Directionally tunable co‐ and counterpropagating photon pairs from a nonlinear metasurface
Maximilian A. Weissflog, Jinyong Ma, Jihua Zhang, Tongmiao Fan, Shaun Lung, Thomas Pertsch, Dragomir N. Neshev, Sina Saravi, Frank Setzpfandt, Andrey A. Sukhorukov
Nonlinear metasurfaces have recently been established as a new platform for generating photon pairs via spontaneous parametric down-conversion. While for classical harmonic generation in metasurfaces a high level of control over all degrees of freedom of light has been reached, this capability is yet to be developed for photon-pair generation. In this work, we theoretically and experimentally demonstrate for the first time precise control of the emission angle of photon pairs generated from a nonlinear metasurface. Our measurements show angularly tunable pair generation with high coincidence-to-accidental ratio for both co- and counterpropagating emission. The underlying principle is the transverse phase matching of guided-mode resonances with strong angular dispersion in a nonlinear metasurface consisting of a silicon dioxide grating on a nonlinear lithium niobate guiding layer. We provide a straightforward design strategy for photon-pair generation in such a device and find very good agreement between the calculations and experimental results. Here, we use all-optical emission angle tuning by means of the pump wavelength; however, the principle could be extended to modulation via the electro-optic effect in lithium niobate. In sum, this work provides an important addition to the toolset of subwavelength thickness photon-pair sources.
Nonlinear metasurfaces have recently been established as a new platform for generating photon pairs via spontaneous parametric down-conversion. While for classical harmonic generation in metasurfaces a high level of control over all degrees of freedom of light has been reached, this capability is yet to be developed for photon-pair generation. In this work, we theoretically and experimentally demonstrate for the first time precise control of the emission angle of photon pairs generated from a nonlinear metasurface. Our measurements show angularly tunable pair generation with high coincidence-to-accidental ratio for both co- and counterpropagating emission. The underlying principle is the transverse phase matching of guided-mode resonances with strong angular dispersion in a nonlinear metasurface consisting of a silicon dioxide grating on a nonlinear lithium niobate guiding layer. We provide a straightforward design strategy for photon-pair generation in such a device and find very good agreement between the calculations and experimental results. Here, we use all-optical emission angle tuning by means of the pump wavelength; however, the principle could be extended to modulation via the electro-optic effect in lithium niobate. In sum, this work provides an important addition to the toolset of subwavelength thickness photon-pair sources.
Entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) are the cornerstone of many quantum optical applications. Nonlinear metasurfaces have recently been introduced as a new class of compact SPDC sources [1], and the next step is to enable active tunability of their resulting quantum state. Here, we demonstrate the generation of narrow-band yet frequency tunable, spectrally entangled photon pairs from a nonlinear metasurface. For this, we use guided-mode resonances (GMR) with high angular dispersion. In previous works that use Mie- or BIC-type resonances, for a fixed pump wavelength, the demonstrated SPDC spectrum remained fixed by the respective resonance wavelength [1], [2]. Here, we theoretically and experimentally show that in a metasurface supporting GMRs, the non-degeneracy of the generated photon pairs can be tuned by tilting the metasurface while the pump wavelength can remain constant, see the conceptual rendering in Fig. 1(a).
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.
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.
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.
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.
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.