Two light beams run side by side along silicon tracks narrower than a human hair. One oscillates up and down, the other left and right. The task is to separate them into different outputs without losing a drop of power. Engineers built a device just 20 micrometers long — roughly a quarter of the thickness of a sheet of paper.
The secret lies in periodically varying the width of the waveguides. Light in this "washboard" behaves differently depending on polarization. Waves with one oscillation direction barely cross from one track to the other, while those with the other direction transfer completely. Thus the two beams are separated like passengers at a turnstile: one goes straight through, the other takes a side passage.
Such splitters are needed in optical chips, where information is transmitted by light rather than electricity. The lower the losses and the wider the operating range, the more data can be pushed through a single channel. The device works from 1483 to 1620 nanometers — nearly the entire range used by telecom lasers.
The 20-micrometer silicon track replaces bulky prisms and polarization filters. It's already printed using standard microelectronics equipment, meaning it can be integrated into chips cheaply and at scale.
A human hair is on average about 80 micrometers thick, so the entire splitter structure is four times thinner than a hair.