Ghost sensing: the rise and role of exceptional points in planar geometry

In optical biosensing, detection of small analytes at ultra-low concentration with a simple sensor device is an important challenging problem. A recent line of research promises enhanced sensitivity through operation based on exceptional points, which are spectral singularities present in open systems. When a sensor operates at an exceptional point it shows strong nonlinear response to external perturbation in contrast to usual linear behavior for traditional sensors. Although increased sensitivity has been demonstrated in several systems, they usually require non-trivial microscale geometry, high-Q resonators and precise control of optical loss and gain -- with a resulting complexity that so far prevented the use of these novel devices for practical biosensing. Here we show this complexity problem can be solved using the recent discovery of ghost waves, which are a special class of non-uniform electromagnetic waves in biaxial anisotropic media. Since these waves can be excited at the planar surface of bulk crystals, the need for complicated fabrication or doping is lifted. In addition to showing high sensitivity and precision, the proposed sensor is also shown to be robust against noise. The resulting ``ghost sensor" which derives its sensing enhancement through exceptional point-based operation and its device simplicity through manipulation of ghost waves in bulk optics, can enable applications ranging from clinical diagnosis and drug discovery to food process control and environmental monitoring to be more accurate and more affordable.