This work presents the CDI experiment with three absorption samples, whose characteristics are similar to biological samples, are investigated using a high harmonic source with wavelength around 30 nm. Three samples were selected from a large number of candidates are used to demonstrate the capability of the table-top CDI experiment. These samples were prepared as a result of a collaboration with the Ultrafast and Microspectroscopy Laboratories at The University of Melbourne. All three samples were fabricated on 30 nm-thick silicon nitride films where the dimension of the window space is around 13 x 13 μm, which is located in the middle of a 200 μm-thick, 4.4 x 4.4 mm frame. Unlike the previous CDI experiment with a transmission sample, the strong absorption objects in this experiment are deposited on a semi-transparent silicon nitride membrane, which has a transmission capability up to 30% at the 30 nm wavelength; as a consequence the images of these objects will appear “black” on a “white” background. This characteristic is very similar when imaging biological samples using x-ray sources, in which the contrast of the image is derived from the difference of the atoms’ absorption edges between the sample and the membrane or between different materials in the sample itself.

The left sample (called sample A) consists of 2 μm-diameter plain silica (SiO2) spheres on a silicon nitride membrane. The middle sample (sample B) consists of similar material to sample A; it has 2 μm-diameter plain silica spheres and 400 nm-diameter amino polystyrene particles.The right sample (sample C) consists of 2 um carboxylated polystyrene (COOHC8H8) and 400 nm amino polystyrene particles.

Using phase correction reconstruction algorithm, image of the sample A is clearly reconstructed. The edges of the 2 μm spheres are clearly resolved in the reconstructed image, and even the small 400 nm particles can also be identified. A knife edge test is also applied to the image to measure the attained resolution, which reveals a resolution of 140 nm (10% – 90% max intensity) is achieved.

a) Scanning electron microscope image of sample A. b) Reconstructed image using a curvature correction phase retrieval algorithm.

The same method is applied to reconstruct the images of samples B and C; however, due to lack of high angle diffraction data, the expected resolution of the recovered images is lower than that of the image of sample A and the objects will appear more blurry. In this experiment, a beam stop is installed and a new approach in processing diffraction patterns is investigated in an effort to increase the resolution of the reconstructed images of samples B and C. For each sample B and C, the diffraction patterns are now taken with a short exposure time without saturation, and with longer exposure time with a saturated region and with a very long exposure time with the beam stop until the highest intensity nearly reaches the maximum value (~65000). The diffraction patterns of these samples without and with a beam stop are shown:

Diffraction pattern of sample B without a beam stop for an exposure time of 40 seconds (a) and with a beam stop for an exposure time of 5 minutes (b). Diffraction pattern of sample C without a beam stop for an exposure time of 45 seconds (c) and with a beam stop for an exposure time of 8 minutes (d).

New reconstruction algorithm is implemented to combine the diffraction data and reconstruct the images of sample B and C, as shown below:

a) Low resolution reconstructed image of sample B without saturation and beam stop. b) Higher resolution image obtained by processing the diffraction pattern with the beam stop and saturation region.

a) Low resolution reconstructed image of sample C without saturation and beam stop. b) Higher resolution image obtained by processing the diffraction pattern with the beam stop and saturation region.

Using a beam stop and a new processing algorithm to produce a high dynamic range diffraction pattern, high resolution images can be reconstructed. Since these samples have similar characteristics to practical biological specimens, the experiment confirms the feasibility of imaging biological samples with a nanometre resolution using an inexpensive table-top high harmonic generation source. The experiment also proves the effectiveness of the curvature correction phase retrieval algorithm to reconstruct a high resolution image of a sample which is illuminated by a focused source to utilize the high photon flux near the focus point and reduce the required exposure time. 

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