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Here we present a method and structures for midinfrared, free-space optical communication using unmodified, commercially available complementary metal-oxide-semiconductor integrated circuits. The modulator is based on the free carrier absorption in parasitic PN junction structures under reverse bias. Measured results demonstrate the proof-of-concept with speeds of 100 b/s (1.55-μm wavelength), but at least two orders of magnitude improvement can be achieved. This technology will enable nongalvanic chip-to-chip and chip-to-package communication as an alternative to wire-bonding in applications that benefit from a planar top chip surface, such as chemical sensing lab-on-chip systems as well as general sensors and midinfrared communication.