Ptychography has won accolades for the ALS recently, as researchers achieved the highest resolution ever recorded in x-ray microscopy, but what exactly is this technique with the catchy name? (For the record, it’s pronounced “tie-cog-raphee.”) We recently sat down with ALS physicist David Shapiro, lead author on the paper reporting this research in Nature Photonics, to talk about ptychography and why the ALS is uniquely suited to work on this next-generation technique.
Shapiro explains that with a conventional scanning microscope, you focus the beam onto a sample and while scanning it, you record the total transmitted x-ray intensity. Since different chemical species will have different soft x-ray transmission this provides the ability to map chemical composition but is limited in resolution to the smallest spot that you can make. Focusing is a challenging process with x-rays because x-ray optics are difficult to make, inefficient, and have very short focal lengths.
With ptychography, you can work with a somewhat larger spot and measure a diffraction pattern rather than the total transmission. A specialized two-dimensional imaging detector measures all the scattered x-rays and gives researchers much more information than traditional x-ray microscopy.
“We record information that’s outside of the numerical aperture of the x-ray optic, this information is always present but conventional microscopes can’t measure it.” says Shapiro. The very large volume of data generated by a ptychographic microscope requires state-of-the-art, high speed imaging detectors developed by the ALS detector group.
The measurement must then be converted into an image, so researchers use high performance computing and phase retrieval algorithms to reconstruct the sample at high spatial resolution from the diffraction information. There are some standard algorithms that are commonly used, but Shapiro and his colleagues are also working with The Center for Applied Mathematics for Energy Research Applications (CAMERA) in Berkeley Lab’s computational research division to develop even faster algorithms and computer code.
“We’re approaching the point where the resolution is comparable to the wavelength of the x-rays,” says Shapiro. “The ultimate goal would be to map chemical composition in three dimensions at that resolution. The ALS is bringing together the technologies which will make that a reality but the very high brightness x-ray beams from the new ultimate storage ring sources is needed.”
Read more about ALS ptychography in the recent Berkeley Lab news release: http://newscenter.lbl.gov/2014/09/10/advanced-light-source-sets-microscopy-record/