Spawning rings of exceptional points out of Dirac cones

The Dirac cone underlies many unique electronic properties of graphene and topological insulators, and its band structure--two conical bands touching at a single point--has also been realized for photons in waveguide arrays, atoms in optical lattices, and through accidental degeneracy. Deformations of the Dirac cone often reveal intriguing properties; an example is the quantum Hall effect, where a constant magnetic field breaks the Dirac cone into isolated Landau levels. A seemingly unrelated phenomenon is the exceptional point, also known as the parity-time symmetry breaking point, where two resonances coincide in both their positions and widths. Exceptional points lead to counter-intuitive phenomena such as loss-induced transparency, unidirectional transmission or reflection, and lasers with reversed pump dependence or single-mode operation. These two fields of research are in fact connected: here we discover the ability of a Dirac cone to evolve into a ring of exceptional points, which we call an "exceptional ring." We experimentally demonstrate this concept in a photonic crystal slab. Angle-resolved reflection measurements of the photonic crystal slab reveal that the peaks of reflectivity follow the conical band structure of a Dirac cone from accidental degeneracy, whereas the complex eigenvalues of the system are deformed into a two-dimensional flat band enclosed by an exceptional ring. This deformation arises from the dissimilar radiation rates of dipole and quadrupole resonances, which play a role analogous to the loss and gain in parity-time symmetric systems. Our results indicate that the radiation that exists in any open system can fundamentally alter its physical properties in ways previously expected only in the presence of material loss and gain

Observation of the exceptional-point-enhanced Sagnac effect

Exceptional points (EPs) are special spectral degeneracies of non-Hermitian Hamiltonians that govern the dynamics of open systems. At an EP, two or more eigenvalues, and the corresponding eigenstates, coalesce. Recently, it was predicted that operation of an optical gyroscope near an EP results in improved response to rotations. However, the performance of such a system has not been examined experimentally. Here we introduce a precisely controllable physical system for the study of non-Hermitian physics and nonlinear optics in high-quality-factor microresonators. Because this system dissipatively couples counter-propagating lightwaves within the resonator, it also functions as a sensitive gyroscope for the measurement of rotations. We use our system to investigate the predicted EP-enhanced Sagnac effect and observe a four-fold increase in the Sagnac scale factor by directly measuring rotations applied to the resonator. The level of enhancement can be controlled by adjusting the system bias relative to the EP, and modelling results confirm the observed enhancement. Moreover, we characterize the sensitivity of the gyroscope near the EP. Besides verifying EP physics, this work is important for the understanding of optical gyroscopes

Outlook for inverse design in nanophotonics

Recent advancements in computational inverse design have begun to reshape the landscape of structures and techniques available to nanophotonics. Here, we outline a cross section of key developments at the intersection of these two fields: moving from a recap of foundational results to motivation of emerging applications in nonlinear, topological, near-field and on-chip optics.Comment: 13 pages, 6 figure

Single-Mode Parity-Time-Symmetric Micro-Ring Lasers

Parity-time (PT) symmetry has been recently emerged as a new paradigm for mode management in micro-cavity lasers. Single-mode lasing is demonstrated in longitudinally and transversely multi-moded PT-symmetric micro-ring arrangements

Second-Order Coherence Measurement Of A Metallic Coaxial Nanolaser

The second-order coherence function is measured for a metallic coaxial nanolaser using a modified Hanbury Brown-Twiss technique. The results indicate that such nanoscale lasers can indeed generate coherent radiation

Pt-Symmetry Breaking Of Topological Defect-States In Ssh Micro-Ring Laser Arrays

The PT-symmetry breaking for topological defectstates are studied in SSH micro-resonator laser arrays. For defect modes, the PT-symmetry breaking threshold reduces when the coupling strength between closely paired elements is increased. Such topological defect-modes are demonstrated in a 16-ring SSH PT-laser arrangement

Towards Electrically Injected Parity-Time-Symmetric Microring Lasers

We present an electrically pumped parity-time-symmetric coupled microring laser. Using the interplay between gain and loss, single mode operation is demonstrated with no penalty in terms of output power or threshold pump intensity

Ultrasensitive Micro-Scale Parity-Time-Symmetric Ring Laser Gyroscope

We propose a new scheme for ultrasensitive laser gyroscopes that utilizes the physics of exceptional points. By exploiting the properties of such non-Hermitian degeneracies, we show that the rotation-induced frequency splitting becomes proportional to the square root of the gyration speed (√Q), thus enhancing the sensitivity to low angular rotations by orders of magnitudes. In addition, at its maximum sensitivity limit, the measurable spectral splitting is independent of the radius of the rings involved. This Letter paves the way toward a new class of ultrasensitive miniature ring laser gyroscopes on chip

Complex Edge-State Phase Transitions In 1D Topological Laser Arrays

We report the first observation of complex lasing transitions in a topological 1D Su-Schrieffer-Heeger active array. The effect of gain saturation nonlinearities and carrier dynamics on the edge-state is investigated both theoretically and experimentally