  Light emitted from single-mode semiconductor lasers generally has large
divergence angles, and high numerical aperture lenses are required for beam
collimation. Visible and near infrared lasers are collimated using aspheric
glass or plastic lenses, yet collimation of mid-infrared quantum cascade lasers
typically requires more costly aspheric lenses made of germanium, chalcogenide
compounds, or other infrared-transparent materials. Here we report mid-infrared
dielectric metasurface flat lenses that efficiently collimate the output beam
of single-mode quantum cascade lasers. The metasurface lenses are composed of
amorphous silicon posts on a flat sapphire substrate and can be fabricated at
low cost using a single step conventional UV binary lithography. Mid-infrared
radiation from a 4.8 $\mu$m distributed-feedback quantum cascade laser is
collimated using a polarization insensitive metasurface lens with 0.86
numerical aperture and 79% transmission efficiency. The collimated beam has a
half divergence angle of 0.36$^\circ$ and beam quality factor of $M^2$=1.02.
