  We report the nanofabrication and characterization of optical spot-size
converters couplers based on curved GaAs cantilever waveguides. Using the
stress mismatch between the GaAs substrate and deposited Cr-Ni-Au strips,
single-mode waveguides can be bent out-of-plane in a controllable manner. A
stable and vertical orientation of the out-coupler is achieved by locking the
spot-size converter at a fixed 90$^\circ$ angle via short-range forces. The
optical transmission is characterized as a function of temperature and
polarization, resulting in a broad-band chip-to-fiber coupling extending over a
200 nm wavelength bandwidth. The methods reported here are fully compatible
with quantum photonic integrated circuit technology with quantum dot emitters,
and open opportunities to design novel photonic devices with enhanced
functionality.
