Filling the terahertz gap

Scientists have now used metamaterials to control terahertz radiation efficiently in real time

© (2006) Nature

Technologically speaking, terahertz (THz) radiation has much to offer. But many materials do not respond to THz waves, which makes it difficult to design devices that operate within this frequency regime. So far, work has focused on generating and detecting THz waves instead of actively controlling and manipulating them. Now studies from Hou-Tong Chen and colleagues¹ show that metamaterials can provide new levels of control over THz light, offering hope of filling the 'terahertz gap'.

Metamaterials are artificial structures that control light in ways not possible with natural materials. Most research has been geared towards making them work at microwave wavelengths, but by developing a metamaterial device that efficiently modulates THz signals at room temperature, Chen et al. have opened the door to a range of further applications.

Their device consists of an array of tiny gold resonator elements (the metamaterial) sitting on top of a semiconductor substrate. The elements are 'split-ring resonators', pairs of concentric metallic rings with gaps that prevent current from circulating through them. Because the rings act both as capacitors (by storing electric charge) and inductors (by inducing magnetic fields that then maintain the current flowing through), the presence of the gaps creates a resonant response.

Chen and co-workers engineer their metamaterial structures so that this resonance occurs when THz-frequency light is shone on the device. By applying an electrical voltage to the semiconductor, they can control the resonance at will — and therefore the amount of radiation allowed to pass through. The authors demonstrate an on-off transmission ratio of 0.5, which is 10 times better than that offered by conventional devices in this frequency range. But more importantly, they reveal a new way of actively controlling THz light.