Effective Wavelength Scaling for Optical Antennas

In antenna theory, antenna parameters are directly related to the wavelength $\ensuremath{\lambda}$ of incident radiation, but this scaling fails at optical frequencies where metals behave as strongly coupled plasmas. In this Letter we show that antenna designs can be transferred to the optical frequency regime by replacing $\ensuremath{\lambda}$ by a linearly scaled effective wavelength ${\ensuremath{\lambda}}_{\mathrm{eff}}={n}_{1}+{n}_{2}\ensuremath{\lambda}/{\ensuremath{\lambda}}_{p}$, with ${\ensuremath{\lambda}}_{p}$ being the plasma wavelength and ${n}_{1}$, ${n}_{2}$ being coefficients that depend on geometry and material properties. It is assumed that the antenna is made of linear segments with radii $R\ensuremath{\ll}\ensuremath{\lambda}$. Optical antennas hold great promise for increasing the efficiency of photovoltaics, light-emitting devices, and optical sensors.