Device configuration

As illustrated in Fig. 1a, the hybrid system consists of a 2H-type WS 2 monolayer on the SiO 2 /Si substrate, a layer of Al 2 O 3 and the silver sawtooth nanoslit array patterned subsequently on top. The structural parameters of the sawtooth nanoslits are shown in Fig. 1b. Different from most of previous works where the WS 2 monolayers were transferred on the plasmonic nanostructure, here in our system they were designed to be beneath the metallic layer. This configuration allows adequate interaction between excitons and the plasmonic metasurface to guarantee an efficient plasmonic modulation. In addition, inserting an Al 2 O 3 layer between the WS 2 and sawtooth nanoslits isolates the WS 2 from dopants in the ambient atmosphere and serves as a control parameter for tuning the spectral position of the plasmonic resonance because different thicknesses of Al 2 O 3 layers would change dielectric environments of the sawtooth nanoslits.

Fig. 1: Schematics of the WS 2 -Ag hybrid nanostructure. a 3D schematic view of the hybrid plasmonic device consisting of silver sawtooth nanoslit array on top of WS 2 monolayer separated by a thin layer of Al 2 O 3 . The device is pumped by a 532 nm laser with the incident polarization fixed in the x direction. b Top view of two unit cells for the sawtooth nanoslit array with geometrical parameters: p x = 200 nm, p y = 400 nm, s = 40 nm, b and θ are two variables. Full size image

For the metasurface on top, the sawtooth rather than line nanoslits are chosen, because the former supports broadband resonant transmission, which could enable efficient excitation and emission of PL (Supplementary Fig. 1). At the same time, the transmission mode is polarized preferentially along the TE direction (Fig. 1b) which is in parallel with the long axis of the sawtooth nanoslit array, hence is able to drive the PL emission to be polarized along the same direction. Furthermore, the resonant transmission modes are sensitive to the geometrical parameters, e.g. the slanting angle θ of the middle slit (Fig. 1b), thereby offering the opportunity for the polarization modulation of the PL emission. To support the above pictures, optical responses of the sawtooth nanoslit array are simulated by using commercial finite-difference time-domain algorithm (FDTD Solution, Lumerical) for x and y polarization excitations along both forward and backward directions46.

Simulations of multiple optical paths

As shown in Fig. 2a, multiple optical paths are depicted in the cross-sectional view of the device to illustrate the laser excitation and the PL emission (The color-coding is the same as that in Fig. 1a). For the excitation path A, to arrive at the surface of the WS 2 monolayer, the incident 532 nm laser (TE polarized) needs to transmit through sawtooth nanoslits and the Al 2 O 3 layer along the backward (BW) direction as indicated by the green arrow. The PL emission occurs around 630 nm which corresponds to the direct band gap transition of neutral excitons of WS 2 monolayers (~2.0 eV) and mainly follows two optical paths as marked by B 1 and B 2 (red arrows). Path B 1 starts from the top surface of the WS 2 monolayer and then transmits through the Al 2 O 3 /sawtooth nanoslits along the forward (FW) direction. For path B 2 , PL is emitted from the bottom surface of the WS 2 monolayer, going through the SiO 2 layer to the top surface of the silicon substrate and then is bounced back to the sawtooth nanoslits along the forward direction, and finally penetrates the sawtooth nanoslits.

Fig. 2: Two optical modes dominating excitation and emission paths of the WS 2 -Ag hybrid nanostructure. a The excitation path of 532 nm pump laser from nanoslits to WS 2 in the backward (BW) direction is denoted as A, the emission path from WS 2 to nanoslits in the forward (FW) direction as B 1 and the emission path from WS 2 to Si and bounced back to the nanoslits as B 2 . t 1 and t 2 indicate the thickness of Al 2 O 3 and SiO 2 , respectively. b The BW transmittance spectra are simulated under the TE polarized light incidence for the excitation path A, i.e. silver nanoslits (b = 200 nm, θ = 45°) on Al 2 O 3 with increasing t 1 from 10 to 40 nm, in which the resonance mode is denoted as Mode I whose electric field distribution is presented in 3D. c The FW transmittance spectra of the emission paths B 1 and B 2 , i.e. silver nanoslits and Al 2 O 3 (t 1 = 30 nm) without (violet curve) and with (red curve) SiO 2 , respectively, are calculated for the TE polarized light incidence, the latter of which exhibits a resonance mode II whose electric field distribution is also depicted in 3D. d–g Electric fields E xx and E yx for resonances I and II in the xz plane cutting along the dashed lines denoted in the 3D view (b, c). Full size image

The excitation of WS 2 flakes is realized by generating a resonant transmission mode around the wavelength of the pump laser, which is represented by the transmission peak of silver nanoslits on Al 2 O 3 as indicated by the black arrows in Fig. 2b. The peak position shifts from 475 to 550 nm when the thickness of Al 2 O 3 increases from 10 to 40 nm. With this continuous shift, one can always find a proper thickness allowing for the coupling of the pump laser with the WS 2 monolayer for an efficient excitation. Here, a 30 nm thick Al 2 O 3 was chosen in the following simulation and experiment whose transmission peak is located at ~530 nm (mode I denoted by the green curve in Fig. 2b) coinciding well with the pump wavelength. The emission of the WS 2 monolayer is determined by the resonant transmission mode at the emission wavelength denoted as mode II in the red curve in Fig. 2c. This mode rises upon a layer of SiO 2 is integrated under the Al 2 O 3 , which fits well with the emission path B 2 . As for path B 1 , a resonance mode is absent at the emission wavelength as depicted by the violet curve in Fig. 2c. Consequently, the emission process is expected to be dominated by path B 2 .

The 3D view of the electric field |E x | for the two modes is shown in Fig. 2b, c, indicating that local fields are primarily concentrated in the Al 2 O 3 and nanoslits, respectively. To reveal the detailed process of the resonant transmission through the nanoslits, the distributions of electric fields for both components E xx and E yx in the xz plane, i.e. cutting in the middle of the unit cell, are shown in Fig. 2d–g. For mode I, E xx is strongly localized in the slanting holes (Fig. 2d) while E yx is oscillating in the Al 2 O 3 layer beneath the vertical holes as shown by the dark blue regions in Fig. 2e, which is indicative of sufficient energy transfer from pump laser to the WS 2 monolayer. For mode II, strong oscillations exists in both slanting and vertical holes for E xx (Fig. 2f) while a relatively weak resonance locates solely in the slanting holes for E yx (Fig. 2g), both of which promote the resonant coupling between the plasmons and the excitons. Importantly, for modes I and II, the far field scattering is dominated by E xx rather than E yx , despite that both components can be excited in the near field (Supplementary Fig. 2). The reason lies on that E yx , which stems from the polarization conversion of the x polarized incidence, is mainly absorbed rather than scattered by the nanostructure.

To evaluate the polarization responses of light propagating through the emission paths B 1 and B 2 , polarization resolved optical spectra of the silver nanoslit array on different substrates are calculated as shown in Fig. 3. In addition, to modulate the resonant transmission properties, the shape of sawtooth nanoslits are engineered by tuning the variable θ, i.e. the angle of the middle slit relative to the x axis.

Fig. 3: Structural dependence of polarization resolved spectra by simulations. a, b Forward transmittance spectra of the nanoslit array on Al 2 O 3 under the TE (top row) and TM (bottom row) polarized light incidence. Structural parameters for three types of sawtooth nanoslit arrays: Black curve θ = 30°, b = 118 nm; red curve θ = 45°, b = 200 nm; blue curve θ = 55°, b = 260 nm. c, d Forward transmittance spectra of the nanoslit array on Al 2 O 3 /SiO 2 . e, f Backward reflectance spectra of the nanoslit array on Al 2 O 3 /SiO 2 /Si. Full size image

For emission path B 1 , the transmittance of silver nanoslits on Al 2 O 3 is ~0.3 and 0.1 around 630 nm for TE (Fig. 3a) and TM (Fig. 3b) polarizations, respectively, which satisfies the requirement of anisotropic transmission. The transmittance is comparable for three types of nanoslit arrays implying that the polarization of light along the emission path B 1 is less influenced by θ. For emission path B 2 , the transmittance spectra of sawtooth nanoslits on Al 2 O 3 /SiO 2 are shown in Fig. 3c, d. The resonant transmission mode is denoted by a broad and high transmission peak (Fig. 3c) locating between two transmission minima, whose full width at half maximum (FWHM) is strongly dependent on the slanting angle, i.e. the larger θ leads to wider FWHM (Supplementary Fig. 3). The minima at 560 and 690 nm probably arise from SPP Bloch waves, which are determined by the high reflectance of SiO 2 /Al 2 O 3 and Al 2 O 3 /Ag interfaces, respectively47. Our simulations show that the spectral positions of the two minima are blue/red shifted accordingly with decreasing/increasing the lattice spacing in the y direction, enabling a high tunability of the resonant transmission mode (Supplementary Fig. 4). The wide transmission peak is, however, advantageous to fit for the wide PL spectra of TMDC monolayers that are usually broadened by the disordered potential of chalcogenide vacancies and other impurities48. The peak position for the TE polarization is changed from 580, 630 to 670 nm when θ is increased from 30°, 45° to 55°, while the transmittance of the TM polarization remains low and flat, as shown in Fig. 3d. This is of utmost importance for the polarization modulation of the PL emission through the geometrical engineering of the plasmonic nanostructure.

To simulate the experimental configuration, backward reflectance spectra of three types of sawtooth nanoslits on Al 2 O 3 /SiO 2 /Si substrates are calculated as shown in Fig. 3e, f. Despite the shift of reflectance dips coincides well with that of the resonant transmission peaks in Fig. 3c, the dips become shaper when the silicon substrate is included, suggesting the enhancement of surface plasmon resonances.

In summary, different optical paths are explicitly analyzed: (1) optical mode around the pump wavelength can be excited in the nanoslits-Al 2 O 3 nanostructure to fit the excitation path, whose spectral position is determined by the thickness of the Al 2 O 3 layer. (2) When the SiO 2 is assembled under the Al 2 O 3 , the optical mode is moved to the emission wavelength of WS 2 excitons and behaves as a broad and high transmission peak polarized in the TE direction. The spectral position and FWHM of the transmission peak change dramatically with the slanting angle θ, while the transmittance keeps low and flat for the TM polarized incidence, resulting in a big transmittance difference between x and y polarizations at the emission wavelength. The polarized resonant transmission properties of the sawtooth nanoslit array allow for manipulating the polarization of the PL emission through a well designed plasmon-exciton hybrid system.

Polarization modulation of the excitonic emission

To verify the polarization responses of the excitonic emission, a WS 2 -Ag hybrid system is fabricated (see the Methods section). The optical image of a CVD grown WS 2 monolayer is shown in Fig. 4a exhibiting a regular triangle shape with a uniform surface. The SEM images of three types of sawtooth nanoslits are shown in Fig. 4b–d in which the slanting angle of the middle slit is 30°, 45°, and 55°, respectively. The hybrid nanostructure was measured by the polarization resolved PL spectroscopy (see the Methods section), which was excited by a 532 nm laser along the backward direction with the incident polarization fixed in the TE (0°) direction. The emitted photons passing through the sawtooth nanoslit array were then collected by the spectrometer.

Fig. 4: Polarization resolved PL characterization. a Optical image of a bare WS 2 monolayer (scale bar is 10 μm). b–d SEM images of silver sawtooth nanoslits with θ = 30° (b), 45° (c) and 55° (d), respectively. Scale bar is 500 nm. The inserts at the top right corner show two unit cells of the corresponding array. e–h Normalized PL spectra of bare WS 2 monolayer and three hybrid nanostructures with a peak at ~636 nm for the detection angle along TE (black curve) and TM (red curve) polarization directions respectively. i–l Normalized PL peak intensity, as a function of the detection angle for a given incident laser polarization (marked by the green arrow) for WS 2 monolayer (i) and three hybrid nanostructures (j–l), respectively. 0 corresponds to the center and 1.0 to the outermost dashed line. Full size image

As a reference, PL spectra of a bare WS 2 monolayer are measured. The emission in TE and TM directions are denoted by the black and red curves in Fig. 4e, respectively, where no difference could be found. The intensity is further characterized as a function of the collection angle of the analyzer. As expected, no anisotropy is observed as shown in Fig. 4i, indicating that the PL of the bare WS 2 monolayer is indeed randomly polarized at room temperature. In contrast, with silver sawtooth nanoslits on top, the difference of the PL intensity between TE and TM is significantly enhanced as shown in Fig. 4f–h, where each graph corresponds to a specified angle of the slanted nanoslit. The angular dependences of the emission intensity are plotted in Fig. 4j–l, respectively. One would find that the difference between TE and TM is relatively small for the sawtooth nanoslit array with θ = 30° (Fig. 4f, j), plausibly because the resonant transmission peak at 580 nm (black curve in Fig. 3c) is far away from the emission wavelength at ~630 nm. So when the resonant transmission peak of the sawtooth nanoslit array (θ = 45°) is shifted to 630 nm (red curve in Fig. 3c), the striking difference is observed as shown in Fig. 4g, k. Further increase of the slanting angle, the difference starts to degrade (Fig. 4h, l). As such, by integrating silver sawtooth nanoslits above the WS 2 monolayer, a linear polarization of the PL emission appears, and can be further optimized by tuning the geometrical parameter, and hence the resonant transmission modes of the sawtooth nanoslit array.

To gain deeper understanding of the modulated polarization responses of the excitonic emission, polarization resolved reflectance spectra of three hybrid nanostructures were measured with a white light illumination. As shown in Fig. 5a, pronounced reflectance dips are observed in the spectra of the TE polarization, suggesting the excitation of the strong resonant transmission modes. In contrast, reflectance spectra along the TM polarization are higher in magnitude and more flat across the measured spectrum (Fig. 5b). Consequently, the PL emission in the TE polarization direction can be enhanced while that of the TM polarization cannot, resulting in the polarized resonant emission. We note that the measured reflectance dips (Fig. 5a) are much broader than the theoretical simulation (Fig. 3e), which can be accounted by the size variations of sawtooth nanoslits owing to the imperfect nanofabrication. In addition, the reflectance dip shifts from 580, 630, to 670 nm when θ is changed from 30°, 45°, to 55° as denoted by the black, red and blue curves in Fig. 5a, respectively. The shift of reflection dips is consistent with the numerical simulations (Fig. 3e), manifesting the desirable structural engineering of the metasurface. The spectral shift can be easier captured, as shown in Fig. 5c, by the reflectance difference defined as (R TM − R TE )/(R TM + R TE ), where the magnitude, i.e. the polarization anisotropy, rises from 0.4, 0.5 to 0.6 as the slanting angle increases.

Fig. 5: Plasmon and exciton coupling in three types of WS 2 -Ag hybrid nanostructures. Reflectance spectra are measured for the TE polarized light incidence in (a) and TM polarized in (b) for varying slanting angles. c Reflectance difference between TE and TM polarizations. The differential reflectivity of the WS 2 monolayer is depicted by the purple curve. d Measured linear dichroism of the PL emission. e, f Intensity comparison among the three hybrid nanostructures for TE and TM polarized PL emissions, respectively. Full size image

To highlight the effect of anisotropic resonances on the excitonic emission, we also plot the differential reflectivity that characterizes the absorption feature of the WS 2 monolayer, as shown by the purple curve in Fig. 5c. The peak at 630 nm matches the direct band gap transition of the WS 2 excitons, coinciding well with the peak of the reflectance difference for the sawtooth nanoslit array with θ = 45° but deviating largely from that of θ = 30° and 55°. Consequently, one would obtain different coupling strength between the plasmon and the exciton, leading to different linear dichroism (LD) of the PL emission \(\left( {{\mathrm{LD}} = (I_{{\mathrm{TE}}} - I_{{\mathrm{TM}}})/(I_{{\mathrm{TE}}} + I_{{\mathrm{TM}}})} \right)\) as shown in Fig. 5d. The highest linear dichroism is generated by the sawtooth nanoslits with θ = 45° rather than 55°, despite the latter has the maximum polarization anisotropy in the plasmon mode. This is because the spectral overlap is much more crucial for the resonant coupling, through which the polarization of plasmons can be thoroughly inherited by the excitons.

To quantify the coupling with plasmonic modes, we investigate the evolution of PL intensity of three hybrid nanostructures for TE and TM polarizations as a function of the slanting angle. For TE polarization, the hybrid nanostructure with θ = 30° has the lowest PL as denoted by the black curve in Fig. 5e. This is due to the fact that the nanoslit array with 30° slanting angle has a high reflectance at the pump wavelength (black curve in Fig. 3a) and a low transmittance at the emission wavelength (black curve in Fig. 3c), leading to the less efficient excitation and emission. The intensity increases dramatically (2.8 times) when the slanting angle θ is changed to 45° (red curve in Fig. 5e), which is attributed to the better coupling of WS 2 excitons with the resonant transmission modes of sawtooth nanoslits at both laser excitation (red curve in Fig. 3a) and PL emission (red curve in Fig. 3c) wavelengths. The PL intensity exhibits the largest enhancement (4.2 times) when θ is increased to 55° as shown by the blue curve in Fig. 5e, even though the simulated transmittance is not increased as much (blue curve in Fig. 3c). The discrepancy can be ascribed to the increase of the slit width in the experiment, which are narrower and fixed for three hybrid structures in the simulation, as wider slits allow more energy passing through therefore leading to stronger emission.

For TM polarization, the PL intensity is much lower than that of the TE polarization. Specifically, the lowest PL intensity is from the nanostructure of θ = 30° due to the large deviation of the transmission mode from the emission wavelength and the lowest transmittance among the three nanostructures as shown in Fig. 3b, d. The PL intensity remains extremely low when θ is increased to 45°, which is six-fold lower than that of the TE polarization (red curve in Fig. 5f). It suggests the emission in the TM polarization direction can be effectively suppressed by choosing a proper geometrical parameter of the sawtooth nanoslit array, while the emission in the TE polarization direction keeps being high, thereby leading to the largest linear dichroism as shown by the red curve in Fig. 5c. In case of θ = 55°, despite the PL intensity in the TM polarization direction is 2.4 times that of θ = 45°, the PL in the TE direction is merely 1.5 times (blue curve in Fig. 5e), hence the linear dichroism is actually decreased. In short, to increase the linearity of the PL emission, one needs to enhance the PL emission in one polarization (TE here), and minimize it in the perpendicular direction at the same time, both of which require the coherent coupling between excitons and anisotropic resonant transmission modes.

Linear polarization from the valley coherence

Besides the anisotropic resonant transmission modes, the other contributor to the large linear polarization is the coherent superposition of the excitonic emission from K/K′ valleys, which is enhanced by coupling of the valley excitons with the ultrafast plasmonic nanocavity. To extract the contribution from the valley coherence, we compare the PL spectra of the sawtooth nanoslit array with and without integration of the WS 2 monolayer. In the bare array, the fluorescent moieties of the embedded PMMA residue show broad PL, whose polarization reflects purely the anisotropic transmittance of the metasurface since valley is absent in this organics. As shown in Fig. 6a, the linear dichroism is compared for both WS 2 -Ag and organics-Ag hybrid nanostructures by measuring the PL spectra along TE and TM polarization directions (Supplementary Fig. 5 and Supplementary Note 1). At the excitonic emission of the WS 2 monolayer, the linear dichroism of the WS 2 -Ag hybrid is clearly higher than that of the organics-Ag hybrid, while they almost coincide with each other elsewhere. It is easy to understand the coincidence across most of the measured spectra, as they both originate from the anisotropic transmittance of the nanostructure. However, the difference—the peak around 636 nm—must be related to the intrinsic valley properties of WS 2 itself, which can be attributed to the plasmon enhanced valley coherence. The polarization from the valley coherence can be evaluated quantitatively by \({\mathrm{LD}}_{{\mathrm{valley}}} = {\mathrm{LD}}_{{\mathrm{WS}}_2/{\mathrm{Ag}}}-{\mathrm{LD}}_{{\mathrm{Organics}}/{\mathrm{Ag}}}\), which is ~0.2 (pink curve in Fig. 6b) and contributes about 30% of the total linear dichroism (green curve).