image: Figure | Working principle of thermoradiative signatureless communication. Thermoradiative signatureless communication achieves covert transmission of data by rapidly modulating mid-infrared photodiodes, balancing forward and reverse bias luminescence. Only fast detectors resolve the signal; slow observers observe no evidence of communication.
Credit: Michael Nielsen et al.
“Covert” optical communication, which hides the existence of a message rather than merely encrypting it, is the most secure form of data transfer. Unlike conventional “bright” communication methods, which are observable even if encrypted, covert communications can be achieved by mixing “bright” and “dark beam” emissions such that the time-averaged output is zero. By burying data in a lossy, noisy channel, like thermal blackbody radiation, the signal becomes indistinguishable from background for detectors without sufficient bandwidth. Unlike bright communication methods that tip off eavesdroppers even if the content remains unreadable, this approach leaves no telltale trace, reducing the chance that adversaries deploy the appropriate sensors to eavesdrop. It can also be layered with conventional encryption for added security.
In a new paper published in Light: Science & Applications, a team of scientists at the University of New South Wales (Dr Michael Nielsen and Professor Nicholas Ekins-Daukes) and Monash University (Professor Michael Fuhrer and Professor Stefan Maier) in Australia have demonstrated a new form of covert communication called thermoradiative signatureless communication. Using mid-infrared light emitting diodes (LEDs), they have achieved thermoradiative signatureless communication by balancing electroluminescence (how conventional visible light LEDs emit) with a phenomena known as negative luminescence. This negative luminescence creates a “darker than usual” state that can perfectly balance the conventional electroluminescent bright state LEDs normally operate in. By rapidly switching between these states, the mid-infrared LED emission perfectly blends into the thermal background we are familiar with when using thermal camera imaging.
The use of thermoradiative diodes for this application was inspired by recent developments at the University of New South Wales in developing “night-time solar”, inverse solar cells that generate power at night by emitting to the cold night-sky.
“While working on the night-time solar project we determined that negative luminescence was the critical determinant of how well thermoradiative diodes performed,” said Dr Michael Nielsen, lead author of the paper, “While comparing their negative and electroluminescent properties we realized that their symmetry could allow covert communications.”
While to date the researchers have only demonstrated covert data communications on the order of 100s of kilobytes per second, in the paper they propose new methods and materials (chiefly graphene) to improve the range and speed of this new communication method.
Article Title
Balancing positive and negative luminescence for thermoradiative signatureless communications