SEM image (a) and real-color PL photograph (b) of a branched nanowire structure with two composition-homogeneous CdSSe nanowires linked to a composition-graded CdSSe nanowire, which can be considered as an integration of three independent asymmetric waveguides, B1F, B2F, B3F, respectively. Scale bar, 30 μm. (c) Real-color PL photograph of the three waveguides' backward actions. The three waveguides have the same input end F but different output ends B1, B2, B3, respectively. (d–f) Real-color PL photographs of the three waveguides' forward actions. The three asymmetric waveguides have different input ends B1, B2, B3 but the same output end F (1, 2, 3). The dashed arrows track the outlines of the nanowires. (g) Corresponding output spectra of the three asymmetric waveguides. Inset in each panel shows the corresponding enlarged backward output spectra.