Germanium infrared absorption chopper

The use of Pyroelectric array detectors in Infrared cameras in the 8 to 14 µm region necessitates a means for area-modulation, or chopping of the infrared scene, thus generating an element of change for measurement by the detector. Currently this is achieved by mechanically chopping the image using a rotating blade, with drawbacks of high electric motor power, blurring as camera motion changes the rotational speed, and the chopper blade largely determining the camera diameter. Adequate modulation using a novel method has been achieved in the 8 to 14 µm region by introducing moderate levels of excess carriers to suitably prepared Germanium. These were introduced via excitation from a diode laser source. The process described uses inter valence band transitions from the light-hole to heavy-hole band, requiring power densities in the order of Watts cm-2. Modulation of from 44% to 6% transmission at 10 µm using a power density of 28.6 W cm2 from an AlGaAs 809nm laser is easily achievable. The on state transmission will then be then increased to essentially 100% by using anti-reflection coatings. We will describe the relative importance of the bulk and surface properties; high bulk purity and low surface recombination velocities being required to make an effective device, although operation is possible with lower quality material. The most suitable material for the device is low carrier density n-Germanium. The modulator does not operate in the much-publicised intra-band transition region which involves generating a free carrier density greater than the critical density, typically requiring laser powers in the order of MW cm-2