Optical immersion of mid-infrared LEDs and photodiodes for gas sensing applications

The high gains in performance predicted for optical immersion are difficult to achieve in practice due to total internal reflection at the lens/detector interface. By reducing the air gap at this interface optical tunnelling becomes possible and the predicted gains can be realised in practical devices. Using this technique we have demonstrated large performance gains by optically immersing mid-infrared heterostructure InAlSb LEDs and photodiodes using hyperspherical germanium lenses. The development of an effective method of optical immersion that gives excellent optical coupling has produced a photodiode with a peak room temperature detectivity (D*) of 5.3 × 10⁹ cmHz^1/2W^-1 at ?_peak=5.4µm and a 40° field of view. A hyperspherically immersed LED showed a 5-fold improvement in the external efficiency, and a 3-fold improvement in the directionality compared with a conventional planar LED for f/2 optical systems. The incorporation of these uncooled devices into a White cell produced a NO₂ gas sensing system with 2 part-per-million sensitivity, with an LED drive current of < 5mA. These results represent a significant advance in the use of solid state devices for portable gas sensing systems.