Structured Single-photon Metasource

Abstract

Structured quantum light is crucial for high-dimensional quantum information processing, yet its direct generation from quantum emitters remains challenging due to their intrinsic locality and omnidirectional radiation. Metasurfaces have been adopted for quantum-light wavefront shaping, typically in cascaded or stacked configurations that suffer from low efficiency and limited resolution. Here, we demonstrate a semiconductor metasource that directly embodies single quantum dots in a nonlocal GaAs metasurface. Spontaneous emission from quantum dot is efficiently funneled into an extended quasi-bound-state-in-the-continuum mode while sustaining strong mode-emitter overlap. A lateral core-barrier heterostructure tunes mode volume and spatial distribution to balance Purcell enhancement and holographic resolution. Using spatially modulated geometric phase, our compact metasource enables deterministic generation of diverse single-photon radiation patterns, including orbital-angular-momentum beams and holographic images. Our work brings versatile single-photon wavefront control into the nanoscale cavity quantum electrodynamics regime, offering a scalable route toward integrated sources of structured quantum light.

Figures (13)

• Figure 1: Arbitrarily structured single photons from a QD-coupled nonlocal heterogeneous metasurface.a, Schematic of a single quantum emitter coupled to a nonlocal heterogeneous metasurface. A single QD at the cavity center is optically excited, and the emitted photons are funneled into the mode and shaped into a designed wavefront. b, Comparison of our embodied quantum metasurface with other active metasurface platforms. Unlike existing structured quantum-light sources based on cascaded or stacked metasurface-emitter integration, our approach waives such integration while achieving strong Purcell enhancement and high holographic resolution in a single-layer device. c, Schematic of a unit cell with perturbation rotation angle $\uptheta$, inducing a phase shift $\phi = -3\uptheta$ in the emitted LCP component. The geometric parameters are $p=382$ nm, $r=118$ nm, $h=140$ nm, $w=115$ nm and $d=39$ nm. d, Simulated phase distribution of the LCP component (left) and electric-field distribution (middle and right) of the unit-cell eigenmode. e, Simulated photonic band structure, with the quasi-BIC at the Brillouin-zone center (blue) selected as the operating mode. f, Top-view SEM image of the fabricated nonlocal metasurface. Lower inset: seven representative unit cells with different rotation angles. Scale bar, 1 $\upmu$m. g, Induced phase retardance $\phi$ of the LCP (blue) and RCP (red) components as a function of perturbation rotation angle $\uptheta$.
• Figure 2: Design and optimization of the nonlocal heterogeneous metasurface.a, Eigenmode band structure (top) and quality factor (bottom) versus perturbation rotation angle $\uptheta$ from 0 to 120 deg. b, Simulated electric-field intensity distribution of the heterostructure, showing effective longitudinal and lateral confinement in the slab core region. By varying the unit-cell number in the core region ($N_u$), the mode volume and cavity NA are flexibly tuned. The intensity profile along $y$ is extracted and fitted with a Gaussian function (white line). Scale bar, 2 $\upmu$m. c,d, Calculated cavity mode volume $V_m$, maximum Purcell factor $F_p$, effective NA, and corresponding holographic resolution $D$ as functions of the core-region unit-cell number $N_u$. Increasing $N_u$ increases $V_m$ and NA, improving holographic resolution at the cost of reduced $F_p$. e-h, Numerically predicted single-photon holographic images of one spot (e), an OAM vortex spot with $l=1$ (f), nine spots (g), and the letter ‘Q’ (h).
• Figure 3: Observation of structured light from the nonlocal heterogeneous metasurface.a,b, Photonic band structures of the metasurface with $N_u=91$ (a) and $N_u=11$ (b), measured by momentum-resolved spectroscopy. A 0.4 nm bandpass filter (yellow shaded area) selects the band-edge component for holographic imaging. White dashed lines: simulated band structures. c, Schematic of the imaging planes. Blue: designed focal plane. Red: device surface plane. d, Measured intensity distribution at the device surface plane. e--i, Measured intensity distributions at the designed focal plane. Five devices with different spatial phase profiles generate an OAM vortex with $l=1$ (e), a single spot (f), five spots (g), a nine-spot square (h), and a circular array (i). Upper-right insets: simulated intensity distributions. Lower-right inset of e: phase map of the vortex spot. Lower inset of f: intensity profile along $y=0$. Scale bar, 5 $\upmu$m.
• Figure 4: Tailoring and enhancing structured single-photon emission from quantum metasurfaces.a, PL intensity map from confocal raster scanning under p-shell excitation, using a 0.4 nm detection window. Scale bar, 5 $\upmu$m. b, Radiative lifetimes of a QD coupled to the resonant mode (blue) and of QD ensemble in bulk (purple), showing a Purcell enhancement factor of 10.1(1). IRF, instrument response function. c, Log-scale second-order correlation function of the QD emission measured in a Hanbury--Brown and Twiss interferometer, yielding $g^{2}(0)=0.018(1)$. d--h, PL spectra of QDs (red) in the nonlocal metasurface and the corresponding resonant modes (green) from five devices with different holographic phase profiles. The cavity modes are fitted with a Voigt function. i--m, Corresponding single-photon holographic images.
• Figure S1: Main fabrication process of the sample. The QD wafer is grown by molecular beam epitaxy. An electron-beam resist (AR-P 6200) is spin-coated onto the sample surface. The designed photonic structures are patterned by EBL and transferred into the membrane using ICP–RIE with a BCl$_3$/N$_2$ gas mixture. The residual resist is removed using N-methyl-2-pyrrolidone (NMP). To release the membrane, the sacrificial $\mathrm{Al_{0.75}Ga_{0.25}As}$ layer is selectively etched using a diluted hydrofluoric acid solution (HF:H$_2$O = 1:20).