Nature Phys. 7, 705–708 (2011)

Conventional imaging with electromagnetic radiation is subject to a diffraction limit that confines the achievable resolution to the scale of the wavelength. Now, Kenji Tamasaku and co-workers from the RIKEN Spring 8 Center, the Japan Science and Technology Agency and Nagoya University in Japan have proposed a method for visualizing the local optical response of a material to extreme-ultraviolet radiation at atomic resolution. The key technique in the study was X-ray parametric downconversion, which allowed the achievable resolution to be separated from the wavelength of the probe radiation. The researchers exploited the nonlinear response of diamond to X-rays to characterize the optical response of diamond at extreme-ultraviolet wavelengths of 103 Å (120 eV) and 206 Å (60 eV) and at a resolution of 0.54 Å. When illuminated with 11.1 keV X-rays, the diamond produced an idler signal through its second-order X-ray nonlinear susceptibility. Because this value also includes the material's linear optical susceptibility, which has a microscopic structure on the atomic scale, Fourier synthesis can be used to reconstruct the structure of the local optical response to the idler light.