Imagine trying to spot a firefly hovering next to a lighthouse from several kilometres away. That's essentially the challenge astronomers face when searching for Earth like planets around other stars. The planet is there, it’s just completely lost in the overwhelming blaze of its host star. Solving that problem is the whole point of a tiny but extraordinarily precise piece of glass called an optical vortex phase mask.

NASA's planned Habitable Worlds Observatory, a future space telescope designed specifically to hunt for signs of life beyond our Solar System, will need to image faint exoplanets directly. To do that, it must suppress incoming starlight by a factor of ten billion. Even a perfect mirror can't achieve that on its own. When light passes through a telescope's circular aperture, it spreads outward into a ringed pattern of light called an Airy pattern, a fundamental consequence of the physics of waves. Those rings can be millions of times brighter than a nearby exoplanet, and they have to go.

That's where the vortex phase mask comes in. Placed at the focal point of the telescope, it applies a carefully engineered delay to the starlight, one that increases continuously as you move around the centre of the mask, like the rising thread of a screw. The result is that the starlight cancels itself out through destructive interference, and what's left can be blocked by a simple aperture stop, leaving only the faint planet light to reach the detector. Light from the exoplanet, arriving at a slight angle, misses the mask's centre and passes through unaffected.

The most promising version of this technology uses a thin layer of liquid crystal polymer, whose long molecular chains can be precisely oriented to manipulate light differently depending on its polarisation direction. Because the delay is produced geometrically rather than by the material's chemical properties, it works across a wide range of wavelengths and that’s crucial for a telescope that needs to analyse the full colour spectrum of a planet's atmosphere.