Photon Pairs from Resonant Metasurfaces

Abstract

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.

Keywords: Mie-type resonances; nonlinear metasurfaces; photon-pair generation; quantum optics; spontaneous parametric down-conversion.

Figure 1

(a) Artist’s view of SPDC from a LN metasurface: the pump is incident from the substrate side, photon pairs are collected in reflection. Both the pump and the SPDC photons are polarized along the LN optic axis z. (b) Scanning electron microscopy (SEM) image of a fabricated metasurface, showing a periodic array of nanoresonators in the shape of truncated pyramids. (c) Electric field E distribution inside such a nanoresonator, as calculated in COMSOL Multiphysics at the electric resonances (λ_ER) (left), and magnetic resonances (λ_MR) (right). The incident field E₀ is polarized along the LN optic axis, z-axis. Arrows show the electric field direction. (d) Experimental (solid lines, shading) and simulated (dashed lines) reflectance spectra of four QOMs with different resonance positions, further labeled as A, B, C, D; vertical black line marks the wavelength of degenerate SPDC. Vertical colored lines mark the positions of the electric resonances.

Figure 2

(a) Correlation experiment. A parabolic mirror focuses a cw pump into the QOM and collects backward-emitted SPDC. A dichroic mirror separates the SPDC radiation from the pump, and a 50 nm FWHM bandpass filter centered at 1575 nm transmits nearly degenerate photon pairs. Another parabolic mirror feeds the SPDC into a Hanbury Brown–Twiss setup formed by a fiber splitter and two superconducting nanowire single-photon detectors (SNSPD). (b) Coincidence histograms of degenerate SPDC from QOMs A and B, shown by red diamonds and blue circles, respectively. The lines are guides to the eye. Gray stars show the coincidence histogram from an unpatterned LN film of the same thickness as the nanoresonators. In all measurements, the pump power is ∼70 mW and the acquisition time is 10 min.

Figure 3

Real coincidence rate measured in QOM A versus the pump polarization angle, with respect to the LN optic axis. The pump power was ∼50 mW. The purple curve shows the theoretical cos²θ dependence.

Figure 4

(a) Measured SPDC spectra from QOMs A (red), B (blue), C (orange), and D (green). Gray stars show the SPDC spectrum from the unpatterned LN film. (b) Spectra obtained through the numerical simulation of SFG at normal incidence. (c) SFG spectra calculated for the signal and idler incident at ±2° to the normal direction in the xy (solid curves) and xz (dashed curves) planes. Vertical colored lines mark the positions of the electric resonances, and the dashed vertical lines mark their conjugate wavelengths. The vertical black lines mark the wavelength of degenerate SPDC.