A laser beam hits the chip vertically, like an arrow striking a target. Ordinary gratings in this position lose half the light: it scatters in both directions, like a crowd rushing through open doors. The new design forces light to go only to the left, into the desired waveguide.

The secret is that the grating stripes are asymmetric — one wall of each stripe is slightly longer than the other. It's like blinds tilted at an angle: from outside they let your gaze through, but from inside they barely do. So does light: it enters freely from above but cannot escape backward or sideways.

Fabricating such a grating is simple: a silicon wafer is etched once, without additional layers. In simulation, the coupling efficiency from a standard optical fiber reached 41% at vertical incidence, and the difference between the desired and undesired directions exceeded 10 dB. The experiment confirmed: unidirectionality 12.8 dB, losses 8.35 dB at a wavelength of 1528 nm.

Such a grating is a ready-made building block for hybrid circuits where a laser sits directly above the chip. This will speed up the assembly of optical transmitters, lidars, and sensors. Light enters strictly from above, and stays in the chip.