  The interplay between charge order and superconductivity remains one of the
central themes of research in quantum materials. In the case of cuprates, the
coupling between striped charge fluctuations and local electromagnetic fields
is especially important, as it affects transport properties, coherence and
dimensionality of superconducting correlations. Here, we study the emission of
coherent terahertz radiation in single-layer cuprates of the
La$_{2-x}$Ba$_x$CuO$_4$ family, for which this effect is expected to be
forbidden by symmetry. We find that emission vanishes for compounds in which
the stripes are quasi-static, but is activated when $c$-axis inversion symmetry
is broken by incommensurate or fluctuating charge stripes, such as in
La$_{1.905}$Ba$_{0.095}$CuO$_4$ and in La$_{1.845}$Ba$_{0.155}$CuO$_4$. In this
case, terahertz radiation is emitted by surface Josephson plasmons, which are
generally dark modes, but couple to free space electromagnetic radiation
because of the stripe modulation.

Superconductivity;Strongly Correlated Electrons