Gallium nitride (GaN) and its related alloys are technologically important for applications in short-wavelength LEDs, lasers, photodetectors, and power electronics. GaN is a piezoelectric material. As a result, when it is strained, a strong built-in electric field is generated which has profound impacts on its applications in LEDs and lasers. In these devices, strained InGaN heterostructures are routinely employed as the light-emitting active region. The strain induced built-in electric field can significantly lower the emitter efficiency, cause efficiency droop and undesirable wavelength drift. Nanowires can effectively relax the strain and nanowire LEDs have attracted substantial interests in recent years. In this work, we showed that by controlling and engineering the strain in GaN nanowires, not only can one realize benefits from strain relaxation, new degrees of control can be obtained which have potential applications for display, communication, and sensing. In this talk, I will discuss two of these applications: (1) on-demand single photon sources with highly pure and programmable polarization states; (2) monolithically integrated multi-color light emitter chips.