  A near threshold all-optical backaction amplifier is realized. Operating near
threshold in an integrated micronscale architecture allows a nearly three
orders of magnitude improvement in both gain and optical power requirements
over the only previous all-optical implementation, with 37 dB of gain achieved
for only 12 uW of input power. Minor adjustments to parameters allows optical
filtering with narrow bandwidth dictated by the mechanical quality factor.
Operation at cryogenic temperatures may enable standard quantum limit
surpassing measurements and ponderomotive squeezing.
