Dirac mass induced by optical gain and loss.

Mass is commonly considered an intrinsic property of matter, but modern physics reveals particle masses to have complex origins 1 , such as the Higgs mechanism in high-energy physics 2,3 . In crystal lattices such as graphene, relativistic Dirac particles can exist as low-energy quasiparticles 4 with masses imparted by lattice symmetry-breaking perturbations 5-8 . These mass-generating mechanisms all assume Hermiticity, or the conservation of energy in detail. Using a photonic synthetic lattice, we show experimentally that Dirac masses can be generated by means of non-Hermitian perturbations based on optical gain and loss. We then explore how the spacetime engineering of the gain and loss-induced Dirac mass affects the quasiparticles. As we show, the quasiparticles undergo Klein tunnelling at spatial boundaries, but a local breaking of a non-Hermitian symmetry can produce a new flux non-conservation effect at the domain walls. At a temporal boundary that abruptly flips the sign of the Dirac mass, we observe a variant of the time-reflection phenomenon: in the non-relativistic limit, the Dirac quasiparticle reverses its velocity, whereas in the relativistic limit, the original velocity is retained.