Unidirectional Perfect Absorber

We show an interplay between Fano resonances and a judicious absorption mechanism leads to a unidirectional perfect absorber, which can be controlled in both direction and frequency. Critical coupling phenomenon created by interference, separates the left- and right-side of the system. At the same time, Fano resonance causes a divergence in the delay time of photons traveling through the loss part of the system, which results in full absorption of the photons from one side. Moreover, we depict that coincidence of the two unidirectional perfect absorber modes from opposite directions results in a perfect absorber mode, which is distinct from the CPA modes. Furthermore, we show that the unidirectional perfect absorber mode is at the same time a spectral singularity and an exceptional point, which makes this point ultrasensitive to any changes in the system. Our results open a direction for designing new type of absorbers, sensors, and switches.