EUV and Hard X-ray Hartmann Wavefront Sensing for Optical Metrology, Alignment and Phase Imaging

Abstract

For more than 15 years, Imagine Optic have developed Extreme Ultra Violet (EUV) and X-ray Hartmann wavefront sensors for metrology and imaging applications. These sensors are compatible with a wide range of X-ray sources: from synchrotrons, Free Electron Lasers, laser-driven betatron and plasma-based EUV lasers to High Harmonic Generation. In this paper, we first describe the principle of a Hartmann sensor and give some key parameters to design a high-performance sensor. We also present different applications from metrology (for manual or automatic alignment of optics), to soft X-ray source optimization and X-ray imaging.

Keywords: EUV wavefront sensor; Hartmann sensor; X-ray sources; X-ray wavefront sensor; metrology; phase imaging.

Figure 1

(a) Schematic representation of the incoming aberrated wave (in green), going through the Hartmann plate and reaching the detector. (b) The white dots correspond to the diffraction spots when a perfect beam reaches the detector, while the green dots correspond to an unknown beam. Courtesy of Dr. Li Lu.

Figure 2

Raw image illustrating the diffraction pattern created by the grid of rotated square holes.

Figure 3

Simulation of wavefront errors (line with triangles) as a function of photon energy for a soft X-ray wavefront sensor design. The blue line corresponds to a wavefront error of λ/20 RMS, while the dashed blue line corresponds to λ/50 RMS.

Figure 4

Soft X-ray wavefront sensor attached to the soft X-ray beamline at the Linac Coherent Light Source (LCLS). Reprinted with permission from [22].

Figure 5

Wavefront maps before (a) and after (c) manual alignment of a Kirkpatrick-Baez. Energy distribution at the focal spot before (b) and after (d) alignment. Adapted with permission from [22].

Figure 6

Wavefront map before (a) and after (b) the correction applied to the mirror.

Figure 7

Wavefront maps in false color of (a) the High Harmonic Generation and (b) the infrared driving laser.

Figure 8

False color maps of the intensity distribution at the position of high harmonic emission for the infrared (a) and High Harmonic Generation (b) beams (adapted with permission from [26] © The Optical Society).

Figure 9

Schematic presentation of the experiment. The infrared laser path is displayed as the red beam while HHG is represented in purple. The picture shows an Extreme Ultra Violet (EUV) wavefront sensor manufactured by Imagine Optic. The image shows the best infrared focal spot achieved during the experiment and measured with the microscope.

Figure 10

Variation of the HHG wavefront versus the IR driving laser wavefront considering pure 45° astigmatism (a) and pure 0° coma (b).

Figure 11

Intensity (a) and wavefront map (b) for the l = −1 mode of the 16th high-harmonic order emission (adapted from [50]).

Figure 12

Shot-to-shot stability of the main aberrations for the High Harmonic Generation seed (a) and the seeded soft X-ray laser (b). Complementing the strong wavefront improvement, the wavefront is much more stable after amplification (reprint with permission from [21] © The Optical Society).

Figure 13

False color map of the 25th harmonic wavefront after passing through the 300 nm aluminum foil (a) before and (b) after the irradiation by the 3 keV FEL.

Figure 14

(a) Simulated hologram. (b) Reconstruction obtained using the general (planar waves) algorithm assuming no aberrations on the reference beam while in (c), the wavefront information was included in the reconstruction. X and Y axis are in µm. Adapted with permission from [72].

Figure 15

Experimental setup for phase imaging showing the Excillum X-ray source, the sample and the hard X-ray wavefront sensor (HASO HXR).

Figure 16

I/I₀ map for the Hartmann mask for (a) the capillary with water, (b) the Poly(methyl methacrylate) (PMMA) and (c) the capillary with oil. An intensity ratio of one corresponds to the air (at the top of the three maps). Deflection (in µrad) maps for (d) the capillary with water, (e) the PMMA and (f) the capillary with oil. Sample magnification of 3.08. X and Y axis are in mm.

Figure 17

For the three samples (water in blue, PMMA in orange and oil in green): (a) average of the transmission map and (b) average of the deflection map along the Y direction.

Figure 18

(a) Transmission map, (b) deflection map along the X direction and (c) deflection map along the Y direction obtained with the sample (polycarbonate tube filled with water) located at ~6 cm from the mask.