  A single electron transistor (SET) consisting of parallel double quantum dots
fabricated in a GaAs/Al$_{x}$Ga$_{1-x}$As heterostructure crystal is
demonstrated to serve as an extremely high sensitive detector of submillimeter
waves (SMMW). One of the double dots is ionized by SMMW via Kohn-mode plasma
excitation, which affects the SET conductance through the other quantum dot
yielding the photoresponse. Noise equivalent power of the detector for
wavelengths about 0.6 mm is estimated to reach the order of $10^{-17}$
W/$\sqrt{Hz}$ at 70 mK.
