Current nonlinear metasurface photon-pair sources face limitations in angular emission range and uniformity due to optical resonance dispersion, potentially hindering their application in quantum imaging, which demands broad-angle emission for a wide field of view and high resolution. Here, we propose a broad-angle photon-pair source using flatband engineering. We demonstrate that tailored partially etched patterns in a lithium niobate metasurface can facilitate a flatband across a wide transverse wavevector range (±0.2 rad/μm) around a 1573 nm wavelength. The flatband metasurface supports enhanced photon-pair generation at a simulated rate of 539 Hz/mW within a cone of 3° apex angle relative to the pump propagation, which is an order of magnitude broader than previously reported metasurfaces. This general approach is also scalable to other wavelengths, opening new possibilities for compact entangled photon-pair sources in quantum imaging.
Current nonlinear metasurface photon-pair sources face limitations in angular emission range and uniformity due to optical resonance dispersion, potentially hindering their application in quantum imaging, which demands broad-angle emission for a wide field of view and high resolution. Here, we propose a broad-angle photon-pair source using flatband engineering. We demonstrate that tailored partially etched patterns in a lithium niobate metasurface can facilitate a flatband across a wide transverse wavevector range (±0.2 rad/μm) around a 1573 nm wavelength. The flatband metasurface supports enhanced photon-pair generation at a simulated rate of 539 Hz/mW within a cone of 3° apex angle relative to the pump propagation, which is an order of magnitude broader than previously reported metasurfaces. This general approach is also scalable to other wavelengths, opening new possibilities for compact entangled photon-pair sources in quantum imaging.
Nonlinear metasurfaces have emerged as a versatile platform for photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the angular dispersion of optical resonances restricts the emission range and momentum-space uniformity of the generated photon pairs, thereby hindering practical applications in quantum sensing and imaging. Here, we propose and numerically investigate an all-dielectric InGaP metasurface that supports angle-robust photon-pair generation enabled by tailored multipolar flatband resonances. By introducing elliptical nanopores into nonlinear nanodisks, we engineer the far-field coupling and polarization response of weakly dispersive flatband modes while preserving strong field confinement. As a result, the tailored structure exhibits a more than one-order-of-magnitude enhancement in second-harmonic generation efficiency compared with the unperforated design. Based on the quantum-classical correspondence between SPDC and sum-frequency generation (SFG), we further reveal a pronounced enhancement of the nonlinear response and demonstrate stable photon-pair generation with rates up to approximately 2×10 5 Hz over an angular range of ±12°. In addition, the generated photon pairs remain robust over a broad momentum range and show tunable polarization properties under different pump wavelengths. These results provide a practical route toward compact and efficient quantum light sources in nonlinear metasurfaces, and highlight the potential of flatband engineering for applications in quantum imaging and integrated quantum photonics.
Abstract We reveal that strongly enhanced generation of photon pairs with narrow frequency spectra and sharp angular correlations can be realised through spontaneous parametric down-conversion in metasurfaces. This is facilitated by creating meta-gratings through nano-structuring of nonlinear films of sub-wavelength thickness to support the extended bound state in the continuum resonances, associated with ultra-high Q-factors, at the biphoton wavelengths across a wide range of emission angles. Such spectral features of photons can be beneficial for various applications, including quantum imaging. Our modelling demonstrates a pronounced enhancement, compared to unpatterned films, of the total photon-pair generation rate normalized to the pump power reaching 1.75 kHz mW−1, which is robust with respect to the angular bandwidth of the pump, supporting the feasibility of future experimental realisations.
Broad-angle photon-pair generation from flatband quasi-BIC resonant 3R-MoS2 metasurfaces
Spontaneous parametric down-conversion (SPDC) in ultrathin optical resonant metasurfaces offers a promising platform for integrated quantum light sources. However, conventional resonances suffer from steep momentum dispersion, limiting photon-pair generation to narrow excitation and emission angles. Here, we demonstrate a flatband-resonant 3R-MoS2 metasurface for high-efficiency, broad-angle SPDC photon-pair generation. By implementing a supercell symmetry-breaking strategy via Brillouin zone folding, we achieve a flatband quasi-bound states in the continuum (quasi-BICs) resonance in the near-infrared regime that maintains a robust quality factor across a wide range of incident angle. Exploiting the strong local electric field enhancement of resonance and the large second-order susceptibility of 3R-MoS2, the metasurface boosts the degenerate SPDC pair-generation rate with orders of magnitude enhancement over an unpatterned film of identical thickness. Crucially, the near-zero band dispersion extends the effective emission angle to ±5∘ with negligible spectral drift ∼0.1 nm and high directional tolerance. Under pulsed excitation with a finite spectral bandwidth, the photon pair yield remains high, demonstrating strong spectral locking to the quasi-BIC resonance against pump detunings. This work establishes a versatile strategy for momentum-independent quantum light sources, opening new avenues for scan-free quantum ghost imaging and spatial-frequency quantum processing.
All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons-a new promising platform for quantum optics experiments.
The ability to generate quantum entangled photon pairs through spontaneous parametric down conversion (SPDC) is playing a pivotal role in many applications in quantum technologies, including quantum communications, quantum computation, and quantum imaging. Metasurfaces, two-dimensional arrays of nanostructures with subwavelength thickness, have recently shown extraordinary capability in manipulating classical and quantum states of light and realizing miniaturized SPDC sources. Whereas previous studies have primarily focused on periodic metasurfaces, we uncover the potential of spatially modulated nanopatterns for further SPDC enhancement and state engineering. Specifically, we propose a nonlinear metasurface featuring a lateral guided-mode cavity formed by two distributed Bragg reflectors. With the quasi-normal mode theory, we predict an SPDC rate up to 157 Hz/mW, which represents about 30 times rate enhancement when compared to the metasurface without a cavity. Furthermore, the cavity supports a continuous set of high-Q resonances across a broad bandwidth, allowing the all-optical tuning of the photon-pair emission directions transverse to the cavity by controlling the pump wavelength, which can find applications in quantum imaging.
The two-photon state with spatial entanglement is an essential resource for testing fundamental laws of quantum mechanics and various quantum applications.Its creation typically relies on spontaneous parametric downconversion in bulky nonlinear crystals where the tunability of spatial entanglement is limited.Here, we predict that ultrathin nonlinear lithium niobate metasurfaces can generate and diversely tune spatially entangled photon pairs.The spatial properties of photons including the emission pattern, rate, and degree of spatial entanglement are analyzed theoretically with the coupled mode theory and Schmidt decomposition method.We show that by leveraging the strong angular dispersion of the metasurface, the degree of spatial entanglement quantified by the Schmidt number can be decreased or increased by changing the pump laser wavelength and a Gaussian beam size.This flexibility can facilitate diverse quantum applications of entangled photon states generated from nonlinear metasurfaces.