Why Lasers Inject Perceived Sound Into MEMS Microphones: Indications and Contraindications of Photoacoustic and Photoelectric Effects

Recent work published in the cybersecurity research community demonstrated a surprising discovery: modulated, low-power lasers can reliably inject falsely-sensed acoustic signals in MEMS microphones. However, the work remained mute on the physics-based causality with only passing conjectures on why the technique works. Until the physics of the energy transfer is understood, it will be difficult to design defenses with convincing evidence of effectiveness and reliability. In this work, we provide a methodology to test the presence and contribution of the photoacoustic and photoelectric effects to laser signal injection in MEMS microphones. Our programmable, precise laser experiments on MEMS devices in a vacuum chamber creates conditions to sufficiently isolate photoacoustic effects from photoelectric effects in a diverse set of microphones. The results indicate a dominance of photoacoustic effects while also providing contraindications of photoelectric effects. This leads to profound implications on laser injection defenses as modern MEMS designs do not consider security requirements to protect against laser signal injection via photoacoustic phenomena.