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Abstract
Photon-exciton coupling behaviors in optical nanocavities attract broad attention due to their crucial applications in light manipulation and emission. Herein, we experimentally observed a Fano-like resonance with asymmetrical spectral response in an ultrathin metal-dielectric-metal (MDM) cavity integrated with an atomic-layer tungsten disulfide (WS2). The resonance wavelength of an MDM nanocavity can be flexibly controlled by adjusting dielectric layer thickness. The results measured by the home-made microscopic spectrometer agree well with the numerical simulations. A temporal coupled-mode theoretical model was established to analyze the formation mechanism of Fano resonance in the ultrathin cavity. The theoretical analysis reveals that the Fano resonance is attributed to a weak coupling between the resonance photons in the nanocavity and excitons in the WS2 atomic layer. The results will pave a new way for exciton-induced generation of Fano resonance and light spectral manipulation at the nanoscale.
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1. Introduction
Fano resonance was derived from the interference effects of continuum and discrete bound states in atomic systems, leading to asymmetric lineshapes [1,2]. Fano resonances with asymmetric spectral response have also been found in optical systems, such as microcavities, photonic crystals, and plasmonic structures [2–13]. For example, Li et al., experimentally observed Fano resonance in a single whispering-gallery microresonator [4]. Gu et al., realized Fano resonance in the side-coupled waveguide of a microring with simply inserting two air-holes in the waveguide [7]. Yang et al., found that Fano resonance could be generated in plasmonic resonators with a nanorod dimer and a nanoemitter [8]. Graphene metasurfaces enable the formation of Fano resonance with the coupling between the bright- and dark-state plasmonic resonances [11]. With asymmetric spectral profile and giant field enhancement, Fano resonances in optical systems presented crucial applications in highly sensitive optical sensors [12–14], nonlinear optical conversion [15], and optical switches [16]. Fano resonances in optical systems generally result from the photonic or plasmonic coupling and interference [4,7,8,11–16]. Recently, the novel mechanisms of Fano resonances have attracted significant attentions in the quantum emitter-involved optical systems [17–20]. The interaction between photons in the single silicon nanogroove and J-aggregate excitons contributes to the realization of Fano resonance [18]. The excitons (electron-hole pairs bound by Coulomb interaction) in quantum emitters offer a fantastic type of particles for enriching light-matter interactions [18–20]. Two-dimensional (2D) transition metal dichalcogenides (TMDs) (e.g., MoS2, WS2, WSe2, and MoSe2) are an emerging kind of atomic-layer semiconductors with excellent electrical, optical, and chemical characteristics. They provide an exciting platform for achieving novel physical phenomena and promising electronic and nanophotonic applications [21–25]. The special layer-dependent bandgap structures of TMDs can contribute to the realization of optoelectronic devices, such as high-performance light emitters, transistors, and modulators [22]. Quite recently, the excitons existing in TMDs even at room temperature have been employed to achieve the formation of Fano resonances in the plasmonic structures [19–21]. Lee et al., reported the plasmon-exciton interference induced Fano reflection spectrum in a silver-bowtie nanoantenna array integrated with chemically grown MoS2 monolayer [19]. Wang et al., observed a tunable Fano resonance with plasmon-exciton coupling at room temperature in single metallic nanoparticle on the WS2 monolayer [20]. Du et al., reported the ultrafast modulation of the plasmon-exciton coupling in a WS2 monolayer on metallic nanodisks with the generation of Fano resonance [21]. TMD exciton-induced Fano resonances in optical systems will be significant for exploring nanoscale photon-exciton coupling phenomena and their functional applications.
Herein, we experimentally and numerically demonstrated an exciton-induced Fano resonance with an obvious asymmetric spectral lineshape in the ultrathin MDM cavity integrated with a WS2 atomic layer. The MDM nanocavity presents Fabry-Perot (F-P) resonance, whose wavelength can be tailored by adjusting the grown thickness of dielectric layer. The temporal coupled-mode theoretical analysis illustrates that the Fano resonance derives from the weak coupling between the nanocavity resonance photons and WS2 excitons. The results will offer a new way for the generation of Fano resonance and light spectral manipulation at the nanoscale.
2. Results and analysis
As shown in Fig. 1(a), the MDM nanocavity consists of silver, indium-tin oxide (ITO), silver layers coated on a silica substrate. The thicknesses of the upper-layer silver, ITO, and lower-layer silver are denoted as d1, d2, and d3, respectively. Figure 1(b) shows the cross-section scanning electron microscopy (SEM) image of a grown MDM nanocavity. Here, the ITO and silver films were grown by using magnetron sputtering technique. The thicknesses of the films were measured by using atomic force microscope (AFM). Figure 1(c) depicts the transmission spectrum of an MDM nanocavity with d1 = 24 nm, d2 = 175 nm, and d3 = 24 nm. The spectrum shows that there exists a transmission peak at the wavelength of 843 nm. Figure 1(d) depicts the intensity distribution of electric field component Ex in the MDM nanocavity at 843 nm wavelength, which reveals the appearance of F-P resonance in the nanocavity. The electric field was numerically calculated by using finite-difference time-domain (FDTD) simulation [26]. In the simulation, the periodic boundary condition was set at the left and right sides of computational space, and the perfectly matched layer absorbing boundary condition was set at the upper and lower sides. The non-uniform mesh was employed, and the maximum mesh steps of ITO and silver layers were set as Δx=Δy = 2 nm. The simulation time is set as 3000 fs. The optical constant of the ITO material was set as the experimental data [27]. The relative permittivity of silver was measured by ellipsometer and fitted with the Drude model ɛ(ω)=ɛ∞-ωp2/(iγω+ω2). Here, ɛ$_{\infty}$=3.893, ωp = 8.575 eV, and γ=0.116 eV. The refractive index of SiO2 is set as 1.457 at the wavelengths of interest.
Fig. 1. (a) Diagram of the MDM nanocavity on a silica substrate. Here, d1, d2, and d3 denote the thicknesses of the upper-layer silver, ITO, and lower-layer silver, respectively. (b) Cross-section SEM image of a grown MDM nanocavity. (c) Transmission spectrum of the MDM nanocavity with d1 = 24 nm, d2 = 175 nm, and d3 = 24 nm. (d) Intensity distribution of electric field component Ex in the MDM nanocavity at the peak wavelength of 843 nm. The arrow denotes the direction of incident light.
To experimentally demonstrate the spectral properties of the MDM nanocavities, we established an optical path (microscopic spectrometer) with the broadband light source (WY Optics HL1000), optical fiber coupler, collimator lens, plane convex lens, linear polarizer, beam splitter, aperture stop, mirror reflector, objective lens, spectrometer, and charge coupled device. As depicted in Fig. 2, the nanocavities can be placed on the sample platform between two 50× objective lens (Nikon LU Plan 0.55 NA) and measure the transmission spectra with a spectrometer (Ocean Optics QE Pro). The transmission spectrum can be obtained by using the ratio of the spectral signal with the sample to the source signal without the sample. We fabricated the MDM nanocavities with different ITO dielectric layer thicknesses d2. Figure 3(a) depicts the transmission spectra of the MDM nanocavities with d2 = 58, 80, 106, and 122 nm. The thicknesses of silver layers are 30 nm (i.e., d1 = d3 = 30 nm). It is shown that the transmission peak presents a red shift with the increase of d2. The insets of Fig. 3(a) show the transmission color images of the MDM nanocavities, which correspond to the transmission spectra. Thus, we can control the resonance wavelength of MDM nanocavity by adjusting the grown thickness of ITO dielectric layer. Meanwhile, we numerically calculated the transmission spectra of MDM nanocavities using the FDTD simulations. As shown in Fig. 3(b), the simulation results are in good agreement with the experimental measurements. This also confirms the reliability of the home-made optical system.
Fig. 2. Optical path for measuring the transmission spectra of the MDM nanocavities. The optical path contains the broadband light source, optical fiber coupler (OFC), collimator lens (CL), plane convex lens (PCL), linear polarizer (LP), beam splitter (BS), aperture stop (AS), mirror reflector (MR), objective lens (OL), spectrometer, and charge coupled device (CCD).
Fig. 3. (a) Experimentally measured and (b) numerically simulated transmission spectra of MDM nanocavities with different ITO layer thicknesses d2 when d1 = d3 = 30 nm. The insets in (a) show the transmission color images of the MDM nanocavities.
Subsequently, we mechanically exfoliated a WS2 atomic layer from the WS2 single-crystal bulk material and integrated it into the MDM nanocavity, as depicted in Fig. 4(a). The thicknesses of upper-layer ITO, WS2, lower-layer ITO are denoted as d21, dw, and d22, respectively. Figure 4(b) shows the transmission spectra of the WS2 atomic layer on the polydimethylsiloxane (PDMS) substrate (before transfer) and ITO/silver layer (after transfer). We can see the obvious transmission dips at the wavelength of ∼623 nm, corresponding to the A exciton position of WS2 atomic layer [28]. Figure 4(c) shows the optical microscopy images of WS2 atomic layer on the PDMS substrate and in the MDM nanocavity. The structural parameters of WS2-involved MDM nanocavity are d1 = 18.5 nm, d21 = 47.5 nm, d22 = 54 nm, and d3 = 30 nm. We measured the transmission spectrum of the MDM nanocavity without the WS2 atomic layer, as shown in Fig. 4(d). The MDM nanocavity exhibits a transmission peak at 623 nm wavelength. At the resonance wavelength, the electric field intensity in the MDM nanocavity can approach the maximum value at the position of WS2 (d21 = 47.5 nm). Figure 4(e) depicts the Raman and photoluminescence (PL) spectra of the WS2 atomic layer on the PDMS substrate, which were measured by WITec confocal Raman spectroscopy with a 532 nm laser. We found that the frequency difference between the 2LA(M) and A1g modes (Raman peaks) of WS2 atomic layer is about 68.6 cm−1, which reveals the trilayer characteristic of WS2 [29]. The inset of Fig. 4(e) shows two PL emission peaks around the wavelengths of 635 and 768 nm, which are attributed to the A exciton emission in WS2 and the indirect bandgap of multilayer WS2, respectively. The PL peak at the wavelength of 768 nm agrees well with the reported result of trilayer WS2 [28]. Moreover, we experimentally measured the transmission spectrum of WS2-involved MDM nanocavity, as shown in Fig. 4(f). There exists an obvious dip at the A exciton emission wavelength (∼635 nm) of WS2 PL. Obviously, the transmission spectrum possesses an asymmetric line profile around the F-P resonance peak, different from the transmission spectrum of MDM nanocavity without WS2 in Fig. 4(d). The theoretical result published during the review process of our work shows that a F-P cavity with the 316 nm ZnS film and WS2 monolayer exhibits asymmetric spectral lineshape [30]. It verifies the reliability of our experimental results.
Fig. 4. (a) Diagram of the MDM nanocavity with a WS2 atomic layer. The thicknesses of upper-layer ITO, WS2, and lower-layer ITO are denoted as d21, dw, and d22, respectively. (b) Transmission spectra of the WS2 atomic layer on the PDMS substrate and ITO/silver layer. (c) Optical microscopy images of the WS2 atomic layer on the PDMS substrate and in the MDM nanocavity with d1 = 18.5 nm, d21 = 47.5 nm, d22 = 54 nm, and d3 = 30 nm. The scale bar is 10 µm. (d) Experimentally measured transmission spectrum of the MDM nanocavity. (e) Raman shift spectrum and PL spectrum (the inset) of WS2 atomic layer on the PDMS substrate. (f) Experimentally measured, theoretically fitted, and numerically simulated (the inset) transmission spectra of the WS2-involved MDM nanocavity.
Moreover, we numerically calculated the transmission spectrum of WS2-involved MDM nanocavity using the FDTD method. Here, the optical constant of WS2 trilayer was set as the experimental data [31]. The thickness of WS2 trilayer was set as 1.854 nm (i.e., dw = 1.854 nm). The maximum mesh step of WS2 layer was set as 0.1 nm. As shown in the inset of Fig. 4(f), the calculated transmission spectrum exhibits a distinctly asymmetric profile, which is consistent with the experimental result. The asymmetric spectrum could be attributed to the coupling between the A excitons in the WS2 atomic layer and F-P resonance photons in the MDM nanocavity.
To clarify the generation mechanism of asymmetric spectral response, we employed the two-oscillator system to describe the coupling behavior between the WS2 excitons and MDM resonance photons. The MDM nanocavity with F-P resonance can be represented by Oscillator 1, which is driven by the incident light launched from the upper port. The WS2 atomic layer with exciton emission can be represented by Oscillator 2. The exciton emission can be excited by the F-P resonance in Oscillator 1. The resonant properties of coupled oscillator systems can be analyzed by the temporal coupled-mode theory [32,33]. Temporal evolution of the modes a and b in Oscillators 1 and 2 can be respectively described as,
Here, γ1=γ11+γ12 denotes the total decay rate of Oscillator 1. Oscillators 1 and 2 can be regarded as the radiative and subradiant elements, respectively. The decay rate γ1 of F-P resonance is about 2.18 × 1014 rad/s according to the measured transmission spectrum of MDM nanocavity in Fig. 4(d). The above theoretical equation can be employed to fit the measured transmission spectrum. As depicted in Fig. 4(f), the fitted spectrum is in accordance with the experimental measurement. According to the fitting result, the decay rate γ2 of WS2 excitons is about 1.13 × 1014 rad/s. The coupling coefficient κ is 0.88 × 1014 rad/s, which satisfies the condition: κ<(γ1+γ2)/2. The formation of asymmetric Fano resonance spectrum stems from the photon-exciton coupling interaction, which doesn’t satisfy the strict strong coupling criterion and still locates in the weak coupling regime [34]. As reported previously, the F-P resonators with dielectric distributed Bragg reflectors (DBRs) or metal-based DBR can achieve strong coupling behaviors due to the high quality factor and light confinement (enhanced field intensity) [35,36]. Their spectral splitting is of dependence on the incidence angle. Our F-P cavity with a simple MDM structure presents relatively low quality factor and light confinement. The spectral lineshape can also be tailored by changing the angle of incident light, which results from the blue shift of F-P resonance wavelength and unchanged exciton wavelength. With the oblique incidence angle, the light confinement (field intensity) will decrease in our F-P cavity, which may be unfavorable to the generation of stronger photon-exciton coupling. But we find that the photon-exciton coupling in the MDM nanocavity is dependent on the structural parameters (e.g., metallic layer thicknesses and dielectric refractive index). With selecting proper metallic layer thickness and dielectric materials, the stronger photon-exciton coupling could be observed, even with Rabi splitting. For example, the quality factor and field intensity of resonant mode can be distinctly improved when the MDM nanocavity is changed as a silver-SiO2-silver structure with d1 = 25 nm and d3 = 35 nm. The higher quality factor and field intensity of resonant mode can contribute to stronger photon-exciton coupling. The WS2-involved MDM nanocavity presents an ultrathin thickness of ∼152 nm for the generation of Fano resonance, comparable to the reported ultrathin films [37].
3. Conclusion
We have experimentally demonstrated the transmission spectral response of silver-ITO-silver (MDM) nanocavity using a home-made microscopic spectrometer system. A distinct red shift is observed for the F-P resonance-induced transmission peak as the ITO layer thickness increases, which agrees well with the FDTD simulations. By integrating a trilayer WS2 into the MDM nanocavity, we have experimentally and numerically achieved an asymmetric Fano resonance spectral lineshape around the F-P resonance. The generation of asymmetric Fano resonance spectrum can be attributed to the coupling between the F-P resonance photons in the MDM nanocavity and excitons in the WS2 atomic layer. The temporal coupled-mode theoretical analysis illustrates that the photon-exciton coupling in the WS2-involved MDM nanocavity locates in the weak coupling region. Our results will offer a new way for the exciton-induced generation of Fano resonance and light spectral manipulation at the nanoscale.
Funding
National Key R&D Program of China (2022YFA1404800); National Natural Science Foundation of China (11974283, 61705186, 11774290); “Double first-class” construction fund project (0206022GH0202); Fundamental Research Funds for the Central Universities (D5000220175).
Acknowledgments
The authors thank the Analytical & Testing Center of Northwestern Polytechnical University (NPU) for the AFM, SEM, and Raman measurements.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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