## Partially coherent nano-focused x-ray radiation characterized by Talbot interferometry |

Optics Express, Vol. 19, Issue 10, pp. 9656-9675 (2011)

http://dx.doi.org/10.1364/OE.19.009656

Acrobat PDF (2375 KB)

### Abstract

We have studied the spatial coherence properties of a nano-focused x-ray beam by grating (Talbot) interferometry in projection geometry. The beam is focused by a fixed curvature mirror system optimized for high flux density under conditions of partial coherence. The spatial coherence of the divergent exit wave emitted from the mirror focus is measured by Talbot interferometry The results are compared to numerical calculations of coherence propagation. In view of imaging applications, the magnified in-line image of a test pattern formed under conditions of partial coherence is analyzed quantitatively. Finally, additional coherence filtering by use of x-ray waveguides is demonstrated. By insertion of x-ray waveguides, the beam diameter can be reduced from typical values of 200 nm to values below 15 nm. In proportion to the reduction in the focal spot size, the numerical aperture (NA) of the projection imaging system is increased, as well as the coherence length, as quantified by grating interferometry.

© 2011 OSA

## 1. Introduction

1. K. Nugent, “Coherent methods in the x-ray sciences,” Adv. Phys. **59**, 8732 (2010). [CrossRef]

3. I. A. Vartanyants and A. Singer, “Coherence properties of hard x-ray synchrotron sources and x-ray free-electron lasers,” N. J. Phys. **12**, 035004 (2010). [CrossRef]

4. J.-P. Guigay, S. Zabler, P. Cloetens, C. David, R. Mokso, and M. Schlenker, “The partial Talbot effect and its use in measuring the coherence of synchrotron X-rays,” J. Synch. Radiat. **11**, 476–482 (2004). [CrossRef]

7. A. Diaz, C. Mocuta, J. Stangl, M. Keplinger, T. Weitkamp, F. Pfeiffer, C. David, T. H. Metzger, and G. Bauer, “Coherence and wavefront characterization of Si-111 monochromators using double-grating interferometry,” J. Synch. Radiat. **17**, 299–307 (2010). [CrossRef]

*j*(

*z,d*) describing the phase correlations between two points separated by a distance

*d*in a plane perpendicular to the optical axis a distance

*z*> 0 behind the focus. We show that by use of an x-ray waveguide the partial coherent wavefront of a Kirkpatrick-Baez (KB) mirror system can be filtered to yield nearly fully coherent exit waves behind the waveguide at the relevant sample distances.

8. J. Miao, P. Charalambous, J. Kirz, and D. Sayre, “Extending the methodology of x-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens,” Nature **400**, 342–344 (1999). [CrossRef]

9. B. Abbey, K. A. Nugent, G. J. Williams, J. N. Clark, A. G. Peele, M. A. Pfeifer, M. de Jonge, and I. McNulty, “Keyhole coherent diffractive imaging,” Nat. Phys. **4**, 394–398 (2008). [CrossRef]

11. K. Giewekemeyer, H. Neubauer, S. Kalbfleisch, S. P. Krger, and T. Salditt, “Holographic and diffractive x-ray imaging using waveguides as quasi-point sources,” N. J. Phys. **12**, 035008 (2010). [CrossRef]

12. J. R. Fienup, “Reconstruction of an object from the modulus of its fourier transform,” Opt. Lett. **3**, 27–29 (1978). [CrossRef] [PubMed]

13. H. M. Quiney, A. G. Peele, Z. Cai, D. Paterson, and K. A. Nugent, “Diffractive imaging of highly focused x-ray fields,” Nat. Phys. **2**, 101–104 (2006). [CrossRef]

14. S. Marchesini, “Invited article: A unified evaluation of iterative projection algorithms for phase retrieval,” Rev. Sci. Instrum. **78**, 011301 (2007). [CrossRef] [PubMed]

15. J. M. Rodenburg, A. C. Hurst, A. G. Cullis, B. R. Dobson, F. Pfeiffer, O. Bunk, C. David, K. Jefimovs, and I. Johnson, “Hard-x-ray lensless imaging of extended objects,” Phys. Rev. Lett. **98**, 034801– (2007). [CrossRef] [PubMed]

16. P. Thibault, M. Dierolf, A. Menzel, O. Bunk, C. David, and F. Pfeiffer, “High-resolution scanning x-ray diffraction microscopy,” Science **321**, 379–382 (2008). [CrossRef] [PubMed]

17. M. Guizar-Sicairos and J. R. Fienup, “Phase retrieval with transverse translation diversity: a nonlinearoptimization approach,” Opt. Express **16**, 7264–7278 (2008). [CrossRef] [PubMed]

18. A. Schropp, P. Boye, J. M. Feldkamp, R. Hoppe, J. Patommel, D. Samberg, S. Stephan, K. Giewekemeyer, R. N. Wilke, T. Salditt, J. Gulden, A. P. Mancuso, I. A. Vartanyants, E. Weckert, S. Schrder, M. Burghammer, and C. G. Schroer, “Hard x-ray nanobeam characterization by coherent diffraction microscopy,” Appl. Phys. Lett. **96**, 091102 (2010). [CrossRef]

19. C. M. Kewish, P. Thibault, M. Dierolf, O. Bunk, A. Menzel, J. Vila-Comamala, K. Jefimovs, and F. Pfeiffer, “Ptychographic characterization of the wavefield in the focus of reflective hard x-ray optics,” Ultramicroscopy **110**, 325–329 (2010). [CrossRef] [PubMed]

20. S. Kalbfleisch, M. Osterhoff, K. Giewekemeyer, H. Neubauer, S. P. Krger, B. Hartmann, M. Bartels, M. Sprung, O. Leupold, F. Siewert, and T. Salditt, “The holography endstation of beamline P10 at PETRA III,” SRI 2009, AIP Conf. Proc. **1234**, 433–436 (2010). [CrossRef]

21. S. P. Krger, K. Giewekemeyer, S. Kalbfleisch, M. Bartels, H. Neubauer, and T. Salditt, “Sub-15 nm beam confinement by twocrossed x-ray waveguides,” Opt. Express **18**, 13492–13501 (2010). [CrossRef]

11. K. Giewekemeyer, H. Neubauer, S. Kalbfleisch, S. P. Krger, and T. Salditt, “Holographic and diffractive x-ray imaging using waveguides as quasi-point sources,” N. J. Phys. **12**, 035008 (2010). [CrossRef]

22. C. Fuhse, C. Ollinger, and T. Salditt, “Waveguide-based off-axis holography with hard x rays,” Phys. Rev. Lett. **97**, 254801 (2006). [CrossRef]

*D*of the WG guiding layer as an ’ultra-narrow slit’ to scan the beam around the KB focus. In the last section, the characteristic mode structure of waveguides is exploited, using the waveguide as a coherence filter. Waveguide mode structure and transmission is calculated using finite-difference (FD) simulations [23

23. C. Fuhse and T. Salditt, “Finite-difference field calculations for two-dimensionally confined x-ray waveguides,” Appl. Opt. **45**, 4603–4608 (2006). [CrossRef] [PubMed]

26. Y. V. Kopylov, A. V. Popov, and A. V. Vinogradov, “Application of the parabolic wave equation to x-ray diffraction optics,” Opt. Commun. **118**, 619–636 (1995). [CrossRef]

## 2. Experimental setup and parameters

*β*configuration consists of a 5 m long undulator with a period of 29 mm, with a source size of 36

*μ*m × 6

*μ*m (1

*σ*, horz. × vert.). The beamline was operated in monochromatic mode using a Si(111) double crystal monochromator, positioned at 35 m behind the source. At a distance of 87.7m the x-ray beam is focused by two elliptically shaped mirrors (fixed shape) in Kirkpatrick-Baez (KB) geometry, contained in a vaccuum vessel. The mirrors are configured to a fixed elliptical shape, corresponding to an incidence angle of

*α*= 4.00 mrad and

*α*= 4.05 mrad, as well as a focal length of 305 mm and 200 mm, for the vertically (v) focusing mirror (WinlightX, France) and the horizontally (h) focusing mirror (JTEC Corporation, Japan), respectively. The mirrors consist of

*Pd*-covered silicon (v-mirror) and

*Pd*-covered silica (h-mirror), respectively. For the h-mirror, which was polished by elastic emission machining [27

27. H. Mimura, H. Yumoto, S. Matsuyama, Y. Sano, K. Yamamura, Y. Mori, M. Yabashi, Y. Nishino, K. Tamasaku, T. Ishikawa, and K. Yamauchi, “Efficient focusing of hard x-rays to 25 nm by a total reflection mirror,” Appl. Phys. Lett. **90**, 051903 (2007). [CrossRef]

20. S. Kalbfleisch, M. Osterhoff, K. Giewekemeyer, H. Neubauer, S. P. Krger, B. Hartmann, M. Bartels, M. Sprung, O. Leupold, F. Siewert, and T. Salditt, “The holography endstation of beamline P10 at PETRA III,” SRI 2009, AIP Conf. Proc. **1234**, 433–436 (2010). [CrossRef]

*xyz*, and two rotations along two directions orthogonal to the optical axis. Alignment of the waveguide as well as the Talbot gratings was facilitated by use of two on-axis optical microscopes whose focal planes coincide with the KB. The two microscopes are directed to the focus with a ’downstream’ and an ’upstream view’, respectively. For x-ray beam path, the ’downstream view’ microscope (Optique Peter) has a hole in the deflecting mirror, while the ’upstream view’ microscope observing the sample at a fixed working distance of 32 mm, has a drilled objective (Bruker AXS), providing in-situ inspection. The sample stage is equipped with an air-bearing rotation (Micos) for ultra-high precision turns needed for nano-tomography. On top of the rotation, a group of

*xyz*piezos (Physik Instrumente) is used for aligning the sample in the axis of rotation. Additional

*xyz*stages (Micos) below the rotation are used for aligning the rotation axis in the x-ray beam and for distance variation between the waveguide and the sample. The detectors were placed at a distance of

*z*

_{1}+

*z*

_{2}= 5.29m behind the sample, see Fig. 1. Along with a direct-illumination CCD (LCX, Princeton Instruments) used primarily for the data analyzed here, a scintillator CCD (SCX, Princeton Instruments) and a single photon counting pixel detector (Pilatus 300K, Dectris) was available, and used to cross-check the far-field signal and/or to determine the integral flux.

**Waveguides:**The beam is coupled into the waveguide by the so-called front-coupling scheme [22

22. C. Fuhse, C. Ollinger, and T. Salditt, “Waveguide-based off-axis holography with hard x rays,” Phys. Rev. Lett. **97**, 254801 (2006). [CrossRef]

11. K. Giewekemeyer, H. Neubauer, S. Kalbfleisch, S. P. Krger, and T. Salditt, “Holographic and diffractive x-ray imaging using waveguides as quasi-point sources,” N. J. Phys. **12**, 035008 (2010). [CrossRef]

20. S. Kalbfleisch, M. Osterhoff, K. Giewekemeyer, H. Neubauer, S. P. Krger, B. Hartmann, M. Bartels, M. Sprung, O. Leupold, F. Siewert, and T. Salditt, “The holography endstation of beamline P10 at PETRA III,” SRI 2009, AIP Conf. Proc. **1234**, 433–436 (2010). [CrossRef]

*E*. An array of (well separated) waveguide channels is first exposed into an e-beam resist layer on a Si wafer by electron beam lithography. After development this resist layer acts as an etching mask for the reactive ion etching (RIE) of the waveguide channels. After the removal of the etching mask, a second (cap) wafer is bonded onto the first (structured) wafer, forming the waveguide chip. The entrances and exits on the cleaved edges of the waveguide chip are then polished by Focused Ion Beam (FIB) polishing (FEI, Nova 600 Nanolab). The cross-section of the channels can be varied, typically in the range between 30 nm×20 nm and approx. 120 nm×60 nm, depending on the specific application of the experiment. (B) For smaller waveguide dimensions, an arrangement of crossed high-transmission planar x-ray waveguides is used similar to [28

28. L. D. Caro, C. Giannini, D. Pelliccia, C. Mocuta, T. H. Metzger, A. Guagliardi, A. Cedola, I. Burkeeva, and S. Lagomarsino, “In-line holography and coherent diffractive imaging with x-ray waveguides,” Phys. Rev. B **77**, 081408 (2008). [CrossRef]

*C*as the guiding layer [21

21. S. P. Krger, K. Giewekemeyer, S. Kalbfleisch, M. Bartels, H. Neubauer, and T. Salditt, “Sub-15 nm beam confinement by twocrossed x-ray waveguides,” Opt. Express **18**, 13492–13501 (2010). [CrossRef]

29. T. Salditt, S. P. Krger, C. Fuhse, and C. Bhtz, “High-transmission planar x-ray waveguides,” Phys. Rev. Lett. **100**, 184801–4 (2008). [CrossRef] [PubMed]

*Ge*/

*Mo*[

*d*

*=30 nm]/*

_{i}*C*[

*D*=35 nm]/

*Mo*[

*d*

*=30 nm]/*

_{i}*Ge*was deposited on a 3 mm thick

*Ge*single crystal substrate (Incoatec GmbH, Germany). A second so-called cap wafer (

*Ge*, 440

*μ*m thickness) was bonded onto the WG wafer by an alloying process to block the beam areas not impinging onto the waveguide entrance. The resulting ‘sandwich’ sample was cut by a dicing saw to the desired lengths

*l*= 200

*μ*m, and cleaned by FIB.

**Simulation of beam propagation and coherence:**In order to simulate the focusing properties and near-field intensities of the KB-focus, wave-optical propagation was carried out numerically. The undulator source at 7.9 keV with the nominal source size as given above was discretized into a set of independent emitters, with Fresnel-Kirchhoff integrals used to calculate the propagation from source to mirror, as well as from mirror to the focus. To this end, the measured height profile of the mirror is taken into account. Partial coherence is modeled by averaging stochastic realizations emitted by the virtual point-sources. The equal time complex degree of coherence

*u*

_{1,2}and intensities

*I*

_{1,2}was computed from a random phase superposition

*u*(

*x*) = Σ

_{n}*w*

_{n}*c*

_{n}*u*

*(*

_{n}*x*), with equally distributed) phases and (real valued) weight factors

*w*

*adjusted to the Gaussian envelope of the source. For each basis field*

_{n}*u*

*, free space propagation and reflection was carried out, and an ensemble average of 8000 stochastic realizations of random field distributions was evaluated in the ’detection’ plane of interest, modeling the time-averaging measurement process. The reflection process was modeled by the Fresnel equations, applied locally to a discretized elliptically shaped curve (in 1+1 dimension) including the measured surface deviation from the perfect ellipse, using 32000 data points for integration, randomly distributed on the curve (surface). Details of this method will be described elsewhere (Osterhoff and Salditt, in preparation). Results for the near-field intensity distribution around the focal point are shown in Fig. 2(e) for the case of the horizontally focusing mirror (JTEC) on logarithmic scale, simulated for the experimental photon energy, horizontal source size, and distances, as given above.*

_{n}**Grating and test structures:**For coherence measurements by the Talbot effect, a high resolution chart (NTT-AT, Japan, model # ATN/XRESO-50HC) consisting of a 500 nm thick nano-structured tantalum layer on a

*Ru/SiC/SiN*membrane was placed in the beam at a distance

*z*

_{1}downstream from the KB focus, as determined by the on-axis optical microscope. For this study, we used three different gratings on this test structure, namely 500 nm lines and spaces, 200 nm lines and spaces, and 50 nm lines and spaces, respectively. Note that for example 500 nm lines and spaces (

*l*&

*s*), means 500 nm lines followed by 500 nm spaces, i.e. the given value is half the grating period. At 7.9 keV, the calculated phase shift of a 500 nm

*Ta*pattern is

*ϕ*= 0.830 rad, and the intensity transmission is

*T*= 0.871. In addition to the grating measurements, the hologram of a double slit test structure was analyzed, based on analytical forward calculations and least square fitting. The double slit test structure was fabricated by FIB etching (FEI, Nova 600 Nanolab). First, a thin

*Au*layer was deposited by e-beam deposition on a 200 nm

*SiN*foil (Silson), coated with a small 5 nm

*Ti*adhesion layer. The

*Au*film thickness was determined to 35 nm by profilometry (Veeco Dektak 6M). The

*Au*was then etched away in a double slit pattern, with a slit length of 4

*μ*m, a width

*w*= 1.6

*μ*m and a distance between the two slits of 6

*μ*m. After fabrication, the geometric layout was checked by e-beam microscopy.

## 3. Intensity distribution of the KB focus

*D*= 35 nm in the horizontal and

*D*= 30 nm in the vertical direction. The lateral and vertical profile of the focal intensity distribution can then be analyzed as a function of

*z*at and around the focal plane, to determine the spot sizes and the field of depth. In (a,b), the beam cross section (FWHM) are plotted as a function of

*z*after batch fitting of scans along the

*y*(vertical) and

*z*(horizontal) directions for each

*z*to a Gaussian peak profile. The over-estimated error bar values drawn correspond to ±50 nm, and may considered an upper bounds for the uncertainties, including systematic errors in parameter initialization during the batch fit. Single profiles in particular in the focal plane have been analyzed by hand with individual initialization of parameters, and fitting range, resulting in much smaller errors. Along with the experimental results (circles), the values determined from the Fresnel-Kirchhoff simulations (solid black line) and an empirical fit to a Gaussian beam profile (solid red line) are shown. The simulations of beam propagation take into account the geometrical parameters (undulator source size, distances) and measured height profiles of the mirrors, as shown in Fig. 2(e), for the case of the vertical direction. The resulting focal distribution is comparable to the experimentally measured 2D intensity distribution, as shown in (f), obtained from a series of waveguide scans (similar to those shown in (a,b)), after normalization to the peak area for each scan. The simulation and the experimental results are in qualitative agreement, as far as beam width and depth of focus is concerned, but show differences concerning the exact lineshapes and side maxima. The functional form of the empirical fit is based on a convolution of a diffraction limited beam with a Gaussian (FWHM value

*ζ*) taking into account finite source size and all other spurious beam broadening effects The finite source size broadens the diffraction limited Gaussian beam with Rayleigh length

*z*

_{0}, but from the

*z*

_{0}value of the fits the depth of focus can still be quantified. The vertical focus series is fitted to

*z*

_{0}= 92.1

*μ*m and

*ζ*= 194 nm (FWHM) with reduced

*χ*

^{2}= 0.43. The horizontal focus series is fitted to

*z*

_{0}= 52.1

*μ*m and

*ζ*= 226 nm (FWHM) with reduced

*χ*

^{2}= 1.46. The Gaussian fits to the focus profiles in the focus plane as determined above yielded a spot size of 203 × 221nm

^{2}(h × v, FWHM). In Fig. 2(d), a vertical focus scan along with a Lorentzian fit is shown yielding a FWHM of 186 nm. This indicates that depending on alignment and lineshape used in fitting, individual scans also show FWHM values below 200 nm in the vertical direction. Note that the exact fitting values also fluctuate slightly in the course of successive scans, possibly also in response to mirror alignment after refilling of the storage ring and drift of optical components. The largest source of errors, however, was due to the measurement itself, i.e. the positioning system, which despite the use of encoders, resulted in stick-slip tilt inaccuracies of the positioning, at least in the horizontal direction, as cross-checked by laser interferometry. At a storage ring current of 60 mA and a photon energy of E = 7.9 keV, the measured flux in the 200 nm focal spot was 2.13 · 10

^{11}cps, as measured with a PIN diode (Canberra PD300-500CB,

## 4. Coherence measured by Talbot effect

1. K. Nugent, “Coherent methods in the x-ray sciences,” Adv. Phys. **59**, 8732 (2010). [CrossRef]

*u*probed at two points (1) and (2), and at times

*t*and

*t*+

*τ*, respectively, and can be written for stationary fields as where

*W*

_{1,2}= ∫

*dτ*Γ exp(

*iωτ*). In the case of stationary and quasi-monochromatic fields, it is sufficient to consider the mutual intensity function or equal time complex degree of coherence

*j*

_{1,2}=

*γ*

_{1,2}(0). Again, before normalization we define

*J*

_{1,2}= Γ

_{1,2}(0). In the following, the

*complex degree of coherence*will always implicitly denote the equal-time function

*j*, which is relevant here. If not stated otherwise, point (1) will be located on the optical axis, point (2) in a plane normal to the optical axis, at a distance

*d*from point (1). The degree of coherence will then be written as a function

*j*(

*d*), for fixed distance

*z*along the optical axis, or as a function of two arguments

*j*(

*z,d*), if its evolution as a function of defocus distance is considered.

*z*

_{1}can be the defocus distance in the divergent beam (laboratory) coordinate system, or

*z*

*=*

_{eff}*z*

_{1}

*z*

_{2}

*/*(

*z*

_{1}+

*z*

_{2}), if the parallel beam equivalent coordinate system is used. According to the so-called Fresnel scaling theorem for paraxial beams, free space propagation and the associated contrast formation is equivalent in the cone beam and parallel beam case, up to a simple coordinate transformation of the detector pixel size by the geometric magnification

*M*= (

*z*

_{2}+

*z*

_{1})

*/z*

_{1}and the defocus distance

*z*→

*z*

*[2*

_{eff}2. D. M. Paganin, *Coherent X-Ray Optics* (Oxford University Press, 2006). [CrossRef]

4. J.-P. Guigay, S. Zabler, P. Cloetens, C. David, R. Mokso, and M. Schlenker, “The partial Talbot effect and its use in measuring the coherence of synchrotron X-rays,” J. Synch. Radiat. **11**, 476–482 (2004). [CrossRef]

*j*| = 1, while partial coherence leads to a loss of contrast. The contrast in the detector plane evaluated for the first order Fourier coefficient

*m*= 1 of a lattice with period

*a*, reflects the degree of coherence between two points separated by

*λz*

_{eff}*/a*. Thus, the diffraction angle of the

*m*= 1 lattice reflection and the distance between object and detector (the defocus distance) determine the argument

*d*of the measured function |

*j*(

*d*)|.

*j*(

*z,y*)|, as well as selected curves |

*j*(

*y*)| for

*z*const., with the corresponding Gaussian fits. Finally, (c) shows the linear scaling of the full width at half maximum (FWHM) of the curves |

*j*(

*y*)| as a function of

*z*, in other words the lateral coherence length

*ξ*

*. The linear scaling of the coherence length and the Gaussian functional form, support the use of a Gaussian Shell-model (GSM) for the coherence properties, as put forward in [3*

_{FWHM}3. I. A. Vartanyants and A. Singer, “Coherence properties of hard x-ray synchrotron sources and x-ray free-electron lasers,” N. J. Phys. **12**, 035004 (2010). [CrossRef]

*W*(or for the quasi-monochromatic the mutual intensity

*J*) for any two points in a plane orthogonal to the optical axis separated by a distance

*ξ*or the FWHM equivalent

*q*. In the horizontal plane,

*q*≃ 0.37 is thus obtained from the constant opening angle of the beam (2 mrad) and the opening angle subtended by the lateral coherence length, which is

*ξ*

_{FWHM}*/z*≃ 0.733 mrad. In the vertical plane, simulations predict full coherence

*q ≥*1.

*l*&

*s*), aligned vertically, i.e. in the case of probing the coherence properties in the horizontal plane. As the sample is scanned in

*z*

_{1}, the visibility of the fringes oscillates between weak or no contrast as shown in (a) to high contrast as shown in (b). After vertical averaging over a region of interest (ROI), the spectral density of the profile is computed. The integrated intensity of the first order (|

*f*

_{m}_{=1}|) is then evaluated as a function of

*z*

_{1}. Since the magnification

*M*also changes with

*z*

_{1}, the position of the

*m*= 1 moves linearly in the PSD curve. An automated peak search function was used to single out the

*m*= 1 peaks. When

*z*

_{1}corresponds to a Talbot minimum, this procedure did not work well for the 200 nm

*l*&

*s*grating. In a refined procedure, the movement of the peak was analyzed by linear regression to predict the peak position, followed by intensity readout, see also Fig. 5(b), comparing the two procedures of simple peak search (open symbols) and of peak search based on regression (solid symbols). According to Talbot theory (Eq. (3) in [4

4. J.-P. Guigay, S. Zabler, P. Cloetens, C. David, R. Mokso, and M. Schlenker, “The partial Talbot effect and its use in measuring the coherence of synchrotron X-rays,” J. Synch. Radiat. **11**, 476–482 (2004). [CrossRef]

*a*is the periodicity of the lattice and

*g*(

*x*) is the complex-valued function describing the transmittance of the lattice. For the particular case of a binary phase grating, the integral can be solved analytically, see Eqs. (4) and (5) in [4

**11**, 476–482 (2004). [CrossRef]

*a*,

*ϕ*and

*T*given above, as well as the duty cycle 0.5, the integral can be evaluated numerically.

*m*= 1 Fourier coefficient extracted from the Talbot scans are fitted to the equation where

*Z*

*=*

_{T}*a*

^{2}

*/λ*is the Talbot (replication) distance, and

*z*

*=*

_{eff}*z*

_{1}

*z*

_{2}

*/*(

*z*

_{1}+

*z*

_{2}) is the effective defocus distance.

*c*= 0.440 is a prefactor which is determined numerically, by solving Eq. (3) of [4

**11**, 476–482 (2004). [CrossRef]

*|j|*.

*l*&

*s*/

*a*= 400 nm grating, and for the 500nm

*l*&

*s*/

*a*= 1

*μ*m grating. The magnitude of the first order (

*m*= 1) maximum in the power spectral density (PSD), as computed by Fourier analysis of the recorded Fresnel diffraction pattern, is plotted as a function of the defocus distance

*z*

_{1}. The fitted Talbot distances for the two gratings of

*Z*

_{200}= 1.020

*μ*m, and

*Z*

_{500}= 6.306

*μ*m correspond well with the expected Talbot distances

*a*

^{2}

*/λ*, within the experimental uncertainties. A small error

*δz*

_{1}of the nominal defocus

*z*

_{1}was allowed as a free parameter in the least-square fit. The resulting value

*δz*

_{1}= −0.62 mm and

*δz*

_{1}= −0.44 mm for the two gratings, respectively, show that the position of the test pattern as determined by the optical microscope was probably not exact. The focal plane of the optical microscope had initially been adjusted to the KB focus, but due to realignment after injection the focal plane of the KB can vary slightly from one ring filling to the next. Finally, the third and last free parameter was the fitted

*|j|*-value associated with the amplitudes of the Talbot curves, which were evaluated to

*j*

_{200}= 0.415 ± 0.003, and

*j*

_{500}= 0.927 ± 0.007, respectively. This measured coefficient

*|j*(

*z*

_{1}

*, d*)

*|*corresponds to the degree of coherence in the sample plane

*z*

_{1}for any two points separated by a lateral distance

*d*=

*λz*

_{eff}*/a*. During the scan,

*z*

_{1}and

*d*both increase linearly. For

*z*

_{1}≪

*z*

_{2},

*z*

_{1}≃

*z*

*, and a constant value*

_{eff}*|j|*=

*const.*is measured along the line forming an angle

*λ/a*with respect to the optical axis. Note that the data sets could of course be fitted with models parameterizing a variation of

*j*along the scan, but the amplitude of the Talbot effect proves constant on visual inspection, and the rather good agreement of the data with the three parameters motivates the conclusion that

*|j*(

*z*

_{1}

*,λz*

_{1}

*/a*)| is constant. This conclusion is plausible, since the coherence length

*ξ*(

*z*

_{1}) increases linearly with

*z*

_{1}due to propagation. The reduced mean square sum is

*χ*

^{2}= 4.0 and

*χ*

^{2}= 0.51, for the two fits. The overall agreement is quite satisfactory of a three parameter fit, and precludes models with more parameters.

*l*&

*s*oriented horizontally were used in analogous scans to determine the

*|j|*values in the vertical (

*yz*)-plane (data not shown). Least-square analysis resulted in

*j*

_{200}= 0.473 ± 0.009, and

*j*

_{500}= 0.948 ± 0.007, for the 200 nm

*l*&

*s*and the 500 nm

*l*&

*s*grating, respectively. The

*|j|*values in the vertical plane are therefore only slightly larger than in the horizontal plane. One would have expected a higher degree of coherence based on the smaller vertical (primary) source size, which leads to a nominally almost fully coherent illumination of the vertically focusing mirror. However, the presence of the double crystal monochromator in the vertical plane was observed to introduce vibrations, leading to a decease in coherence. This effect does not seem to be as systematic as the finite horizontal source size. Thus, apart from the reduced amplitude of the Talbot scans, which already disprove full coherence, the Talbot self image quality seems somewhat inferior and more irregular. In the Talbot scans,

*|f*

_{m}_{=1}

*|*never decreased to zero, and the quality of the fits was worse than in the horizontal direction. In any case, the vertical coherence length

*ξ*

*(*

_{x}*z*

_{1}) was thus also smaller than the beam diameter.

*j*(

*z*

_{1}

*, d*)| in the

*xz*-plane and illustrates the geometry of the probing scans. (a) shows the two experimental

*j*values corresponding to the two gratings, as a function of normalized lateral distance

*X*= (

*x/z*). A one-parameter fit to a Gaussian of unit amplitude exp(

*–X*

^{2}/2Σ

^{2}) yields a normalized coherence length Σ = 0.30±0.02, or correspondingly a Full Width at Half Maximum (FWHM)

*yz*plane are Σ = 0.33 ± 0.02, and

*ξ*

*= 0.78 · 10*

_{FWHM}^{–3}

*z*, respectively. Along with the experimental values, the resulting

*|j*(

*X*)

*|*curve of the coherence simulations is shown (thin black line), as calculated for the plane

*z*

_{1}= 10 mm, scaled to the

*X*=

*x/z*coordinate, showing good agreement. In (b), The function

*|j*(

*z*

_{1}

*, x*)

*|*= exp(–(

*x/z*

_{1})

^{2}/2Σ

^{2}) is shown as a contour plot, with selected contour levels. The two oblique lines (white dotted lines on both sides of the optical axis ) in the (

*xz*)-plane indicate the measurement points probed by the two defocus scans. The solid white lines indicate the divergence of the KB-beam in the vertical (

*xz*) plane. Finally, the lateral coherence length

*ξ*

*(*

_{FWHM}*z*

_{1}) is plotted in (c), for the experimental values (open circles) with errors corresponding to the Σ fit, along with the numerical coherence simulations (solid red squares), and the analytical results for an equivalent fully incoherent source at distance

*z*

_{1}. The only parameter in this expression is the ’effective’ horizontal source size, which best approximates the experimental results when set to

*s*= 194.6 nm (FWHM). In other words, all observations at the respective distances probed experimentally are equivalent to the coherence properties of an incoherent source with the above cross section. The corresponding treatment for the vertical plane yields an effective vertical source size of

*s*= 179.0 nm (FWHM). In conlcusion, the coherent opening angle of the beam was determined to 0.71 mrad in the horizontal and 0.78 mrad in the vertical plane, which divided by the total beam opening angle, 1.97 mrad and 1.15 mrad, fixes the coherence parameter

*q*= 0.36, and

*q*= 0.68, for the two planes, respectively. Throughout this treatment we have assumed that the focusing, propagation and coherence properties factorize in the two orthogonal directions. The results are in excellent agreement in the horizontal plane, but deviate from expectation in the vertical plane, for which

*q*≥ 1 was expected, since the vertical mirror should have been illuminated coherently due to the smaller vertical undulator source.

## 5. Holographic imaging of a double slit

*ζ*= Δ

*s z*

_{2}

*/z*

_{1}[32], and the source size Δ

*s*≃ 200 nm (FWHM), a FWHM value of 2.64 pixel is expected, substantially smaller then the fit result of 4.03 pixel. One may be tempted to readily attribute the discrepancy to all other smearing effects aside from coherence, such as vibrations between beam and sample and/or detector point spread (PSF) function. However, the PSF of the direct illumination CCD was measured, confirming essentially the 1 pixel value expected for a CCD with negligible cross-talk. Vibration amplitudes on the order of 200 nm are also unlikely. Instead, investigating the error returned for each parameter by the least-square fit, we see that the lower bound of the confidence interval defined by the increase of

*χ*

^{2}→

*χ*

^{2}+1 is

*ζ*≃ 1.4, and that the expected value would thus still be within the confidence interval. Accordingly, the smearing of the Fresnel diffraction pattern is not in disagreement with the experimental coherence length or source size. At the same time, we also see that imaging a test structure at fixed defocus is not sensitive enough to determine

*|j|*. At the same time, we can conclude that imaging under given parameters and conditions of partial coherence is well described by the ideal propagation contrast followed by an semi-empirical Gaussian convolution, and that imaging by the KB beam alone without further coherence filtering already gives quantitatively tractable results.

## 6. Coherence filtering by a waveguide

*Ge*/

*Mo*[30 nm]/

*C*[35 nm]/

*Mo*[30 nm]/

*Ge*with a waveguide length of 200

*μ*m was used as described above. Simulation and previous experiments have shown that very small near-field beam cross sections (FWHM) in the range of 10–15 nm can be obtained in theses waveguides by multi-modal interference and damping of higher orders [21

21. S. P. Krger, K. Giewekemeyer, S. Kalbfleisch, M. Bartels, H. Neubauer, and T. Salditt, “Sub-15 nm beam confinement by twocrossed x-ray waveguides,” Opt. Express **18**, 13492–13501 (2010). [CrossRef]

23. C. Fuhse and T. Salditt, “Finite-difference field calculations for two-dimensionally confined x-ray waveguides,” Appl. Opt. **45**, 4603–4608 (2006). [CrossRef] [PubMed]

*z*

_{1}value with respect to the waveguide exit plane was determined in the x-ray beam by vertical translations of the grating, yielding a magnification of

*M*= 8997 and correspondingly a defocus of

*z*

_{1}= 0.5880 mm, in perfect agreement with the thickness of the waveguide. Figure 8(c) shows the far-field intensity pattern of the 50 nm

*l*&

*s*grating at 1 mm defocus with respect to the KB focus. The intensity matrix was divided first by the empty waveguide far-field matrix, and then divided by the average horizontal (vector) intensity profile (mean over all rows). Finally, (d) shows the result of the Talbot scan (solid circles) with a least-square fit to Eq. (5) (solid line). Given the significantly reduced Talbot replication period

*Z*

*=*

_{T}*a*

^{2}

*/λ*, the curve is not sampled finely enough, but a least-square fit is nevertheless possible, if the

*Z*

*is kept constant, or fixed within a small range of values. The resulting amplitude of the Talbot curve is significantly increased with respect to the results without waveguide. The amplitude fit parameter of 0.445 directly illustrates the gain in coherence, since within error it equals the prefactor in Eq. (5), with*

_{T}*c*= 0.440 calculated for the given optical constants of the grating and photon energy. Accordingly, this indicates a degree of coherence

*|j|*≃ 1 over the measured range, in line with the coherence simulations, which predict a fully coherent waveguide exit beam.

## 7. Summary and conclusion

^{11}cps. To characterize the lateral coherence in particular of the defocused beam behind the mirror focus, grating interferometry measurements as well as numerical wave propagation were employed. Defocus values have been chosen in view of propagation imaging. This study is part of a long term goal to relax coherence requirements in imaging applications, i.e. to develop advanced reconstruction algorithms which take a (measured) finite degree of coherence into account [1

1. K. Nugent, “Coherent methods in the x-ray sciences,” Adv. Phys. **59**, 8732 (2010). [CrossRef]

*ξ*

*was essentially indistinguishable from that of an incoherent secondary source of the same size and distance. In other words the effect of partial coherence simply changes the ratio of beam diameter and coherence length, but not the coherence length in absolute value. This is true for defocus distances larger than the Rayleigh length, in line with simple wave optical coherence propagation, both in analytical approximation as in a more complete numerical treatment. The agreement in the horizontal directions with the numerical results was excellent. In the vertical direction, the degree of coherence was very similar to the horizontal values, in contrast to expectation based on the smaller vertical (primary) source size. The small source size (*

_{x,y}*σ*= 6

*μ*m) in the vertical direction should have led to a nearly fully coherent vertical beam, over the acceptance of the KB mirror. This was, however, not the case, and the discrepancy can be attributed to the double nitrogen cooled crystal monochromator which is known to introduce vibrations, and to compromise coherence. Possible improvements are now under close investigation at PETRA III.

## Acknowledgments

*HASYLAB/DESY*for support and commissioning of the low emittance radiation storage ring PETRA III. We acknowledge financial support by Deutsche Forschungsgemeinschaft through

*SFB*755

*Nanoscale Photonic Imaging*and the German Ministry of Education and Research under Grants No. 05KS7MGA and 05K10MGA.

## References and links

1. | K. Nugent, “Coherent methods in the x-ray sciences,” Adv. Phys. |

2. | D. M. Paganin, |

3. | I. A. Vartanyants and A. Singer, “Coherence properties of hard x-ray synchrotron sources and x-ray free-electron lasers,” N. J. Phys. |

4. | J.-P. Guigay, S. Zabler, P. Cloetens, C. David, R. Mokso, and M. Schlenker, “The partial Talbot effect and its use in measuring the coherence of synchrotron X-rays,” J. Synch. Radiat. |

5. | T. Weitkamp, A. Diaz, C. David, F. Pfeiffer, M. Stampanoni, P. Cloetens, and E. Ziegler, “X-ray phase imaging with a grating interferometer,” Opt. Express |

6. | F. Pfeiffer, O. Bunk, C. Schulze-Briese, A. Diaz, T. Weitkamp, C. David, J. F. van der Veen, I. Vartanyants, and I. K. Robinson, “Shearing interferometer for quantifying the coherence of hard x-ray beams,” Phys. Rev. Lett. |

7. | A. Diaz, C. Mocuta, J. Stangl, M. Keplinger, T. Weitkamp, F. Pfeiffer, C. David, T. H. Metzger, and G. Bauer, “Coherence and wavefront characterization of Si-111 monochromators using double-grating interferometry,” J. Synch. Radiat. |

8. | J. Miao, P. Charalambous, J. Kirz, and D. Sayre, “Extending the methodology of x-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens,” Nature |

9. | B. Abbey, K. A. Nugent, G. J. Williams, J. N. Clark, A. G. Peele, M. A. Pfeifer, M. de Jonge, and I. McNulty, “Keyhole coherent diffractive imaging,” Nat. Phys. |

10. | G. J. Williams, H. M. Quiney, A. G. Peele, and K. A. Nugent, “Coherent diffractive imaging and partial coherence,” Phys. Rev. B |

11. | K. Giewekemeyer, H. Neubauer, S. Kalbfleisch, S. P. Krger, and T. Salditt, “Holographic and diffractive x-ray imaging using waveguides as quasi-point sources,” N. J. Phys. |

12. | J. R. Fienup, “Reconstruction of an object from the modulus of its fourier transform,” Opt. Lett. |

13. | H. M. Quiney, A. G. Peele, Z. Cai, D. Paterson, and K. A. Nugent, “Diffractive imaging of highly focused x-ray fields,” Nat. Phys. |

14. | S. Marchesini, “Invited article: A unified evaluation of iterative projection algorithms for phase retrieval,” Rev. Sci. Instrum. |

15. | J. M. Rodenburg, A. C. Hurst, A. G. Cullis, B. R. Dobson, F. Pfeiffer, O. Bunk, C. David, K. Jefimovs, and I. Johnson, “Hard-x-ray lensless imaging of extended objects,” Phys. Rev. Lett. |

16. | P. Thibault, M. Dierolf, A. Menzel, O. Bunk, C. David, and F. Pfeiffer, “High-resolution scanning x-ray diffraction microscopy,” Science |

17. | M. Guizar-Sicairos and J. R. Fienup, “Phase retrieval with transverse translation diversity: a nonlinearoptimization approach,” Opt. Express |

18. | A. Schropp, P. Boye, J. M. Feldkamp, R. Hoppe, J. Patommel, D. Samberg, S. Stephan, K. Giewekemeyer, R. N. Wilke, T. Salditt, J. Gulden, A. P. Mancuso, I. A. Vartanyants, E. Weckert, S. Schrder, M. Burghammer, and C. G. Schroer, “Hard x-ray nanobeam characterization by coherent diffraction microscopy,” Appl. Phys. Lett. |

19. | C. M. Kewish, P. Thibault, M. Dierolf, O. Bunk, A. Menzel, J. Vila-Comamala, K. Jefimovs, and F. Pfeiffer, “Ptychographic characterization of the wavefield in the focus of reflective hard x-ray optics,” Ultramicroscopy |

20. | S. Kalbfleisch, M. Osterhoff, K. Giewekemeyer, H. Neubauer, S. P. Krger, B. Hartmann, M. Bartels, M. Sprung, O. Leupold, F. Siewert, and T. Salditt, “The holography endstation of beamline P10 at PETRA III,” SRI 2009, AIP Conf. Proc. |

21. | S. P. Krger, K. Giewekemeyer, S. Kalbfleisch, M. Bartels, H. Neubauer, and T. Salditt, “Sub-15 nm beam confinement by twocrossed x-ray waveguides,” Opt. Express |

22. | C. Fuhse, C. Ollinger, and T. Salditt, “Waveguide-based off-axis holography with hard x rays,” Phys. Rev. Lett. |

23. | C. Fuhse and T. Salditt, “Finite-difference field calculations for two-dimensionally confined x-ray waveguides,” Appl. Opt. |

24. | L. D. Caro, C. Giannini, S. D. Fonzo, W. Yark, A. Cedola, and S. Lagomarsino, “Spatial coherence of x-ray planar waveguide exiting radiation,” Opt. Commun. |

25. | I. Bukreeva, A. Popov, D. Pelliccia, A. Cedola, S. B. Dabagov, and S. Lagomarsino, “Wave-field formation in a hollow x-ray waveguide,” Phys. Rev. Lett. |

26. | Y. V. Kopylov, A. V. Popov, and A. V. Vinogradov, “Application of the parabolic wave equation to x-ray diffraction optics,” Opt. Commun. |

27. | H. Mimura, H. Yumoto, S. Matsuyama, Y. Sano, K. Yamamura, Y. Mori, M. Yabashi, Y. Nishino, K. Tamasaku, T. Ishikawa, and K. Yamauchi, “Efficient focusing of hard x-rays to 25 nm by a total reflection mirror,” Appl. Phys. Lett. |

28. | L. D. Caro, C. Giannini, D. Pelliccia, C. Mocuta, T. H. Metzger, A. Guagliardi, A. Cedola, I. Burkeeva, and S. Lagomarsino, “In-line holography and coherent diffractive imaging with x-ray waveguides,” Phys. Rev. B |

29. | T. Salditt, S. P. Krger, C. Fuhse, and C. Bhtz, “High-transmission planar x-ray waveguides,” Phys. Rev. Lett. |

30. | E. Wolf, |

31. | S. Mayo, T. Davis, T. Gureyev, P. Miller, D. Paganin, A. Pogany, A. Stevenson, and S. Wilkins, “X-ray phase-contrast microscopy and microtomography,” Opt. Express |

32. | T. Weitkamp, “Imaging and tomography with high resolution using coherent hard synchrotron radiation,” Ph.D. thesis, Universitt Hamburg (2002). |

**OCIS Codes**

(070.6760) Fourier optics and signal processing : Talbot and self-imaging effects

(340.7440) X-ray optics : X-ray imaging

(340.7450) X-ray optics : X-ray interferometry

**ToC Category:**

X-ray Optics

**History**

Original Manuscript: March 9, 2011

Revised Manuscript: April 24, 2011

Manuscript Accepted: April 25, 2011

Published: May 3, 2011

**Citation**

T. Salditt, S. Kalbfleisch, M. Osterhoff, S. P. Krüger, M. Bartels, K. Giewekemeyer, H. Neubauer, and M. Sprung, "Partially coherent nano-focused x-ray radiation characterized by Talbot interferometry," Opt. Express **19**, 9656-9675 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-9656

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### References

- K. Nugent, “Coherent methods in the x-ray sciences,” Adv. Phys. 59, 8732 (2010). [CrossRef]
- D. M. Paganin, Coherent X-Ray Optics (Oxford University Press, 2006). [CrossRef]
- I. A. Vartanyants and A. Singer, “Coherence properties of hard x-ray synchrotron sources and x-ray free-electron lasers,” N. J. Phys. 12, 035004 (2010). [CrossRef]
- J.-P. Guigay, S. Zabler, P. Cloetens, C. David, R. Mokso, and M. Schlenker, “The partial Talbot effect and its use in measuring the coherence of synchrotron X-rays,” J. Synch. Radiat. 11, 476–482 (2004). [CrossRef]
- T. Weitkamp, A. Diaz, C. David, F. Pfeiffer, M. Stampanoni, P. Cloetens, and E. Ziegler, “X-ray phase imaging with a grating interferometer,” Opt. Express 13, 6296–6304 (2005). [CrossRef] [PubMed]
- F. Pfeiffer, O. Bunk, C. Schulze-Briese, A. Diaz, T. Weitkamp, C. David, J. F. van der Veen, I. Vartanyants, and I. K. Robinson, “Shearing interferometer for quantifying the coherence of hard x-ray beams,” Phys. Rev. Lett. 94, 164801 (2005). [CrossRef] [PubMed]
- A. Diaz, C. Mocuta, J. Stangl, M. Keplinger, T. Weitkamp, F. Pfeiffer, C. David, T. H. Metzger, and G. Bauer, “Coherence and wavefront characterization of Si-111 monochromators using double-grating interferometry,” J. Synch. Radiat. 17, 299–307 (2010). [CrossRef]
- J. Miao, P. Charalambous, J. Kirz, and D. Sayre, “Extending the methodology of x-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens,” Nature 400, 342–344 (1999). [CrossRef]
- B. Abbey, K. A. Nugent, G. J. Williams, J. N. Clark, A. G. Peele, M. A. Pfeifer, M. de Jonge, and I. McNulty, “Keyhole coherent diffractive imaging,” Nat. Phys. 4, 394–398 (2008). [CrossRef]
- G. J. Williams, H. M. Quiney, A. G. Peele, and K. A. Nugent, “Coherent diffractive imaging and partial coherence,” Phys. Rev. B 75, 104102 (2007). [CrossRef]
- K. Giewekemeyer, H. Neubauer, S. Kalbfleisch, S. P. Krger, and T. Salditt, “Holographic and diffractive x-ray imaging using waveguides as quasi-point sources,” N. J. Phys. 12, 035008 (2010). [CrossRef]
- J. R. Fienup, “Reconstruction of an object from the modulus of its fourier transform,” Opt. Lett. 3, 27–29 (1978). [CrossRef] [PubMed]
- H. M. Quiney, A. G. Peele, Z. Cai, D. Paterson, and K. A. Nugent, “Diffractive imaging of highly focused x-ray fields,” Nat. Phys. 2, 101–104 (2006). [CrossRef]
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- M. Guizar-Sicairos and J. R. Fienup, “Phase retrieval with transverse translation diversity: a nonlinearoptimization approach,” Opt. Express 16, 7264–7278 (2008). [CrossRef] [PubMed]
- A. Schropp, P. Boye, J. M. Feldkamp, R. Hoppe, J. Patommel, D. Samberg, S. Stephan, K. Giewekemeyer, R. N. Wilke, T. Salditt, J. Gulden, A. P. Mancuso, I. A. Vartanyants, E. Weckert, S. Schrder, M. Burghammer, and C. G. Schroer, “Hard x-ray nanobeam characterization by coherent diffraction microscopy,” Appl. Phys. Lett. 96, 091102 (2010). [CrossRef]
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- S. P. Krger, K. Giewekemeyer, S. Kalbfleisch, M. Bartels, H. Neubauer, and T. Salditt, “Sub-15 nm beam confinement by twocrossed x-ray waveguides,” Opt. Express 18, 13492–13501 (2010). [CrossRef]
- C. Fuhse, C. Ollinger, and T. Salditt, “Waveguide-based off-axis holography with hard x rays,” Phys. Rev. Lett. 97, 254801 (2006). [CrossRef]
- C. Fuhse and T. Salditt, “Finite-difference field calculations for two-dimensionally confined x-ray waveguides,” Appl. Opt. 45, 4603–4608 (2006). [CrossRef] [PubMed]
- L. D. Caro, C. Giannini, S. D. Fonzo, W. Yark, A. Cedola, and S. Lagomarsino, “Spatial coherence of x-ray planar waveguide exiting radiation,” Opt. Commun. 217, 31–45 (2003). [CrossRef]
- I. Bukreeva, A. Popov, D. Pelliccia, A. Cedola, S. B. Dabagov, and S. Lagomarsino, “Wave-field formation in a hollow x-ray waveguide,” Phys. Rev. Lett. 97, 184801 (2006).
- Y. V. Kopylov, A. V. Popov, and A. V. Vinogradov, “Application of the parabolic wave equation to x-ray diffraction optics,” Opt. Commun. 118, 619–636 (1995). [CrossRef]
- H. Mimura, H. Yumoto, S. Matsuyama, Y. Sano, K. Yamamura, Y. Mori, M. Yabashi, Y. Nishino, K. Tamasaku, T. Ishikawa, and K. Yamauchi, “Efficient focusing of hard x-rays to 25 nm by a total reflection mirror,” Appl. Phys. Lett. 90, 051903 (2007). [CrossRef]
- L. D. Caro, C. Giannini, D. Pelliccia, C. Mocuta, T. H. Metzger, A. Guagliardi, A. Cedola, I. Burkeeva, and S. Lagomarsino, “In-line holography and coherent diffractive imaging with x-ray waveguides,” Phys. Rev. B 77, 081408 (2008). [CrossRef]
- T. Salditt, S. P. Krger, C. Fuhse, and C. Bhtz, “High-transmission planar x-ray waveguides,” Phys. Rev. Lett. 100, 184801–4 (2008). [CrossRef] [PubMed]
- E. Wolf, Introduction to the Theory of Coherence and Polarization of Light (Oxford University Press, 2007).
- S. Mayo, T. Davis, T. Gureyev, P. Miller, D. Paganin, A. Pogany, A. Stevenson, and S. Wilkins, “X-ray phase-contrast microscopy and microtomography,” Opt. Express 11, 2289–2302 (2003). [CrossRef] [PubMed]
- T. Weitkamp, “Imaging and tomography with high resolution using coherent hard synchrotron radiation,” Ph.D. thesis, Universitt Hamburg (2002).

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