Observing exceptional point degeneracy of radiation with electrically pumped photonic crystal coupled-nanocavity lasers

TL;DR

This paper demonstrates the first on-chip electrically pumped photonic crystal lasers exhibiting exceptional point degeneracy, revealing enhanced radiation and unique spectral features, thus opening new avenues for non-Hermitian photonics and reconfigurable laser arrays.

Contribution

It reports the experimental observation of EP degeneracy in electrically pumped nanocavity lasers with independent gain control, and confirms the associated spectral and emission enhancements both experimentally and theoretically.

Findings

Observation of EP phase transition in laser emission

Spectral coalescence of coupled modes at EP

Four-fold enhancement of local density of states near EP

Abstract

Controlling gain and loss of coupled optical cavities can induce non-Hermitian degeneracies of eigenstates, called exceptional points (EPs). Various unconventional phenomena around EPs have been reported, and expected to incorporate extra functionalities into photonic devices. The eigenmode exactly under the EP degeneracy is also predicted to exhibit enhanced radiation. However, such responses have yet to be observed in on-chip lasers, because of both the limited controllability of their gain and loss and the lifting of degeneracy by pump-induced cavity detuning. Here, we report the first non-Hermitian nanophotonic platform based on two electrically pumped photonic crystal lasers and its spontaneous emission at an EP degeneracy. Systematically tuned and independent current injection to our wavelength-scale active heterostructure cavities enables us to demonstrate the clear EP phase transition of their spontaneous emission, accompanied with the spectral coalescence of coupled modes and reversed pump dependence of the intensity. Furthermore, we find experimentally and confirm theoretically the peculiar squared Lorentzian emission spectrum very near the exact EP, which indicates the four-fold enhancement of the photonic local density of states induced purely by the degeneracy. Our results open a new pathway to engineer the light-matter interaction by non-Hermiticity and explore larger reconfigurable laser arrays for further non-Hermitian features and physics.