Integrated on-chip quantum light sources on a van der Waals platform

TL;DR

This work demonstrates a fully van der Waals–based integrated quantum photonic platform by coupling strain-engineered bilayer WSe2 quantum emitters to multimode WS2 waveguides with optimized grating couplers. It achieves bright, waveguide-coupled, high-purity single-photon emission and validates on-chip quantum statistics via Hanbury Brown–Twiss measurements, including an on-chip configuration that uses the waveguide as a beam splitter. The results yield MHz-level waveguide-coupled count rates and a ~9% overall quantum efficiency, establishing a viable path toward monolithic 2D-material quantum photonic circuits. The study also provides detailed numerical modelling and nanofabrication strategies to scale vdW quantum photonics toward fully integrated devices with detectors and modulators.

Abstract

Scalable photonic quantum information technologies require a platform combining quantum light sources, waveguides, and detectors on a single chip. Here, we introduce a van der Waals platform comprising strain-engineered bilayer WSe$_2$ quantum emitters, integrated on multimode WS$_2$ waveguides with optimized grating couplers, enabling efficient on-chip quantum light sources. The emitters exhibit bright, highly polarized emission that couples efficiently into WS$_2$ waveguides. Under resonant p-shell excitation, we observe high-purity, waveguide-coupled single-photon emission, measured using both an off-chip Hanbury Brown-Twiss configuration ($g^{(2)}(0) = 0.003^{+0.030}_{-0.003}$) and an on-chip configuration ($g^{(2)}(0) = 0.076\pm0.023$). For a single output, the out-coupled single-photon count rate at the first lens reaches approximately 320 kHz under continuous-wave p-shell excitation, corresponding to an estimated waveguide-coupled rate of 1.7 MHz. These results demonstrate an efficient, integrated single-photon source and establish a pathway toward scalable photonic quantum information processing centered around nanoengineered van der Waals materials.

Figures (3)

• Figure 1: Nanostructured WS$_2$ for on-chip waveguides. (a) Dispersion relation of a $500 \times 150$ nm WS$_2$ slab waveguide on a SiO$_2$ substrate, showing all propagating modes with a cut-off wavelength above $\lambda = 800$ nm. The dashed black lines mark the propagation constant in bulk WS$_2$ and SiO$_2$. (b) Electric field intensity profile at $\lambda = 802$ nm for each propagating mode shown in (a); the waveguide and substrate cross-sections are outlined in white. (c) Atomic force microscopy trace of the $500 \times 175$ nm waveguide used for scanning near field optical microscopy, showing a slab waveguide with minimal surface roughness. (d) Scanning electron microscopy image of short $500 \times 150$ nm slab waveguides with variable grating coupler pitch or no grating coupler (bottom waveguide). (e) Optical microscopy image of two of the waveguides shown in (d) under illumination with a broadband source; the empty orange circle indicates the beam spot. Light can be coupled in the waveguide by edge scattering (i, iv) or grating coupling (iii) and observed on the opposite end; no light is coupled by irradiating the surface of the waveguide (ii). (f) Dispersion relation of the TE modes measured with near-field microscopy (sSNOM) over the waveguide in (c) at decreasing photon wavelengths, from 1190 to 1120 nm. The two dashed lines matching the spectral features correspond to an effective refractive index of 2.90 (propagating mode) and 1.44 (SiO$_2$ substrate mode). (g) Far-field distribution of the upward-coupled fundamental TE mode at $\lambda = 800$ nm from the proposed grating coupler design, obtained via FDTD modelling; the fundamental TM mode shares a similar pattern. All the light within a half-angle of $54.1^\circ$ can be collected with an objective with $\text{NA} = 0.81$. (h) FDTD modelling of the collection efficiency spectra for the propagating modes with the proposed grating coupler design. (i) Comparison of the total transmittance between grating coupling (blue) and side-scattering coupling (yellow) through a $\approx60$ µ m-long WS$_2$ waveguide. The spectra are ten point-averaged and normalized by the grating-coupled transmittance value at 800 nm. The inset shows an illustration of the waveguides characterized in the measurement.
• Figure 2: Integration of WS$_2$ waveguides with hBN/WSe$_2$ heterostructures for on-chip coupling of single photons. (a) Artistic illustration of the characterized device, consisting of a 60 µ m-long WS$_2$ waveguide with a $90^\circ$ bend and grating couplers, with a thin ($<5$ nm) hBN layer and a WSe$_2$ bilayer transferred on top. By optically exciting a quantum emitter in the WSe$_2$ bilayer, single-photon states couple to a propagating waveguide mode and are then collected via the grating couplers. (b) Photoluminescence emitted by the transferred bilayer flake with the excitation laser centered on a quantum emitter; the waveguide and a portion of the bilayer are overlapped in the image for reference. Photoluminescence is observed on both the excitation spot and the two grating couplers. (c) Simulated coupling efficiency between an electric dipole 5 nm above the waveguide and a propagating mode as a function of the distance from the vertical symmetry axis of the waveguide cross-section; the coupling is calculated with the dipole oriented along each main axis. (d) Photoluminescence spectral map of a quantum emitter at $\lambda = 802$ nm, collected at the upper grating coupler; the emitter can be excited via p-shell excitation, showing peak emitter brightness with excitation laser at $\lambda_p = 783$ nm. The partially-filtered laser line is intentionally shown. (e) Emission spectra at the upper grating coupler obtained via p-shell and above-band excitation of the quantum emitter; p-shell excitation suppresses the majority of the background emission and increases the emitter brightness. (f) Excitation power-dependent emission intensity (integrated counts per second at the spectrometer, logarithmic scale) and spectral linewidth of the quantum emitter at the upper grating coupler under the two excitation schemes. (g) Polarization-dependent emission intensity at the upper grating coupler or confocal to the excitation laser under p-shell excitation, fitted with a sinusoidal curve. (h) Additional examples of spectral lines coupled through the waveguide and collected at a grating coupler.
• Figure 3: On-chip waveguide-coupled single-photon purity. (a-c) Simplified illustration of the Hanbury Brown and Twiss (HBT) experimental setup realized with (a) the collection spot confocal to the excitation source and an external beam splitter before the detectors; (b) the collection spot at one grating coupler with an external beam splitter before the detectors; (c) a collection spot at each grating coupler, so that the HBT effect can be observed without an external beam splitter. (d-f) Continuous-wave second-order autocorrelation traces obtained with the respective setup; the traces are plot with a three-point moving average, and show signatures of photoluminescence blinking. All configuration exhibit high single-photon purity at zero time delay.