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Topology optimization techniques have been applied in integrated optics andnanophotonics for the inverse design of devices with shapes that cannot beconceived by human intuition. At optical frequencies, these techniques haveonly been utilized to optimize nondispersive materials using frequency-domainmethods. However, a time-domain formulation is more efficient to optimizematerials with dispersion. We introduce such a formulation for the Drude model,which is widely used to simulate the dispersive properties of metals,conductive oxides, and conductive polymers. Our topology optimization algorithmis based on the finite-difference time-domain (FDTD) method, and we introduce atime-domain sensitivity analysis that enables the evaluation of the gradientinformation by using one additional FDTD simulation. The existence ofdielectric and metallic structures in the design space produces plasmonic fieldenhancement that causes convergence issues. We employ an artificial dampingapproach during the optimization iterations that, by reducing the plasmoniceffects, solves the convergence problem. We present several design examples of2D and 3D plasmonic nanoantennas with optimized field localization andenhancement in frequency bands of choice. Our method has the potential to speedup the design of wideband optical nanostructures made of dispersive materialsfor applications in nanoplasmonics, integrated optics, ultrafast photonics, andnonlinear optics.

Topology optimization of dispersive plasmonic nanostructures in the time-domain