  Transistor structures comprising graphene and sub-wavelength metal gratings
hold a great promise for plasmon-enhanced terahertz detection. Despite
considerable theoretical effort, little experimental evidence for terahertz
plasmons in such structures was found so far. Here, we report an experimental
study of plasmons in graphene-insulator-grating structures using Fourier
transform spectroscopy in 5-10 THz range. The plasmon resonance is clearly
visible above the Drude absorption background even in chemical vapor deposited
(CVD) graphene with low carrier mobility $\sim 10^3$ cm$^2$/(V s). We argue
that plasmon lifetime is weakly sensistive to scattering by grain boundaries
and macoscopic defects which limits the mobility of CVD samples. Upon placing
the grating in close proximity to graphene, the plasmon field becomes tightly
bound below the metal stripes, while the resonant frequency is determined by
the stripe width but not by grating period. Our results open the prospects of
large-area commercially available graphene for resonant terahertz detectors.
