Depletion-mode polysilicon optical modulators in a bulk complementary metal-oxide semiconductor process

We demonstrate depletion-mode carrier-plasma optical modulators fabricated in a bulk complementary metal-oxide semiconductor (CMOS), DRAM-emulation process. To the best of our knowledge, these are the first depletion-mode modulators demonstrated in polycrystalline silicon and in bulk CMOS. The modulators are based on novel optical microcavities that utilize periodic spatial interference of two guided modes to create field nulls along waveguide sidewalls. At these nulls, electrical contacts can be placed while preserving a high optical $Q$. These cavities enable active devices in a process with no partial silicon etch and with lateral $p–n$ junctions. We demonstrate two device variants at 5 Gbps data modulation rate near 1610 nm wavelength. One design shows 3.1 dB modulation depth with 1.5 dB insertion loss and an estimated $160\mathrm{fJ}/\mathrm{bit}$ energy consumption, while a more compact device achieves 4.2 dB modulation depth with 4.0 dB insertion loss and $60\mathrm{fJ}/\mathrm{bit}$ energy consumption. These modulators represent a significant breakthrough in enabling active photonics in bulk silicon CMOS—the platform of the majority of microelectronic logic and DRAM processes—and lay the groundwork for monolithically integrated CMOS-to-DRAM photonic links.