Large-format quantum-well infrared photodetector arrays for astronomical instrumentation

Quantum Well IR Photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors. The wavelength of the peak response and cutoff can be continuously tailored over a range wide enough to enable light detection at any wavelength range between 6-20 micrometers . The spectral band width of these detectors can be tuned from narrow to wide allowing various applications. Also, QWIP device parameters can be optimized to achieve extremely high performance sat lower operating temperatures due to exponential suppression of dark current. Furthermore, QWIPs offer low cost per pixel and highly uniform large format focal plane arrays (FPAs) mainly due to mature GaAs/AlGaAs growth and processing technologies. The other advantages of GaAs/AlGaAs based QWIPs are higher yield, lower 1/f noise and radiation hardness. Recently, we operated an IR camera with a 256 by 256 QWIP array sensitive at 8.5 micrometers at the prime focus of the 5-m Hale telescope, obtaining the images. The remarkable noise stability - and low 1/f noise - of QWIP focal plane arrays enable camera to operate by modulating the optical signal with a nod period up to100 s. A 500 s observation on dark sky renders a flat image with little indication of the low spatial frequency structures associated with imperfect sky subtraction or detector drifts.