Perfect Single-Sided Radiation and Absorption without Mirrors
  Highly directional radiation from photonic structures is important for many applications, including high power photonic crystal surface emitting lasers, grating couplers, and light detection and ranging devices. However, previous dielectric, few-layer designs only achieved moderate asymmetry ratios, and a fundamental understanding of bounds on asymmetric radiation from arbitrary structures is still lacking. Here, we show that breaking the 180$^\circ$ rotational symmetry of the structure is crucial for achieving highly asymmetric radiation. We develop a general temporal coupled-mode theory formalism to derive bounds on the asymmetric decay rates to the top and bottom of a photonic crystal slab for a resonance with arbitrary in-plane wavevector. Guided by this formalism, we show that infinite asymmetry is still achievable even without the need of back-reflection mirrors, and we provide numerical examples of designs that achieve asymmetry ratios exceeding $10^4$. The emission direction can also be rapidly switched from top to bottom by tuning the wavevector or frequency. Furthermore, we show that with the addition of weak material absorption loss, such structures can be used to achieve perfect absorption with single-sided illumination, even for single-pass absorption rates less than $0.5\%$ and without back-reflection mirrors. Our work provides new design principles for achieving highly directional radiation and perfect absorption in photonics. 
Hengyun Zhou, Bo Zhen, Chia Wei Hsu, Owen D. Miller, Steven G.   Johnson, John D. Joannopoulos, Marin Soljacic
physics.optics