  In this work, we demonstrate a compact toolkit of inverse-designed
topologically optimized silicon-photonic devices that are arranged in a
plug-and-play fashion to realize many different photonic integrated circuits,
both passive and active, each with a small footprint. The silicon-on-insulator
1550-nm toolkit contains a 2x2 3dB splitter-combiner, a 2x2 waveguide crossover
and a 2x2 all-forward add-drop resonator. The resonator can become a 2x2
electro-optical crossbar switch by means of the thermo-optical effect or
phase-change cladding or free-carrier injection. For each of the ten circuits
demonstrated in this work, the toolkit of photonic devices enables the compact
circuit to achieve low insertion loss and low crosstalk. By adopting the
sophisticated inverse-design approach, the design structure, shape, and sizing
of each individual device can be made more flexible to better suit the
architecture of the greater circuit. For a compact architecture, we present a
unified, parallel waveguide circuit framework into which the devices are
designed to fit seamlessly, thus enabling low-complexity circuit design.
