For centuries, lens-based microscopy, such as optical, phase-contrast, fluorescence, confocal, and electron microscopy, has played an important role in the evolution of modern science and technology. In 1999, a novel form of microscopy, i.e., coherent diffraction imaging (also termed coherent diffraction microscopy or lensless imaging), was developed and transformed our conventional view of microscopy, in which the diffraction pattern of a noncrystalline specimen or a nanocrystal was first measured and then directly phased to obtain a high-resolution image. The well-known phase problem was solved by combining the oversampling method with iterative algorithms. In this paper, we will briefly discuss the principle of coherent diffraction imaging, present various implementation schemes of this imaging modality, and illustrate its broad applications in materials science, nanoscience, and biology. As coherent X-ray sources such as high harmonic generation and X-ray free-electron lasers are presently under rapid development worldwide, coherent diffraction imaging can potentially be applied to perform high-resolution imaging of materials/nanoscience and biological specimens at the femtosecond time scale.

Coherent X-Ray Diffraction Imaging

The advent of X-ray free-electron lasers has granted researchers an unprecedented access to the
ultrafast dynamics of matter on the nanometre scale(1-3). Aside from being compact, seeded
plasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich(4) sources (up to 10(15)
per pulse) that display high-quality optical properties(5,6). Hitherto, the duration of these sources was
limited to the picosecond range(7), which consequently restricts the field of applications. This bottleneck
was overcome by gating the gain through ultrafast collisional ionization in a high-density plasma
generated by an ultraintense infrared pulse (a few 10(18) W cm(-2)) guided in an optically pre-formed
plasma waveguide. For electron densities that ranged from 3 x 10(18) cm(-3) to 1.2 x 10(20) cm(-3), the
gain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which paves
the way to compact and ultrafast SXRL beams with performances previously only accessible in
large-scale facilities.

Table-top femtosecond soft X-ray laser by collisional ionization gating

X-ray holography offers the potential for obtaining high resolution three-dimensional images of in vitro biological microstructures. Significant progress toward this goal has been achieved with holography systems using synchrotron x-ray sources and recently spatial resolutions as small as 40 nm have been demonstrated. These experiments required x-ray exposures of an hour or longer, which makes high spatial resolution difficult to achieve in live biological specimens because of blurring of the image. This blurring is caused by specimen motion and will prohibit the imaging of dynamical processes within the specimen. A possible solution to this problem is to exploit the extremely high brightness and long coherence lengths produced by x-ray lasers and create the hologram with exposure times of less than 1 nsec. This report presents the results from an experiment in which an x-ray laser was used to produce x-ray holograms. The holography geometry used was a Gabor in-line type modified by the inclusion of a high reflectivity multi-layer x-ray mirror used as a narrow bandpass filter. The x-ray mirror had a flatness and roughness of less than X/100 (where X is the wavelength) at the x-ray laser wavelengths.

Demonstration Of X-Ray Holography With An X-Ray Laser

We demonstrate the first real-space recording of nanoscale dynamic interactions using single-shot soft x-ray (SXR) full-field laser microscopy. A sequence of real-space flash images acquired with a table-top SXR laser was used to capture the motion of a rapidly oscillating magnetic nanoprobe. Changes of 30 nm in the oscillation amplitude were detected when the nanoprobe was made to interact with stray fields from a magnetic sample. The table-top visualization of nanoscale dynamics in real space can significantly contribute to the understanding of nanoscale processes and can accelerate the development of new nanodevices.

https://www.engr.colostate.edu/ece///faculty/rocca/pdf/journals/carbajo_etal_2012.pdf

Sequential single-shot imaging of nanoscale dynamic interactions with a table-top soft x-ray laser

Single-shot nanometer-scale imaging techniques have become important because of their potential application in observing the structural dynamics of nanomaterials. We report here the image reconstruction results obtained using single-shot Fourier transform x-ray holography with an x-ray laser driven by a table top laser system. A minimum resolution of 87 nm was obtained from the reconstructed image. We could also discriminate the aggregates of carbon nanotubes, which shows the feasibility of single-exposure nanoimaging for real specimens using a laser-driven x-ray laser.

Single-shot nanometer-scale holographic imaging with laser-driven x-ray laser

A strongly saturated waveguide-based optical-field-ionization soft-x-ray laser seeded by high harmonic generation was demonstrated for Ni-like Kr lasing at 32.8 nm. Compared with the same laser seeded only with spontaneous emission, seeding with high harmonics yields much smaller divergence, enhanced spatial coherence, and controlled polarization. The integration of high harmonic seeding, optically preformed plasma waveguide, and optical-field-ionization pumping forms one of the optimal archetypes of an ultrashort-pulse soft-x-ray laser.

Seeding of a soft-x-ray laser in a plasma waveguide by high harmonic generation.

Single-shot digital holographic microscopy with an adjustable field of view and magnification was demonstrated by using a tabletop 32.8 nm soft-x-ray laser. The holographic images were reconstructed with a two-dimensional fast-Fourier-transform algorithm, and a new configuration of imaging was developed to overcome the pixel-size limit of the recording device without reducing the effective NA. The image of an atomic-force-microscope cantilever was reconstructed with a lateral resolution of 480 nm, and the phase contrast image of a 20 nm carbon mesh foil demonstrated that profiles of sample thickness can be reconstructed with few-nanometers uncertainty. The ultrashort x-ray pulse duration combined with single-shot capability offers great advantage for flash imaging of delicate samples.

Single-shot soft-x-ray digital holographic microscopy with an adjustable field of view and magnification

Growth of strongly textured $\mathrm{FeCO}_{3}$
 thin films on substrates was achieved with ultrashort-pulsed laser deposition using 810-nm, 46-fs ablation pulses. The crystallinity and composition were verified with X-ray diffraction and Raman spectroscopy. Using Mossbauer spectroscopy, it is shown that the deposited $\mathrm{FeCO}_{3}$
 thin films possess the film quality required for application in research of nuclear quantum optics. It is found that a relatively low substrate temperature is crucial for growing a strongly textured film of $\mathrm{FeCO}_{3}$
 while avoiding decomposition of $\mathrm{FeCO}_{3}$
 into $\mathrm{Fe}_{2}\mathrm{O}_{3}$
 and $\mathrm{CO}_{2}$
 . This supports the importance of the use of ultrashort-pulsed laser deposition in providing adatoms with high mobility for attaining good crystallinity. The surface morphology was characterized by surface profilometry, scanning electron microscopy and atomic force microscopy. It is found to be significantly affected by changing the ablation laser parameters, including laser fluence, pulse duration, and on-target spot size. The results show that the peak deposition flux must be below approximately 0.03 nm/pulse in order to grow a flat film.

Growth of strongly textured FeCO3 thin films for application in research of nuclear quantum optics with ultrashort-pulsed laser deposition

Efficient soft x-ray lasing was achieved by using plasma waveguide to confine the pump beam. With a 9-mm-long pure krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400 folds. An output level of 8 × 10 10 photon/shot is reached at an energy conversion efficiency of 2 × 10 -6 . Seeding the laser with high-harmonic generation yields small divergence, high coherence, and controlled polarization. Application in digital holographic microscopy was demonstrated.

Ping-Hsun Lin

Papers

Seeding of a soft-x-ray laser in a plasma waveguide by high harmonic generation.

Single-shot soft-x-ray digital holographic microscopy with an adjustable field of view and magnification

Growth of strongly textured FeCO3 thin films for application in research of nuclear quantum optics with ultrashort-pulsed laser deposition

Development and application of plasma-waveguide based soft x-ray lasers

Development and application of plasma-waveguide based soft x-ray lasers