The current-voltage characteristics of Josephson junctions exhibit a subharmonic gap structure (SGS), denoting jumps at specific voltages. While the prevalent multiple Andreev reflection theory matches the experimentally observed SGS, it is limited to a dc voltage bias. For a dc current bias, existing theories are restricted to low-transparency junctions and fail to capture the full SGS. We introduce a microscopic Floquet approach applicable for arbitrary transparencies, and recover the correct SGS for a dc current bias. We provide a comprehensive understanding of SGS for a dc current bias, which entails two-quasiparticle tunneling processes absent in existing theories, via two complementary perspectives: in the frequency domain, as generalized Andreev reflections absorbing multiple energies, and in the time domain, as the interference of nonequilibrium current pulses.

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• Josephson effect
• Quantum transport
• Josephson junctions
• BCS theory
• Green's function methods
• Methods in transport