A seam that tells light which way to go
A coin lands on a table and drops into the narrow seam between two matching ridged guides. Rails sit above and below on both sides, so the coin could run along four edges, yet its spin sends it cleanly one way. This chip is built around that same sort of seam.
Getting light to move like that on a chip usually needs extra bits to shove it into place, then more bits to check where it ended up. The awkward part here is a light ripple that skims along a surface. It already carries a built-in sideways twist, so the trick was to use that rule instead of forcing it.
Here the seam does the launch. Light with no left or right twist feeds both mirrored sides. Light twisting left goes into the right patterned side, and light twisting right goes into the left. In the coin picture, the guides are the patterned sides, the four rails are the four paths, and the coin's spin picks the route. That's the takeaway.
The handoff depends on which way the twist turns compared with the direction the energy is travelling. Think of the coin again: what matters is how the rim turns against its roll, not which face you happen to notice first. That lets the incoming twist stay recognisable while the light hugs the surface and runs on.
At the far end, rows of tiny silver posts act like comb teeth and flick the surface-running light back out where you can see it. One post pattern gave a very clean twist, but not in the strongest exits, so a smaller partner pattern was added. Then the main exits carried opposite twists at opposite ports.
When the mirrored grooves and those posts were cut into a silver layer on glass, the route map held up. Straight light fed both sides. Left-twisting and right-twisting light came out at opposite ports, and the light leaving through the glass showed the twist of the path it had taken. No bulky steering pieces, just the seam doing the sorting.