Photon-pair generation via bound states in the continuum in nonlinear metasurfaces

Abstract

In recent decades metamaterials and their two-dimensional form, metasurfaces, have generated a great deal of scientific and industrial interest due to the novel optical responses that they enable. Importantly, it has been shown that metasurfaces provide a low weight, small footprint device for nonlinear optical generation. At the same time there has been recent interest in Bound States in the Continuum (BICs) for their ability to create ultra-high quality factor resonances, or quasi-BICs, with recent work demonstrating that harmonic generation can be enhanced by orders of magnitude using such BIC derived resonances. The enhancement of higher order nonlinear processes, such as the classical process of Sum Frequency Generation (SFG) and the quantum process of photon-pair generation, however require enhancement at multiple frequencies. This thesis is an investigation of the uses of quasi-BICs in higher order nonlinear generation and especially in entangled photon-pair generation.

I will first demonstrate the ability to tune quasi-BICs dynamically by using liquid crystals to tune the refractive index of the medium surrounding the meta-atoms of a metasurface.

I will then present a novel design for metasurfaces that contain multiple reconfigurable BICs, which I have dubbedGhost Oligomer metasurfaces. The platform allows for a great degree of control over the eigenspace of the modes of the metasurface as well as over the frequency of the BICs that form within it. It thus allows us to generate nonlinear responses in a narrow bandwidth and at engineered frequencies that are determined by the design parameters of the metasurface.

I will demonstrate that Second Harmonic Generation (SHG) and SFG can be enhanced by many orders of magnitude by placing quasi-BICs at both the signal and idler wavelengths, with a linewidth in the picometer range and wavelengths that are reconfigurable over a broadband range. I then show that this means that photon-pairs can be generated over a broadband with picometer linewidths. Further I show that the symmetry of the metasurface has a profound effect on the distribution of photon-pairs in k-space via a novel form of the transverse phase matching requirement of Spontaneous Parametric Down Conversion (SPDC) in metasurfaces. I will also show that the entanglement of the photon-pairs can be tuned to any value via the polarisation of the pump beam.

Finally, I will analyse some of the properties of BICs in a one-dimensional grating on a LiNbO3 thin film. Show more