When we observe interactions between light and biological matter at the nanoscale, we are almost always watching a one-sided effect. The field of optical biosensing, encompassing established technologies ranging from surface plasmon resonance (SPR) to guided-mode resonance (GMR) sensors and photonic crystal (PC) platforms, has achieved remarkable milestones in recent decades. These platforms can now detect trace amounts of cancer biomarkers, viral particles and nucleic acids. Yet despite this progress, the underlying physics of these optical biosensors has remained fundamentally asymmetric: They rely almost entirely on the enhancement and modulation of the electric-field component of electromagnetic radiation.