The collimation slit, double-slit, and mask were made by FIB milling into three 100 nmthin
silicon-nitride membrane windows. The FIB milling was performed on a 30 keV Ga+
system (FEI Strata 200xp). After milling, each membrane was coated with approximately 2 nm
of gold. The double-slit consists of two 62-nm-wide slits with a center-to-center separation of
272 nm (see inset 1 in figure 1). Each slit is 4μm tall and has a 150 nm support midway along its
height. The mask is 4.5μm wide ×10μm tall (see inset 2 in figure 1), and was placed 240μm
away from the double-slit. The mask was held securely in a frame that could slide back and
forth and was controlled by a piezoelectric actuator. For a more detailed description of the setup
and analysis see supplementary information.