  Non-Hermitian degeneracies reveal intriguing and non-trivial behaviors in
open physical systems. Examples like Parity-Time (PT) symmetry breaking,
topological encircling chirality, and enhanced sensing near an exceptional
point (EP) are often associated with the abrupt nature of the phase transition
around these degeneracies. Here we experimentally observe a cavity-enhanced
second-harmonic frequency (SHG) conversion on a PT symmetry line, i.e. a set
consisting of open-ended isofrequency or isoloss lines, both terminated at EPs
on the Riemann surface in parameter space. The enhancement factor can reach as
high as 300, depending on the crossing point whether in the symmetry or the
broken phase of the PT line. Moreover, such enhancement of SHG enables
sensitive distance sensing with a nanometer resolution. Our works may pave the
way for practical applications in sensing, frequency conversion, and coherent
wave control.
