EUV­FEL diraction imaging of nanostructuresat SCSS
Jaehyun PARK, Yoshiki KOHMURA and Changyong SONG
RIKEN SPring­8 Center, 111 Kouto, Sayo, Hyogo 6795148, JapanAbstract

We report on a successful demonstration of coherent diraction imaging (CDI) by using the EUV­FEL atthe SPring­8 Compact SASE Source (SCSS). Strong speckle patterns were obtained from a holey carbon mesh, used asa test specimen, from a single pulse exposure. The reconstructed image, a complex density object, well reproduced thestructural features of the test specimen at a few hundred nm resolution detail.

1. Introduction
The X­ray diraction (XRD) is a well accepted tech­nique in characterizing structures of crystalline speci­
mens at atomic scale. However, it is not always
amendable for the XRD technique to study biological
specimens and nanostructures samples, especiallywhen a crystal specimen is not available. CDI is apromising technique to investigate structures of non­crystalline samples18). It provides high resolution andhigh contrast images of non­crystalline specimens withparticular advantages in studying biological single cells
or organelles as well as functional nanostructures. To
achieve high resolution, strong incident beam becomes
indispensible, which accompanies signicant radiation
damages to the samples at the same time. As one way
to deal with this x­ray radiation damage, acquiring adiraction pattern by using intense and short pulse(º10 fs) x­rays was proposed9). X­ray free electron
lasers (XFELs) provide bright, ultra­short pulse, co­herent x­rays enabling high resolution 3D imaging of
biological specimens with a single­particle approach.
As a prototype XFEL source, the SCSS has been con­structed. We have developed an EUV­FEL coherentdiraction microscope (CDM) and carried out CDI ex­
periments for a test sample at the SCSS. We have suc­cessfully demonstrated that high quality speckle pat­tern can be obtained even from a single shot (1012 pho­tons).

2. Experimental
The SCSS provides laser lights of the wavelength be­
tween 51 and 61 nm operating up to 60 Hz repetition
rate. The SCSS delivers about 1012 photons per single
pulse10). Higher photon density is achievable by using apair of elliptical and cylindrical focusing­mirrors. TheFig. 1

Schematic drawing of coherent diraction microscope.

pair of focusing­mirrors is installed before the dirac­tion microscope delivering 1012 photons at 30 microns
by 30 microns focal spot at 1 m downstream from thecenter of the second cylindrical mirror.
We have built a CDM for EUV­FEL at the SCSS.
The microscope hosts submicron precision linear mo­tion stages to manipulate a pinhole aperture, a sample
holder and a beamstop as shown in Fig. 1. The beam­stop is made of 2 mm~2 mm square tungsten blockhold by 50 microns diameter tungsten wires. Speckle
patterns are recorded on an in­vacuum CCD detector(PI­MTE 1300) housed inside the diraction chamber.
During the data acquisition, the CCD chip of the de­tector was kept at |409C cooled by Peltier circuit
reducing thermal noises.
We have carried out CDI experiments by using aholey carbon mesh (SPI, Quantifoil). The thickness ofthe carbon lm is 10 nm. The holey carbon mesh has arepeated pattern composed of circular and ellipticalholes in various sizes as shown in Fig. 2(a). The circularholes have diameters of 1.0, 1.4 and 2.0 mm. The ellip­
tical holes measure 1 mm~4 mm and 2 mm~8 mmalong the shortest and longest axes, respectively11). The
mesh is attached to 12 micron thick Cu pinhole aper­úËõ Sept. 2010 Vol.23 No.5  325(C) 2010 The Japanese Society for Synchrotron Radiation Research3. Results
Fig. 2(b) shows a measured speckle pattern from thesample. Non­negligible absorption of EUV lights bythe carbon lm results in non­centrosymmetric dirac­tion pattern. Focused single EUV­FEL pulse wassucient to acquire a strong speckle with saturated
pixels beyond the dynamic range of the CCD at a low
Q region. During the acquisition, the beamstop wasemployed around the center of the pattern. The two
dark blue lines of the vertical and horizontal directions
are the regions that the diraction signals have been
blocked by the tungsten wires holding the beamstop.
Later by removing the beamstop, we have also ac­
quired the data for the low­Q region: portion of data
blocked by the beamstop before. To avoid the damageon the CCD from the beam intensity uctuation espe­
cially without using a beamstop, incident beam ux
was reduced by using an Ar gas attenuator on thebeamline. The data was taken by accumulating 2000
frames of single­shot data increasing the S/N ratio.
Fig. 2(c) shows an image reconstructed through itera­tive phase retrieval processes1218) from the oversam­
pled diraction pattern. Complex image reconstruc­tion was carried out and only the magnitude is dis­played in Fig. 2(c). All the features of the mesh de­scribed above are clearly resolved in the obtained im­age. The eld of view is 30 mm in diameter, the size ofemployed pinhole aperture. The pixel resolution of theimage in Fig. 2(c) is 161 nm. We note that the 1.0 mm
diameter circular holes are not equidistance in theregion A and B indicated by red and yellow dotted cir­cles in Fig. 2(c), respectively, which is consistent with
the SEM image of the mesh in Fig. 2(a).

4. Summary and perspective

Fig. 2(a) SEM image of holey carbon lm11), (b) Coherent dirac­tion pattern from the test sample, high q region is obtainedby a single shot, low q region is obtained by 2000 pulses withgas attenuator (c) Reconstructed image.

In this work, we have developed a CDM for EUV­
FEL. We have obtained strong speckle patterns from aholey carbon lm. The reconstructed image well repro­
duced the structural features in submicron scale of thelm in detail. We have successfully demonstrated theCDI experiments via a single pulse exposure of EUV­FEL are feasible at the SCSS at a few hundred nmresolution. Since the resolution of CDM is limited onlyby the wavelength of incident light in principle, XFELwith wavelength of angstrom order will enable to get
the 3D single particle image at near­atomic resolution
based on this experimental scheme.

Acknowledgement
ture with 30 mm in diameter dening a eld of view.
The wavelength of incident EUV­FEL beam was tuned
at 51 nm. The distance between the sample and theCCD detector was 80 mm.

326  úËõ Sept. 2010 Vol.23 No.5

This study is collaboration with experimental facili­ty group at XFEL project head oce, RIKEN. We areparticularly grateful to M. Nagasono, M. Yabashi, T.
Ishikawa and the SCSS test accelerator operationgroup, RIKEN for their stimulating discussions andEUV SASE_FEL pÌWJ ¡ EUV­FEL diraction imaging of nanostructures at SCSS

technical support.

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 ÒÐî Jaehyun Park

Postdoctoral Researcher, Song Initia­
tive Research Unit, RIKEN SPring­8CenterE­mail: jhparkspring8.or.jp
Specialty: coherent x­ray imaging, x­
ray microdiractionmªðn
2008, Ph. D. in experimental condensed
matter physics, POSTECH (Korea)
2008­present, Postdoctoral researcher at
Song IRU, RIKEN

Yoshiki Kohmura

Unit Leader, SR Imaging Instrumenta­tion Unit, RIKEN SPring­8 CenterE­mail: kohmuraspring8.or.jp
Specialty: coherent x­ray imagingmªðn
1994, Ph. D. in Science at Faculty ofScience, Univ. Tokyo
19942010, Research Scientist inRIKEN
2010­present, Unit leader, SR Imaging
Instrumentation Unit, RIKEN

Changyong Song

Unit Leader, Song Initiative Research
Unit, RIKEN SPring­8 CenterE­mail: cysongspring8.or.jp
Specialty: coherent x­ray imaging,resonant x­ray scatteringmªðn
2001, Ph.D. in experimental condensed
matter physics, Iowa State University(USA)20012004, Postdoctoral researcher at
POSTECH (Korea)
20042007, Postdoctoral researcher at
UCLA (USA)2008­present, Unit leader, Song IRU,RIKEN (Japan)

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