Visualizing the local optical response to extreme-ultraviolet radiation with a resolution of λ/380

Scientists have continually tried to improve the spatial resolution of imaging ever since the invention of the optical microscope in around 1610 by Galileo(1). Recently, a spatial resolution near lambda/10 was achieved in a near-field scheme by using surface plasmon polaritons(2,3). However, further improvement in this direction is hindered by the size of metallic nanostructures(2). Here we show that atom-scale resolution is achievable in the extreme-ultraviolet region by using X-ray parametric down-conversion, which detaches the achievable resolution from the wavelength of the probe light. We visualize three-dimensionally the local optical response of diamond at wavelengths between 103 and 206 angstrom with a resolution as fine as 0.54 angstrom. This corresponds to a resolution from lambda/190 to lambda/380, an order of magnitude better than ever achieved. Although the present study focuses on the relatively high-energy optical regions, our method could be extended into the visible region using advanced X-ray sources(4-7), and would open a new window into the optical properties of solids.