Abstract

It is demonstrated that in a zone plate based scanning X-ray microscope, used to image low absorbing, heterogeneous matter at a mesoscopic scale, differential phase contrast (DPC) can be implemented without adding any additional optical component to the normal scheme of the microscope. The DPC mode is simply generated by an appropriate positioning and alignment of microscope apertures. Diffraction from the apertures produces a wave front with a non-uniform intensity. The signal recorded by a pinhole photo diode located in the intensity gradient is highly sensitive to phase changes introduced by the specimen to be recorded. The feasibility of this novel DPC technique was proven with the scanning X-ray microscope at the ID21 beamline of the European Synchrotron Radiation facility (ESRF) operated at 6 keV photon energy. We observe a differential phase contrast, similar to Nomarski’s differential interference contrast for the light microscope, which results in a tremendous increase in image contrast of up to 20 % when imaging low absorbing specimen.

© 2002 Optical Society of America

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References

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  1. G. Schneider, "Cryo X-ray microscopy with high spatial resolution in amplitude and phase contrast," Ultramicroscopy 75, 85-104 (1998)
    [CrossRef] [PubMed]
  2. U. Bonse and M. Hart, "An X-ray interferometer," Appl. Phys. Lett. 6, 155-165 (1965)
    [CrossRef]
  3. G. Schmahl and D. Rudolph, "Proposal for a phase contrast X-ray microscope," in X-ray Microscopy Instrumentation and Biological Applications, P. C. Cheng and G. J. Jan, 231-238 (Springer, Berlin, 1987)
  4. F. Polack, D. Joyeux, and J. Svaloš, "Applications of wavefront division interferometers in soft X-rays," Rev. Sci. Instrum. 66, 2180-2183 (1995)
    [CrossRef]
  5. D. Joyeux, F. Polack, and D. Phallipou, "An interferometric determination of the refractive part of optical constants for carbon and silver across soft X-ray absorption edges," Rev. Sci. Instrum. 70, 2921-2926 (1999)
    [CrossRef]
  6. G. Morrison, and B. Niemann, "Differential phase contrast X-ray microscopy," in X-ray microscopy and Spectromicroscopy, J. Thieme, G. Schmahl, D. Rudolph, and E. Umbach, I-95 - I-105 (Springer, Berlin, 1998)
  7. H. N. Chapman, C. Jacobsen, and S. Williams, "Applications of a CCD detector in scanning-transmission X-ray microscopy," Rev. Sci. Instrum. 66, 1332-1334 (1995)
    [CrossRef]
  8. T. Wilhein, B. Kaulich, E. Di Fabrizio, F. Romanato, S. Cabrini, and J. Susini, "Differential interference contrast X-ray microscopy with submicron resolution," Appl. Phys. Lett. 78, 2082-2084 (2001)
    [CrossRef]
  9. B. Kaulich, T. Wilhein, E. Di Fabrizio, F. Romanato, M. Altissimo, S. Cabrini, B. Fayard, and J. Susini, "Differential interference contrast X-ray microscopy with twin zone plates," J. Opt. Soc. Am A 19, 797-806 (2002)
    [CrossRef]
  10. R.Barrett, B. Kaulich, M. Salome, and J. Susini, "Current status of the scanning X-ray microscope at the ESRF," AIP Conf. Proc. 507, 458-463 (2000)
    [CrossRef]
  11. J. Susini, R. Barrett, B. Kaulich, S. Oestreich, and M. Salomé, "The X-ray microscopy facility at the ESRF: A status report," AIP Conf. Proc. 507, 19-26 (2000)
    [CrossRef]
  12. E. Di Fabrizio, F. Romanato, M. Gentili, S. Cabrini, B. Kaulich, J. Susini, and R. Barrett, "High-efficiency multilevel zone plates for keV X-rays," Nature 401, 895-898 (1999)
    [CrossRef]
  13. E.M. Waddell and J.N. Chapman, "Linear imaging of strong phase objects using asymmetrical detectors in STEM," Optik 54, 83-96 (1979)

AIP Conf. Proc.

R.Barrett, B. Kaulich, M. Salome, and J. Susini, "Current status of the scanning X-ray microscope at the ESRF," AIP Conf. Proc. 507, 458-463 (2000)
[CrossRef]

J. Susini, R. Barrett, B. Kaulich, S. Oestreich, and M. Salomé, "The X-ray microscopy facility at the ESRF: A status report," AIP Conf. Proc. 507, 19-26 (2000)
[CrossRef]

Appl. Phys. Lett.

U. Bonse and M. Hart, "An X-ray interferometer," Appl. Phys. Lett. 6, 155-165 (1965)
[CrossRef]

T. Wilhein, B. Kaulich, E. Di Fabrizio, F. Romanato, S. Cabrini, and J. Susini, "Differential interference contrast X-ray microscopy with submicron resolution," Appl. Phys. Lett. 78, 2082-2084 (2001)
[CrossRef]

J. Opt. Soc. Am A

B. Kaulich, T. Wilhein, E. Di Fabrizio, F. Romanato, M. Altissimo, S. Cabrini, B. Fayard, and J. Susini, "Differential interference contrast X-ray microscopy with twin zone plates," J. Opt. Soc. Am A 19, 797-806 (2002)
[CrossRef]

Nature

E. Di Fabrizio, F. Romanato, M. Gentili, S. Cabrini, B. Kaulich, J. Susini, and R. Barrett, "High-efficiency multilevel zone plates for keV X-rays," Nature 401, 895-898 (1999)
[CrossRef]

Optik

E.M. Waddell and J.N. Chapman, "Linear imaging of strong phase objects using asymmetrical detectors in STEM," Optik 54, 83-96 (1979)

Rev. Sci. Instrum.

H. N. Chapman, C. Jacobsen, and S. Williams, "Applications of a CCD detector in scanning-transmission X-ray microscopy," Rev. Sci. Instrum. 66, 1332-1334 (1995)
[CrossRef]

F. Polack, D. Joyeux, and J. Svaloš, "Applications of wavefront division interferometers in soft X-rays," Rev. Sci. Instrum. 66, 2180-2183 (1995)
[CrossRef]

D. Joyeux, F. Polack, and D. Phallipou, "An interferometric determination of the refractive part of optical constants for carbon and silver across soft X-ray absorption edges," Rev. Sci. Instrum. 70, 2921-2926 (1999)
[CrossRef]

Ultramicroscopy

G. Schneider, "Cryo X-ray microscopy with high spatial resolution in amplitude and phase contrast," Ultramicroscopy 75, 85-104 (1998)
[CrossRef] [PubMed]

Other

G. Morrison, and B. Niemann, "Differential phase contrast X-ray microscopy," in X-ray microscopy and Spectromicroscopy, J. Thieme, G. Schmahl, D. Rudolph, and E. Umbach, I-95 - I-105 (Springer, Berlin, 1998)

G. Schmahl and D. Rudolph, "Proposal for a phase contrast X-ray microscope," in X-ray Microscopy Instrumentation and Biological Applications, P. C. Cheng and G. J. Jan, 231-238 (Springer, Berlin, 1987)

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Figures (6)

Fig. 1:
Fig. 1:

Optical scheme of a scanning X-ray microscope operated in differential interference contrast (DPC) mode: The specimen is raster scanned across a microprobe formed by a coherently illuminated zone plate (ZP). Order sorting aperture (OSA) and central stop aperture eliminate spurious diffraction orders of the ZP only +1. order is used for imaging. Diffraction from both apertures modulates the wavefront. A detector shaped with a pinhole aperture is highly sensitive to phase slopes in the specimen and movements of fringes in the modulated wave front.

Fig. 2:
Fig. 2:

First diffraction order of the microprobe forming ZP scanned by a photodiode shaped with a 10 μm pinhole. The images with 200×200 pixel and 2 μm × 2 μm pixel size were acquired with a dwell time of 50 ms/ px. (a) Raster scan without central stop with Fresnel diffraction fringes from the OSA in the central part overlapped to a speckle pattern resulting from X-ray source and ZP diffraction efficiency inhomogeneities. (b) Raster scan with central stop in the optical scheme.

Fig. 3:
Fig. 3:

Horizontal line scan through the center of the raster images in Fig. 2 without and with central stop. The line scan is dominated by the on-axis zero order light passing through the OSA, resulting in the central peak, which is surrounded by OSA diffraction. In the outer part Speckle pattern dominates. The convolution of diffraction from OSA and central stop results in a central double peak and a broad fringe slope in the outer part. The arrow indicates the detector position on this fringe slope.

Fig. 4:
Fig. 4:

Phase signal OTF of the DPC set-up using a small off-axis detector. Calculations have been performed with the measured pupil functions of figure 3; only the cut in the direction of the detector offset is shown. Red line: the wavefont is shaped by a central stop. Blue line: the central stop is taken out.

Fig. 5.
Fig. 5.

Comparison of bright-field imaging with DPC X-ray imaging under different conditions. The X-ray images show 2 μm thick PMMA grating structures with a transmission of 99.6 % transmission at 6 keV photon energy. The acquisition time was in all cases 50 ms/ px. (a) is acquired in bright-field imaging mode without pinhole shaping the detector and integrating over the entire detector area. (b) DPC X-ray image with a 10 μm pinhole shaping the detector, but without central stop. (c) Is the corresponding DPC X-ray image with central stop introduced into the optical scheme. (d) DPC X-ray image, where the 10 μm pinhole in front of the detector was replaced by a 50 μm pinhole.

Fig. 6.
Fig. 6.

Comparison of (a) bright-field imaging and (b) DPC imaging of cell membranes of maize plant cells. The acquisition time of the 400 × 300 pixel images was 50 ms/ px. A 50 μm pinhole was used to shape the detector.

Equations (2)

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F ( k ) = P * ( k ) [ P ( k ) D ( k ) ] P ( k ) P * ( k ) D ( k )
F ( k ) = P 0 ( k 0 k ) P 0 ( k 0 + k )

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