Plasmonic nanoantennas for efficient control of polarization-entangled photon pairs

We suggest a source of polarization-entangled photon pairs based on a cross-shaped plasmonic nanoantenna driven by a single quantum dot. The integration of the nanoantenna with a metal mirror overcomes the fundamental tradeoff between the spontaneous emission (SE) enhancement and the extraction efficiency typical of microcavity- and nanowire-based architectures. With a very high extraction efficiency of entangled photons ($\approx$$90\%$) at $1.55$ $\mu$m and large SE enhancement ($\approx$90) over a broad 330-nm spectral range, the proposed design will pave the way toward reliable integrated sources of nonclassical light.

Figure 1

(a) Schematic of the cross-shaped plasmonic nanoantenna on a pedestal glass substrate of height $h$ and side size $L$ with an underlying gold mirror. The mirror is on the top of an infinitely wide glass bulk. The left inset sketches the radiative cascade of a single QD placed in the common gap of the cross (the red arrow in the main panel). The right inset defines the dimensions of the cross. (b) The directivity of the far-field radiation from the QD at $\lambda =1550$ nm, $L=2000$ nm, and gold mirror height of 150 nm with the cross (red solid line) and without the cross (blue dashed line). The characteristic far-field patterns are shown for three values of $h$. The inset shows the reflectivity at normal incidence of an infinitely wide glass layer of thickness $h$ placed over a gold bulk.

Figure 2

Spectral performance of the nanoantenna. (a) Total $\Gamma$, radiative ${\Gamma }^{\mathrm{R}}$, and nonradiative ${\Gamma }^{\mathrm{NR}}$ normalized decay rates due to absorption by the cross and the mirror. ${\Gamma }_{\mathrm{mirror}}^{\mathrm{NR}}$ is enlarged for the sake of illustration. (b) Extraction efficiency for several opening angles $\theta$. Owing to the symmetry of the cross, the same result holds for both $H$- and $V$-polarized dipoles.

Figure 3

Influence of the axial $b$ and diagonal $b^{\prime }$ off-axis dipole displacement at $\lambda =1550$ nm. Red left-pointing and green right-pointing triangles denote $\Gamma$ of the $H$ and $V$ dipoles. Dashed lines are the guide to the eye. Inset: $|E_{\mathrm{x}}|$ field profile. By virtue of symmetry, $|E_{\mathrm{y}}|$ equals to $|E_{\mathrm{x}}|$ rotated by 90 deg.

Figure 4

Fidelity of entanglement for the axial (black solid and red dotted lines) and diagonal (blue dashed line) off-axis QD locations at $\lambda =1550$ nm, for two cross-gap dimensions $g$ of 75 nm and 150 nm. The $0.85$ threshold is indicated.