  The material and exact shape of a nanostructure determine its optical
response, which is especially strong for plasmonic metals. Unfortunately, only
a very few plasmonic metals are available, which limits the spectral range
where these strong optical effects can be utilized. Alloying different
plasmonic metals can overcome this limitation, at the expense of using a high
temperature alloying process, which adversely destroys the nanostructure shape.
Here, we develop a low temperature alloying process at only 300{\deg}C and
fabricate Au-Ag nanostructures with a broad diversity of shapes, aspect ratios
and stoichiometries. EDX and XPS analyses confirm the homogeneous alloying
through the entire sample. Varying the alloy stoichiometry tunes the optical
response of the nanostructure and controls spectral features such as Fano
resonances. Binary metasurfaces that combine nanostructures with different
stoichiometries are fabricated using multiple-step electron beam lithography,
and their optical function as hologram or Fresnel zone plate is demonstrated at
the visible wavelength of 532 nm. This low temperature annealing technique
provides a versatile and cost-effective way of fabricating complex Au-Ag
nanostructures with arbitrary stoichiometry.
