Certain LEDs, when heated and then run at a very low power, exhibit wall-plug efficiencies of more than 100%; this effect is explained by the existence of thermoelectric pumping of the LED. (If the LED is run at higher powers, this effect is overwhelmed by normal LED inefficiencies.) Now, Rajeev Ram and colleagues at the Massachusetts Institute of Technology (MIT; Cambridge) have demonstrated ultralow-power free-space communication (over a short distance in the lab) using a thermoelectrically pumped mid-IR indium gallium arsenide antimonide (In0.15Ga0.85As0.13Sb0.87) LED heated to 167°C. The setup transmitted 3 kbit/s at a bit-error rate of 3 x 10-3 and an energy expenditure per bit of only 40 fJ.
The photodetector was cooled to -20°C to reduce its noise-equivalent power (NEP). Light detection was performed with the aid of an analog-to-digital converter (ADC) to allow fast sampling; a discrete Fourier transform applied to the data enabled measurement of the modulation frequency of the signal, allowing the signal to be separated from photocurrent due to background thermal radiation (noise that has no preferential phase relationship to the source modulation). As a result, the setup was able to detect light power of only 7.66 ±1.6 pW at an LED wallplug efficiency of 8000 ±1700%. The picowatts-level power consumed by the data-transmitting LED could enable ultra-low-power remote sensing. Reference: D. Huang et al., Opt. Express 22, S7, A1650 (Dec. 2014); doi:10.1364/OE.22.0A1650.