We applied nonlinear optimization to design nanophotonics-based metamaterials for efficient free-space-to-waveguide coupling. Three devices were designed, fabricated and characterized. The first device couples incident light into a multi-mode waveguide, the second device couples incident light into a single-mode waveguide directly, and the third device couples and separates two orthogonal polarizations into two multi-mode waveguides. All devices offer comparable or higher coupling efficiencies, are easier to fabricate, and demonstrate higher bandwidth when compared to conventional devices. Furthermore, each device is at least an order of magnitude smaller in area than previously reported devices. The highly efficient single-mode waveguide-coupler is a unique device that has not been experimentally demonstrated before. We further performed careful simulations to underscore the tolerance of these devices to fabrication errors. Their robustness is primarily a result of the large number of coupled guided-mode resonances that are responsible for each device performance.