High-purity and stable single-photon emission in bilayer WSe$_2$ via phonon-assisted excitation

Abstract

The excitation scheme is essential for single-photon sources, as it governs exciton preparation, decay dynamics, and the spectral diffusion of emitted photons. While phonon-assisted excitation has shown promise in other quantum emitter platforms, its proper implementation and systematic comparison with alternative excitation schemes have not yet been demonstrated in transition metal dichalcogenide (TMD) quantum emitters. Here, we investigate the impact of various optical excitation strategies on the single-photon emission properties of bilayer WSe$_2$ quantum emitters. Based on our theoretical predictions for the exciton preparation fidelity, we compare excitation via the longitudinal acoustic and breathing phonon modes to conventional above-band and near-resonance excitations. Under acoustic phonon-assisted excitation, we achieve narrow single-photon emission with a reduced spectral diffusion of 0.0129 nm, a 1.8-fold improvement over above-band excitation. Additionally, excitation through breathing-phonon mode yields a high purity of $ 0.947\pm 0.079\,$ and reduces the decay time by over an order of magnitude, reaching $(1.33 \pm 0.04)\,$ns. Our comprehensive study demonstrates the crucial role of phonon-assisted excitation in optimizing the performance of WSe$_2$-based quantum emitters, providing valuable insights for the development of single-photon sources for quantum photonics applications.

Figures (5)

• Figure 1: Quantum emitters in mono- and bilayer WSe2.(a) Sketch of mono- (1L) and bilayer (2L) flake deposited on top of star-shaped nanostructures leading to the formation of nanowrinkles nearby their vertex. The nanowrinkles host the quantum emitters. Inset: atomic force microscopy (AFM) image of a star-shaped nanostructure covered by a bilayer flake with the visible presence of a nanowrinkle originating from the top vertex. (b) µ PL spectra of the two bright spots in 1L WSe2 (grey) and 2L WSe2 (red) encircled in the inset image, integrated over 1s. They show typical emission imprints of the two different thicknesses' flakes. Inset: color-coded PL image of the sample taken at $T$ = 4. The white dashed lines highlight the contours of the 1L and 2L flakes. (c) Second-order correlation measurement of emitter Q1 from the bilayer spot (red dashed circle in inset in (b)) under CW LED excitation at 470 nm. The fit reveals a $g^{(2)}(0)$ value of $0.098 \pm 0.045$.
• Figure 2: Above-band and near-resonant excitations. Emission spectra of emitter Q1 under pulsed above-band excitation at 532 nm (a) and emitter Q2 under near-resonant pulsed excitation at 785 nm (d). The graphs present fittings to the raw data (solid purple line) with a sum of a Lorentzian (red area) and a Gaussian (yellow area) accounting for the contribution from the ZPL and the PSB, respectively. The inset schemes on the right show a simplified band diagram of the emitters as a two-level system ($\ket{g}$ and $\ket{e}$) with the respective pumping energy, while A represents the energy level of the planar WSe2 A-exciton. (b) Semi-logarithmic plot of the time-resolved PL measurement from Q1 (grey circles) with relative single decay exponential fitting function (red line) with a constant $\tau_1 = (16.65 \pm 2.39)$ ns. (c) Second-order intensity correlation measurement ($g^{(2)}(\tau)$) of Q1 under above-band pulsed excitation. From the double exponential fit (red line), we extract an antibunching value of $g^{(2)}(0) = 0.079\pm 0.015$. (e) Semi-logarithmic plot of the time-resolved PL measurement from Q2 (grey circles) with relative double decay exponential fitting function (red line). The two extracted time constants are $\tau_{1} = (12.62 \pm 1.18)$ ns and $\tau_2 = (1.14 \pm 0.21)$ ns, accounting for 96.5% and 3.5% of the fit curve, respectively. (f) Second-order intensity correlation measurement of Q2 under near-resonant excitation. The $g^{(2)}(0)$ value extracted is $0.057 \pm 0.018$. The measurements in (b) and (e) are integrated over 3 minutes, while those in (c) and (f) over 8 hours.
• Figure 3: DFT-based phonon signatures on the population inversion for mono- and bilayer WSe$_{2}$.(a) Calculated phonon dispersion in mono- and bilayer WSe$_{2}$ around the $\Gamma$ point. For the bilayer, the equilibrium value $d_{\rm W-W} = 6.5$ Å is used for the vertical distance between W atoms in different layers. The inset shows the phonon dispersion across the full Brillouin zone. (b) Energy of interlayer SMs and BM at $\Gamma$ as a function of the interlayer W-W distance $d_{\rm W-W}$. (c) Calculated population inversion in the presence of LA phonon coupling, as a function of wavelength detuning $\lambda_{\rm laser} - \lambda_X$ from the exciton and pulse area $\Theta$. The laser pulse has a FWHM of 0.30 nm ($t_p$ = 2.65 ps in our notation). (d) Same as in (c), including also the coupling to SMs and BM. Here, we use $d_{\rm W-W} = 6.1$ Å for the interlayer W-W distance, and the dimensionless BM coupling weight is set to $\xi_{\rm BM} = 5$ (see Methods). Dashed orange and red lines are placed at detunings of -2.7nm and -0.3nm, respectively.
• Figure 4: Longitudinal acoustic phonon-assisted excitation and spectral diffusion.(a) Coarse PLE spectrum of the quantum emitter, showing two absorption maxima centered at detuning $\Delta \lambda\!=\,$-1.7nm and $\Delta \lambda\!=\,$-0.3nm, respectively. (b) Power dependence of the peak intensity evaluated at a detuning $\Delta \lambda\!=\,$-1.4nm. High-resolution PL spectra at time 1s (c top, d top) and their time evolution (c bottom, d bottom) for above-band excitation and phonon-assisted excitation, respectively. (e) and (f) show the statistical analysis of the central wavelength of the ZPL as extracted from the fitting of each line in the map. The red curves are Gaussian fittings of the obtained distributions. The mean value and the standard deviation in (e) are µ ZPL = 800.49 nm and ZPL = 0.0232 nm, respectively. At the same time, their values for (f) are µ ZPL = 800.62 nm and ZPL = 0.0129 nm.
• Figure 5: Phonon-assisted excitation.(a) PL spectrum of emitter Q3 under pulsed excitation at detuning of -2.7 from the quantum emitter. The fit function on the raw data (purple) is a sum of a Lorentzian (red area) and a Gaussian (yellow area) accounting for the contribution from the ZPL and the PSB, respectively. The inset shows a simplified band diagram of an emitter as a two-level system highlighting the pumping energy detuning. (b) Second-order autocorrelation measurement of Q3. From the double exponential fit (red line), we extract an antibunching value of $g^{(2)}(0) = 0.053 \pm 0.079$. (c) Time-resolved PL measurement from Q3 in a semilogarithmic plot with relative double exponential fitting function with extracted time constants of $\tau_{1} = (1.33 \pm 0.04)$ ns (80.4%) and $\tau_{2} = (8.31 \pm 6.29)$ ns (19.6%). (d) Zoom-in of $g^{(2)}(\tau)$ in (c) for time delays up to $\pm 50$ ns.