Light Concentration at the Nanometer Scale

Visible and near-infrared optical excitations are common currency in the biological world, and consequently, they are routinely used to investigate microscopic phenomena taking place in living organisms and their environment. However, the wavelength of light within that energy region is above hundreds of nanometers, thus averting the possibility of direct nanometer-scale resolution. We show in this Perspective that narrow gaps between metals and sharp tips in colloidal gold particles constitute excellent “light confiners” that permit solving this problem, and in particular, they lead to record levels of surface-enhanced Raman scattering. Our results are framed in the context of a historical quest toward achieving optical focusing in the near field, and we offer a tutorial explanation of why evanescent waves such as plasmons are needed for deep-subwavelength focusing. These results provide the required elements of intuition to understand light concentration at the nanometer scale and to design optimized systems for application in ultrasensitive optical analyses and nonlinear photonics.