From X-ray grating to X-ray speckle: one global approach of wavefront modulation
The high energy and short wavelength of X-rays make them ideal for imaging inner features of samples at the sub-micrometer scale. Over the past twenty years, imaging techniques exploiting the phase of X-rays have progressively developed, pushed by the desire of achieving higher resolution, and to image light materials such as biological soft tissue. X-ray grating interferometry (XGI) is one such successful technique that has seen its user community growing both at synchrotrons and lab-based sources.
A team from Diamond Light Source has used the B16 Test beamline to demonstrate an extension of the X-ray grating interferometry technique to a general scheme based instead on a random phase object. Just like the X-ray grating interferometer, they managed to recover the absorption, scattering and two dimensional phase gradient images of the sample from a raster scan of the wavefront modulator. Moreover, this new method offers several advantages: i) an improved spatial resolution, ii) no need for expensive and sometimes hard to source X-ray gratings and iii) only small requirements on the coherence of the X-ray source. Their study has been recently published in the journal Physical Review A.
X-ray multimodal imaging provides multiple information maps of a sample, namely, its absorption, its refraction index (phase image) and its homogeneity at the nanometer scale (darkfield or scattering image). The principle of this imaging method relies on wavefront modulation which is to insert in the beam and analyse a pattern with spatial frequencies much higher than the one of the sample under study. In XGI, a phase grating is placed downstream of the sample to modulate the wavefront and create an interference pattern. The difference between this pattern and the pattern obtained when there is no sample in the beam enables determination of the phase shift induced by the sample. The Diamond team replaced this phase grating with a simple biological filtering membrane with random features.
Interference arising from spatially uncorrelated features such as those from the biological filtering membrane generates a random interference pattern called speckle. With hard X-rays the intensity pattern obtained close to the scattering object has a specific form and propagation behavior, known as the “near field speckle”. This speckle pattern is closely related to the form and structure of the scattering object. The membrane, used as a wavefront phase modulator, is mounted on a piezo motor that allows it to move in the two transverse directions relative to the X-ray beam, to perform a 2D raster scan with and without the sample in the beam. When the sample is introduced into the beam, the speckle pattern is slightly distorted and the calculation of this distortion permits recovering information about the sample.
Dr Kawal Sawhney, Diamond Light Source
X-ray multimodal imaging using a random-phase object Sebastien Berujon, Hongchang Wang, Kawal Sawhney, Physical Review A 86 DOI: 10.1103/PhysRevA.86.063813