  The use of metals of nanometer dimensions to enhance and manipulate
light-matter interactions for a range of emerging plasmonics-enabled
nanophotonic and optoelectronic applications is an interesting, yet not highly
explored area of research outside of plasmonics1,2. Even more importantly, the
concept of an active metal, i.e. a metal that can undergo an optical
non-volatile transition has not been explored. Nanostructure-based applications
would have unprecedented impact on both the existing and future of optics with
the development of active and nonlinear optical tunabilities in single
elemental metals3-5. Compared to alloys, pure metals have the material
simplicity and uniformity; however single elemental metals have not been viewed
as tunable optical materials, although they have been explored as viable
electrically switchable materials. In this paper we demonstrate for the first
time that antimony (Sb), a pure metal, is optically distinguishable between two
programmable states as nanoscale thin films. We then show that these states are
stable at room temperature, and the states correspond to the crystalline and
amorphous phases of the metal. Crucially from an application standpoint, we
demonstrate both its optoelectronic modulation capabilities as well as speed of
switching using single sub-picosecond (ps) pulses. The simplicity of depositing
a single metal portends its potential for use in applications ranging from high
speed active metamaterials to photonic neuromorphic computing, and opens up the
possibility for its use in any optoelectronic application where metallic
conductors with an actively tunable state is important.
