Amorphous silicon subwavelength gratings have been fabricated. Their use as quarter-wave plates in polarization switching vertical-cavity surface-emitting lasers with tunable oscillation frequency as high as several terahertz is proposed. The substantial increase in frequency comes from a reduction in the oscillator cavity length due to the small thickness of the grating (only about one-third of the wavelength thick). The frequency tuning is provided by fabricating the wave plate and a partial reflector monolithically on a movable microcantilever.