Efficient photon-pair generation empowered by dual quasibound states in the continuum

Here we demonstrate the efficient photon-pair generation via spontaneous parametric down conversion from a semiconductor metasurface supporting dual quasibound states in the continuum (quasi-BICs). In a simple metasurface composed of AlGaAs elliptical nanocyclinders, the two high-$Q$ quasi-BIC resonances that coincide with the generated signal and idler frequencies significantly boost the local electric field. This leads to a substantial enhancement in the reverse classical nonlinear process of sum-frequency generation and subsequently the remarkably high generation rate of photon pairs under the quantum-classical correspondence principle. Within a narrowband wavelength regime around the quasi-BIC resonances, the rate of pair production is enhanced up to $\sim {10}^4$ Hz, two orders of magnitude larger than that in the Mie resonant AlGaAs nanoantennas. Moreover, the photon pair emission is mainly concentrated in the normal direction with respect to the metasurface, and shows a tunable rate with the $Q$ factor by engineering the rotation angle of nanocylinders. The presented work enables nanoscale sources of high-quality entangled photons which will find applications in advanced quantum imaging and communications.

Figure 1

Dual quasi-BIC metasurface for nonlinear processes. (a) The schematic of the designed metasurface for SPDC and its reverse SFG process. (b) The unit cell consists of four AlGaAs elliptical nanocylinders on the ${\mathrm{AlO}}_x$ substrate. The refractive index of AlGaAs is taken from experimental measurements and plotted in Fig. S1(a) [49] (see also Ref. [50] therein). The index of ${\mathrm{AlO}}_x$ is set to 1.6. (c) The energy diagram of SFG and SPDC processes.

Figure 2

Transmission spectra of the dual quasi-BIC metasurface under normal incidence of $x$-polarized plane waves propagating along the $z$ axis. (a) The transmission spectra as a function of rotation angle of the elliptical nanocylinders. (b) The transmission spectrum around the two quasi-BIC resonances with a rotation angle of ${10}^{\circ }$. The insets are the electric field distributions at the resonant wavelengths.

Figure 3

Enhanced SFG efficiency in the dual quasi-BIC metasurface. The SFG conversion efficiency as a function of (a) the pump wave incident wavelengths around the resonances, (b) the input intensity of the pump waves, (c) the pump wavelength ${\lambda }_1$ for the case of ${\lambda }_2$ fixed at Q-BIC 2, and (d) the pump wavelength ${\lambda }_2$ for the case of ${\lambda }_1$ fixed at Q-BIC 1.

Figure 4

Enhanced photon-pair generation rate of SPDC in the dual quasi-BIC metasurface. The SPDC photon-pair rate as a function of (a) the wavelengths of signal and idler beams and (b) the signal wavelength when the pump wavelength is constant at ${\lambda }_{\text{p}}=760$ nm.

Figure 5

SFG efficiency and photon-pair rate of SPDC at quasi-BIC resonances as a function of the rotation angle of the metasurface.

Figure 6

Photon-pair generation rate of SPDC at the quasi-BIC resonances in the metasurface at different observation angles, with the ${\lambda }_{\text{s}}=1471$ nm, ${\lambda }_{\text{i}}=1571$ nm, and ${\lambda }_{\text{p}}=760$ nm. The SPDC photon-pair rates as a function of (a) the signal and idler wave observation angles (${\theta }_{\text{s}}$ and ${\theta }_{\text{i}}$) and (b) the observation angles of signal beam when the idler beam is observed with ${\theta }_{\text{i}}=0^{\circ }$ at ${\lambda }_{\text{p}}=760$ nm.