Tailoring Polarization in WSe$_2$ Quantum Emitters through Deterministic Strain Engineering

TL;DR

This work addresses the challenge of achieving deterministic polarization control for single-photon emitters in tungsten diselenide monolayers by engineering directional strain with novel polygonal nanopillars. The authors demonstrate that orientation-controlled nanowrinkles can align SPE dipoles, achieving high degrees of linear polarization and maintaining high single-photon purity. By comparing cylindrical pillars (random wrinkle directions) with three polygonal designs (TS, FS, BT), they show near-ideal polarization and improved emitter localization, notably a BT design delivering $g^{(2)}(0) = 0.030 \pm 0.025$ and robust polarization. The results pave the way for integrating TMD-based quantum emitters into on-chip photonic structures, although environmental stabilization and emission stability remain important future considerations.

Abstract

Quantum emitters in transition metal dichalcogenides (TMDs) have recently emerged as a promising platform for generating single photons for optical quantum information processing. In this work, we present an approach for deterministically controlling the polarization of fabricated quantum emitters in a tungsten diselenide (WSe$_2$) monolayer. We employ novel nanopillar geometries with long and sharp tips to induce a controlled directional strain in the monolayer, and we report on fabricated WSe$_2$ emitters producing single photons with a high degree of polarization $(99\pm 4 \%)$ and high purity ($g^{(2)}(0) = 0.030 \pm 0.025$). Our work paves the way for the deterministic integration of TMD-based quantum emitters for future photonic quantum technologies.

Figures (5)

• Figure 1: WSe2 quantum emitters with cylindrical pillars. (a) Bright-field (BF) image of monolayer WSe2 deposited on cylindrical nanopillars on a Si/SiO2 substrate. The white outline indicates the WSe$_2$ monolayer region. (b) Photoluminescence (PL) image of the WSe2 monolayer flake taken at T = 4 K. The green circle indicates the region around the nanopillar. (c) SEM image of the nanopillar region circled in (a) and (b), revealing four nanowrinkles that are formed WSe2 around the pillar. These nanowrinkles are likely to host the potential quantum emitters due to the strain. The crosses indicate the centers of the collection spots for each PL spectra presented in (d). (d) PL spectra collected from WSe2 sample at T = 4 K around the five crosses depicted in (c). The obtained PL spectra from S2-5 show narrow emission lines, in contrast to a broad PL emission signal collected from the planar region (S1). (e) Exemplary second-order correlation measurement ($g^{(2)}(\tau)$) of an isolated peak on the presented sample (S5), resulting in $g^{(2)}(0) = 0.303\pm0.035$.
• Figure 2: Highly polarized emission from WSe2 quantum emitters with three-pointed star (TS) shaped pillars. (a) A sketch of monolayer-based WSe2 quantum emitters with a star-shaped nanopillar design with a high aspect ratio. (b) AFM image of a WSe2 monolayer flake deposited on the TS nanostructure depicted in (a). The inset in (b) shows the corresponding SEM image. Both images reveal the nanowrinkles forming along the vertices of the TS. The directionality of the formed nanowrinkles mostly follows the directions of the vertices. (c) Corresponding PL image taken at T = 4 K of the fabricated sample exhibits brighter localized emission signals along the nanowrinkles, in contrast to the planar region. The two orthogonal axes are given as a reference for the subsequent polarization-resolved spectra and polar plots. In both (b) and (c), the three collection regions (P1-3) are highlighted. (d-f) In-plane polarization-resolved PL spectra taken from three highlighted regions (P1-3), respectively. The presented PL spectra in (d-f) are measured under parallel and orthogonal in-plane polarization with respect to the direction of each nanowrinkle. (g-i) Corresponding polar plots of the polarization-resolved PL intensities for each peak, color-coded to match the graphs in (d-f). The solid lines represent sinusoidal fits koudinov2004optical of the data, revealing a degree of linear polarization of 99 ± 4%, 82 ± 14%, and 93 ± 3%, respectively.
• Figure 3: WSe2 quantum emitters with a five-pointed star (FS) shaped pillars. (a) AFM image of a WSe2 monolayer flake deposited on the FS nanostructure and false-shading representation (inset). Both the main image and the inset reveal nanowrinkles forming at the vertices, mostly following their directionality. (b) The corresponding PL image at T = 4 K reveals increased photon emission along the nanowrinkles. The two orthogonal axes are given as a reference for the upcoming polarization-resolved polar plots. Both (a) and (b) highlight the collection spots (P1-4) for the polarization-resolved polar plots to follow. (c-f) Corresponding polar plots of the polarization-resolved PL intensities collected from P1-4, fitted with a sinusoidal function. The calculated degree of linear polarization is $86 \pm 2\%$, $81 \pm 2\%$, $76 \pm 3\%$, and $85 \pm 11\%$ for each peak respectively.
• Figure 4: WSe2 quantum emitters with bowtie (BT) shaped pillars. (a) AFM image of a WSe2 monolayer flake deposited on a bowtie pair of triangular nanostructures. A single nanowrinkle connects the two structures and is visible both in the main image and in the SEM image of the corresponding inset. (b) The corresponding PL image, taken at T = 4 K, exhibits increased photon emission along the nanowrinkle connecting the triangles. The two orthogonal axes are given as a reference for the upcoming polarization-resolved spectrum and polar plot. Both (a) and (b) highlight the collection spot (Q1) for the characterized emitter. (c) The presented PL spectra are measured from emitter Q1 under parallel and orthogonal in-plane polarization with respect to the direction of the nanowrinkle, which coincides with the two orthogonal axes given in (b). (d) Corresponding polarization-resolved polar plot of the PL intensities from emitter Q1. The sinusoidal fitting reveals a degree of linear polarization of 92 $\pm 12\%$.
• Figure 5: Single-photon characterization. (a) PL spectrum (4 K) taken from an exemplary single localized quantum emitter with high-degree polarization of 92 $\pm 12\%$ under the above-band excitation of 532 nm femtosecond pulsed laser (80 MHz). The inset shows a high-resolution PL spectrum with a pronounced zero-phonon line (ZPL) and a low-energy broader phonon-side band. The fitting results exhibit the ZPL located at 806.1 nm, with a linewidth of $0.35 \pm 0.08$ nm. The broader PSB with a linewidth of $0.91 \pm 0.04$ nm is red-detuned by 0.6 nm to the ZPL, resulting in an asymmetric PL emission spectrum. (b) Semi-logarithmic plot of the lifetime measurement of the WSe$_2$ quantum emitter fitted with a biexponential function, resulting in a lifetime of $\tau_\text{fast} = 0.16 \pm 0.24$ ns and $\tau_\text{slow} = 13.3 \pm 0.2$ ns. (c) Time trace of PL emission signal, recorded using a high-resolution spectrometer with an integration time of 0.2 seconds per frame. The time trace data reveals a maximum wobbling span of approximately 1 meV for the ZPL. Fitting the distribution of the energy of the maxima for each line in the spectral map gives an average central position of the peak at 1.5365 eV with a standard deviation of 0.267 meV. (d) Corresponding second-order autocorrelation measurement ($g^{(2)}(\tau)$) of the emitter under the pulsed above band excitation, revealing an antibunching value of $g^{(2)}(0) = 0.030 \pm 0.025$.