Balancing positive and negative luminescence for thermoradiative signatureless communications

Ambient-temperature thermal infrared radiation is an underutilised resource for secure communications. Demonstrations of free-space data transfer using thermal radiation have been few, and have relied on intrinsically slow modulation of either the emissivity and/or physical temperature of a broadband blackbody emitter, severely limiting data transfer rates ( < 1 kHz). Here, we demonstrate a covert communications method in which photon emission is rapidly electrically modulated both above and below the level of a passive blackbody at the emitter temperature. The time-averaged emission can be designed to be identical to the thermal background, realizing communications with zero optical signature for detectors with bandwidth lower than the modulation frequency. We demonstrate this scheme using both electro- and negative luminescence in thermoradiative diodes, enabling data rates up to at least 100 kbps and modulation rates above 1 MHz. Future prospects for ultra-high-bandwidth (up to THz) emitters and detectors utilising meta-optics and 2D materials are discussed.