In the 23rd century, the Starship Enterprise cannot boast Star Trek’s infamous camouflage, the cloaking device – only the Federation’s arch enemies the Klingons and the Romulans have this. But in the present day, an accidental discovery by the UK’s Defence Evaluation Research Agency may render tanks and other army vehicles ‘invisible’ to infra-red sensors sooner.
Dera has discovered that, by applying a bias voltage to an indium antimonide LED, the diode can be made to appear hot or cold to infra-red detectors, independent of its real surface temperature.
When forward biasing is applied, the LED emits infra-red radiation, thus appearing to be hotter than it really is. This effect has been known about for some time. But Dera discovered the unexpected effect that reverse biasing suppresses infra-red emission, making a surface appear colder. Dera has termed this negative luminescence.
The potential defence implications for the technology are self-evident, given the use of infra-red for night-time surveillance. However, it is likely to be some time before any systems will be deployed in the battlefield.
Currently it is impossible to produce a big enough array of the LEDs, and even if it were, the level of power required to achieve the effect across a sufficient number would be too great to disguise a person, never mind a tank or personnel carrier.
Instead, Dera has concentrated its efforts on possible civilian uses for negative luminescence.
Foremost among these is creation of reference sources for calibration of infra-red detection systems.
Such systems need to be reset hourly using an internally generated reference surface, and the process can take some time because of difficulties in maintaining a surface at a uniform temperature. Ideally, at least two reference surfaces at different temperatures are needed – or more if the system is being used to watch several scenes at a range of different temperatures – and each takes several minutes. By contrast, panels of the biased LEDs could precisely mimic the infra-red output of a range of temperatures, according to the current passed across them, in quick succession.
Also, in detection systems that are required to monitor small differences in temperature across a ‘hot’ surface, another panel of LEDs could be inserted into the equipment to provide a constant reference point.
Other potential applications include gas detection systems, test panels for infra-red missile guidance systems, and free space communications over infra-red wavelengths.
A linear scale of infra-red detectability developed by Dera suggests that 0.17 electron volts passed across a suitably doped diode produces infra-red photons of around 5 microns wavelength.
The greatest challenge facing development of the LEDs has been reducing the current so that larger panels can be produced. Dera has so far achieved a tenfold reduction for inputs and panels of up to 1cm2.
The LEDs are epitaxial layers of indium antimonide doped at different levels in silicon and beryllium. Similar effects have been observed with mercury cadmium telluride.