Spin angular momentum-encoded single-photon emitters in a chiral nanoparticle-coupled WSe2 monolayer

Abstract

Spin angular momentum (SAM)-encoded single-photon emitters, also known as circularly polarized single photons, are basic building blocks for the advancement of chiral quantum optics and cryptography. Despite substantial efforts such as coupling quantum emitters to grating-like optical metasurfaces and applying intense magnetic fields, it remains challenging to generate circularly polarized single photons from a subwavelength-scale nanostructure in the absence of a magnetic field. Here, we demonstrate single-photon emitters encoded with SAM in a strained WSe₂ monolayer coupled with chiral plasmonic gold nanoparticles. Single-photon emissions were observed at the nanoparticle position, exhibiting photon antibunching behavior with a g⁽²⁾(0) value of ~0.3 and circular polarization properties with a slight preference for left-circular polarization. Specifically, the measured Stokes parameters confirmed strong circular polarization characteristics, in contrast to emitters coupled with achiral gold nanocubes. Therefore, this work provides potential insights to make SAM-encoded single-photon emitters and understand the interaction of plasmonic dipoles and single photons, facilitating the development of chiral quantum optics.

Fig. 1.. SAM-encoded single-photon emission from a WSe₂ monolayer coupled with a plasmonic cNP.

(A) Schematic of the design and operation of circularly polarized single-photon emission. A monolayer of WSe₂ is placed onto a cNP. (B) Tilted-view SEM image of the fabricated sample showing the monolayer WSe₂ flake transferred to the cNP. Scale bar, 200 nm. Inset, top-view SEM image. Scale bar, 200 nm. (C) Measured spectrum of the PL emission from the sample. The inset shows the integrated PL intensity as a function of the pump laser power. A 10-nm-wide spectral filter was used for the measurement. The fitted red curve indicates a saturation pump power of 280 nW and a saturation emission intensity of 1.69 × 10³ counts/s. (D) Measured photon correlation function g⁽²⁾(τ) of the peak (λ = 738 nm) in (C). The fitted red curve indicates photon antibunching behavior with g⁽²⁾(0) = 0.286 ± 0.063.

Fig. 2.. Array of SAM-encoded single-photon emitters.

(A) Optical microscope image captured after the monolayer WSe₂ flake was transferred to the cNP array. Scale bar, 10 μm. (B) Measured PL intensity map for the strained WSe₂ monolayer from the dashed box in (A). Photon antibunching behaviors were observed from the emitters denoted by the dotted circles. Scale bar, 5 μm. (C) Representative PL emission spectra measured from dotted circles 1, 2, and 3 in (B). (D) Measured photon correlation functions g⁽²⁾(τ) of the arrow-marked peaks in (C). The fitted red curves indicate photon antibunching behavior with g⁽²⁾(0) = 0.196 ± 0.091, 0.249 ± 0.093, and 0.108 ± 0.034, respectively (from top to bottom). Data from the remaining dotted circles are shown in fig. S4.

Fig. 3.. Polarization characterization of single-photon emissions.

Measured properties in two different samples [(A) to (D) and (E) to (H)]. (A and E) PL spectra of right-handed (σ+; black) and left-handed (σ−; red) circularly polarized single-photon emissions. Insets, measured photon correlation function g⁽²⁾(τ). g⁽²⁾(0) is 0.213 ± 0.086 (A) and 0.286 ± 0.063 (E). (B and F) Polar plots of the integrated intensity of the emission measured with an HWP and a linear polarizer, yielding Stokes parameters S₁ and S₂. Inset, schematic of the setup (H: HWP and L: linear polarizer). (C and G) Polar plots of the integrated intensity of the emission measured with a QWP, an HWP, and a linear polarizer, yielding a Stokes parameter S₃. Inset, schematic of the setup (Q: QWP, H: HWP, and L: linear polarizer). (D and H) Polarization ellipses reconstructed from the measured S₁, S₂, and S₃, demonstrating the polarization orientations of LCP-dominant (D) and RCP-dominant (H) emissions. Dashed lines represent major/minor axes of the ellipses.

Fig. 4.. WSe₂ monolayer coupled with a Au nanocube and comparison with cNP-coupled samples.

(A) Tilted-view and top-view (inset) SEM images of a monolayer of WSe₂ transferred to a Au nanocube. Scale bar, 100 nm. (B) Polar plots of linear (red) and circular (blue) polarization measurements, yielding the Stokes parameters S₁, S₂, and S₃ of the emission. For the linear polarization (LP) data (red), a sinusoidal function with a direction of 51° was fitted. (C) Polarization ellipse derived from the polarization measurement results in (B), exhibiting almost pure LP. (D) Distribution of the absolute values of the degree of circular polarization (DOCP) for cNP (black bar) and nanocube (red bar) samples. (E) Ratio of the LCP-dominant, LP, and RCP-dominant cNP (orange bar) and nanocube (green bar) samples. Raw data for (D) and (E) are from tables S1 and S2.

Fig. 5.. Theoretical analysis.

Full-wave optical simulations of the polarization of the radiation field in a single cNP depending on dipole orientation. (A) Schematic of the cNP with a dipole at a vertex (black arrow). The dipole source excites the instantaneous induced current, J_d (red curved arrows), and the retarded induced current, J_r (blue curved arrow). (B) Magnetic field normal to the surface of the cNP, directed either outward (red) or inward (blue). The white dashed circle represents the magnetic field generated by J_r. (C) Calculated Stokes parameters, S₁ (black), S₂ (red), and S₃ (blue), as a function of the dipole orientation with varying azimuthal angle, ϕ. The polar angle of dipole orientation is set to π/2 (inset). The asterisks at ϕ = 0° and 90° indicate LCP-dominant and RCP-dominant polarization cases, respectively. (D and E) Calculated polarization ellipses at ϕ = 0° (D) and 90° (E). The corresponding Stokes parameters (S₁, S₂, and S₃) and schematics of dipole orientation are shown in the insets.