  In most superconductors, optical excitations require impurity scattering or
the presence of multiple bands. This is because in clean single-band
superconductors, the combination of particle-hole and inversion symmetries
prevents momentum-conserving transitions. In this work we show how the flow of
supercurrent can lead to new contributions to optical conductivity. As
supercurrent breaks inversion symmetry, transitions across the superconducting
gap become allowed even in clean superconductors and dominate over
impurity-induced contributions for energies comparable to the gap width. The
response is dependent on the nature of the underlying normal state as well as
on the type of superconducting order. Through use of the screening supercurrent
with controllable magnitude and direction, that arises from an external
magnetic field, this enables a detailed investigation of the superconducting
state and possible gap symmetry determination in unconventional superconductors
for which other techniques have not been practicable.

Superconductivity