  We present a mid-IR ($\lambda \approx$ 8.3 $\mu$m) quantum well infrared
photodetector (QWIP) fabricated on a mid-IR transparent substrate, allowing
photodetection with illumination from either the front surface or through the
substrate. The device is based on a 400 nm-thick GaAs/AlGaAs semiconductor QWIP
heterostructure enclosed in a metal-insulator-metal (MIM) cavity and hosted on
a mid-IR transparent ZnSe substrate. Metallic stripes are symmetrically
patterned by e-beam lithography on both sides of the active region. The
detector spectral coverage spans from $\lambda \approx 7.15$ $\mu$m to $\lambda
\approx 8.7$ $\mu$m by changing the stripe width L - from L = 1.0 $\mu$m to L =
1.3 $\mu$m - thus frequency-tuning the optical cavity mode. Both micro-FTIR
passive optical characterizations and photocurrent measurements of the two-port
system are carried out. They reveal a similar spectral response for the two
detector ports, with an experimentally measured T$_{BLIP}$ of $\approx$ 200K.
