This simple way to create a X-ray standing wave in a paraxial geometry opens up the opportunity to develop new X-ray interferometry techniques to study natural and advanced man-made nanoscale materials, such as self-organised biosystems, photonic and colloidal crystals, and nanoelectronic materials. [Snigirev et al., Phys. Rev. Lett. (2009), 103] present a new and simple way to generate an X-ray periodic interference field such as a standing wave with variable period ranging from tens of nanometres to tens of micrometres. The proposed interferometer thus occupies the place between crystal and grating interferometers. The silicon bilens can generate standing waves with a 40 nm period above 50 keV. If diamond lenses were employed, then we could expect the smallest pitch to be 30 nm at energies E less than 12 keV. Contrary to Bonse-Hart interferometers and X-ray standing wave techniques, the bilens interferometer generates an interference pattern without the requirement of additional optics like crystals or multilayers. The interference occurs in air at a reasonable distance from the device itself allowing great flexibility in sample size and environment.

Such coherent spatially harmonic illumination can be used for new diffraction and imaging methods to study mesoscopic materials. A phase contrast imaging technique is feasible whereby a sample is inserted into one of the beams while they are separated, as in the case of a classical interferometer. Any interaction with that beam will induce significant changes in the interference pattern, allowing the extraction of high resolution information on the sample from the new phase pattern produced. A second technique would be Moiré imaging whereby the sample is placed behind the bilens within the interference field. Standing wave techniques are evident: a sample could be scanned across the periodic interference field and secondary processes (fluorescence, secondary electrons, etc.) would be detected by a detector placed to one side of the beam.

In addition to imaging applications, the bilens interferometer could also be used for coherence diagnostics at existing synchrotrons. Silicon bilenses are stable under extremely powerful beams and relatively insensitive to mechanical vibrations. We expect that they will also be widely used for beam characterisation for future free electron X-ray laser sources.