Ellipsoidal mirror offers an alternative method to focus soft x-ray

Fresnel zone plates are used to focus soft x-ray beams via diffraction in devices with spatial resolution of less than 100 nanometers. Unfortunately, these optics have low efficiency and can result in chromatic aberration.

Takeo et al. built an x-ray focusing system using an ellipsoidal mirror fabricated by electroforming based replication of a master mandrel. They found that the mirror, which focuses beams via reflection, has a higher efficiency than Fresnel zone plates with no chromatic aberration.

The mirror allows researchers to illuminate samples with greater intensity of nanobeams compared to Fresnel zone plates. Once the sample is aligned in the incident beam, it doesn’t need to be readjusted when the wavelength is changed. It has a working distance of 30 millimeters with a theoretical spot size of 50 nanometers.

“The extremely intense beam formed by an ellipsoidal mirror would reveal new physical phenomena like nonlinear optical effects. The focusing system without chromatic aberration would free researchers of the need to adjust sample position to the focal point during an energy scan,” said author Yoko Takeo.

The authors fabricated the mirror with an entrance aperture of 8.2 millimeters. They measured it’s focusing performance using ptychographic measurement. The mirror successfully focused soft X-rays onto sizes of less than 300 nanometers at wavelengths of 2.4, 3.1 and 4.1 nanometers.

“By using an ellipsoidal mirror, unrevealed characteristics might be observed in the realm of material science or images of biological cells could be obtained with better contrast,” said Takeo.

The authors are currently working to resolve the fabrication errors of the mirror. They look to improve the reflectivity of the mirrors by altering the material on the inner surface.

Source: “Soft x-ray nanobeam formed by an ellipsoidal mirror,” by Yoko Takeo, Akihiro Suzuki, Hiroto Motoyama, Yoshinori Takei, Takehiro Kume, Yusuke Matsuzawa, Yasunori Senba, Hikaru Kishimoto, Haruhiko Ohashi, and Hidekazu Mimura, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/1.5144932 .