Optical cavity cooling of mechanical resonators has recently become a research frontier. The cooling has been realized with a metal-coated silicon microlever via photo-thermal force and subsequently with dielectric objects via radiation pressure. Here we report cavity cooling with a crystalline semiconductor membrane via a new mechanism, in which the cooling force arises from the interaction between the photo-induced electron-hole pairs and the mechanical modes through the deformation potential coupling. The optoelectronic mechanism is so efficient as to cool a mode down to 4 K from room temperature with just 50 uW of light and a cavity with a finesse of 10 consisting of a standard mirror and the sub-wavelength-thick semiconductor membrane itself. The laser-cooled narrow-band phonon bath realized with semiconductor mechanical resonators may open up a new avenue for photonics and spintronics devices.
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