2 nd Swedish-German Workshop on X-Ray Optics HZB Berlin-Adlershof, 28-30 April 2015 Wavefront metrology and beam characterization in the EUV/soft X-ray spectral range K. Mann J.O. Dette, J. Holburg, F. Kühl, U. Leinhos, M. Lübbecke, T. Mey, M. Müller, M. Stubenvoll, B. Schäfer Laser-Laboratorium Göttingen e.v. Hans-Adolf-Krebs Weg 1 D-37077 Göttingen Laser- Laboratorium Göttingen e.v.
Dept. Optics / Short Wavelengths Optics test (351 193 nm) (Long term) degradation (10 9 pulses) Non-linear processes LIDT Absorption / Scatter losses Wavefront deformation / Thermal lensing Beam propagation Wavefront coherence M² EUV/XUV source LPP / pulsed gas jet compact low debris long-term stable
Table-top spectroscopy XAFS = x-ray absorption fine-structure ~ 1 20nm, broad-band (polychromatic) o Accessible x-ray absorption edges: C, N, O, Cl, Ca, Mn, Fe, Pr, o Pump-probe experiments on Pr 1-x Ca x MnO 3 Optically induced phase transition : Pr, Ca : O : Mn Großmann, Techert, Mann et al., Rev. Sci. Instr. 83 (2012)
Monochromatic radiation @ =2.88nm Table-top soft x-ray microscope 1 m o Compact soft x-ray microscope o Spatial resolution 50 nm (up to now) Bacterium radiodurans M. Müller, K. Mann Optics Express 2014
Definition: Wavefront w(x,y) / ISO 15367 w(x, y; z m ) = Surface orthogonal to energy flux of a radiation field in measurement plane z m = Surface Poynting vector S(x,y) Coherent radiation: w(x,y) is surface of constant phase: 2 ( x, y) w( x, y) w(x,y) Representation of wavefronts: Polynomials w( x, y) N a k 1 k P ( x, y) k
Hartmann-Shack Wavefront Sensor: Wavefront w(x,y): Zernike analysis Directional Distribution Intensity Distribution Advantages: Compact, robust High dynamics + sensitivity Low requirements for - spatial coherence - spectral purity
EUV Hartmann Wavefront Sensor: Optics Adjustment at FLASH FLASH / DESY @ =13.5nm Hartmann plate 0.5 Yaw [mrad] w rms =15.5nm before adjustment after adjustment w rms =3.5nm w rms =2.6nm w rms =2.6nm w rms =5.9nm Wavefront 0.0 w rms =13nm -0.5 Pitch [mrad] 0.21 0.29 0.36 0.53 B. Flöter, K. Mann, K. Tiedke et al., Nucl. Instrum. Meth. (2011) w rms ~ 10nm w rms ~ 2.5nm ~ /10 Intensity
Hartmann wavefront sensor (VUV, EUV, soft x-rays) High pulse-to-pulse stability! Adjustment of active KB-system 10µm x 10µm focal size Online optics alignment at MLS synchrotron at 13.5 nm 8
Beam parameters from Hartmann data FLASH BL2 @ =7nm Beam profiles and wavefronts of single pulses (no focusing mirror) Evaluation of: Intensity profile / Beam size Pointing stability Wavefront / Zernike coefficients Waist position and size Beam divergence Beam parameter product M 2 Beam parameters X Y w pv [nm] 5.3 ± 0.69 w rms [nm] 0.67 ± 0.09 Beam propagation parameter M 2 1.15 ± 0.08 Beam propagation parameter M 2 i 1.23 ± 0.1 1.1 ± 0.1 Beam width d [mm] 6 ± 0.2 4.4 ± 0.1 Waist position z 0,i [m] -109.2 ± 0.9-99.2 ± 1.4 Rayleigh length z R [mm] 3760 ± 484 5090 ± 731 Waist diameter d 0,i [µm] 2 nd moment 200 ± 20 223 ± 25 Divergence [µrad] 55 ± 2 44 ± 2 Spatial coherence?
Motivation 1. Coherent diffractive imaging H. Chapman, Nature Phys. (2006) 1µm Decreasing coherence 2. EUV Lithography 2µm [5] B. Chen et al., Diffraction imaging: The limits of partial coherence, Phys. Rev. B 86, 235401 (2012)
Spatial coherence: Young s experiment: interference of elementary waves M. Born and B. Wolf, Principles of Optics (1980) Cambridge University Press 11
Wigner distribution y v z x h = Fourier transform of Mutual Coherence Function: Wigner distribution mutual coherence function M. J. Bastiaans, 1986, Opt. Acta 28 1215-24
Determination of Wigner distribution: Tomographic analysis Caustic of FLASH @ =13.5nm: Phosphor screen microscope Intensity distribution Translation stage CCD camera Mapping data into 4D Wigner Fourier space Wigner function = Fourier transform of Mutual Coherence Function comprehensive beam characterization beam parameters angular characteristics spatial coherence wavefront B. Schäfer, T. Mey, K. Mann, K. Tiedtke et al., Nucl. Inst. Meth. 2011 E. Plönjes, F. Siewert, T. Mey, K. Mann et al., in preparation
Coherence properties of FLASH Literature data: Coherent area Hanbury Brown-Twiss (Singer, 2013) Double pinhole (Singer, 2012) Beam area Michelson interferometer (Hilbert, 2014) Double slit (Singer, 2008) Wigner (Mey, 2014) Wigner [1] 24.7 67 / 53 5.5 / 7.2 0.032 Double pinhole [2] 8.0 17 / 17 6.2 / 8.7 0.42 [1] T. Mey et al., Wigner distribution measurements of the spatial coherence properties of the free-electron laser FLASH, Opt. Expr. 22, 16571-16584 (2014) [2] A. Singer et al., Spatial and temporal coherence properties of single free-electron laser pulses, Opt. Expr. 20, 17480-17495 (2012) 14
Wavefront Curvature Sensor Wavefront from transport-of-intensity equ.: I / z I w I w w( x, y) Vis: Thermal lens in BK7 glass laser-irradiated 1070nm, 100W Advantages: Wavefront with high spatial resolution (Pixel) Self-referencing compact 15
Wavefront at waist of FLASH beam FLASH BL2 @ =13.5nm: Wigner: 45µm -13nm 10nm 40 mm 90 mm 130 mm 200 mm 240 mm Transport Equ. ( z=1mm): 45µm good agreement high resolution strong astigmatism EUV, soft (hard) x-rays -10nm 8.2nm
Summary: Wavefront sensing (EUV / soft x-ray) o Hartmann sensor Real-time alignment of optics Beam characterization / propagation o Curvature sensor Wigner distribution Full characterization of partially coherent beams Mutual coherence (FEL @13nm) 2 nd Swedish-German Workshop on X-Ray Optics
Thank You! Coworkers: Dr. B. Schäfer Dr. U. Leinhos Dr. T. Mey M. Müller M. Stubenvoll F. Kühl J. Holburg J.O. Dette M. Lübbecke