We present an original design of a nonlinear metasurface featuring symmetry protected bound states in the continuum (BICs) which enable enhanced photon pair generation via the process of Spontaneous Parametric Down-Conversion (SPDC). We establish both analytical and numerical methods for the optimization of BIC modes which enables the simultaneous enhancement of non-degenerate photon frequencies. We achieve this by inserting oligomer holes into the AlGaAs nanodisks which, along with the symmetry of the lattice, allow us to select from a range of unit cell symmetries. The non-Mie eigenfunctions of the chosen symmetry group will form symmetry protected BICs for zero transverse momentum (Gamma point of the Brillouin zone). Away from the Gamma point, these BICs become high-quality factor Fano resonant modes, which can significantly enhance the photon pair generation. Because the BICs are symmetry protected, we are able to tune the design parameters of the metasurfaces to select pairs of wavelengths for which a non-degenerate SPDC process is enhanced.
We further utilize an analytical analysis of the classical-quantum correspondence between sum frequency generation (SFG) and SPDC for metasurfaces and thus predict the SPDC generation of our metasurface via numerical simulations of SFG over the first Brillouin zone.
The ultra-thin metasurface thickness removes the conventional restrictions associated with bulk phase-matching. This opens the potential for generation of photons with tailored quantum entanglement at ultra-short time-scales for photons across the visible and infrared spectral regions. Such features of quantum states can underpin advances in nonlinear quantum spectroscopy, low-light sensing, and ghost imaging.