High-Q-Factor Chiral Metasurfaces for Spin Angular Momentum-Encoded Photon-Pair Generation

Photonic quantum states that involve entanglement of multiple degrees of freedom (e.g. energy, momentum, and polarization) underpin devices and systems for quantum communication, sensing, and metrology. Spin angular momentum (SAM) of light is an important degree of freedom that can be entangled at the same time along with photon energy. SAM-encoded entangled photons, i.e. circularly polarized photon pairs, encode and transmit quantum information more efficiently and amplify sensitivity in quantum metrology. Among a wide variety of approaches for photon-pair generation, spontaneous parametric downconversion (SPDC) stands out because of room-temperature operation, the ease of photon pair extraction and entangling several degrees of freedom, and strong coherent emission. Conventionally, nonlinear crystals that are used to realize the down-conversion require the stringent phase matching and significant volume footprint for light-matter interaction, challenging current efforts in on-chip quantum light sources. Despite some successful demonstrations of SPDC in ultrathin films of nonlinear materials by relaxing the phase-matching condition, the photon-pair generation rates are modest due to weak parametric amplification of the vacuum field and free of SAM encoding. Engineering high-quality-factor chiral photonic modes in miniaturized SPDC-based sources is an exciting route to strongly enhance chiral light-matter interactions and thus the down-conversion process for high chiral photon-pair generation rates.<br/>Here, we present a high-Q-factor chiral metasurface to generate photon pairs encoded with SAM. Our chiral metasurface is composed of periodically arranged AlGaAs nano-discs, which have a large second-order nonlinear coefficient (100 pm/V). In our design, the nano-discs are cut in the diagonal direction to break both in-plane inversion and mirror symmetries, forming a nonlocal chiral quasi-bound state in the continuum. Through numerical simulations, a high-Q chiroptical resonance is observed at 1452 nm, with Q's exceeding 7200, when the nano-disc has a radius of 280 nm and height of 350 nm. This high-Q resonance is designed for coinciding with both signal and idler photon energies so that signal and idler emissions are strongly enhanced. Our simulations demonstrate a strong electric field enhancement (E/E0: 600), which largely boosts the reverse classical nonlinear process of sum-frequency generation. Our calculations give rise to a significantly high photon-pair generation rate of up to 106 Hz at 1452 nm using the quantum-classical correspondence, four orders of magnitude higher than Mie-type resonant AlGaAs nano-antennas. We further examine the power-dependence of photon-pair generation rate and engineer dual chiral q-BIC modes in the metasurface for realizing signal and idler emission at different resonant wavelengths. Moreover, the generation rate is tunable with the Q-factor by engineering the symmetry perturbation strength. This SAM-encoded quantum light source holds promise for advancing chiral quantum optics, quantum sensing, and communication.