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Non-Hermitian optics is a burgeoning field at the intersection of quantum physics, electrodynamics, and nanophotonics. It provides a new perspective of the role of gain and loss in optical systems. Leveraging the advanced designs inspired by non-Hermitian physics, classical optical platforms have been widely investigated to unveil novel physical concepts, such as parity-time symmetry and exceptional points, which have no counterparts in the conventional Hermitian settings. These investigations have yielded a plethora of new phenomena in optical wave scattering, optical sensing, and nonlinear optical processes. Non-Hermitian effects also have a profound impact on the lasing behaviors in the semiclassical framework of lasers, allowing for novel ways to engineer single-mode lasers, chiral laser emission, laser noise, linewidth, etc. Furthermore, over recent years, there has been increasing interest in the explorations of non-Hermitian physics in quantum optics, which addresses photon statistics, entanglement, decoherence, and quantum sensing in non-Hermitian systems. In this review, we review the most recent theoretical and experimental advances in non-Hermitian optics and photonics, covering the significant progress in both classical and quantum optics regimes.

 

On‐chip optical nonreciprocity is one of the essential functions to fully advance the development of integrated optical systems, which remains technically challenging in many aspects. There is a great need for mechanisms and approaches to facilitate the large‐scale implementation of nonreciprocal light propagation. Recently, unconventional phenomena, such as chiral optical modes and directional light propagation, have been unraveled at exceptional points (EPs), which are unique degeneracies in the energy spectrum and eigenspace of non‐Hermitian systems. Here, this work theoretically and experimentally demonstrates that by steering a single microresonator with thermo‐optic nonlinearity to chiral EPs, nonreciprocal light propagation is achieved with an isolation ratio up to 24 dB and insertion loss less than 0.5 dB. The nonreciprocity is dependent on the chirality and could be optimized near the EPs. Their results pave new avenues for the nonreciprocal control of light propagation enabled by non‐Hermitian degeneracies and hold great potential for microscale and nanoscale on‐chip nonreciprocal devices.

 

Harnessing parity–time symmetry with balanced gain and loss profiles has created a variety of opportunities in electronics from wireless energy transfer to telemetry sensing and topological defect engineering. However, existing implementations often employ ad hoc approaches at low operating frequencies and are unable to accommodate large-scale integration. Here we report a fully integrated realization of parity–time symmetry in a standard complementary metal–oxide–semiconductor process technology. Our work demonstrates salient parity–time symmetry features such as phase transition as well as the ability to manipulate broadband microwave generation and propagation beyond the limitations encountered by existing schemes. The system shows 2.1 times the bandwidth and 30% noise reduction compared to conventional microwave generation in the oscillatory mode, and displays large non-reciprocal microwave transport from 2.75 to 3.10 GHz in the non-oscillatory mode due to enhanced nonlinearities. This approach could enrich integrated circuit design methodology beyond well-established performance limits and enable the use of scalable integrated circuit technology to study topological effects in high-dimensional non-Hermitian systems.

 

Recently, exceptional points, a degeneracy of open wave systems, have been observed in photonics, acoustics, and electronics. They have mainly been realized as a degeneracy of resonances; however, a degeneracy associated with the absorption of waves can exhibit distinct and interesting physical features. Here, we demonstrate such an absorbing exceptional point by engineering degeneracies in the absorption spectrum of dissipative optical microcavities. We experimentally distinguished the conditions to realize an absorbing exceptional point versus a resonant exceptional point. Furthermore, when the optical loss was tuned to achieve perfect absorption at an absorbing exceptional point, we observed its signature, an anomalously broadened line shape in the absorption spectrum. The distinct scattering properties of the absorbing exceptional point create opportunities for both fundamental study and applications of non-Hermitian degeneracies. 

Polarization of optical fields is a crucial degree of freedom in the all-optical analogue of electromagnetically induced transparency (EIT). However, the physical origins of EIT and polarization-induced phenomena have not been well distinguished, which can lead to confusion in associated applications such as slow light and optical/quantum storage. Here we study the polarization effects in various optical EIT systems. We find that a polarization mismatch between whispering gallery modes in two indirectly coupled resonators can induce a narrow transparency window in the transmission spectrum resembling the EIT lineshape. However, such polarization-induced transparency (PIT) is distinct from EIT: It originates from strong polarization rotation effects and shows a unidirectional feature. The coexistence of PIT and EIT provides additional routes for the manipulation of light flow in optical resonator systems.