**2. Materials and Methods**

Both the above devices were processed from a planar AlGaAs heterostructure consisting of a 123 nm thick film of Al0.19Ga0.81As on top of a 4 μm thick Al0.8Ga0.2As layer, grown on a GaAs {001} substrate by molecular-beam epitaxy.

Suspended nanowires 1 μm wide and 1 mm long (Figure 2a) were patterned along with their anchoring points by e-beam lithography followed by Ar/SiCl4-assisted inductively coupled plasma reactive-ion etching (ICP-RIE). The anchoring points were pairs of 100 nm wide and 1 μm long lateral tethers placed every 50 μm along the wire. A 1 mm wide, 100 μm deep mesa was then defined in the GaAs substrate by optical lithography and wet etching, giving access to the input and output ends for butt coupling. Finally, the Al0.8Ga0.2As layer was underetched with 1% HF solution at 4 ◦C for 6 minutes without stirring before sample CO2 critical point drying.

**Figure 2.** Scanning electron microscope (SEM) images of the suspended nanowire (**a**) and nanorib (**b**) waveguides.

Suspended nanorib waveguides (Figure 2b) were patterned by means of a two-step e-beam lithography plus ICP-RIE process: the former defined a 1 μm wide, 200 μm long and 80 nm thick rib in the Al0.19Ga0.81As layer, while the latter opened two lines of 2 μm × 2 μm square windows through the same layer, 2 μm away from the strip. The windows allowed wet isotropic underetching (10 min in 1% HF at room temperature with moderate stirring) of the underlying Al0.8Ga0.2As layer, which thus liberated a suspended 40 nm thick, 15 μm wide and 200 μm long Al0.19Ga0.81As membrane supporting the guiding rib. It is worth noticing that rib waveguides, due to intrinsic robustness, do not require critical point drying at the end of processing but can be simply flash dried (isopropanol evaporation on a hot plate at 270 ◦C).

Both types of waveguides were terminated with inverted tapers designed for efficient input/output coupling at fundamental frequency ω and second-harmonic 2ω.

All devices were tested using two continuously tunable laser sources: aCW external cavity laser diode emitting between 1.5 and 1.6 μm and a single mode CW Ti:sapphire tunable between 0.7 and 1 μm. Both laser beams butt coupled at the input and the output with microlensed, single mode optical fibers. Linear and nonlinear spectra have been recorded by injecting and tuning the laser sources while detecting the outcoupled light either by an InGaAs or an Si photodiode.
