Nuclear Instruments and Methods in Physics Research A 347 (1994) 170-176
North-Holland
NUCLEAR
INSTRUMENTS
& METHODS
IN PHYSICS
RESEARCH
Section A
The future of X-ray holography
1. McNulty
Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA
X-ray holography is a promising technique for high resolution study of biological, microelectronics and materials science specimens . It
offers absorption and phase contrast, it is amenable to flash sources, and it is capable of three-dimensional imaging when coupled with
tomography . Soft X-rays are suited to microscopy of thin organic specimens and wet biological objects such as subcellular organelles . Major
advances in X-ray sources, optics, and detectors have made possible Gabor and Fourier transform holography with 50 to 60-nm transverse
resolution using 350-600 eV photons; 100-nm depth resolution by holographic tomography has recently been demonstrated . X-ray
holography at higher energies (1-4 keV) may be feasible soon using third-generation synchrotron sources. These methods might profitably
be used to investigate denser objects than are accessible to soft X-rays, for example defects in microcircuits and adsorbates in zeolites . The
future is likely to see progress in X-ray holography over an increasing range of photon energies, with elemental and chemical specificity,
and with three-dimensional resolution below one micron .
1. Introduction
The last decade has seen X-ray holography progress
from proof-of-principle experiments disappointingly shy of
expectation, to dramatic results with biological and artificial test objects at an order of magnitude better resolution
than those obtainable using visible light. This progress has
largely taken place with soft X-rays in the "water window" between the oxygen K-edge at 532 eV and the
carbon K-edge at 284 eV [1]. Soft X-rays offer suboptical
resolution and lower radiation dose than electron probes
for hydrated biological objects, and thus are well suited to
the study of thick, unstained and possibly living specimens
under physiologically natural conditions [2]. High brightness undulators at the newly-commissioned third-generation Advanced Light Source (ALS) storage ring [3] will
enable soft X-ray holography experiments to be performed
at higher resolution, with the potential for time-resolved
experiments . In the near future, third-generation sources of
higher energy X-rays at the APS, ESRF and SPRing-8
storage rings [4] will, for the first time, allow X-ray
holography in the intermediate energy (1-4 keV) region .
These more penetrating photons are likely to be useful for
imaging denser, thicker microelectronics and materials sciences specimens, and for phase contrast imaging of biological objects [5].
Holographic imaging involves two steps. A recording is
first made of the interference pattern that results when a
reference wave is mixed with the wave scattered by an
illuminated object. The object wave is then reconstructed
from the interference pattern or hologram. The hologram
intensity I is the squared sum of the complex amplitudes
of the object and reference waves
I = a. a* + a, a* + a, a* + a, a* ,
where a. and a r are the object and reference wave amplitudes . Re-illumination of the hologram by a, produces
image terms proportional to ao and a** , and two noise
terms representing the diffraction patterns of the object and
the reference source. Reconstructing the object wave from
the image-forming terms with minimal added noise is the
challenge in refining a particular holographic technique
from a novel demonstration to a practical tool .
Holography is a coherent process in which the object
wave amplitude is, in principle, fully recoverable . This
feature resolves the famous phase problem of crystallography, enables image formation by both absorption and
phase contrast, and is the basis for three-dimensional (3D)
wavefrom reconstruction. Elemental specificity is possible
in the X-ray region due to the existence of absorption
edges at the electronic binding energies of atoms [1].
Chemical selectivity in the vicinity of absorption edges is
also possible ; chemical contrast has been shown in recent
X-ray microscopy experiments [6]. Phase contrast, which
is sensitive to the real part of the object's refractive index,
can be exploited to obtain information about membranes,
interfaces and regions of anomalous dispersion that are
inaccessible to methods based solely on absorption .
The thrust of this paper is to review the origins, current
status, and methods of X-ray holography and to extrapolate
some of the directions it will take in the near future . These
include techniques for obtaining 3D information, extension
to higher energies, and the use of flash sources.
2. Historical background
The first X-ray hologram, of a thin wire, was unintentionally recorded by Kellstr6m with Al-K (1 .5 keV) X-rays
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved
SSD70168-9002(94)00269-D
I. McNulty /Nucl. Instr. and Meth. in Phys. Res. A 347 (1994) 170-176
in 1932 [7]. Holography was unknown until Gabor invented it in 1948 as a lensless means of high resolution
imaging in order to circumvent the aberrations of electron
optics of the day [8]. Although Gabor's experiments were
conducted in the optical region, Baez envisioned using
shorter wavelength radiation and proposed ways to construct a holographic X-ray microscope [9]. About this time,
El-Sum was able to reconstruct Kellstr8m's hologram with
visible light [10] . Leith, Stroke, Winthrop, and others
[11-14] subsequently explored the theoretical foundations
for high resolution X-ray holography . The next few years
saw modest results by Giles [15] and Bjorklund [16] with
vacuum-ultraviolet light. Significant advances were not
made until Aoki and Kikuta [17], then Reuter and Mahr
[18], recorded and reconstructed X-ray holograms to a
resolution of a few microns. Progress was slow, however,
because bright enough X-ray sources, efficient wide-aperture X-ray optics, and high resolution detectors did not yet
exist. Extension of Gabor's approach to the X-ray region
had the advantage of not requiring X-ray optics, but a
fine-grained detector was necessary to obtain high resolution images . The Fourier transform holography geometry
[12,13] was hailed as the solution because it decoupled the
hologram resolution from the detector resolution, but it
required a strong point reference source. In both cases,
there still was no X-ray source with sufficient coherent
flux to form holograms with a resolution beyond that
obtainable with visible light.
This situation changed dramatically with the advent of
high brightness X-ray sources on synchrotron storage rings
and demonstration of the first X-ray lasers (XRLs) . It
became evident that synchrotron-based sources could provide the requisite coherent flux [19,20] and that short-pulse
100
w
ô
ôfN
001
-
105
107
109
10 11
,,Ô O
1013
10 15
Fig. 2 . Gabor in-line (a) and Fourier transform (b) holography
geometries .
lasers held promise for the future [21] . Concurrently, many
of the difficulties of making diffraction-limited diffractioe
and reflective X-ray optics were overcome by advances in
microfabrication technology, surface polishing, thin film
deposition and precision metrology. This progress was
paralleled by the introduction of sensitive electronic array
detectors and high resolution resists for X-rays .
Soft X-ray bending magnet and undulator sources at the
Photon Factory, NSLS and LURE have since been used to
record Gabor and Fourier transform X-ray holograms [2227]. The Livermore XRL was used to make Gabor holograms with 20 .6-nm X-rays and 200-ps exposures [28] .
Fig. 1 shows the downward trend in imaging resolution
by X-ray holography with increasing source brightness,
since El-Sum . The most significant work has been realized
using undulators, the brightest continuous sources of partially coherent, tunable x-radiation available. Undulators,
at least until XRLs become competitive, are still the
sources of choice for X-ray holography experiments . Currently, the brightest soft and hard X-ray sources, such as
the X1 soft X-ray undulator at the NSLS and undulators
planned for the ALS and APS storage rings, have a
spectral brightness of 10 17 to 10 18 photons/ s/mm 2 /
mrad2 per 0.1% bandwidth. If this trend is extrapolated to
a resolution of 10 nm, it appears that one would require a
time-averaged source brightness of 10 21 (in these units)!
3. State of the art
1017
1019
1021
Source Brightness (ph/s/mm 2/mrad2 /0 .1 % BW)
Fig. 1. Progress in two-dimensional imaging resolution by X-ray
holography (circles) as a function of estimated source brightness,
from 1952 to the present [10,17,18,22-281 . X-ray energies used in
the work shown are between 300 and 1500 eV . Historically, the
resolution has improved approximately with the fourth power of
the brightness (dashed-line fit to data).
Sub-100-nm holography has been demonstrated with
soft X-rays using both the Gabor in-line and the Fourier
transform methods (Fig . 2) . The two approaches are complementary and offer a different mix of capabilities .
In the Gabor geometry, a plane wave both illuminates
the object and provides the reference wave [8]. The spatial
frequencies of the Fresnel fringes in a Gabor hologram
extend to the frequency limit of the object . Consequently,
X-ray Gabor holograms are recorded on high resolution
X-ray resists, magnified by atomic force or transmission
electron microscopy (TEM), then reconstructed optically
with ultraviolet light or numerically by computer . The
resolution of this technique depends on the detector resoluVl . COHERENCE
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I. McNulty/Nucl. Instr. and Meth. in Phys . Res. A 347 (1994)170-176
tion and means of readout. As the spatial coherence width
must only be as large as the object under study, the field of
view can be as large as the beam size, detector size, and
optical path differences will permit . Effective use can thus
be made of a multimode beam . The main advantage of
Gabor holography is simplicity : apart from a monochromator to provide sufficient temporal coherence (typically, a
monochromaticity of a few hundred), no optics, no prefocusing, and minimal alignment are required in the recording step .
On the other hand, off-line processing is necessary to
enlarge and read out the hologram, which slows the
turnaround from hologram recording to reconstruction .
One must also deal with the "twin-image" inherent to
in-line holography, whereby the out-of-focus object image
from the last term in Eq. (1) overlaps the primary image in
the reconstruction . Twin-image effects can be alleviated by
iterative phase retrieval [29] .
Gabor holography is the most well developed and
applied in the X-ray region . Joyeux and Polack recorded
Gabor holograms of diatoms with 10 nm X-rays at LURE
and optically reconstructed them to submicron resolution
[24] . Jacobsen and others obtained Gabor holograms at the
NSLS of dry rat pancreatic zymogen granules, critical-point
dried Chinese hamster ovarian cells, and hydrated fixed
hippocampal cells with 2.0 to 2.5-nm undulator radiation.
PMMA resist was used to record the holograms, which
were numerically reconstructed to a resolution of 60 nm
[25,27]. Information down to 20 nm is indicated in the
exposed resists; optical distortions in the TEM used to read
them are believed to have limited the reconstructed resolution . Typical exposure times are several minutes using the
X1 soft X-ray undulator.
TEM hologram readout requires coating of the resists
with metal for contrast, leaving no opportunity for further
development. Lindaas [30] has built a wide-field, high
linearity, scanning atomic force microscope to read out the
exposed resists directly without the need for metallization,
which permits additional development if desired and should
reduce aberrations, leading to better image resolution .
In high resolution Fourier transform holography, a
spherical reference wave originating from a point near the
object is made to interfere with the illumination scattered
by the object [12,13]. The point reference source can be
formed with a focusing optic or by a point scatterer.
Because the object wave has approximately the same
curvature as the reference wave, the hologram fringes are
of low spatial frequency at the expense of object field of
view . The imaging resolution is limited by the precision
with which the reference wavefront is known and the
angular extent over which the hologram is recorded . Due
to the off-axis location of the reference source with respect
to the object, both primary and conjugate images of the
object are reconstructed, one to either side of the optic
axis . This avoids the twin-image problem, provided that
the object and reference are sufficiently separated such that
the reconstruction of the first and last two terms in Eq . (1)
does not overlap.
Because of the low fringe frequencies, the Fourier
transform geometry is ideal for digital hologram recording
with coarse-grained electronic detectors such as CCDs .
This provides a swift route to numerical reconstruction .
Being spatially separated, the relative strength of the object
and reference waves can also be balanced for optimum
fringe contrast . Moreover, this geometry may be best for
high power sources due to the large area over which the
hologram intensity is distributed.
A disadvantage is that the intensity is very strong at the
center and weak near the hologram periphery, demanding
considerable dynamic range of the detector. In X-ray experiments, it is usually necessary to employ a beam stop to
attenuate this bright central peak (principally the zerothorder beam) to avoid detector saturation . Because the
lower spatial frequencies in the object are encoded near the
hologram center, these may be blocked by the stop, lending a high-pass-filtered appearance to the reconstruction .
The reference wave and object illumination were derived with a Fresnel zone plate in Fourier transform holography experiments with 3.4-nm soft X-rays by McNulty
[26,27]. The NSLS X1 undulator provided the coherent
X-ray beam . The object was situated a few microns from
the first-order focus of the zone plate such that it was
illuminated by the zeroth and other diffraction orders ; this
beam-splitting geometry was suggested as a way to generate a strong reference wave adjacent to the object [14,20].
The zone plate, fabricated by e-beam lithography, was
made of gold and had a finest zone width of 50 nm . Gold
patterns with 50-125 nm features, fabricated by the same
method, were used as test objects. A CCD camera was
used to record holograms of the test patterns . The CCD (a
576 X 384 array of 23 p,m-square pixels) was cooled with
liquid nitrogen for low dark current and was coated with a
thin phosphor layer to convert incident X-rays to visible
light for better quantum efficiency . The digitally-recorded
Fourier transform holograms were reconstructed by computer to the 60-nm diffraction limit of the zone plate lens
that formed the reference source .
At present, the reference source size limits the resolution by this technique. Although soft X-ray zone plates
with finest zone widths of 25 nm have now been made
[31], this limit will probably not decrease significantly over
the next few years. However, it should be possible to
improve the resolution by extended source compensation
[13] and by accounting for nonplanarity of the object
illumination in the reconstruction .
Mechanical and thermal stability of the apparatus during the recording step is crucial in view of the minutes-long
exposures currently necessary to obtain X-ray holograms
with synchrotron sources. Considerable effort must be
invested to achieve the necessary stability for good fringe
visibility . In Fourier transform X-ray holography, the specimen position in relation to the reference source must be
I. McNulty/Nucl. Instr. and Meth . in Phys. Res. A 347 (1994) 170-176
maintained to a precision comparable to the desired resolution. In the Gabor case, the object and detector must stay
stable to within this limit.
Numerical reconstructions of X-ray holograms to date
have mostly been based on digital implementations of the
Fresnel-Kirchhoff diffraction integral [32] . The object
wave amplitude at the detector is represented in the Fresnel approximation by
e ,ks
ao(x, y)
a,( e, "!) =
i.tz
Az
ff
2z
[(x-6)2+(y-n)2I) dx dy,
where a o (x, y) is the object transmittance, k=2,rr/.l,
(x,y) and (6, i7) are space coordinates in the object and
hologram planes, respectively, and z is the distance from
the object plane to the detector . The object transmittance is
reconstructed by applying the inverse of Eq . (2) to the
product of the hologram intensity and a numerical representation of the reference wave . This procedure, easily and
rapidly implemented with a fast Fourier transform algorithm, has been successfully used to reconstruct X-ray
Gabor and Fourier transform holograms to a numerical
aperture of about 0.05. Reconstructions of digitized 512 X
512-pixel holograms can now be performed by this method
in less than a minute on a fast RISC workstation computer .
If substantially wider-aperture holograms are recorded to
reach better resolution, it will be necessary to go beyond
the Fresnel approximation to reconstruct them . One possibility is to use the limited basis set algorithm of Haddad
[33] .
4. Three-dimensional X-ray holography
Holography is not by definition a three-dimensional
form of imaging despite its popular reputation . Indeed,
173
much work in optical, acoustic and microwave holography,
as well as nearly all work in the X-ray region, has been
one- or two-dimensional (a notable exception can be found
in ref. [22]). Nonetheless, the 3D capability of holographic
methods is both tantalizing and opportune. In holography,
as well as in other types of imaging, the depth resolution
depends on the inverse-square of the numerical aperture of
the optical system, i.e . the largest angle over which fringes
in the hologram are recorded . But because of the small
cross-section in matter for coherent scattering, X-rays are
scattered weakly at large angles, making it difficult to
achieve a numerical aperture of more than - 0.1 and,
therefore, high depth resolution with a single X-ray hologram .
This limitation can be overcome by recording several
holograms of the object from various directions then reconstructing them according to hmographic principles with
the effects of diffraction included [34] . Holography may be
regarded as a scattering process in which the incident,
scattered and transferred momenta satisfy Bragg's law
k = kmc - kscat'
The scattering angle defined by Eq . (3) limits the range of
spatial frequencies that are accessible to the hologram, as
shown in Fig. 3a. By recording an ensemble of holograms
from various angles of incidence, the object is sampled
over a wider spatial frequency range, giving a larger
effective numerical aperture and better depth resolution
(Fig. 3b). This is particularly useful in cases which, as for
X-rays, the object scatters predominantly in the forward
direction. A potential advantage of holographic tomography (HT) over conventional projection tomography is that
fewer views should be required because each hologram
contains some depth information about the object. In addition, HT can help eliminate speckle that may degrade
reconstructions of successive depth planes from a single,
wide-aperture hologram [35] . Although this approach to
3D imaging is well known [36,37] and its application to
Fig. 3. (a) View in reciprocal space (k x and k. axes shown) indicating the range of accessible spatial frequencies (shaded region) for the
scattenng angle NA . (b) Use of three different incidence angles in holographic tomography to cover wider range in kr to obtain better depth
resolution.
Vl. COHERENCE
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I. McNulty/Nucl. Instr. and Meth. in Phys. Res. A 347 (1994) 170-176
the X-ray region has been suggested before [20,38], it has
not been tested with X-rays until recently.
We performed a demonstration experiment to explore
the feasibility of HT [39]. Using coherent 3 .2-nm X-rays
and a CCD camera, we recorded Fourier transform holograms of a microfabricated test object from various illumination angles. The object, consisting of two parallel gold
bars (2.5 p,m apart, 130 nm wide and 90 nm thick), was
oriented normal to the beam and rotated such that the bars
were in various depth planes . X-rays coherently scattered
by the bars interfered to produce a Young's fringe pattern
that is the hologram of one bar, phase-shifted by the
difference in depth (the other bar served as the reference
source). Numerical reconstructions of a 1-wm segment of
one bar were obtained from seven holograms covering an
angular range of -45° to +45°. Three-dimensional phase
recovery and inverse Fourier transformation were used to
recover the object scattering potential
F(x, y, z) = - k2 [ n'(x, y, z) - 1]
(4)
and therefore its 3D index of refraction n(x, y, z) [40] .
The bar is localized in the reconstructions to better than
100 nm in both the transverse and longitudinal directions .
We have also applied an algebraic reconstruction technique [41] that effectively reduces artifact arising from
using a limited hologram set.
Object complexity and the number, aperture, and registry of the holograms used in the reconstruction determine
the resolution attainable by HT . Misregistration of the
holograms due to object rotational errors can be partially
corrected a posteriori by aligning the images to an identifiable feature in the object . When the depth resolution per
view is insignificant, the number of views required for a
densely packed object is equal to the object radius divided
by the desired depth resolution . Clearly this is impractical
for a radiation-sensitive specimen (e .g ., 100 views to
image a 10-wm diameter volume with a depth resolution
of 50 nm), although a substantially smaller number of
views may be sufficient for sparse objects. Moreover,
incorporation of a priori information about the object into
the reconstruction could drastically reduce this number.
The CAD drawing that would necessarily accompany an
integrated circuit is such an example.
Holographic tomography may be a practical approach
to 3D imaging provided the number of holograms needed
to achieve satisfactory depth resolution with an acceptable
signal-to-noise ratio and radiation dose can be determined
for a given class of objects. We have shown that the depth
resolution can approach the transverse resolution in simple
objects with a small number (- 10) of holograms provided
that the angular coverage approaches 90 °. Additionally,
there must be enough coherent flux available to record the
tomographic ensemble, which becomes more restrictive at
higher photon energies .
5. Higher energies
By contrast to the soft and hard X-ray regions, the 1-4
keV intermediate energy region has seen comparatively
few spatially resolved applications . This energy range,
noteworthy for the wide variety of elements in the middle
of the periodic table whose K, L, and M absorption edges
fall within it, has attracted increasing attention in the past
few years [42] . The interaction lengths of such X-rays are
suited to investigation of dense microscopic objects, with
natural elemental and chemical absorption contrast for the
heavier elements and phase contrast for the lighter elements. Owing to the unprecedented coherent flux that will
be available from undulators on third-generation storage
rings, X-ray holography at these energies might be feasible
for the first time .
Intermediate energy X-rays are potentially useful for
holographic microscopy of microstructures important to
the materials sciences and to the microelectronics sector,
especially those containing aluminum, silicon, nickel, copper, gallium and arsenic. Tomographic X-ray analysis of
composites is already an active field [43] . Molecular sieves
(e .g ., zeolites), used extensively for shape-selective catalysis and adsorption, figure prominently in environmental
cleanup research [44] . Nondestructive in-situ imaging of
imbedded microcircuits is another potential application .
For instance, holography could be used to pinpoint microdefects in 100-nm-wide aluminum interconnects sandwiched between micron-thick layers of silicon within a
microcircuit, without disturbing its mechanical or electrical
integrity.
The 1-4 keV range is also attractive for holography of
biological specimens. Biological structures can have significant phase-shifting yet relatively weak absorptive effects
on intermediate energy X-rays . By comparison to soft
X-ray and charged-particle probes, these energies could
permit imaging with better phase contrast, less dose and
through thicker specimens [5].
The coherent flux Fc = B(A/2) Z expected at intermediate X-ray energies using several of the brightest existing
and planned undulators [3,4] is illustrated in Fig. 4, where
B is the spectral brightness per bandwidth t1 A/A. At an
energy of 3 keV and bandwidth of 0.1%, the APS U5 .5
undulator will deliver an anticipated coherent flux of 5 X
10' ° photons/s. For example, we can expect enough
signal with a beamline efficiency of several percent to
record holograms of a 10-micron-cube volume with 10 3
coherent photons/s per 50-nm resolution element. For
fixed temporal coherence length 1. _ AZ/t> A and the
brightness B per unit bandwidth, Fc decreases even more
swiftly with A according to
F~ =BA3/41C .
In addition to brighter sources, high resolution optics
have also become available for harder X-rays . Bionta
fabricated zone plates made by the sputtered/ sliced tech-
L McNulty /Nucl. Instr. and Meth . in Phys. Res. A 347(1994)170-176
m
ô
fn
â
10 12
10 11
1010 _
d
9
108
n=3
ALS U3 9
n=5
NSLS X1
n=5
050
10
15
20
25
30
35
40
Photon energy (KeV)
Fig. 4. Coherent flux at 1-4 keV produced by the NSLS X1 (8 .0
cm period), ALS U3.9 and APS U5 .5 undulators [3,41. To reach
above 1 keV, the higher harmonics of the X1 and U3 .9 devices
must be used because of the lower storage ring energy (2 .5 and
1 .5 GeV for the NSLS and ALS respectively, as compared to 7
GeV for the APS).
nique [45] . Yun demonstrated submicron focusing with
near-ideal diffractive efficiency using phase zone plates
made by X-ray lithography [46] . These optics have been
used in scanning transmission and fluorescence microscopy experiments near 8 keV [47], but they could be
adapted to work at lower energies . Grazing incidence
Wolter and multilayer Schwartzschild mirrors, previously
well behind diffractive optics in achieving theoretical resolution limits, are now competitive with zone plates in the
soft X-ray region and show promise for extension to
higher energies .
6. Flash sources
Perhaps the greatest challenge facing application of
X-ray holography to the life sciences is the problem of
radiation damage . To a lesser degree, radiation damage
may complicate holography of inanimate but fragile objects, for example the gate electrode of a working MOS
field-effect transistor. Radiation dose increases like S-z"
with the imaging dimension n and resolution 8 [2].
Flash sources potentially offer the greatest reward in
this regard . The key to flash X-ray holography will lie in
the ability of the source and beam transport optics to
deliver sufficient coherent flux to the experiment in a short
enough time . Onset of hydrodynamic blurring scales as the
inverse third power of the resolution and is of order 100 ps
for a resolution of 50 run [48] . The time for radiation
damage to be manifest in living specimens following
exposure is much greater, on millisecond or longer scales
175
[49] . To avoid artifact due to irreparable radiation damage,
a holographic snapshot of the specimen must be captured
with low incident intensity in less time than it takes the
damage to appear, or with high incident intensity in less
time than hydrodynamic blurring occurs . Synchrotron
sources do not posses the peak brightness necessary to
record high resolution flash holograms, therefore bright
XRLs are the only alternative.
While XRLs have steadily improved in brightness and
exhibited significant gain at water-window wavelengths
[501 since their inception in 1985, their coherent output is
still too limited for submicron X-ray holography . Nevertheless, high resolution, direct-imaging experiments have
been conducted with 4.48-nm X-rays from the Livermore
XRL [51], suggesting that use of these sources for holography, when they become bright enough, is around the
corner .
The proposed Stanford soft X-ray free-electron laser
[52], if realizable, is one of the most exciting future
prospects. This extremely short pulse (150 fs), high peak
brightness (1031 photons/s/mm2/mrad2 per 0.1% bandwidth) XRL would make possible an array of X-ray holography experiments that could never be attempted with an
undulator. The immense number of coherent photons per
pulse, 10 14 , is four orders of magnitude greater than that
currently needed to form a single high resolution Gabor
[25] or Fourier transform X-ray hologram [26] . Moreover,
the short pulse duration could allow biological X-ray
microscopy that is truly free of radiation damage artifact .
Down the line, 3D X-ray holography may be possible with
this source if means can be found to record several simultaneous holograms of the specimen with a single pulse.
7. Future outlook
The potential of X-ray holography has only recently
been explored and the many uses to which it could be put
are clearly far from exhausted. Certainly, development of
3D flash holography with 10-nm resolution would have
dramatic implications for structural biology. Higher energies are attractive in view of the third-generation X-ray
sources due to come on-line soon . Other possible applications include interferometric and microdifferential holography [53], development of holographic optical elements,
and holographic lithography [54] . An intriguing possibility
is atomic resolution by X-ray fluorescence holography
with a local reference source [55] . Synchrotron sources are
invaluable for developing these applications and for refining the techniques that will be needed to utilize high
brightness XRLs . The richest rewards of X-ray holography
will likely be realized in combination with other methods.
Even at this juncture, it is clear that holography has just
begun to find application in the X-ray domain. There are
sure to be many interesting developments to come .
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I. McNulty/Nucl. Instr. and Meth . in Phys. Res . A 347 (1994) 170-176
Acknowledgements
I wish to thank J . Kirz, M . Howells, D . Sayre, C .
Jacobsen, J . Trebes and W. Haddad for fruitful collaboration and many helpful discussions . This work was supported under contract W-31-109-ENG-38 by the U .S . Department of Energy, BES-Materials Sciences .
References
[1] B .L. Henke, in : Encyclopedia of Microscopy, ed . G .L . Clark,
(Reinhold, New York, 1961) p. 675 .
[2j D . Sayre, J . Kirz, R . Feder, D.M. Kim and E . Spiller,
Science 196 (1977) 1339 .
[3] An ALS Handbook (Advanced Light Source, Berkeley, 1989)
p . 40 .
[4] G .K. Shenoy and D .E . Moncton, Nucl . Instr. and Meth. A
266 (1988) 38 ;
Foundation Phase Report (European Synchrotron Radiation
Facility, Grenoble, 1987) ; SPring-8 Project, Part 1, Facility
Design 1990 (SPring-8 Project, Japan, 1991).
M .R . Howells, Lawrence Berkeley Laboratory Report LBL27420 (1989) .
[6] H. Ade et al ., Science 258 (1992) 972 .
[7] G. Kellstr6m, Nova Acta Reg . Soc . Sci. Upsaliensis 8 (1932)
61 .
[8] D . Gabor, Nature 161 (1948) 777 .
[9] A.V . Baez, J. Opt . Soc . Am . 4 2 (1952) 756.
[10] H .M .A . EI-Sum and P . Kirkpatrick, Phys . Rev . 85 (1952)
763 .
[ll] E .N Leith, J . Upatnieks and K .A. Haines, J . Opt . Soc. 55
(1965) 981 .
[12] J .T. Winthrop and C .R . Worthington, Phys . Lett . 15 (1965)
124 ;
J .T . Winthrop and C .R . Worthington, Phys . Lett 21 (1966)
413 .
[13] G .W. Stroke, AppL Phys . Lett . 6 (1965) 201 ;
G .W. Stroke, R . Restrick, A. Funkhouser and D . Brumm,
Phys. Lett . 18 (1965) 274 .
[14] G .L . Rogers and J . Palmer, J . Microse . 89 (1969) 125 .
[15] J .W . Giles, J . Opt . Soc. Am 59 (1969) 778 .
[16] G .C . Bjorklund, Appl . Phys . Lett. 25 (1974) 451 .
[17] S . Aoki and S . Kikuta, Jpn. J Appl . Phys . 13 (1974) 1385 .
(18] B . Reuter and H . Mahr, J. Phys. E 9 (1976) 746 .
[19] A .M . Kondratenko and A.N . Skrinsky, Opt . Spectrosc . 42
(1977) 189 .
[20] M . Howells and J . Kirz, m : AIP Proc . No . 118, Free Electron
Generation of Extreme Ultraviolet Coherent Radiation, eds .
J .M .J . Madey and C . Pellegrini, (Am. Inst. Phys, New York,
1984) p . 85.
[21] J .C. Solem and G .C. Baldwin, Science 218 (1982) 229 .
[22] S . Aoki and S . Kikuta, in : AIP Proc . No. 147, Short Wavelength Coherent Radiation : Generation and Applications, eds.
D.T. Attwood and J . Bokor, (Am . Inst. Phys ., New York,
1986) p . 49.
[23] M . Howells et al ., Science 238 (1987) 514.
[24] D. Joyeux and F. Polack, m : OSA Proc on Short Wavelength Coherent Radiation : Generation and Applications vol .
2, eds. R W . Falcone and J. Mrz, (Opt. Soc . Am., Wash .,
DC, 1988), p. 295 .
[25] C . Jacobsen, M. Howells, J . Kirz and S . Rothman, J . Opt .
Soc . Am . A 7, (1990) 1847 .
[26] l. McNulty et al ., Science 256 (1992) 1009 .
[27] C. Jacobsen, S . Lindaas, M. Howells and 1 . McNulty, Inst
Phys . Conf. Ser. 130 (1992) 547 .
[28] J .E . Trebes et al ., Science 238 (1987) 517 .
[29] G . Lm and P .D. Scott, J. Opt . Soc . Am. A 4, (1987) 115 .
[30] S Lindaas, C . Jacobsen, M . Howells and K. Frank, SPIE
Proc. 1741 (1992) 213 .
[31] G . Schmahl et al ., Proc . 4th Int . Conf. on X-ray Microscopy
(Moscow, 1993), in press .
[32] J W. Goodman, Introduction to Fourier Optics (McGraw
Hill, San Francisco, 1968) pp . 57-62
[331 W.S . Haddad, D . Cullen, J.C. Solem, K . Boyer and C .K.
Rhodes, in : OSA Proc . on Short Wavelength Coherent Radiation : Generation and Applications, vol . 2, eds . R .W. Falcone
and J . Kirz, (Opt . Soc . Am ., Wash ., DC, 1988) p . 284 .
[34] E. Wolf, Opt . Commun . 1 (1969) 153 .
[35] E. Spiller, in : X-Ray Microscopy, eds P .C . Cheng and G .J .
Jan (Springer, Heidelberg, 1987) p. 224 .
[36] R . Dändliker and K . Weiss, Opt. Commun . 1 (1970) 323
[37] A .J . Devaney, Phys . Rev . Lett 62 (1989) 2385 .
[38] M.R . Howells and C .J . Jacobsen, Synchrotron Radiation
News 3 (1990) 23 .
[39] 1 . McNulty et a, SPIE Proc . 1741 (1992) 78 .
[40] J. Brase, T . Yorkey, J . Trebes and 1 . McNulty, SPIE Proc .
1741 (1992) 234.
[41] R. Gordon, Appl . Opt . 24 (1985) 4124 .
[42] For example, see G . van der Laan et al., Phys. Rev . B 34
(1986) 6529 .
[43] J H Kinney et al ., J . Mater. Res . 5 (1990) 1123 .
[44] S . Kesraoui-Ouki, C . Cheeseman and R . Perry, Environ . Sci.
Technol . 27 (1993) 1108 .
[45] R .M . Bionta et al ., Opt . Eng. 29 (1990) 576.
[46] W.B Yun, P .J . Viccaro, J . Chrzas and B. Lai, Rev . Sci . Instr.
63 (1992) 582 .
[47] B Lai, W .B. Yun, D . Legnini, Y .H . Xrao and J . Chrzas,
SPIE Proc 1741 (1992) 180.
[48] J .C Solem, J Opt . Soc. Am . B 3 (1986) 1551 .
[49] R .A . London, J .E . Trebes and C .J . Jacobsen, SPIE Proc.
1741 (1992) 333 .
[50] B .J . MacGowan et al ., Phys . Rev. Lett . 64 (1990) 420 .
[51] L.B . DaSilva et al ., Opt . Lett. 17 (1992) 754.
[52] C . Pellegrini et al ., in : Proc. 13th Int Free-Electron Laser
Conf., C . Yamanaka and K . Mima, eds ., Nucl . Instr . and
Meth . A 331 (1993) 223 .
[53] M . Sharnoff, J . Opt . Soc. Am . A 2 (1985) 1619.
[54] C . Jacobsen and M .R. Howells, J . Appl . Phys . 71 (1992)
2992
[55] A. Sz6ke, in : AIP Proc . No . 147 Short Wavelength Coherent
Radiation : Generation and Applications, eds . D .T . Attwood
and J . Bokor, (Am . Inst . Phys., New York, 1986) p . 361 .