Unidirectional light propagation at exceptional points.

The magic garment worn by the fictional wizard Harry Potter makes him invisible and simultaneously allows him to see the outside world. The concept of this type of invisibility cloak has captured the imagination for centuries and has inspired many recent explorations in the field of optics. Making such a fictional garment is indeed a grand challenge: when invisible to the outside observer, the concealed individual cannot see through the present-day cloaks due to the reciprocity of light. The present cloaking schemes hide an object by redirecting the flow of light smoothly around it without reflection and scattering, much like water flowing around a rock in a stream, rendering the object invisible to the downstream observer1–3. To realize the magic Harry Potter cloak, however, one has to break the reciprocity of light — a seemingly impossible task. Recently, manipulating parity-time symmetry in a synthetic photonic medium with a judicious arrangement of the dielectric constants of the constituent materials has led to intriguing optical phenomena, including the one-way reflectionless propagation of light and the ‘unidirectional invisibility’4–6. But can it cast new light on accomplishing the fictional garment worn by Harry? By designing the spatial distribution of materials, extraordinary electromagnetic properties may be obtained. An example is a photonic crystal, which is typically created by periodically arranged non-absorbing dielectrics at the wavelength scale. Photonic crystals not only allow for manipulating the propagation of light owing to the existence of ‘forbidden-gaps’, but also enable controlled light–matter interactions7,8. Introducing a dispersive and nonperturbative imaginary part to the dielectric constant profile, however, destroys the mode orthogonality across different bands, leaving the system open and sensitive to the complex dynamics of gain and/or loss9,10. Parity-time synthetic matter, on the other hand, modulates the materials’ refractive index, gain and loss in a strong but delicate and balanced manner (Fig. 1). This leads to interesting optical phenomena and devices, such as asymmetric power oscillations between two waveguides and unidirectional reflectionless light propagation4–6,11–14.