  There is a demand for the manufacture of ultra low-loss metallic films with
high-quality single crystals and surface for quantum optics and quantum
information processing. Many researches are devoted to alternative materials,
but silver is by far the most preferred low-loss material at optical and
near-IR frequencies. Usually, epitaxial growth is used to deposit
single-crystalline silver films, but they still suffer from losses and
well-known deweting effect. Here we report the two-step approach for e-beam
evaporation of atomically smooth single-crystalline metallic films. The
proposed method is self-controlled by quantum size effects and is based on the
step switch of film growth kinetics between two deposition steps, which allow
to overcome the film-surface dewetting. Here we have used it to deposit 35-100
nm thick single-crystalline silver films with sub-100 pm surface roughness and
extremely low losses. We anticipate that the proposed approach could be readily
adopted for the synthesis of other low-loss single-crystalline metallic thin
films.
