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Panoramic images are captivating in any form, be it of a city sky line or a
mosaic of micrographs. They feature a wide field of view and uncompromised
resolution. Forming such extraordinary images can be achieved with oversized
film or by stitching together smaller images. But these options are not always
practical for microscopy with x-rays especially for imaging with ultrashort
pulses of x-rays. We have demonstrated a new holographic technique, called
extended field of view Fourier transform holography (FTH), for imaging isolated
areas of a nanoscale sample without compromising spatial resolution. This
technique overcomes one of the central challenges in imaging, namely to
simultaneously obtain high spatial resolution and a large field of view. The
method is compatible with single shot imaging—a feature particularly important
for studying dynamics at upcoming X-ray free electron lasers like the Linac
Coherent Light Source (LCLS) at SLAC.
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Figure 1: Illustration of the Extended Field of View. (a) A cartoon of
the scattering plane profile of a holographic mask where the sample objects are
represented by the letters F and G and the reference sources by dots. Note
spatial arrangement of each element. The spatial autocorrelation of the
pattern in (a) is presented in (b) and contains images of the objects. To
record as much information as possible the references can be placed such at the
images in the Autocorrelation tightly packed or tiled, but not overlapping.
The duplicate images are characteristic of the holographic imaging process.
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In FTH, the light scattered from a coherently illuminated sample object and
reference source is detected as a far field diffraction pattern or hologram.
[Eisebitt] To reconstruct the image, the spatial Fourier transformation of
the hologram is calculated. An image of the sample object is embedded in the
result of the calculation, which is called the autocorrelation. The position
and field of view of the image within the autocorrelation is controlled by the
respective placement of the reference source and extent of the object. To
extend the field of view our method applies multiple objects and reference
sources to x-ray Fourier transform holography. The key is the strategic
arrangement of the reference and object structures such that the individual
images tile without overlapping in the autocorrelation as explained in Fig 1.
But this concept is not new, it was realized first at Stanford over 50 years
ago using visible wavelengths as a method to improve holographic images
distorted by atmospheric disturbances. [Goodman] Though fluctuations in the
atmosphere blur images of distant objects it is the finite size of the pixels
in a x-ray detector that blur the spatial resolution of a hologram. However,
because the holograms represent reciprocal space the blurring translates to a
smaller field of view in the real space reconstruction. By strategically
arranging holographic references on the sample the effective field of view can
be extended, thus circumventing the smearing caused by the detector.
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Figure 2: Extended Field of View FTH Experiment. (a) An illustration of
the holography sample spanning 160 µm. (b) Arrow
structures and reference holes were milled into the x-ray opaque 1um thick Au
film. (c) The coherent scattering pattern contains streaks from the heads and
tails of the arrow structures. (d) The magnitude of the spatial Fourier
transform of the hologram results in the autocorrelation which is shown. Here
the arrows tile nicely because of the arrangement of the references with
respect to the arrows, thus resulting in an image with high resolution of
isolated regions of the sample.
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To test the LCLS readiness of our method, we used the soft x-ray coherent
scattering beamline at SSRL, which is now Beamline 13-3. Our sample is a
nanostructured 1 µm thick Au film supported by the
silicon nitride membrane shown in Fig 2(a). Four isolated arrow shaped
structures were milled through the soft x-ray opaque film, and next to each
arrow is a strategically placed reference, see Fig 2(b). The entire structure,
which spans 180 µm, was illuminated with a spatially coherent soft x-ray beam
(lambda=1.58nm) to generate the hologram in Fig. 2(c).
The Fourier reconstruction in Fig. 2(d) shows that all four arrows are imaged
at a resolution set by the size of the reference source, ~50nm. Since there
is a unique relationship between each arrow on the mask, the specific arrow or
region of the sample can be identified in the reconstruction.
In addition to demonstrating extended field of view FTH this experiment can be
used to envision a novel method for recording the evolution of ultrafast
processes. This method will be enabled by x-ray free electron laser sources
like the LCLS. Just one of the ultrashort x-ray pulses generated by the LCLS
is bright enough to acquire an entire hologram. As a result the entire
ultrafast temporal response across different locations on the sample can to be
captured simultaneously. But recording the response of the sample is only
part of the method, the sample must also be synchronously excited.
Cross-beam pump-probe experiments exploit the angle between an excitation
(pump) pulse and a readout (probe) pulse to map temporal dynamics onto the
extent of a sample. [Lindenberg] In this way, spatial images can record
temporal dynamics, and thus the temporal resolution is limited by the spatial
resolution. We therefore envision applying the cross-beam geometry to
spatially multiplexed FTH as depicted in Fig. 3. The pump pulse sweeps across
the sample exciting it while the probe, x-ray pulse, captures the evolution
after excitation. Because of the geometry of this holographic mask the
temporal evolution follows the arrows like the hand on an ultrafast stopwatch.
Using this realistic geometry it is possible to study sub-femtosecond dynamics
over a picosecond timescale.
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Figure 3: Proposed Ultrafast Stop Watch (a) The experimental geometry shown
in Fig 2. is combined with a pump laser pulse to excited the sample from an
angle. (b) An ultrafast x-ray pulse illuminates the sample and the scattering
is detected as a hologram. Note that this illustration is drastically not to
scale, the entire sample should only span a few pixels on the detector. (c)
The reconstruction now provides temporal information with each arrow
corresponding to a progression of time just as on a stopwatch.
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Robust experimental techniques are essential to make meaningful measurements at
x-ray free electron lasers. The advanced sample preparation for extended
field of view FTH assures experimental success for ultrafast dynamics at an
FEL.
Primary Citation
W. F. Schlotter, J. Lüning, R. Rick, K. Chen, A. Scherz, S. Eisebitt, C. M.
Günther, W. Eberhardt, O. Hellwig, J. Stöhr, "Extended field of view soft x-ray
Fourier transform holography: toward imaging ultrafast evolution in a single
shot" Optics Letters, 32 (21) p3110 (2007)
References
S. Eisebitt, et al., Nature, 432 pp 885-887, 2004
J. W. Goodman and J. D. Gaskill, Proc. IEEE 57, 823 (1969)
A. M. Lindenberg et al., Science 308, 392 (2005)
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