After many years of preliminary studies, researchers have announced the first demonstration of a Fabry-Perot (FP) interferometer for hard x-rays. Visible-wavelength FPs have been used for more than 100 years in the physical and life sciences but x-ray wavelengths have proven to be a challenge, mostly because x-rays do not reflect from surfaces as easily as do visible wavelengths.
Fabry-Perot interferometers (resonators) can be used to measure wavelengths at high resolutions in atomic spectroscopy, astrophysics, and laser physics, even in the infrared. An x-ray FP interferometer could be used for phase-contrast imaging of nanometer-scale objects, for studying dynamics of solids, liquids, and as interference filters with microelectronvolt spectral resolution. The advance also paves the way for novel x-ray optical devices
Until now, back-reflection mirrors with high enough reflectivity to cause interference at x-ray wavelengths had not been successful. Yuri Shvyd'ko and colleagues at the University of Hamburg (Hamburg, Germany) and the Argonne National Laboratory (Argonne, IL) used two back-reflecting sapphire crystal mirrors to solve this problem.
In the prototype x-ray FP interferometer, 14.315-keV x-rays were reflected backwards via (0 0 0 30) Bragg reflection in two single sapphire (α-Al2O3) crystals placed 53.7 mm apart. Backscattering caused the x-rays to bounce back and forth between the two parallel mirrors, causing an interference pattern with sharp FP resonances. The x-rays were constrained in a bandwidth of 2 meV and had a beam divergence of 15 × 40 µrad. The time response of the interferometer measured a finesse of 15- and 0.76-µeV-wide FP transmission resonances—a spectral resolution 100 times better than existing x-ray crystal monochromators. Because x-ray monochromators cannot resolve the narrow peaks, the energy resonances were detected in the time response of the FP by an x-ray detector with 120-ps time resolution.
Because the sapphire crystals are transparent at visible wavelengths, the researchers point out that a combined optical/x-ray interferometer may be realized by coating the sapphire x-ray mirrors to make optical mirrors. Such combined optical/x-ray FPs could be used to directly measure x-ray-to-visible wavelength ratios.
- Y. V. Shvyd'ko et al., Phys. Rev. Lett. 90, 013904 (2003).