Novel germanium infrared modulator

Pyroelectric array detectors in 8.0-14.0 mum thermal imaging cameras requir e a means of area modulation to generate a change in signal intensity on th e detector. Currently this is achieved by mechanically 'chopping' the image using a rotating blade, which introduces moving parts to the system and ha s the drawbacks of size and image degradation as camera motion changes the blade's rotational speed. Adequate modulation has been achieved using a nov el method in the 8.0-14.0 mum region by introducing moderate levels of exce ss carriers to suitably prepared germanium by excitation from a diode laser source. The modulation produced is greatly superior to previously publishe d attempts to produce a broad optical bandwidth, polarization-insensitive l arge-area modulator. The process utilizes intervalence band transitions fro m the light-hole to heavy-hole band, requiring excitation power densities i n the order of W cm(-2). A depth of modulation of from 95.4 to 4.7% transmi ssion at 10 mum using a total power density of 5.6 W cm(-2) from 980 nm dio de lasers has been achieved over a 1 cm(2) aperture. The 'on' state loss at this wavelength is limited by the single-layer quarter-wavelength AR coati ng properties, and may be eliminated through use of a multilayer coating. T echniques used to optimize the induced absorption are detailed, and a carri er diffusion model is presented which is used in temporal transmission deca y experiments to extract the surface recombination velocity and bulk carrie r Lifetime parameters.