Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source

Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.

Figure 1

Tabletop soft x-ray diffractive imaging beam line. The soft x-ray laser harmonics are produced by the interaction of the loose focused infrared laser beam in an argon gas cell. The infrared beam is removed after the soft x-ray generation by a ${\mathrm{SiO}}_2$ beam splitter that reflects most of the soft x ray. An antireflective coating rejects most of the infrared laser light. The residual infrared light is totally suppressed with a 150 nm thin aluminum filter. The 25th harmonic order, at a wavelength of 32 nm, is selected and focused onto the sample using a multilayer coated off axis parabola. The sample is positioned at the focus where it scatters the soft x-ray beam. The diffraction pattern is collected on a backilluminated soft x-ray CCD at 20 mm distance from the sample. With a $3\mu \mathrm{m}$ sample size the diffraction pattern is measured in the Fraunhofer regime.

Figure 2

Multishot coherent soft x-ray diffraction patterns and the reconstructed image. (a) Measured diffracted intensity (magnitude square of diffracted wave) from the sample, in logarithmic scale, obtained in 40 shots acquisition (2 s exposure time). The maximal spatial frequency (${\sigma }_{\max }$) at the edge of the diffraction pattern is $10.8\mu {\mathrm{m}}^{-1}$. (b) Reconstructed object amplitude obtained with a 46 nm pixel size. The reconstruction reveals the modification due to the damage that occurred to the object during the experimental campaign, in particular, at the round of the left harmonic note. (c) PRTF function of the image reconstructed in which the criteria value of $1/e$ provides a half-period resolution length image of 62 nm.

Figure 3

Single-shot coherent soft x-ray diffraction patterns and the reconstructed image. (a) Measured diffracted intensity from the sample, in logarithmic scale, obtained in single-shot acquisition (20 fs exposure time). The maximal spatial frequency at the edge of the diffraction pattern is $8.4\mu {\mathrm{m}}^{-1}$. (b) Image of reconstructed object amplitude obtained with 59 nm pixel size. Despite less quality, the single-shot object reconstruction shows a qualitative agreement with the multishot reconstruction. In particular, the amplitude transmission is lower on the round of the left harmonic note. (c) PRTF function of the reconstructed image. The $1/e$ criteria value provides a half-period resolution length image of 119 nm.

Figure 4

“Harmonic notes” SEM images and details on the structure modification. Panel (a) shows the initial test object used at our table-top coherent imaging beam line. Panels (b)–(d) show a detail of the structure modification of the object at different steps during the experiment. These steps, respectively, correspond to a low (b), medium (c), and high (d) soft x-ray dose. The dark (purple) area corresponds to a full transmission of the soft x-ray whereas the bright (purple) area corresponds to a partial transmission. Panel (c) corresponds to the sample used to collect the diffraction patterns shown in Figs. 2a, 3a.