All real-world systems are open. Non-Hermitian physics is a quantum mechanical framework to study open systems. Since the experimental realization of non-Hermitian potentials is much easier in optics than in other systems, non-Hermitian optics and photonics have garnered much attention, especially in the recent decade. Several works proved the impact of non-Hermitian optics and photonics by demonstrating novel physics, unprecedented functionalities, and novel design approaches [1]. More recently, non-Hermtitian principles are being examined for designing meta-devices [2]. This rapidly growing field holds many opportunities for scientific discoveries and technological inventions. This special issue, consisting of eight research articles and one review, highlights a few current research directions in non-Hermitian optics and photonics and presents a perspective of future research topics in the field. This special issue was inspired by a workshop on non-Hermitian optics organized jointly by the US Army Research Office and Rice University in August of 2021.

While conventional optical and photonic devices focus on the optimal distribution of the real refractive index, non-Hermitian optical and photonic devices engineer the distribution of both the real and imaginary parts of the refractive index. The imaginary index distribution is an additional powerful design dimension available exclusively to non-Hermitian devices. As a consequence, non-Hemitian devices can achieve functionalities that are not easy to access with conventional optics and photonics designs.

One of the interesting aspects of non-Hermitian devices is their topological properties. Non-Hermiticity allows for the emergence of a new class of symmetries, expanding the existent symmetry classifications substantially – an aspect enriching topological phases beyond their Hermitian counterparts. Nasari et al. review recent developments in non-Hermitian topological photonics and present their perspective on future directions and potential challenges [3].

Highlighting exceptional points (EPs) in non-Hermitian devices, Herrero-Parareda et al. demonstrate lasing at a third-order EP formed in a frozen mode associated with a stationary inflection point (SIP) [4]. The SIP lasing mode has a lower lasing threshold than that of a photonic regular band-edge mode. Further, the SIP mode is robust to small perturbations in gain making it promising for the next-generation EP-based lasers. Another promising approach for EP-based lasers is presented by Ghaemi-Dizicheh and Ramezani [5]. They propose a temporal modulation scheme to circumvent the challenge of precisely maintaining the balance between gain and loss in parity-time (PT) symmetric devices. Their temporal modulation scheme is equivalent to a balanced gain and loss situation under unitary transformation.

Simonson et al. develop a mathematical framework for modeling input/output scattering in non-Hermitian systems [6]. Their resolvent expansion explains non-Lorentian lineshapes in non-Hermitian systems. Further, they provide a unifying mathematical framework for studying resonant non-Hermitian systems. Ye et al. study mode localization and phase transition behavior in a parity-time symmetric two-leg ladder lattice structure [7]. Considering a different geometry – an annular structure – Bochin and Nalitov used a mean-field model to show nonequilibrium polariton condensation in annular effective non-Hermitian potential traps [8]. They solved the linearized extended Gross-Pitaevskii equation to reveal a complex map of condensate quantum number transitions and topological charge increments in the multi-dimensional parameter space of the traps. A similar circular geometry is considered by Izadparast et al. for enhancing the sensitivity of lattice ring gyroscopes [9]. Compared to an individual EP-aided ring gyroscope composed of two coupled rings with balanced gain and loss, a suitably-designed lattice of rings can achieve substantially higher responsivity by increasing the effective coupling between the gain and loss subsystems. Such EP-aided lattice ring gyroscopes are promising for GPS-free navigation systems.

Extending non-Hermitian optics to meta-devices, Butler and Argyropoulos show that a plasmonic Huygens’ metasurface composed of active metal-dielectric core-shell nanoparticles exhibits reflectionless transmission at an EP [10]. Such metasurfaces are extremely important for ultracompact perfect transmission optical filters with subwavelength thickness. In another work, Ha et al. study a low-index metachannel composed of parity-time-reciprocal scaling (PTX)-symmetric metasurfaces operating at the coherent perfect absorber-laser point [11]. This metachannel supports nearly lossless, ultra-directive leaky-wave radiation, with reconfigurable and tunable radiation angle and beamwidth based on the reciprocally scaled gain-loss parameter. Such meta-devices are promising for applications in high-directionality antennas, emitters, and simulating extreme material properties such as epsilon-near-zero.

Non-Hermitian optics and photonics is a vibrant research area with research opportunities spanning across disciplines. This special issue is in no way an exhaustive representation of the research topics in this field. Nevertheless, this special issue captures some important topics to highlight the need for cross-disciplinary teams of scientists and engineers to work together to revolutionize optics and photonics. We are grateful to all the authors, reviewers, and Optica staff members for their contributions and efforts to make this issue possible.