Novel germanium infrared modulator

Pyroelectric array detectors in 8.0-14.0 µm thermal imaging cameras require a means of area modulation to generate a change in signal intensity on the detector. Currently this is achieved by mechanically 'chopping' the image using a rotating blade, which introduces moving parts to the system and has the drawbacks of size and image degradation as camera motion changes the blade's rotational speed. Adequate modulation has been achieved using a novel method in the 8.0-14.0 µm region by introducing moderate levels of excess carriers to suitably prepared germanium by excitation from a diode laser source. The modulation produced is greatly superior to previously published attempts to produce a broad optical bandwidth, polarization-insensitive large-area modulator. The process utilizes intervalence band transitions from the light-hole to heavy-hole band, requiring excitation power densities in the order of W cm-2. A depth of modulation of from 95.4 to 4.7% transmission at 10 µm using a total power density of 5.6 W.cm-2 from 980 nm diode lasers has been achieved over a 1 cm2 aperture. The 'on' state loss at this wavelength is limited by the single-layer quarter-wavelength AR coating properties, and may be eliminated through use of a multilayer coating. Techniques used to optimize the induced absorption are detailed, and a carrier diffusion model is presented which is used in temporal transmission decay experiments to extract the surface recombination velocity and bulk carrier lifetime parameters.