  The phase diagrams of correlated systems like cuprates or pnictides
high-temperature superconductors are characterized by a topological change of
the Fermi surface under continuous variation of an external parameter, the
so-called Lifshitz transition. However, the large number of low-temperature
instabilities and the interplay of multiple energy scales complicate the study
of this phenomenon. Here we first identify the optical signatures of a
pressure-induced Lifshitz transition in a clean elemental system, black
phosphorus. By applying external pressures above 1.5 GPa, we observe a change
in the pressure dependence of the Drude plasma frequency due to the appearance
of massless Dirac fermions. At higher pressures, optical signatures of two
structural phase transitions are also identified. Our findings suggest that a
key fingerprint of the Lifshitz transition in solid state systems, and in
absence of structural phase transitions, is a discontinuity of the Drude plasma
frequency due to the change of Fermi surface topology.
