Key Laboratory of Light-Field Manipulation and Information Acquisition, Ministry of Industry and Information Technology, and Shaanxi Key Laboratory of Optical Information Technology, School of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710129, China
The coupling between surface plasmons and excitons in transition metal dichalcogenides (TMDs) plays crucial roles in light emission, nonlinear optics, and quantum information processing. However, the intermediate plasmon–exciton coupling has not been reported in the TMD-integrated metallic nanoarray. Herein, we demonstrate the intermediate coupling behavior between surface plasmons in the silver nanogroove array and excitons in 2D layered tungsten disulfide (). The results show that the reflection spectra of the silver nanogroove array possess an obvious reflection dip at the wavelength of due to the generation of surface plasmons. The experiment results are well consistent with the numerical simulations. When the silver nanoarray is integrated with a trilayer , there exists a distinct coupling between surface plasmons and A excitons in . The temporal coupled-mode theory analysis shows that the plasmon–exciton coupling locates in the intermediate plasmon–exciton coupling region. The intermediate coupling can give rise to the strong photoluminescence (PL) enhancement of 48-fold in . The wavelength of the PL peak presents a red shift with the increase of the temperature. This work paves a new pathway for the generation of plasmon–exciton coupling and the PL enhancement in atomic-layer TMDs.
【AIGC One Sentence Reading】:Intermediate plasmon-exciton coupling in WS2 on silver nanogroove array boosts photoluminescence by 48-fold.
【AIGC Short Abstract】:This study reveals intermediate plasmon-exciton coupling in a WS2 atomic layer integrated on a silver nanogroove array, leading to a 48-fold enhancement in photoluminescence. The coupling is analyzed using temporal coupled-mode theory and confirmed by experiments and simulations, opening new avenues for plasmon-exciton interactions and light emission enhancement in TMDs.
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1. INTRODUCTION
The coupling effects between the photons and excitons in microscale/nanoscale structures with light emitters currently attract broad attention due to their promising applications in quantum information, nonlinear optics, light emission, optoelectronic integrated devices, etc. [1–6]. Surface plasmons are light-driven free electron oscillation generated at the interface between the metal and dielectric with the excellent advantages of overcoming the diffraction limit, enhancing the near-field light intensity, and confining light at the subwavelength scale [7–11]. Nowadays, surface plasmons play important roles in the development of microphotonics/nanophotonics and find crucial applications in enhancing PL emission [12], photocatalysis [13], highly sensitive sensing [14,15], photonic integration [16], light-field manipulation [17], and super-resolution optical imaging [18]. Metallic structures provide the significant and effective platform for the coupling interactions between photons and excitons in light emitters (e.g., 2D semiconductors) due to the generation of surface plasmons with the small mode volume and strong field enhancement [19,20]. Currently, TMDs as typical 2D semiconductors attract extensive attention due to their unique electronic, chemical, and mechanical properties [21–25]. TMDs with the molecular formula (M is a transition metal element like W or Mo and X is a chalcogen element like S or Se) possess controllable bandgap property that depends on their layer number [22–26]. In particular, TMDs present a large exciton binding energy owing to the quantum confinement effect within the 2D structure, which significantly facilitates the coupling interaction between the photons and excitons [27,28]. Until now, the coupling interactions between surface plasmons and excitons in TMDs have been generally observed in weak and strong coupling regimes [29–35]. Initially, the weak plasmon–exciton coupling was employed to achieve the enhancement of PL emission from the TMD atomic layers due to the Purcell effects in the metallic particles and nanoarrays [34,35]. The strong plasmon–exciton coupling reported in metallic nanoparticle/mirror and nanoarrays with 2D TMDs presents the distinct Rabi splitting at room temperature with the generation of the novel half-light and half-matter hybrid state (i.e., plexciton) [29–31,36]. Currently, strong plasmon–exciton coupling receives special attention due to the application respects in room temperature Bose–Einstein condensates [37], single-photon source [38], quantum information processing [39], etc. Recently, the plasmon–exciton coupling in the intermediate regime has been reported in metallic nanoparticle/mirror structures with the generation of Fano resonance interference, which is particularly significant for extremely improving the PL enhancement of TMD atomic layers [40,41]. However, the intermediate plasmon–exciton coupling interactions have not been reported in metallic nanoarrays with TMD atomic layers.
Herein, we demonstrate, for the first time to our knowledge, the intermediate coupling between surface plasmons in the silver nanogroove array and excitons in the atomic layer. The simulated reflection spectrum of the silver nanogroove array exhibits a distinct reflection dip at the wavelength of 630 nm with the generation of surface plasmons, which is in excellent agreement with the experimental measurement. When the silver nanogroove array fabricated by focused ion beam (FIB) lithography is integrated with a trilayer , the coupling interactions can be generated between the surface plasmons and A excitons. The coupling locates in the intermediate coupling regime according to the theoretical analysis. Moreover, we find that the intermediate coupling can contribute to the strong PL enhancement of 48-fold in and the red shift of the PL peak wavelength with the rise of the temperature. This work will open a new door for the intermediate plasmon–exciton coupling and the PL enhancement in atomic-layer TMDs.
2. SILVER NANOGROOVE ARRAY FOR SURFACE PLASMONS
Figure 1(a) shows the schematic of the silver nanogroove array on a substrate, where , , and denote the pitch, width, and depth of the silver nanogroove array, respectively. is the thickness of the silver film. To explore the optical response of the nanogroove array, the finite-difference time-domain (FDTD) method is used to numerically simulate the reflection spectra and field distributions [42]. In the simulation model, the planes vertical to the axis are set as perfectly matched layer boundary conditions, and the other planes are set as periodic boundary conditions. The plane light wave is impinged onto the silver nanogroove array, as shown in Fig. 1(a). The mesh sizes are set as . The dielectric constant of silver is set as the measured data [43]. The refractive index of is set as 1.46 [44]. Figure 1(b) depicts the reflection spectrum of a silver nanogroove array with , , , and . It shows that there exists a distinct reflection dip at the wavelength of 630 nm. As depicted in Fig. 1(c), the electric field is mainly localized on the silver nanogroove with the generation of surface plasmons and strong field enhancement.
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Figure 1.(a) Schematic diagram of the silver nanogroove array. (b) Reflection spectrum of the silver nanogroove array with pitch , width , and depth . (c) Distribution of electric field in the silver nanogroove array at the reflection dip wavelength of 630 nm.
To verify the simulation result, we deposited the silver film on a silica substrate with the magnetron sputtering technique. The deposition pressure and power were set as 0.6 Pa and 50 W, respectively. The nanogroove array was prepared on the silver film by using FIB lithography (FEI Helios G4 CX). A scanning electron microscope (SEM, FEI Verios G4) and an atomic force microscope (AFM, Bruker Dimension FastScan) were used to characterize the surface topography and structural parameters of the silver nanogroove array. Figures 2(a) and 2(b) depict the SEM and AFM images of a part of the FIB-fabricated silver nanogroove array, respectively. Figure 2(c) shows the AFM-measured height profile along the white line in Fig. 2(b). It shows that the pitch, width, and depth of the silver nanogroove array are , , and , respectively. Figure 2(d) shows the reflection spectrum of the fabricated silver nanogroove array by using the confocal Raman microscope system (WITec alpha300R) with the objective lens () and white light source. The reflection spectrum was obtained by using the ratio of reflection signals of the nanogroove array to that of a metallic (silver) mirror [31]. It can be found that the spectrum possesses an obvious reflection dip at the wavelength of 630 nm, which is well consistent with the simulation result. The reflection dip possesses a good robustness to the fabrication deviation of nanogrooves.
Figure 2.(a) SEM image of the fabricated silver nanogroove array. (b) AFM image of the silver nanogroove array. (c) AFM-measured height profile along the white line in (b). (d) Experimentally measured reflection spectrum of the silver nanogroove array with , , , and .
Subsequently, we integrated a atomic layer onto the silver nanogroove array to utilize the strong field enhancement of surface plasmons for the improvement of light–matter interactions. Figure 3(a) shows the schematic diagram of the nanogroove array hybrid structure. The atomic layer can be prepared by the mechanical exfoliation method, and then transferred onto the silver nanogroove array using a three-dimensional fixed-point transfer system. Figure 3(b) shows the optical microscope image of the atomic layer transferred on the silver nanogroove array. We measured the Raman spectrum of on the silver nanogroove using the confocal Raman microscope system with the 532 nm laser.
Figure 3.(a) Schematic diagram of the nanogroove array hybrid structure. (b) Optical microscope image of the atomic layer transferred on the silver nanogroove array. (c) Raman spectrum of the atomic layer. (d) Measured and fitting reflection spectra of the nanogroove array structure with , , , and . Here, the red curve denotes the reflection spectrum fitted by using temporal coupled-mode theory.
As shown in Fig. 3(c), there are two obvious Raman peaks at 419.9 and , which correspond to the out-of-plane vibration mode and in-plane vibration mode of , respectively. The frequency difference between the Raman modes and is , which agrees well with the reported value of a trilayer [45]. The reflection spectrum of the hybrid structure was measured by the confocal Raman microscope system, as shown in Fig. 3(d). Compared with the spectrum in Fig. 2(d), there is an obvious reflection peak at the wavelength of 635 nm in the original reflection dip. This is attributed to the coupling interaction between the A excitons in trilayer and surface plasmons on the silver nanogroove array.
To analyze the coupling interactions between surface plasmons and excitons in the atomic layer, we can consider the surface plasmons and excitons as two oscillators. According to the temporal coupled-mode theory, the temporal equations of amplitudes and for plasmonic and exciton resonance modes can be respectively described as [46,47] where and stand for the resonance frequencies of surface plasmons in the silver nanogroove array and excitons in , respectively. and are the decay rates of surface plasmons in the silver nanogroove array caused by radiation and loss, respectively. denotes the total decay rate of surface plasmons. represents the decay rate caused by the loss of exciton resonance in , and is the coupling strength between surface plasmons and excitons. and represent the amplitudes of input and reflection waves above the silver nanogroove array, which satisfy the relationship . When and , the reflection spectrum of the hybrid structure can be expressed as [47]
By using the above formula, we fitted the experimentally measured reflection spectrum, as depicted in Fig. 3(d). The theoretical fitting result is in good agreement with the measurement. The fitting results show that the coupling strength, decay rate of surface plasmons, and decay rate of excitons are , , and . The coupling parameters satisfy the relationship and . According to the coupling criterion in Refs. [40,41], the coupling interaction between surface plasmons in the silver nanogroove array and excitons in the atomic layer locates in the intermediate coupling region. Meanwhile, we demonstrated the dependence of the plasmon–exciton coupling response on the temperature. From Fig. 4, we can see the measured reflection spectra from 293 to 343 K. It is found that the wavelengths of reflection peak and dips gradually red shift with the rise of temperature. This may be attributed to the red shift of A excitons in with the increasing temperature [48]. With the increase of temperature from 293 to 343 K, the decay rates and gradually increase and decrease, respectively.
Figure 4.Experimentally measured reflection spectra of the nanogroove array hybrid structure with different temperatures.
Finally, we experimentally measured the PL spectra of the atomic layer by using the confocal Raman microscope system, as shown in Fig. 5. We find that the PL spectra of present an obvious PL emission peak in the visible region. Simultaneously, the observed PL intensity of the can be reinforced on the silver nanogroove array. The inset of Fig. 5(a) shows the PL intensity mapping image of the marked region in Fig. 3(b) with the spectral integration from 620 to 640 nm. The PL emission of can be strongly enhanced by 48-fold compared to the atomic layer without the silver nanogroove array. This enhancement is remarkably comparable when compared with the previous reports in plasmonic systems [49–52]. Subsequently, we demonstrated the PL emission spectra from the atomic layer on the silver nanogroove array with different temperatures. Figure 5(b) shows the PL spectra from the on the silver nanogroove array from 293 to 343 K. The PL emission peak presents a gradual red shift with the rise of temperature. This result verifies the alteration of coupling spectra in Fig. 4 with the red shift of the exciton wavelength. We also find that the PL emission can be further improved with the higher temperature (e.g., 343 K). At this temperature, there still exists the strong enhancement of PL on the silver nanogroove array. The PL intensities of the CVD-grown and layers on dielectric substrates present a decrease with the increasing temperature [53,54]. But the enhanced PL intensity in the exfoliated on the silver nanogrooves depends on the plasmon–exciton coupling [55]. The PL intensity has an increase with the increasing temperature from 293 to 343 K.
Figure 5.(a) PL spectra of the atomic layer without and with the nanogroove array. The inset shows the PL intensity mapping image of the marked region in Fig. 3(b) spectrally integrated from 620 to 640 nm. (b) PL spectra of on the silver nanogroove array with different temperatures.
We designed and fabricated the nanogroove array structure on the silver film by using the magnetron sputtering and FIB lithography techniques. The experimental measurement shows that the reflection spectrum presents the obvious dip at the wavelength of 630 nm, which is well consistent with the FDTD simulation. The reflection dip contributes to the generation of surface plasmons on the silver nanogroove array with the strong near-field enhancement. To explore the plasmon-enhanced light–matter interactions, we integrated the atomic layer onto the silver nanogroove array and observed the distinct coupling between surface plasmons and A excitons in the atomic layer. According to the theoretical analysis, the coupling interaction between surface plasmons and excitons locates in the intermediate coupling region. The plasmon–exciton coupling spectra are dependent on the temperature, whose rising can induce the red shift of reflection peak and dips. We also found that the plasmon–exciton coupling gives rise to the strong PL enhancement of 48-fold from the TMDs. Moreover, the PL peak wavelength and position can be effectively tuned by controlling the temperature. With a higher temperature (e.g., 343 K), the PL emission of can be further enhanced on the silver nanogroove array. The PL peak wavelength possesses a red shift with the rise of temperature. This work will provide a novel approach for realizing the intermediate coupling between surface plasmons and excitons in metallic arrays and improving the PL enhancement in atomic-layer TMDs.
Acknowledgment
Acknowledgment. The authors thank the Analytical & Testing Center of Northwestern Polytechnical University (NPU) for the AFM, SEM, and Raman measurements and FIB fabrications.