The refractive index of a liquid crystal depends on the polarization of the light travelling through it. This means that horizontally polarized light is transmitted when vertically polarized light is absorbed, and vice versa. Since the orientation of liquid crystals can be controlled by an applied voltage, the light travelling through a crystal can be switched on and off. This is the basis of liquid crystal displays.

Scientists have long known that this effect also works at microwave frequencies, but Yang and Sambles are among the first to exploit it. The device is based on a grating made from 55 one-mm-thick aluminium sheets interleaved with layers of liquid crystal 75 micrometres thick. This number of slits per millimetre is chosen to make the structure act as a ‘zero-order’ grating – which does not diffract the microwaves – for wavelengths greater than two millimetres. Yang and Sambles used microwaves with wavelengths between seven and 12 millimetres.

When the microwaves reach the grating, they create electromagnetic surface waves in the aluminium. These produce standing waves in the liquid-crystal-filled cavities of the grating, and microwaves with the same frequencies as these standing waves are transmitted. “Were it not for the resonant modes in the small gaps there would be no transmission”, Sambles told PhysicsWeb.

By varying the voltage applied across the liquid crystals between the aluminium slats, the researchers could adjust their orientation. This alters the frequency of the standing waves that arise in the slits, and the frequency of the microwaves that are transmitted.

Scientists realised only recently that these standing waves allow microwaves to pass through such narrow slits. “It had not been appreciated before that one could easily transmit so much energy through such a tiny area”, says Sambles. Yan and Sambles are optimistic that their device will prove useful in the communications industry, which is heavily dependent on high-frequency microwaves.