In optics, we can sometimes find individual devices that have desirable characteristics, like sharp transmission peaks or a negative index of refraction, and it would be useful to scale them up somehow into a bulk material. Some groups have sought to construct such materials – termed “meta-materials” - by making huge arrays of these tiny devices, which are typically patterned metals, such as gold. But unless the devices are made with excellent precision and arranged in perfect arrays, the attractive characteristics of the individual devices will be weak or nonexistent in the bulk material.
The tiny devices in question must always be smaller than the wavelength of light for which they are intended, or else the light would diffract significantly from the bulk material, resulting in poor transmission, shadows, or laser “speckle.” For visible light, that implies feature sizes on the scale of tens of nanometers. Such exacting tolerances require the use of expensive and time-consuming etching methods, like focused-ion-beam or electron-beam lithography. Right now, working optical metamaterials can only be made as large as a couple hundred microns before becoming prohibitively expensive. There is therefore an effort to find cheap, parallelizable methods for the construction of optical metamaterials, which is the focus of this Spotlight on Optics paper.
Liu et al. have found a way to use fast, inexpensive photolithography to turn a solution of gold nanoparticles into an array of nanoscopic rings with interesting optical properties. Lithography is useful for making features as small as about 100nm, and the authors use physical properties of the nanoparticle solution to make the smaller features. After etching an array of 100nm-diameter pits in a surface, they cover the surface with the nanoparticles, which tend to collect inside the pits, near the edges, because of surface tension. Then they heat the sample past the melting point of gold, so that the particles join, forming a ring. By adjusting the diameter and depth of the pits, or the concentration of particles, they can control the inner and outer diameter of the rings.
The rings respond to incident light by reflecting or transmitting, depending on the wavelength. Electrons in the gold follow the electromagnetic field in time, resulting in resonances that depend on the size of the rings. Liu et al. measure the optical transmission to show that the bulk material behaves the same way as the original rings. By tuning the manufacturing process, they can customize the transmission of the metamaterial’s surface, to a certain extent. Since photolithography is already widespread, and solutions of gold nanoparticles are quite easy to acquire, the authors believe that this technique should be useful in practical applications like sensing, optical switching, or microscopy.
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