
One implementation of blazed X-ray optics is a nano-focusing module based on two linear Fresnel zone plates in tilted geometry. Adjustment of the tilt angle enables tuning of the setup for optimal efficiency over a wide range of photon energies. The zone plates are specially designed to generate a blazed phase profile in tilted geometry, thus overcoming the efficiency limitations of binary diffractive structures. The tilted arrangement has the important advantage, that the acceptance of the lenses is not limited to the height of the individual lenslets. We achieved diffraction efficiencies of >50% for two Nickel lenses in series (>70% for a single lens) at 8 keV photon energy. The lenses have an acceptance of 400 µm × 440 µm and provide spot sizes of ~100 nm FWHM .
Schematics of the lens specially designed for blazed profile in tilted geometry. Left: a single lenslet. Right: the principle of a full lens.
The second approach discussed is stacking two FZPs. Previously stacking of two identical FZPs has been used to double the height of the structures and thus the induced phase shift. Our approach is to design the individual FZPs to form a multilevel phase profile when stacked, effectively approximating the optimal continuous blazed profile.
Principle of stacking two Fresnel zone plates to form an effectively blazed staircase profile.
In a collaboration funded by the Nanoscopium beam line of Synchrotron SOLEIL, we have developed schemes for the blazed stacking of zone plates with 3, 4, and 6 height levels by aligned electron-beam exposures and electroplating of Nickel. The measured efficiencies reached 54% at 6.5 keV photon energy .
A three level nickel zone plate with 200 μm diameter and effective 200 nm smallest zone width.
- P. Karvinen, C. Borca, M. Willimann, B. Meyer, M. Birri, D. Grolimund, J. Patommel, G. Wellenreuther, G. Falkenberg, M. Guizar-Sicairos, A. Menzel and C. David, Kinoform diffractive lenses for efficient nano-focusing of hard X-rays Optics Express 22 (2014) p. 16676
- I. Mohacsi, P. Karvinen, I. Vartiainen, V.A. Guzenko, A. Somogyi, C. Kewish, P. Mercere, and C. David, High efficiency X-ray nanofocusing by multilevel zone plates Journal of Synchrotron Radiation 21 (2014) p. 497
- I. Mohacsi, I. Vartiainen, M. Guizar-Sicairos, P. Karvinen, V.A. Guzenko, E. Müller, C.M. Kewish, A. Somogyi and C. David Fabrication and characterization of high efficiency double-sided blazed X-ray optics Optics Letters 41 (2016) p. 281