  The concept of mass-generation via the Higgs mechanism was strongly inspired
by earlier works on the Meissner-Ochsenfeld effect in superconductors. In
quantum field theory, the excitations of longitudinal components of the Higgs
field manifest as massive Higgs bosons. The analogous Higgs mode in
superconductors has not yet been observed due to its rapid decay into
particle-hole pairs. Following recent theories, however, the Higgs mode should
decrease below the pairing gap $2\Delta$ and become visible in two-dimensional
systems close to the superconductor-insulator transition (SIT). For
experimental verification, we measured the complex terahertz transmission and
tunneling density of states (DOS) of various thin films of superconducting NbN
and InO close to criticality. Comparing both techniques reveals a growing
discrepancy between the finite $2\Delta$ and the threshold energy for
electromagnetic absorption which vanishes critically towards the SIT. We
identify the excess absorption below $2\Delta$ as a strong evidence of the
Higgs mode in two dimensional quantum critical superconductors.
