  We propose a monolithic optical bandpass filter with a transmission bandwidth
tunable by the incident angle in a tri-mode high-contrast grating (HCG). We
attribute this extraordinary phenomenon to the destructive interference between
the TM$_{0}$ and TM$_{2}$ modes and the Fabry-P\'erot resonance of the TM1
mode, which can only be excited at the oblique incidence. The transmission
bandwidth can be tuned from zero at normal incidence and increases
quadratically with incident angle, while the center wavelength reds shifts
slightly. The corresponding Q factor can be continuously tuned from 10$^{6}$ to
10$^{2}$ as the incident angle increases from nearly 0$^{\text{o}}$ to
10$^{\text{o}}$.
