  Zero modes are symmetry protected ones whose energy eigenvalues have zero
real parts. In Hermitian arrays, they arise as a consequence of the sublattice
symmetry, implying that they are dark modes. In non-Hermitian systems, that
naturally emerge in gain/loss optical cavities, particle-hole symmetry prevails
instead; the resulting zero modes are no longer dark but feature ${\pi}/2$
phase jumps between adjacent cavities. Here we report on the direct observation
of zero modes in a non-Hermitian three coupled photonic crystal nanocavity
array containing quantum wells. Unlike the Hermitian counterparts, the
non-Hermitian zero modes can only be observed for small sublattice detuning,
and they can be identified through far-field imaging and spectral filtering of
the photoluminescence at selected pump locations. We explain the zero mode
coalescence as a parity-time phase transition for small coupling. These zero
modes are robust against coupling disorder, and can be used for laser mode
engineering and photonic computing.
