  By preparing a sensor system around isolated exceptional points, one can
obtain a great enhancement of the sensitivity benefiting from the
non-Hermiticity. However, this comes at the cost of reduction of the
flexibility of the system, which is critical for practical applications. By
generalizing the exceptional points to exceptional surfaces, it has been
theoretically proposed recently that enhanced sensitivity and flexibility can
be combined. Here, we experimentally demonstrate an exceptional surface in a
non-Hermitian photonic sensing system, which is composed of a
whispering-gallery-mode microresonator and two nanofiber waveguides, resulting
in a unidirectional coupling between two degenerate counter-propagating modes
with an external optical isolator. The system is simple, robust, and can be
easily operated around an exceptional surface. On the one hand, we observe
sensitivity enhancement by monitoring the resonant frequency splitting caused
by small perturbations. This demonstration of exceptional-surface-enhanced
sensitivity paves the way for practical non-Hermitian sensing applications. On
the other hand, we also show the suppression of frequency splitting around the
exceptional surface for the first time.
