Subwavelength devices, in which light is controlled on scales much smaller than its wavelength, are of interest for applications such as imaging and sensing. Researchers from Seoul National University, Korea University and Sun Moon University in Korea, and the Delft University of Technology in the Netherlands have now demonstrated that slits only a few tens of nanometers wide can significantly enhance radiation at wavelengths 30,000 times longer, in the terahertz (THz) range1.

'Metamaterials' are a well-known example of subwavelength devices, in which feature sizes smaller than the wavelength may lead to intriguing applications such as the cloaking of objects. However, even in these metamaterials, feature sizes are generally larger than the metal skin depth—the depth to which electrical currents induced by electromagnetic light waves penetrate into the metal. Typically, skin depths are of the order of 10–100 nm.

Fig. 1: Enhancement of THz waves at a metal nano slit. a, Energy distribution. b, Electrical current, surface charge and electric field distribution around the nano slit.

The researchers studied how narrow gaps of only a few tens of nanometers in width influence the transmission of THz radiation, and found that the light intensity was significantly increased (Fig. 1a). Although enhanced light transmission through arrays of holes in metal plates is known to be mediated by surface plasmon resonance (a type of surface electromagnetic wave), the enhancement mechanism in the present case is fundamentally different; the THz wavelengths are many times longer than the typical wavelengths of surface plasmons.

Instead, the THz field enhancement is based on the electrical currents generated by electromagnetic waves impinging on the metal surface. Whereas in homogeneous media these currents are not directly related to surface charges, the situation changes profoundly for small gaps. “Surface charges are accumulated by the light-induced currents on both sides of the nanogap. The small separation between the positive and negative charges across the slits leads to large enhanced electric fields outside the metal,” says Dai Sik Kim, who led the research team (Fig. 1b).

Consequently, these finding establish subwavelength nano-optics not only for wavelengths close to the surface plasmon frequency, but also at much longer length scales. “Our study shows that nanophotonic terahertz devices will not only have scientific but also technological impact, for example in the imaging of small structures,” says Kim.