1. INTRODUCTION

Modern photoelectronic and quantum applications, such as nanolasers, photodetectors, and ultrafast optical switches, heavily depend on the precise manipulation and enhancement of light–matter interactions [1–5]. The interaction between natural materials and light is inherently constrained by their atomic composition and their arrangement, which limits their performance in practical applications. A promising solution lies in integrating materials with photonic nanostructures that support highly resonant modes, enabling spatial and spectral overlap to tailor and amplify these interactions [6–8]. Among various photonic modes, the bound states in the continuum (BIC) have garnered significant attention due to its unique properties. BIC describes nonradiative photonic states that exist within the radiation continuum (inside the light cone), offering exceptional quality () factors, tightly confined near fields, and rich topological characteristics [9–12]. To date, experimental demonstrations have successfully hybridized BIC modes with materials such as dye molecules [11,13], showing promising influences in the areas, such as biodetection [14,15], enhanced light harvesting and emission [11,13], and nonlinear optics [16] for quantum technologies.

The family of two-dimensional (2D) materials, spanning insulating hexagonal boron nitride (hBN), semiconducting transition metal dichalcogenides (TMDCs), and semimetallic graphene, provides a versatile platform for exploring light–matter interactions across a broad spectral range [17]. These materials consist of covalently bonded atomic layers held together by weak van der Waals (vdW) forces, enabling precise control over layer thickness (from monolayers to the bulk counterpart with the thickness exceeding hundreds of nanometers) and offering dangling-bond-free surfaces. This unique structural flexibility makes 2D materials ideal for integration with a wide range of photonic structures, such as waveguides [18] and photonic crystals [19,20], and allows them to be transferred onto virtually any flat substrate. Moreover, the inherent 2D nature of vdW materials grants them extraordinary photonic and electronic properties that are unattainable in their 3D counterparts, such as silicon. For example, monolayer (ML) TMDCs exhibit room-temperature excitons with direct bandgap emission and strong absorption rates of approximately 10% [21,22], while hBN and graphene support intrinsic polaritons enabled by phonons and plasmons [23]. These distinctive features significantly expand the toolkit for manipulating light–matter interactions, paving the way for groundbreaking scientific discoveries and advanced applications.

The emerging field of hybridizing BIC modes in dielectric photonic structures with vdW materials has recently achieved significant advancements in various areas, including enhanced linear and nonlinear light sources [16,24], vortex beam generation [25], and strongly coupled exciton–polaritons [26–29]. Maximizing the light–matter interactions requires both strong field confinement provided by the BIC mode and placement of the material at the field maximum [30]. However, in many hybrid systems, 2D materials are positioned adjacent to the external photonic structure, often separated by a buffer layer of hexagonal boron nitride (hBN) [16], interacting merely with the evanescent field near the surface.

In contrast to using the external cavity, the nanophotonic structures fabricated directly from the vdW materials themselves enable the intrinsic light–matter interaction. That is, the highly resonant mode supported by the structure would intrinsically couple to the host material, which helps to achieve, for instance, self-hybridized polaritons in the monolithic vdW structures [31–36]. The light–matter interaction is thus enhanced when the maximum field inside the structure inherently couples with the material transitions.

So far, the Mie particles and metasurfaces made of TMDC materials such as  have been demonstrated, while only showing relatively low -factors () [31–36]. By using the concept of quasi-BIC through precisely controlling the asymmetric geometric parameters, the vdW metasurfaces with controlled  factors () and enhanced intrinsic light–matter interactions have been realized in the visible or the near-infrared range [37,38]. Nevertheless, achieving this level of control demands high fabrication precision. The anisotropic lattice symmetry and varying enthalpies of formation across different materials pose significant fabrication challenges, often resulting in uncontrolled shapes and low precision in the final nanostructures after standard etching processes [39]. These issues are even exacerbated for smaller dimensions and thicker materials. Additionally, the chemically active sidewalls created by etching are more reactive than the pristine vdW surfaces, leading to undesirable chemical reactions in materials such as  and a reduction in excitonic oscillator strength [36,40]. Together, these factors impose severe limitations on etched vdW metasurfaces, restricting material choices [37,38] and requiring stringent fabrication conditions. These constraints hinder broader applications where many other vdW materials will be involved.

In this work, we propose an alternative to the traditional “real etching” procedure by introducing the etch-free vdW metasurfaces that achieve intrinsic light–matter interaction through a perturbed layer made of a low-refractive-index (LRI) photoresist. This approach mimics the optical behavior of shallow-etched vdW metasurfaces with reduced scattering loss by the periodic structures. As a proof of concept, we implement a grating structure that supports guided mode resonance (GMR) and BIC modes. Because of the reduced scattering by the LRI nanostructures, the measured  factor of the GMR mode at normal incidence approaches 348, comparable to the highest values reported in the etched TMDC metasurfaces [37,38] in the near-infrared range. Based on this etch-free platform, the self-hybridized exciton–polaritons are demonstrated in four representative TMDCs (, and ) through angle-resolved transmission measurements. An unambiguous anti-crossing behavior of exciton-BIC polaritons is revealed in  and  samples, with their Rabi splittings approaching 80 and 72 meV, respectively. Furthermore, the high- GMR mode enables over 25 times polarization-dependent enhancement and reduced linewidth to  for indirect bandgap emission from bulk , which has a low quantum yield due to the phonon assistance process. The lifetime and angle-resolved photoluminescence (PL) measurements provide a reasonable illustration for this polarization-dependent enhancement. Last but not least, an etch-free heterostructure is demonstrated with a ML  encapsulated in hBN layers, achieving plexcitons through weak coupling and demonstrating polarization-dependent direct bandgap exciton emission in ML . The proposed etch-free structure overcomes the limitation by traditional etch process in vdW nanophotonic. It provides a universal structure for achieving high- resonances in arbitrary vdW materials and their homo-/heterostructures, unlocking new possibilities in vdW nanophotonics and light–matter interactions.

2. RESULTS

A. Concept and Basic Optical Properties

As schematically illustrated in Fig.  1 (a), the etch-free structure consists of a top layer made of a LRI photoresist (e.g., ZEP 520A in this work) and a bottom vdW layer (such as bulk ) possessing higher refractive index. The periodic structure formed by the LRI polymer perturbs the in-plane continuous translation symmetry along the vdW surface, converting the guided mode within the dielectric layer (i.e., the vdW layer) into a leaky mode, known as guided mode resonance or GMR. The leaky rate, or scattering radiation, is determined by the perturbation introduced by the periodic structure. Supplement 1, Fig. S2 demonstrates the evolution of the transmission with the etching depth of a general dielectric grating. As the etching depth increases, the scattering radiation is significantly enhanced, leading to a pronounced reduction in the  factor (or the increase of the linewidth). In this context, the introduction of the LRI nanostructured layer serves a similar function as the shallow etching, as it minimally perturbs the bottom HRI dielectric layer.

Fig. 1. Optical properties of an etch-free vdW grating structure. (a) Schematic illustration of the etch-free structure with a grating structure made of the LRI ZEP 520A on top of a vdW layer on a fused silica substrate. The direction perpendicular (parallel) to the grating bar is defined as - (-)direction. (b) Comparison of TE (orange curves, mode 2 and mode ) and TM (purple curves, mode 1 and mode ) modes in a etch-free and a shallow-etched  gratings. The thickness of the bottom  and geometry of the grating structure are the same for both structures. (c) Corresponding near-field distributions for the modes indicated in (b). The scale bar represents 100 nm. (d) The photonic band structures of TE modes in the  space. The thickness of the bottom layer is set as 60 nm, and the thickness of top ZEP is set as 300 nm to match the parameters in the experiment. (e) Calculated angle-resolved transmission spectra by the RCWA method using the same parameters in (d). (f) Corresponding  factor distributions calculated by eigenvalues in (d). (g) Experimentally measured angle-resolved transmission spectra of a  etch-free grating structure with the period  and filling factor . The calculation results in (d–f) adopt the same geometric parameters as (g).

To illustrate this, Fig.  1 (b) shows the transmission spectra of an etch-free structure and a shallow-etched structure made from 60 nm . The polymer thickness in the etch-free structure is set as 50 nm, while the dielectric structure is shallowly etched by 3 nm for comparison. Both structures are of the identical geometric parameters, including the same period and filling factor. The transmission spectra for TE and TM GMR modes exhibit nearly identical responses in terms of resonant frequencies, linewidths, and corresponding near field distributions [Fig.  1 (c)] where the maximum field is confined inside the HRI layer. Moreover, they exhibit almost the same dispersion in the  ( component of the incident wave vector) and  directions (Supplement 1, Fig. S4).

Figure  1 (d) exhibits the calculated photonic bands in the  direction for TE modes. According to perturbation theory [40,41], the formation of the GMR (blue line) and the BIC (red line) modes arises from the diffractive coupling of band-folded propagating modes across the Brillouin zone (see Supplement 1, Fig. S1 for details). As a result, these two bands show an anti-crossing behavior in the vicinity of  point where the quadratic dispersion is presented. With a larger , the dispersion becomes linear with , approaching the behavior of the original propagating mode.

The corresponding near-field distributions [insets of Fig.  1 (d)] illustrate the distinct photonic instincts of these modes. The BIC mode exhibits an asymmetric near-field distribution, leading to total destructive interference in the far field and rendering it optically dark (infinite  factor) under normal incidence. In contrast, the GMR mode exhibits a fully symmetric distribution, which is optically accessible. The corresponding angle-resolved transmission spectra [Fig.  1 (e)] by the rigorous coupled-wave analysis (RCWA) method unambiguously exhibit the different behaviors for these two modes. The transmission of the BIC branch vanishes when it approaches the  point, with the  factor approaching infinite [Fig.  1 (f)]. It evolves into quasi-BIC mode at oblique incidence, acquiring a finite linewidth (with  factor reduced). In comparison, the GMR mode exhibits finite linewidth at normal incidence [i.e., modes shown in Fig.  1 (b)], and the linewidth of the GMR mode approaches that of BIC with the increase of  [Fig.  1 (f), also see the measured result in Supplement 1, Fig. S5]. Figure  1 (g) shows a measured angle-resolved transmission spectrum (TE) from one of our  etch-free samples. The corresponding dispersions of BIC and GMR modes show a great match to the simulation in Fig.  1 (e). In contrast, the A exciton of  shows no dispersion. The measured result can be well reproduced by the RCWA method (see Fig. S6 in Supplement 1). In addition to TE mode, the TM photonic band structure, presented in Supplement 1, Fig. S7, exhibits a similar dispersion behavior and  factor distribution with the TE mode. Throughout this work, TE and TM modes are selectively adopted based on their respective operating frequencies.

Fig. 2. Evolution of optical properties of the etch-free structures with geometric parameters. (a) Illustration of fabrication procedures for the etch-free vdW structures. (b) Microscopic image of one of the representative etch-free TMDC samples and the schematic illustration of the structures. The scale bar is 50 µm. The period of grating, width of grating bar, and thickness of ZEP and TMDCs are defined as , , , and  separately. (c) SEM image of one of grating structures showing period . (d,e) Measured (d) and simulated (e) transmission spectra as a function of the period under TM polarization. Other geometric parameters are kept the same. (f,g) Evolution of TM (f) and TE (g) modes with TMDC thickness . The period is set as 450 nm and the filling factor is of 0.5. (h,i) Evolution of TM (h) and TE (i) modes with the filling factor . The thickness of the TMDC is set as 60 nm. (j) Evolution of  factors with the filling factor :  factors extracted from experiment measurements (orange squares),  factors extracted from the simulation results (pink curve). All the simulations and measurements are based on the etch-free  structures.

B. Fabrication and Analyses on the Geometric Parameters

As shown in Fig.  2 (a), to fabricate etch-free vdW metasurfaces, bulk (from tens to hundreds of nanometers) vdW materials such as  are first exfoliated onto a PDMS substrate using scotch tape and transferred to a marked area indicated by Au markers on a  (300 nm)/Si substrate. The thickness of the exfoliated material is confirmed through atomic force microscopy (AFM) measurements (Supplement 1, Fig. S24). Then, the sample is spin-coated with a layer of ZEP 520A photoresist at 3500 rpm for 1 min. Subsequently, a standard electron beam lithography (EBL) is performed, followed by a developing process to create the final nanopattern on the target vdW layer. This process results in a final ZEP pattern thickness of approximately 300 nm. The standard EBL procedures show a negligible influence on the optical properties of the TMDCs (as indicated in Supplement 1, Fig. S28). Figure  2 (b) shows a representative sample with the square areas indicating the grating structures. Due to the simplified fabrication procedure, the ZEP nanostructure pattern maintains a good shape as shown in the scanning electron microscope (SEM) image in Fig.  2 (c). With the period varied from 430 to 490 nm, the GMR mode (with etch-free grating structures based on ) shows a (almost) linear red shift with period in both the measured [Fig.  2 (d)] and simulated results [Fig.  2 (e)].

The thickness of the dielectric material is a key parameter to determine the optical properties. We find that the influence of thickness is varied for TM and TE modes. As exhibited in the simulated results in Fig.  2 (f), for the TM mode when the thickness is less than 35 nm, only Rayleigh anomalies (RAs) are observed [42]. While with the thickness increased beyond the critical thickness (i.e., 35 nm for the geometric parameters considered here), an unambiguous red shift with the thickness is observed. This can be understood that the magnetic mode originates from the circulation electric field where a phase retardation is required along the propagation of the incidence wave. Thus, a necessary thickness is required for the generation of the TM mode. The measured results for TM mode (Supplement 1, Fig. S8) show a good agreement of the thickness-dependent behavior indicated by the simulated results. In comparison, there is almost no thickness limitation for the generation of the TE mode [24] [Fig.  2 (g)].

Another parameter, the filling factor, defined as , determines the  factor of the GMR mode. Deviation of the value of 0.5 results in the decrease of the scattering radiation since  and  correspond to two extreme conditions, respectively when there is no perturbation (i.e., without any ZEP nanostructures or with a flat and continuous ZEP layer on the top). As clearly shown in Figs.  2 (h) and 2 (i), the linewidth is reduced when  approaches 0 or 1 for both TM and TE modes. We also notice that although TM modes show an unambiguous red shift with the increase of , the resonant frequency of TE mode is almost independent on . The measured  factor [Fig.  2 (j)] and resonance frequency (Supplement 1, Fig. S11) match the tendency indicated by the simulations. In the meanwhile, we also found that filling factor  determines the diffractive coupling in the formation of GMR and BIC mode. A band inverse takes place when  is varied across 0.5. When  is in the vicinity of 0.5, exceptional point is emergent where GMR and BIC are degenerate as indicated in Supplement 1, Fig. S9.

Although the experimental samples maintain a uniform ZEP thickness of 300 nm in this work, the influence of the ZEP thickness () is also worth analyzing. As the simulation result shown in Supplement 1, Fig. S10 indicates that, intuitively, reducing the ZEP thickness increases the  factor due to lower scattering radiation. Recent study [43] using a 50 nm thick PMMA etch-free structure on SiN has achieved  factors exceeding . However, a thinner LRI layer poses experimental challenges for the measurements due to the decreased measurement contrast of the resonant mode [44]. A trade-off between  factor and measurement contrast needs to be considered when selecting the optimal LRI thickness. Additionally, we found that for the thicknesses of LRI layer exceeding 150 nm, the optical mode becomes almost unchanged. Further optimization of the thickness of photoresist could be made to enhance the  factor.

Fig. 3. Self-hybridized polaritons in four TMDC samples. (a–d) Measured and simulated angle-resolved transmission spectra for the etch-free , and  metasurfaces. The right panel for each image shows their corresponding refractive index () and extinction coefficient (). (e–h) Calculated polaritonic dispersions [with (e) corresponding to (a), (f) corresponding to (b), (g) corresponding to (c), and (h) corresponding to (d)] based on Eq. ( 2 ), where the colormap indicates their separate photonic and excitonic constituents in the polaritonic states. The  mode is removed for the clarity of image display as it is damped in all the samples. For  and  samples, the  is grayed out as it is damped by the absorption of excitons. All the results are based on the TM modes.

Thanks to the low radiation loss in an etch-free structure, the  factors for our vdW structures reach  for TM GMR modes (Supplement 1, Fig. S21) and several hundred for TE GMR modes [e.g.,  in Fig.  4 (b)]. The highest value is comparable to the  factor () achieved in recent etched symmetry-protected BIC  metasurfaces [38] where very delicate () asymmetric structures have been made. Deviations between measured and simulated results are discussed in Supplement 1, Fig. S12, which primarily arise from (i) fabrication imprecision, (ii) material loss in the vicinity of the absorption by A exciton, and (iii) the use of an incoherent light source. Previous studies on plasmonic surfaces suggest that coherent source excitation could double the measured  factor compared to incoherent excitation [45].

C. Self-Hybridized Polaritons in Various TMDC Materials

Polaritons are the half-matter-half-light quasi-particles which enable a plethora of advanced applications in contemporary quantum technologies. By choosing suitable geometric parameters, the optical resonances by the TMDC photonic nanostructures can be tuned in the vicinity of excitonic resonances, allowing formation of self-hybridized polaritons through intrinsic light–matter interaction. Here, we prepare four types of TMDC materials, namely , and , with thicknesses ranging from  to  (as shown in the AFM measurements in Supplement 1, Fig. S21). These vdW materials all possess high refractive index () with their excitonic response varied from visible () to the near-infrared range () as indicated in the right panels of Figs.  3 (a)– 3 (d). Through angle-resolved transmission measurements, the dispersion of these hybridized modes can be unveiled, as shown in left panels of Figs.  3 (a)– 3 (d).

The strong couplings between photonic modes with A excitons can be described by a eigenfunction:

where  is the eigenstate of the polariton state and  is the corresponding eigen energy.  describes the strong coupling Hamiltonian for the hybrid system whose explicit form can be expressed as follows:

where  represents the complex eigen frequency of BIC or GMR modes (noted by its subscript, respectively).  () is the exciton-BIC (exciton-GMR) coupling strength (according to previous workwork [41] ).  is the complex eigen frequency of A exciton. The total Hamiltonian can be decomposed into two  block matrixes, each representing the interaction of the A exciton with the BIC and GMR modes, respectively.

The eigenstate is the hybrid state composed of photonic and excitonic constituents, i.e.,  where  and  are the Hopfield coefficients. Figures  3 (e)– 3 (h) exhibit the calculated dispersions of polaritons corresponding to the angle-resolved spectra in Figs.  3 (a)– 3 (d), which include BIC upper polariton (), BIC lower polariton (), and GMR lower polariton (). Because of the large material loss due to the large absorption by A exciton, the GMR upper polariton () is damped for all the samples (in both simulated and measured results), as exhibited in Supplement 1, Fig. S13. The dispersions of  are thus not exhibited in Figs.  3 (e)– 3 (h).

An unambiguous anti-crossing behavior of  and  is found in both the  and  etch-free structures, giving Rabi splitting (approximated as ) equal to 80 meV and 72 meV, respectively (also see Supplement 1, Fig. S30 for the detailed analysis), significantly exceeding the linewidths of their respective A excitons [46]. However, we failed to observe the dispersions of  branches in  and  samples in both the simulated and measured results. As shown in Supplement 1, Figs. S13(e)–13(h), different from the conditions in the  and  samples, the dispersions of  for  and  samples are damped by the absorption of A exciton (similar as the case for ), thus hindering the direct observation of the dispersion of  in both the simulated and measured angle-resolved spectra.

D. Enhancement of the Indirect Bandgap Emission

Most bulk TMDCs exhibit indirect bandgaps due to momentum mismatch between the valence band maximum and conduction band minimum, as illustrated in Fig.  4 (a). This results in the indirect bandgap phonon-assisted emission [Fig.  4 (c)], while with extremely low quantum yield compared to their monolayer counterparts. Consequently, prior researches on TMDC photonic structures primarily focus on passive properties such as transmission or scattering while the indirect bandgap emission is usually ignored. However, the strong field enhancement and confinement provided by GMR and BIC modes offer the potential to brighten these dark (low quantum yield) modes, enabling novel applications such as lasing, as demonstrated in whispering-gallery modes supported in  disk [47].

Fig. 4. Enhancement of indirect bandgap emission by the etch-free structures. (a) A schematic illustration of direct and indirect bandgap transitions in the bulk . (b,d) Transmission spectra for the etch-free  structure with period of  (b) and the zoom-in spectrum (d) showing the high- GMR mode (TE). The  factor is extracted by the Fano fitting [the orange curve in (d)]. (c) The PL spectrum for the bare  flake where the red and green areas indicate the direct and indirect bandgap emissions. (e) Comparison of the PL spectrum (with electric field parallel to the grating bar) of indirect bandgap emission for  without (grey curve) and with the top etch-free structure. (f) Double-peak fitting of the enhanced PL spectrum in (e). (g,h) Polarization-dependent angle-resolved PL measurement for the etch-free structure: electric field perpendicular to grating bar (g) and parallel to the grating bar (h). (i) Extracted PL spectrum (pink curve) from (h) at the normal direction (0°) and the corresponding Lorentzian fitting (green curve).

To enhance the indirect bandgap transition, the  etch-free grating [Fig.  4 (b)] with GMR mode (TE) in the vicinity of 890 nm (i.e., 1.393 eV, close to the peak of indirect bandgap emission) is chosen, where the  factor approaches 348 [as shown in Fig.  4 (d), the highest among our samples]. We find that the indirect emission is enhanced by around 25 times in  etch-free grating around GMR mode resonance [Fig.  4 (e)]. Through fitting the measured PL spectrum, a broad peak (close to the original indirect bandgap emission spectrum from the bare  flake) and a narrowed peak are extracted, whose summation gives a great match to the measured result, as shown in Fig.  4 (f). The narrow peak is centered around 1.393 eV with linewidth approximately of 16 meV.

There are majorly three factors that result in the PL enhancement by the optical cavity, i.e., (1) enhancement of the excitation field by the cavity, (2) enhanced quantum yield through increase of local density of states (LDOS) by Purcell effects [48], and (3) the increase of collection efficiency through redirection of radiation in the far field. The GMR mode is far detuned from the excitation wavelength (532 nm) in the measurements, such that the first factor can be neglected. For the second factor, we perform the time-resolved PL (TRPL) measurements using a bandpass filter () to select the photon energy range, enabling a comparison of the photoexcited carrier lifetimes of the bulk  with and without top ZEP grating structures. As shown in Supplement 1, Fig. S16(a), the photoexcited carrier decays involve (1) the phonon-related nonradiative decay pathway and (2) the phonon-assisted indirect radiative recombination. A biexponential function  is adopted to fit the photoexcited carriers decay curves measured in the experiment, where  () and  () represent the weight and decay rate of the nonradiative (photon-assisted radiative) pathway, respectively, with . Through temperature-dependent PL decay measurements, these two decay pathways can be well confirmed in the bare bulk  (Supplement 1, Fig. S17), with the nonradiative decays dominating over the radiative recombination, resulting in the low quantum yield for the indirect bandgap emission. In comparison, the resonant TE mode by etch-free structure greatly increases the LDOS [47,48] around the resonance () within the bulk  flake, accelerating the phonon-assisted radiative recombination process. As a result, the radiative weight  is increased from  to , and the decay rate  is accelerated from  to  at 300 K, leading to the enhanced indirect bandgap emission. Detailed analyses can be found in Supplement 1, Figs. S16–S18.

To unveil the redirection of the indirect emission by etch-free structures, we perform the polarization-dependent angle-resolved PL measurements, as shown in Figs.  4 (g) and 4 (h). The emission with the electric field perpendicular [also refer to the schematic illustration in Fig.  5 (a)] to the grating bar shows negligible dispersion [Fig.  4 (g)]. In comparison, the emission parallel [Fig.  4 (h)] to the grating bar exhibits the unambiguous dispersion of BIC and GMR modes, akin to the angle-resolved transmission spectra in Fig.  1 (g). Around the TE mode resonance [ where 8 meV is half of the linewidth of the narrow peak shown in Fig.  4 (f)], the emission angle is within the range of  to the normal direction to the sample surface. As a result, the collective efficiency by the objective ( with ) is significantly enhanced, resulting in the increased PL signal around TE mode resonance. Figure  4 (i) exhibits the PL emission spectrum in the normal direction extracted from Fig.  4 (h), showing linewidth (or full width at half-maximum, i.e., FWHM) of around 5 meV which is close to the linewidth shown in Fig.  4 (d) (). The observed sharp peak (peak 1) shown in Fig.  4 (e) originates from accumulated contributions of enhanced indirect bandgap emission by etch-free structures from different emission angles [Fig.  4 (h)], resulting in the ultimate linewidth approximating around 16 meV.

Fig. 5. Modulation of direct A exciton emission from ML . (a) Schematic structure of the heterostructure where a ML  layer is sandwiched between a bottom () and a top () hBN layer. (b) Microscopic image of the etch-free heterostructure metasurfaces. Note that the filling factor has approximate  fabrication errors. (c) Measured transmission spectra (green to yellow curves) for the metasurfaces of different periods and filling factors, and the PL spectrum (red curve) for the uncoupled . (d,e) PL (integrated from  to ) mapping of the electric field parallel and perpendicular to the grating bar.

E. Intrinsic Light–Matter Interaction in Heterostructure

Maximizing the interaction between photonic modes in dielectric cavities and direct excitons in ML TMDCs or Moiré excitons in TMDC heterostructures has profound implications for quantum and photoelectronic applications. Previous simulations have shown that placing ML TMDCs inside dielectric cavities significantly enhances light–matter interaction [30], though experimental realization remains challenging [49]. Here, we demonstrate an etch-free structure based on a heterostructure, as illustrated in Fig.  5 (a). The heterostructure consists of a ML  layer sandwiched between top and bottom hBN layers.

Figure  5 (c) exhibits the transmission spectra (GMR mode of TE polarization) of five grating structures [Fig.  5 (a)], as well as the PL emission measured from the bare ML  (without top grating structure). The transmission spectrum of the grating of  broadens as it is spectrally overlapped with the A exciton of . In the meanwhile, an unambiguous dip (indicated by a red arrow) can be observed in other samples, indicating weak coupling between light and matter. Figure  5 (d) exhibits the PL mapping with the emission electric field parallel to the grating bar, showing an enhancement of around 3–5 times [see also in Supplement 1, Fig. S26(a)]. The enhancement shows a monolithic increase when the GMR resonance is tuned close to the A exciton emission of . In addition, at the same period (), the mode () with a higher  factor shows a relatively higher enhancement as the Purcell factor  where  is the mode volume. In comparison, the emission with the field perpendicular to the grating bar shows a negligible PL enhancement [see also in Supplement 1, S26(b)] due to the polarization-dependent behavior of the GMR mode.

Although ML  is placed inside the hBN etch-free structure, the strong coupling regime is not achieved. We attribute this majorly to three factors. (i) As shown in Fig.  5 (b), the interface of heterostructure is not even, leading to a reduction of  factor of the cavity. (ii) The large area (µ) of ML  is exfoliated by Au-assisted method. However, the residual left on the surface after removing Au broadens the linewidth of excitons (55 meV is extracted in Fig. S15(a), compared with the 34–36 meV in previous report [50] for ), hindering the realization of strong coupling. (iii) Last but not least, compared with HRI TMDC, hBN is of lower refractive index (). The field confinement of the hBN etch-free structure is much lower than that of the TMDC counterpart for a similar structure [Fig. S15(b)] since the coupling strength  where  is the dipole moment and E is the local electric field strength. In order to address the above issues, the sample fabrication process should be modified to reduce the uneven interface and contamination of the sample. Moreover, the cavity mode can be further designed to improve the  factor of the etch-free cavity, for example, by optimizing the hBN thickness and redesigning the etch-free structure [49].

3. CONCLUSION

In summary, we propose an etch-free structure for vdW materials, eliminating the need for etching procedures in traditional vdW-based photonic structures. This approach is applicable to arbitrary materials and their heterostructures in principle. High- GMR modes are demonstrated both in simulation () and experimentally () using HRI TMDC materials, achieving  factors comparable to the record values in TMDC nanocavities (Supplement 1, Table S2) in the near-infrared range. Using this structure, self-hybrid polaritons are observed in four TMDC materials, with the UP and LP energy splitting reaching 80 meV, exceeding the linewidths of both the exciton and quasi-BIC modes. Beyond passive devices, we demonstrate the modulation of indirect bandgap emission in bulk  and direct bandgap emission in hBN-encapsulated ML . Compared with the previous work on light–matter interaction, our work demonstrates a simple structure which shows comparable or even better performance in terms of coupling strength and modulation of light emission (Supplement 1, Table S2). The proposed etch-free structure preserves the integrity of vdW materials (Supplement 1, Fig. S25), enabling the photoelectronic devices with modulated photonic response based on vdW materials.

Recently, the emergent vdW magnets CrSBr [51], 3R , and  with intrinsic nonlinearity [52,53], and triclinic materials  and  [54] present unprecedented opportunities in vdW-based nanophononics for quantum technologies and applications. The proposed etch-free vdW structure would expand toolkits in nonlinear optics, photonic spins, and entangled quantum sources with intrinsic light–matter interaction.

During the submission of this work, we noticed that an independent published study using an etch-free structure on InSe achieved remarkable enhancements () in out-of-plane dipole emission and second harmonic generation [55]. While a similar structure was employed, our work distinguishes itself by (i) realizing high- GMR and (quasi-)BIC modes, (ii) demonstrating self-hybrid polaritons through intrinsic light–matter interaction across multiple TMDCs (not limited to a single material), and (iii) modulating both indirect bandgap emission in bulk TMDCs and direct bandgap emission in hBN-encapsulated ML TMDCs.

4. METHODS

A. Simulations

The normal and angle-resolved transmission spectra are calculated on the home-built Rigorous coupled-wave analysis (RCWA) algorithm. The eigenmode analysis and band structure calculations are solved by the commercial software COMSOL. The Purcell factor is calculated based on the finite-difference time-domain (FDTD) method by the commercial software Lumerical-FDTD Solutions. The dielectric functions of TMDCs are extracted from the previously published work [46].

B. Fabrications

The fabrication of etch-free vdW structures on  (300 nm)/Si is well illustrated in the main text and Supplement 1, S12. For the transmission measurements, the vdW structures are wet etched in buffered HF (15%) solvent for 15 min and then transferred to the transparent substrates. Details can be referred to Ref. [7] in our previous work.

To fabricate the heterostructure shown in Fig.  5 , a large (µ) ML  is exfoliated by the Au-assisted method [7], and hBN layers are exfoliated by the scotch tape as mentioned above. Utilizing the standard dry transfer method, different layers are aligned and integrated to form the heterostructure.

C. Optical Measurements

The transmission measurements are based on the combination of the home-built setup and commercial spectrometer (Supplement 1, Fig. S14). The angle-resolved transmission measurements are realized by the step-motor to precisely control the rotation angle of the sample. The PL measurements are based on the commercial spectroscopic system (HORIBA LabRAM HR Evolution system) where an  objective with  is used for the measurements in Figs.  4 and 5 , as detailed in our previous work [56].

For the angle-resolved PL measurements, the home-built Fourier transform setup is adopted to obtain the PL intensity distribution in the momentum space [57].