Simultaneously enhanced linear and nonlinear photon generations from WS₂ by using dielectric circular Bragg resonators

1 Introduction

The monolayer transition metal dichalcogenides (TMDs) [1], [2], [3], [4], [5], such as MoS₂, MoSe₂, tungsten disulfide (WS₂) and WSe₂, are emerging platforms to study light–matter interaction at nanoscale. The exceptional optical properties, such as near-unity high quantum yield [6], nonblinking photon emission [7], valley polarization [8], stimulated emission [9], single-photon emission [10] and exciton–polariton emission [11], make them promising candidates for next-generation integrated optoelectronics [5]. Moreover, the monolayer TMDs have also exhibit superior performances in nonlinear optics [12]. A recent study on WS₂ has revealed large second-order nonlinear susceptibility due to the lack of central-symmetry [13], [14]. The second-order nonlinear susceptibility of pure WS₂ has been reported to be as large as 4.5 nm/V, several orders of magnitude larger than that of a conventional nonlinear crystal [13]. The giant nonlinear susceptibility together with the strong emission makes monolayer WS₂ an outstanding platform for frequency doubling, ultra-low threshold lasing and solar cells. However, the performances of atomically thin TMDs as light-emitting devices and non-linear optical materials are significantly limited by the weak light–matter interaction due to their atomic thickness.

The photonic waveguides [15], [16], optical fibers [17], [18], plasmonic structures [19], [20], [21] and microcavities [22], [23], [24], [25] have been successfully demonstrated previously to manipulate the photon emission from TMDs. Among them, circular Bragg resonator (CBR) [26] features a small effective mode volume and can produce high photon extraction efficiency over a broad bandwidth. CBRs are straightforward to fabricate and have been widely employed to enhance the single-photon collection from quantum dots [27], [28] and nitrogen vacancy centers in diamond [29]. In addition, the CBRs have also been recently shown to be particularly attractive and promising for the integration with 2D materials [30].

Here, we demonstrate a monolayer WS₂ coupled to a silicon nitride (SiN) CBR. This hybrid system simultaneously experiences pronounced enhancements of both photo-luminescence (PL) and second harmonic generation (SHG) at the CBR resonance when injecting low-power of green laser and high-power of near infrared laser, respectively. This work may offer a viable solution of building compact and efficient linear and nonlinear optoelectronic devices by the fusion between 2D material of integrated photonic devices.

2 Materials and methods

Within monolayer (1L) WS₂ crystal, as shown in Figure 1(a), sulfur (S) and tungsten (W) atoms are covalently bonded and alternatively arranged in a honeycomb structure. Monolayer WS₂ is a direct band gap material, exhibiting strong PL [4]. It also has a large second-order nonlinearity for the generation of SHG [13] owe to its non-centrosymmetric crystalline structure. A schematic in Figure 1(a) lower panel shows the operation mechanism of PL and SHG. In our experiment, monolayer WS₂ crystals were obtained by mechanically exfoliating its naturally occurring bulk crystalline counterpart using scotch tape and subsequently released onto a SiO₂/Si substrate [31]. The optical image of an exfoliated flake is shown in Figure 1(b), in which the dashed line highlights the monolayer WS₂. A fluorescent image of the flake, excited by ultra-violet (UV) light (A Tungsten-Halogen Lamp [400–2000 nm] filtered by a 450 nm cut-off short pass filter.), is shown in the inset, where the monolayer WS₂ exhibits significantly stronger intensity. The PL spectrum is collected from a home-built confocal microscope. A 532 nm continuous wave laser (∼30 μW) is used to excite the WS₂ via a 50× air objective (numerical aperture, NA = 0.9). The PL spectrum is fitted with a Lorentzian model and is comprised of two components, i.e. neutral exciton and trion emission [32]. As shown in Figure 1(c), the PL spectrum consists of a strong neutral exciton emission centered at ∼614 nm and a weak trion emission peaked at ∼626 nm.

Figure 1:

Optical characterizations of monolayer WS₂. (a) Schematic illustration of non-centrosymmetric monolayer WS₂ crystal lattice structure. (b) Optical image of a WS₂ mechanically exfoliated sample sitting on top of a SiO₂/Si substrate, dash line highlights the monolayer region. Scale bar: 5 μm. Inset: fluorescent image of the WS₂ flake. The red area in the inset, showing significantly brighter emission denotes the monolayer WS₂ emission illuminated by UV light. (c) Typical monolayer PL spectrum under 532 nm excitation showing a single strong neutral exciton emission centered at ∼614 nm and a weak trion emission peaked at ∼626 nm.

We design SiN (Si₃N₄) CBR structures to match the monolayer WS₂ emission. The resonator has a sub-micro disk surrounded by a few concentric rings, acting as antennas. The circular gratings operate under the second order Bragg condition and provide reflective feedback to the cavity, leading to near-vertical upward scattering of the photons, as schematically illustrated in Figure 2(a). Compared to the integrated photonic structures in literature [9], [22], [23], [33], the CBRs are easy to fabricate and can simultaneously enhance the spontaneous emission rate and improve the emission directionality to the free-space over a broad bandwidth. To get a suitable dimension of the CBR, we first simulate the intrinsic optical properties of the CBR. The simulated device consists of a 160 nm thick Si₃N₄ (n = 2.02), a 580 nm thick SiO₂ layer (n = 1.45) and a silicon substrate. The period and the filling faction are set to 375 and 100 nm, respectively, to meet the second-order Bragg condition. The calculated Purcell factors corresponding to the cavity resonances and the reflection band of 1D grating are presented in Figure 2(b). A clear resonance peak centered at 615 nm with a maximum Purcell factor of ∼7 is shown when the central disk radius is 250 nm. The gratings also have a broadband reflection of up to 0.8. Using these parameters, we further construct 3D far-field intensity distribution by finite difference time domain (FDTD). Figure 2(c) plots the electric field profile within X-Y plane and more than 90% of the emissions are confined within an azimuthal angle of 20 degrees for the dipole on the CBR structure. Figure 2(d) shows the electric field profile of X-Z plane in logarithm scale for a CBR with a resonant wavelength of 615 nm which perfectly matches the emission wavelength of monolayer WS₂.

Figure 2:

The design and the characterization of SiN CBRs. (a) A schematic illustration of the CBR. Inset: a schematic cross-sectional view of the substrate used to fabricate the CBRs. The simulated device consists of a 160 nm Si₃N₄ (n = 2.02), a 580 nm thick SiO₂ layer (n = 1.45) and a 500 μm thick Si. (b) Calculated Purcell factors for the CBRs with different cavity radius. The period and filling fraction are 375 and 100 nm, respectively. The blue dashed line denotes the reflection band of 1D grating structure. Lateral far-field (c) and vertical (d) intensity distribution at the cavity resonant wavelength of 615 nm. R, w and p denote the radius of the center disk, the filling fraction and the period, respectively. (e) SEM image of a fabricated CBR resonator. Scale bar: 1 μm. (f) The measured radiation spectrum of the CBR mode, showing a Q factor of ∼180.

The CBR structures were patterned on SiN with standard electron-beam lithography (Raith Vistec EBPG5000 + 100 kV) followed by a reactive ion etching (RIE) system (Oxford PlasmaPro System100). The E-beam photoresist was removed by an oxygen plasma RIE process. After examining the CBR with a scanning electron beam (SEM), shown in Figure 2(e), we confirm the feature size of the CBR structure perfectly matches with our design. As shown in Figure 2(f), the cavity mode of CBR is visible at ∼614.4 nm, consistent with the simulation results discussed above. The quality factor (Q) is ∼180, defined as Q=λ/Δλ, where λ is the cavity resonance wavelength and Δλ is the full-width at half-maximum (FWHM) of the resonance peak, respectively.

3 Enhanced PL

To enhance the PL of the monolayer WS₂, we then deterministically transferred it onto the center of fabricated CBR structure with an all-dry transfer technique [31]. In order to prevent the degradation of the material [34], monolayer WS₂ is encapsulated by hexagonal boron nitride (hBN) flakes. A schematic of the fabricated structure is shown in Figure 3(a) and the heterostructure is confirmed with an optical microscope. An optical image is shown in Figure 3(b), where the black dashed line outlines monolayer WS₂ and the white line defines hBN flakes, respectively. To evaluate the coupling of the WS₂ emission to the CBR, optical characterizations of the device were performed with a home-built scanning confocal microscope. As plotted in Figure 3(c), the PL spectrum collected from the center of the CBR at ambient condition shows two emission peaks located at 620.5 and 632.6 nm, which are attributed to the PL of neutral exciton and trion enhanced by the CBR mode, respectively. The PL emission and CBR resonant wavelength are red-shifted by ∼6 and ∼18 nm respectively after the transfer and introduction of hBN. We further measured the PL intensity over the CBR area, as shown in inset Figure 3(c), the emission are concentrated at the center of the CBR.

Figure 3:

The characterization of the monolayer WS₂ coupled to SiN CBR. (a) Schematic diagram of the hBN-WS₂-hBN sandwich structure placed on the CBR. (b) Optical microscope image of the fabricated structure. The WS₂ and hBN layers are marked by black and white dashed lines, respectively. Scale bar: 5 μm. (c) The PL spectrum obtained from the center of the CBR at room temperature. Inset: optical micrograph of the PL intensity mapping over the area shown in (b). (d) PL emission of WS₂ on the center of the CBR structure measured from 300 to 4 K, the dot dashed line indicates the WS₂ and the long dashed line denotes the cavity mode. (e) The PL emission for WS₂ on the center of CBR measured at 4 K. The inset shows the polar plot of the PL (fixing the pumping at horizontal [x] polarization). (f) Time-resolved measurements for the WS₂ on-center and off-grating, revealing a ∼2 times reduction of the radiative lifetime.

To maximize the coupling, we tune the PL and the cavity mode of CBR by changing the sample temperature [32], [35]. The normalized temperature-dependent PL spectra of the WS₂-CBR hybrid system are plotted in Figure 3(d). The PL emission and the CBR’s resonance wavelength are highlighted with blue and purple dash lines, respectively. The PL spectrum starts to split into two peaks (trion and exciton) at 200 K. The trion peak shifts to 613 nm at 4 K. Meanwhile, the resonant wavelength of the cavity mode blue-shift from 632 nm at 300 K to 613 nm at 4 K as a result of the change of dielectric constant, which brings the trion emission into the resonance. As the tuning brings the emission line into the resonance with cavity mode, a 34 times emission enhancement is achieved. Figure 3(e) plots the PL for WS₂ on the center of CBR measured at 4 K, in which one great emission peak is observed at 613 nm. The emission is linear polarized, which is consistent with the polarization characteristic of the simulated cavity mode. Time-resolved PL measurements were carried out for the on-center and off-grating WS₂ using a 532 nm pulsed laser excitation (repetition rate 86 MHz, pulse width 12 ps) at 4 K and plotted in Figure 3(f). The fitted lifetime of the WS₂ on- and off-resonance is determined to be 24(2) and 47(3) ps, respectively, leading to a Purcell factor of ∼2 for the WS₂ emission when coupled to the cavity.

4 Enhanced SHG

We further studied SHG once the WS₂ and CBR were coupled. SHG is a fundamental second-order nonlinear optical process, in which the frequency of the electromagnetic waves doubles by traveling through a nonlinear medium. The setup for measuring the SHG characteristics is schematically shown in Figure 4(a). Briefly, a tunable ps laser (∼30 mW) was coupled onto the sample via a 50× objective. The SHG signal was filtered by a short-pass dichroic mirror and then passed by a half-wave plate and a polarizer before coupling into the CCD. Since the efficiency of grating in the spectrometer is polarization dependent, we rotated the half-wave plate instead of rotating the polarizer itself to analyze the polarization of SHG in our experiment. We fixed the excitation polarization to x-axis and scan the laser wavelength from 1214 to 1240 nm and collect signals from 607 to 620 nm. Figure 4(b) shows representative spectra of the fundamental signal (red line) and SHG signal (orange line), a clear emission at 613.5 nm (FWHM is 2.43 meV) is collected when the pumping is tuned to 1233 nm (FWHM is 5.18 meV), which is a clear evidence of the frequency doubling. The FWHM of the emission is 2.43 meV which is less than half of its excitation. We further compare the intensity of the SHG signal and a normalized intensity is plotted in Figure 4(c). The SHG signals reached the maximal values at the resonant wavelength and dramatically reduced on the off-resonant conditions. The on-resonance SHG signal (at ∼613 nm) is over five times higher than the off-resonance which proves the enhancement due to the cavity resonance. The polarization dependent SHG is presented in Figure 4(d). The resonant SHG intensity is determined to be linearly polarized and is parallel to the polarization of the incident laser beam confirming that the SHG is governed by symmetry-breaking-induced nonlinearity in the monolayer WS₂ [36].

Figure 4:

The characterization of the SHG enhancement for WS₂ coupled to CBR. (a) Schematic of the setup for SHG measurement. M₁, M₂: mirrors, FM: flip mirror, DMSP: shortpass dichroic mirror for 950 nm cutoff. BS: beam splitter, HWP: half-wave plate, P: polarizer, and L₁, L₂: lenses. (b) The normalized fundamental laser spectrum and a typical normalized spectrum of the SHG signal generated from a WS₂ monolayer at the CBR cavity mode. (c) The wavelength dependence of SHG by scanning the pumping wavelength at 4 K. (d) Polar plot of the measured intensity the SHG emission as a function of the detection angle for a given incident laser polarization (marked by the arrow).

5 Conclusion

In summary, we have experimentally demonstrated enhancements of both PL and SHG on a single layered WS₂ coupled to a SiN CBR. Approximately a 34 fold of PL enhancement and a 5 times enhancement in SHG are achieved when the coupling is realized. The higher PL enhancement is attributed to the enhanced emission rate (Purcell effect) and the stronger trion emission at cryogenic temperature. The temperature-dependent emission measurement reveals the operation mechanism of the enhancement and offers a strategy that can be applied to hybrid 2D photonics systems. Our results provide a powerful tool for creating compact and efficient optoelectronic devices for future on-chip nanophotonics and non-linear optics.