Exceptional Points Enhance Sensing at Nanoscale

A discovery regarding the sensing capability of microresonators could impact the creation of biomedical devices, electronics and biohazard detection devices. The novel sensing scheme for enhancing the sensitivity of optical microcavities was demonstrated using two light scatterers to tune sensors to exceptional points.

Two light scatterers, represented by the blue spheres, are utilized to tune the sensors to exceptional points, at which light propagates in one direction. Courtesy of W. Chen and L. Yang.

The two nanoscale scatterers were used to tune a whispering-gallery-mode (WGM) micro-toroid cavity. In the cavity, light propagated along a concave surface by continuous total internal reflection, in a precise and controlled manner, to exceptional points. The nanoscale scatterers tuned various parameters in the system to influence function. Using nanopositioning, researchers could move the scatterers and introduce another medium — a virus particle, for example — into the field to perturb the field and engage an exceptional point.

A target nanoscale object that subsequently entered the evanescent field of the cavity perturbed the system from its exceptional point, leading to frequency splitting (i.e., the sensing signal).

“The so-called ‘exceptional point’ endows a whispering-gallery sensor with exceptional performance for detecting nanoscale objects, surpassing that of conventional whispering-gallery sensors,” said researcher Weijian Chen. “Strikingly, the smaller the target object is, the better the performance of our new sensor will be.”

Owing to the complex-square-root topology near an exceptional point, this frequency splitting scaled as the square root of the perturbation strength and was therefore larger (for sufficiently small perturbations) than the splitting observed in traditional nonexceptional-point sensing schemes.

“It's challenging to detect nanoscale objects, such as nanoparticles,” professor Lan Yang said. “If the object is very small, it introduces little perturbation to a sensing system. We utilize an unusual topological feature associated with exceptional points of a physical system to enhance the response of an optical sensor to very small perturbations, such as those introduced by nanoscale objects. The beauty of the exceptional point sensor is the smaller the perturbation, the larger the enhancement compared to a conventional sensor.”

The research team, from Washington University in St. Louis, is exploring the use of the exceptional point in photoacoustic imaging studies and other scenarios where they seek development of “unconventional light transport modes,” according to Yang.

Yang added,“There should be many applications arising from that.”

Exceptional-point-enhanced sensitivity could pave the way for sensors with unprecedented sensitivity.

The research was published in Nature (doi:10.1038/nature23281).