On-demand polarization by a vertical-cavity surface-emitting laser with two tilted sub-wavelength gratings

We show that each polarization state on the Poincaré sphere (PS) can be accessed on-demand (Poincaré sphere tailoring) by a semiconductor-based vertical-cavity surface-emitting laser (VCSEL) with two tilted sub-wavelength gratings (SWGs). We develop a vectorial Barkhausen criterion that answers the question: what conditions must the cavity fulfill to support a given desired polarization state? Addressing this inquiry leads to a completely different strategy based on the entangled interplay between two tilted SWGs, resulting in an overall chiral cavity, whose features depend on the gratings and their mutual rotation. This leads to the emission of a well-controllable polarization state based on standard technologies used in polarization-stable VCSELs, which paves the way for inspiring several new potential applications.