Robust exceptional point chains and chirality switch in a vast optical parameter space

While the special properties of exceptional points (EPs) may improve application relevant features such as hypersensing, often their realization requires a precise fine-tuning of parameters. Here we report the robust formation of EP chains in coupled optical microcavities over intercavity distances <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>D</a:mi></a:math> on the order of the cavity size. We find that coupling, or more generally, the availability of an outer (coupling) channel, is sufficient to induce EPs in the parameter space spanned by the refractive index <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>n</b:mi></b:math> along with <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>D</c:mi></c:math>. This occurs even at <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>D</d:mi></d:math> of several wavelengths, yielding a network of EPs in parameter space with a periodicity of the order of the wavelength in the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mi>D</e:mi></e:math> direction. Upon the increase of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mi>n</f:mi></f:math>, we observe a sharp transition in the sign of the chirality measured in one of the cavities that further structures the system properties. While the details of the Riemannian surfaces associated with the formation of exceptional points are mode dependent and allow for a variety of single-cavity chiralities, we find the underlying mechanism to be universal, robust, applicable to other and more general coupled systems, and promising for potential applications such as optical measurements of pressure, voltages, or distances.