Tunable light–matter interaction

Light–matter-interaction in optical microcavities has been the subject of nearly three decades of research, involving all kinds of quantum emitters and cavity types1,2,3. The coupling of excited matter states and photon modes can be characterized by two regimes. The weak-coupling regime, leading to altered radiative excitation lifetimes and enhanced or suppressed spontaneous emission due to the Purcell effect4, and the strong-coupling regime. This regime is manifested by a normal mode splitting — known as (vacuum) Rabi splitting — and coherent reversible energy exchange between light and matter states, ultimately leading to the formation of hybrid bosonic quasiparticles5,6. A requirement is that the coupling strength is high enough and decay rates sufficiently low. Besides quantum information and light-source developments on the application side3 it is the field of fundamental cavity quantum electrodynamics studies7 which demands optical high-quality microcavities. The topic highly benefits from research on various confinement concepts and has strongly driven the overall developments8,9,10.

Since their discovery by Weisbuch et al. in 1992, exciton–polaritons in optical microcavities, also known as cavity–polaritons, have attracted considerable attention by the research community, particularly for the first experimental realization of a Bose–Einstein condensate (BEC)11,12 and later for the first achievement of novel practical devices13,14, for room temperature condensate15,16 as well as polariton-lasing17,18 demonstrations, owing to the light effective mass of the hybrid bosonic quasiparticles. In this context, two-dimensional semiconductors such as WS2 and WSe2 from the family of transition-metal dichalcogenides are currently strongly under consideration for the next material system in optical cavities19,20,21,22,23,24 with which practical room-temperature BEC experiments could be carried out.

For Fabry–Pérot microcavities, several parameters such as the mirror distance in combination with the angle of incidence for the incoming light or the emitter position inside of the cavity provide the possibility of continuous tuning of the coupling situation in light–matter interactions. In this work, we propose two ideas how to tailor the coupling strength by the relative field strength at the exciton position. Firstly, a more intuitive approach is considered, i.e. displacing a 2D material with respect to the resonator field inside an open planar microcavity with fixed mirror separation. Secondly, based on this, an open cavity with extra spacing layer between 2D material and one mirror is introduced. This alternative setup provides the advantage that it consists only of two independent movable parts and is therefore presumably easier to realize in an experimental setup. Furthermore it is independent from the choice of the active material. The cavity length is changed to set the excitons’ position within the standing-wave field pattern. At the same time, spectral overlap between cavity and exciton modes occurs at different angles of incidence for the incoming cavity light. We have analyzed this structure configuration by calculating the angle-resolved reflectivity spectra as well as the field distribution for a monolayer WS2 using the transfer-matrix method. Thereby, we show that a transition between the weak and strong coupling regime can be observed. The coupling strength is completely tunable in our proposed setup by the choice of the spacer layer’s thickness. These findings could enable the exploration of the transition between weak and strong coupling — especially the exceptional point — by prospective actively tunable experiments. Furthermore, the material independent continuous adjustability could be utilized for polariton-based chemistry allowing a precise adjustment of the polariton energy levels25. This could provide an experimental stage for an efficient study of chemical reactions influenced by different light–matter coupling situations.

Theoretical Background

The Rabi splitting is the figure of merit in strong-coupling experiments and is written for energetic resonance between cavity and exciton as ħΩRabi = EUP − ELP = 2g0, whereas EUP and ELP are the resulting energy levels of the upper and lower exciton–polariton modes, respectively. For clarity, the momentum-space dependencies of the modes, which lead to angle-dependent emission properties, are blanked out for a moment in our discussion, while coupling does not violate energy and momentum conservation.

The maximal coupling strength g0 is determined, besides the physical constants for the elementary charge e, the vacuum permittivity ϵ0 and the electron mass me, by the cavity’s permittivity ϵ, the exciton’s oscillator strength f as well as the cavity’s mode volume Veff,

$${g}_{0}=\sqrt{\frac{1}{4\pi {\epsilon }{{\epsilon }}_{0}}\frac{\pi {e}^{2}f}{{m}_{e}{V}_{eff}}}\,$$

In general, the physical coupling strength g is described by the maximal possible coupling strength g0, the relative field strength reflecting the position inside the field pattern ψ(z) and the angle between the electric field and the exciton’s dipole moment ξ2. This accounts for the anisotropy of WS2. An explicit consideration of the out-of-plane dark exciton is not necessary, because of its weak oscillator strength26,27 as discussed in the supplement.

$$g={g}_{0}\psi \,(z)cos(\xi )$$
$$\psi (z)=\frac{E(z)}{|{E}_{max}|}$$

The light–matter coupling is modelled by the following Hamiltonian.

$$\hat{H}=\hslash \left({\omega }_{cav}-\frac{i{\gamma }_{cav}}{2}\right){a}^{\dagger }a+\hslash \left({\omega }_{ex}-\frac{i{\gamma }_{ex}}{2}\right){\sigma }^{\dagger }\sigma +\hslash g({\sigma }^{\dagger }a+{a}^{\dagger }\sigma )$$

with the creation and annihilation operators of both species a and a/σ, respectively. The first term describes the cavity field with its resonance frequency ωcav, the second one the exciton with its resonance frequency ωex and the last one the coupling between both.

Based on this, the eigenvalues represent the resonance frequencies of the coupled system,

$${\omega }_{\pm }=\frac{1}{2}\left({{\rm{\omega }}}_{{\rm{ex}}}+{\omega }_{cav}+\frac{i({\gamma }_{ex}+{\gamma }_{cav})}{2}\right)\pm \frac{1}{2}\sqrt{{(2{\rm{g}})}^{2}+{\left({\omega }_{cav}-{\omega }_{ex}+i\frac{({\gamma }_{cav}-{\gamma }_{ex})}{2}\right)}^{2}}$$

The coupling regime is mainly given by the coupling strength as well as the cavity’s linewidth γcav and the exciton’s linewidth γex. In the literature, often two criteria appear: 4 g²>|γcav - γex |2 as condition for a splitting of the eigenvalues in Eq. (1) (e.g. in28) and 2 g²>|γ2cav + γ2ex | as requirement for a resolvable splitting, for which the frequency splitting is larger than the polariton linewidth (cf.24,29). By tuning the coupling, it becomes possible to especially investigate the transition between weak and strong coupling. Between the two regimes lies an exceptional point (EP). At this position, the coupled system only features one complex solution28,30,31. This gives rise to a variety of physical phenomena like chiral behaviour32 as well as an energy transfer between the two modes of the strongly coupled system33 under encirclement of the EP. Due to the sharp transition between the coupling regimes, an EP is also suitable for the design of sensors34.

In order to tune from one to another coupling regime, either the excitons’ or the cavity’s properties can be altered. Tunable coupling has been demonstrated with WS2 by applying a current35 or varying the temperature36, causing a change of the exciton oscillator strength. Furthermore, chemical reactions of molecules can be used to switch between the coupling regimes37. Moreover, in monolithic cavities, the dependency of the coupling situation on the relative field strength has been demonstrated for molecules38. Recently, continuously tunable coupling was shown for carbon nanotubes by utilizing the polarization of the light39.

All mentioned methods above either depend on the choice of the material, need multiple growth processes or are not actively tunable over a wide range. Here, we use properties of an open cavity to continuously tune the coupling strength and thereby are giving the perspective of real-time switching between the strong and weak coupling regime without influencing the active region. Besides this, the presented method keeps the spectral position of the cavity mode resonant to the exciton. Our suggested structure is a priori not limited to a special material. For the active medium providing the excitons only a high enough oscillator strength to reach strong coupling as well as a sub-wavelength thickness compared to the emission wavelength is required. With the aim in mind to control the coupling situation without changing the setup in between experiments, we chose a planar open microcavity. In combination with monolayers of transition-metal dichalcogenides such as WS2, the excitons of which exhibit a large oscillator strength, the strong coupling regime can be reached with an open cavity already at room temperature20.

2D–Microcavity Structure

All simulations are done for s-polarized light in order to exclude the influence of a misalignment between the dipole moment of excitons and the cavity mode’s electric field and thereby to ensure that they are perpendicular to each other for every angle of incidence. The simulated Fabry–Pérot (FP) cavity consists of two dielectric mirrors (distributed Bragg reflector, short DBR), which comprise alternating layers of SiO2 and Si3N4, with their wavelength dependent refractive indices extracted from40 and41, respectively. A similar configuration was already presented in the literature to show room-temperature polaritons for WS220. The high-reflectivity mirrors end with the low refractive index material, namely SiO2, to ensure the field maximum at the DBRs’ surface for normal incidence. Moreover, directional leakage of cavity photons is enabled by one side featuring less mirror pairs, resulting in a lower reflectivity for the corresponding mirror. The two mirrors therefore consist of 11.5 and 12.5 SiO2/Si3N4 layer pairs. Acting as an active medium, a monolayer of the transition-metal dichalcogenide WS2 is – in the simulations – brought into coupling with the cavity mode of the open microresonator. Its optical behavior is modeled by an effective optical thickness of 6.18 Å42. The wavelength-dependent complex refractive index is derived from experimental data (from ref. 42) together with an approximation of that data which consists of a summation over Lorentzian peaks according to ref. 43 (see Supporting Information Fig. SI.1). A scheme of the simulated structure is shown in Fig. 1a.

Figure 1
figure 1

Schematic drawing representing a tunable-cavity coupling approach for (a) a structure without spacer layer incorporating a free-standing monolayer and (b) a structure with intra-cavity spacer layer, on which a monolayer is placed.

For the spacer layer in our second tunable-coupling approach, we simulated PMMA on top of one mirror (Fig. 1b). This material is already used in waveguide applications44 or as spacer layer in monolithic cavities38 due to its transparency for visible light45. Further, it enables a wide range of possible thicknesses, from the nm-range up to several 100 nm. The tunable coupling is achieved by variation of the angle of incidence for the incoming light combined with an adjustable cavity length. Thus, we simulated angle-resolved reflectivity spectra to identify the coupling situation.

Simulations and Discussion

Variation of the 2D-material’s position

As an intuitive approach, the excitons’ position with respect to the field pattern z can be varied by shifting the material inside of an open microcavity with fixed cavity length. As a result of the adjusted position, the Rabi splitting ħΩRabi can be varied, which can lead to a change in the coupling regime. Figure 2a displays a plot of the system’s resonance frequencies based on the calculated reflectivity spectra of the complete microresonator structure. At positions in the cavity corresponding to a strong coupling regime, characterized by a mode splitting, an asymmetric energy splitting into a photonic lower polariton branch (red triangles) and an excitonic upper polariton branch (blue squares) around the exciton’s energy (dashed horizontal line) is noticeable. This is a hint, that the system is not yet completely resonant. Here, the cavity length is determined in a way, that the resonance energy of the empty cavity without 2D material overlaps with the bare exciton mode (dashed horizontal line), which is estimated at a wavelength of 617.2 nm (rounded) by the imaginary part of the refractive index representing the absorption. In Fig. 2a the solid black curve represents a cavity mode with WS2 at different spatial positions for which the imaginary part of the 2D material’s refractive index is neglected. Consequently, only the reflectivity of WS2 is included for the solid black line but not its absorption. The simulated curve indicates that the 2D material’s reflectivity already detunes the spectral position of the cavity mode without coupling considerations. This can result in a position dependent energetic offset between photon and exciton mode within the final setup. The cavity’s resonance has no offset with respect to the exciton mode at the minima of the relative field strength (\(z\approx 0.15\,\mu m)\). At this position, the cavity is not coupled to the excitonic mode. It is worth keeping in mind that the empty uncoupled-cavity mode has not the same spectral position as the calculated cavity for a system with active region. Beside this unreachable zero-detuning configuration for each monolayer position (at fixed cavity length and fixed incidence angle), this approach has from the experimental point of view also the drawback of requiring a complex, rigid and perforated holder to incorporate the free-standing material into the resonator. The calculated energy splitting normalized to the maximum (shown as dots in Fig. 2b) is nearly proportional to the theoretical curve representing the absolute electric field strength of this cavity configuration (black line). For clarity, also here the refractive index modulation of the structure is displayed in the background.

Figure 2
figure 2

(a) Eigen-energies of the upper and lower polariton states (blue squares and red triangles, respectively) as well as of the weakly coupled cavity system (green circles) based on calculated reflectivity spectra (corresponding to a setup like in Fig. 1a). For each simulation, the 2D material WS2 is placed at the corresponding distance z towards the left mirror. The black dashed line marks the exciton’s energy (left scale). The solid black line represents the cavity’s mode for which the imaginary part of the refractive index of WS2 is neglected (corresponding to an uncoupled system, i.e. Eres = ħωcav). (b) Calculated absolute relative electric field strength of the empty cavity (black line) with dots representing the calculated coupled-mode splitting normalized to its maximum (left axis). In all graphs, the gray dotted line in the background marks the refractive index modulation (right scale) of the empty cavity consisting of DBRs and an air gap.

Concept making use of an additional spacing-layer

The optical phase shift \({\varphi }_{cav}\) between two plane parallel mirrors depends on the geometric path difference \(\Delta {s}_{cav}\), given by the angle of incidence ϑ for the incoming light, the refractive index n and the cavity length Lcav, as well as the wavelength \(\lambda \).

$${\varphi }_{cav}=\frac{2\pi \Delta {s}_{cav}}{\lambda }=2\pi \frac{(n\cos (\vartheta )2{L}_{cav})}{\lambda }=\,2\pi \frac{(n\cos (\vartheta )2{L}_{cav})\,{E}_{cav}}{h\,c}.$$

For the resonant FP cavity mode, \({\varphi }_{cav}\) is a multiple of 2π. Waves with another wavelength λ for which \({\varphi }_{cav}\) is not a multiple of 2π interfere completely or partly destructively. If the angle of incidence \(\vartheta \) is increased, the path difference and thereby the phase shift decrease. Consequently, the wavelength of the cavity mode is reduced and the energy of the (uncoupled) cavity mode Ecav is increased (see Eq. (6)). For a longer cavity the path difference is increased leading to a reduction of the cavity mode’s energy. Hence, the influence of ϑ on Ecav could be compensated by changing Lcav (see Fig. 3). By adding an additional spacer layer, the optical path difference between the two mirror surfaces gets the sum of the path difference inside of the PMMA-layer \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) and inside of the air gap \({\varphi }_{{\rm{A}}{\rm{i}}{\rm{r}}}\),

$${\varphi }_{cav}={\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}+{\varphi }_{{\rm{A}}{\rm{i}}{\rm{r}}}=\frac{2\pi }{\lambda }2({n}_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\,\cos \,(\vartheta ){d}_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}+{n}_{{\rm{A}}{\rm{i}}{\rm{r}}}\,\cos \,(\vartheta ){d}_{{\rm{A}}{\rm{i}}{\rm{r}}})$$
Figure 3
figure 3

Optical path between two plane parallel mirrors.

For simplicity, the influence of the mirrors’ reflectivity on the path difference is neglected here. Increasing ϑ shortens the geometric path within the air gap \(\Delta {s}_{{\rm{A}}{\rm{i}}{\rm{r}}}\) as well as the PMMA layer \(\Delta {s}_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\). Thus, the corresponding phase shifts and thereby \({\varphi }_{cav}\) are decreased. Consequently, Ecav is increased. Again, this could be in principle compensated by changing Lcav. The cavity length is given by the layer thicknesses, \({L}_{cav}={d}_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}+{d}_{{\rm{A}}{\rm{i}}{\rm{r}}}\). Here, only the gap size \({d}_{{\rm{A}}{\rm{i}}{\rm{r}}}\) is adjustable, while \({d}_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) stays constant. Increasing the mirror separation leads to an increased gap size \({d}_{{\rm{A}}{\rm{i}}{\rm{r}}}\). Accordingly, \({\varphi }_{cav}\) is increased and Ecav is decreased. Hence, also an increased gap size could be used to compensate the phase shift induced by an increased ϑ. In summary, increasing the cavity length leads to a spectral overlap between exciton resonance and cavity mode at higher angles of incidence. Here, this angle is denoted as resonant angle ϑres. In the following, we consider only the case of spectral resonance (i.e. zero-detuning: Ecav (ϑres) − Eex = 0) and thereby each change in the cavity length corresponds to a change in ϑres. The cavity mode’s spectral position is determined by including the monolayer’s reflectivity while neglecting its absorption to exclude the detuning observed for the structure in Fig. 1a (see Supplement Fig. SI. 6). Again, the exciton wavelength in our simulation is estimated as rounded to 617.2 nm by the peak position within the imaginary part of the dielectric function.

However, \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) and \({\varphi }_{{\rm{A}}{\rm{i}}{\rm{r}}}\) are now changed in a different manner, although the overall phase shift \({\varphi }_{cav}\) stays the same. Increasing ϑ causes a decreased \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) as well as \({\varphi }_{{\rm{A}}{\rm{i}}{\rm{r}}}\). At the same time, the additionally increased \({L}_{cav}\) leads to an increased phase shift only within the air layer. Due to the required spectral resonance between exciton and cavity, the total phase shift \({\varphi }_{cav}\) of the structure is kept constant. In sum, \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) is reduced whereas \({\varphi }_{{\rm{A}}{\rm{i}}{\rm{r}}}\) is increased. Together with this mismatch between the change of phase shift inside PMMA and air, it gives rise to a shift of the field pattern relative to the PMMA–air interface. The phase shift \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) determines the interface’s position inside of the standing wave pattern and thereby the corresponding field amplitude. By placing the 2D material at this interface, these conditions can be utilized to adjust the coupling strength by tuning the relative field strength. This effect is illustrated within an additional video (see Supplementary Information), showing the field pattern as well as the angle-resolved reflectivity spectra for the suggested structure. For the field calculation the reflectivity of WS2 is again considered while its absorption is neglected. Figure 4 shows three representative settings for such a structure (see Fig. 1b). The field pattern is moreover simulated for ϑres. Different energy splittings can be obtained in the angle-resolved reflectivity spectra. Furthermore, the angle of incidence ϑ thereby not only alters the cavity mode’s spectral position as well as the relative field strength but in general also the cavity’s linewidth γcav, because modifying ϑ influences the reflectivity of the DBR mirrors. For this setup the change is less than 0.1% within the region of experimentally interesting angles (<40°) and is thus not further discussed as impact on the coupling regime. Nevertheless, the mirror contributes an additional phase shift for higher angle of incidences (see Supplement Fig. SI 7). Thus, spectral resonance is determined by iteratively simulating the reflectivity spectra for varied mirror separations. However, the reflection dip in the lower row within the last image is slightly shifted with respect to the exciton energy indicating an influence of the monolayer’s absorption on the cavity’s spectral position.

Figure 4
figure 4

Impact of a changed cavity length on an open planar microcavity with a 178.2 nm thick PMMA spacer layer on one mirror (structure depicted in Fig. 1b). The second cavity mode corresponds from left to right to an air gap of 360.8 nm, 425.6 nm and 575.7 nm, respectively. The upper row shows the simulated field distribution of a cavity at resonance (Ecav (ϑres) − Eex = 0), which is marked in the angle-resolved reflectivity spectra below by a dotted vertical line, with neglected absorption of WS2, while the horizontal dashed line indicates the exciton’s emission energy. The exciton’s spectral position is determined by the peak position within the imaginary refractive index. The influence of the first three cavity modes could be found in the Supporting Information (Figs. SI. 4 and 5). Further, the corresponding transmission and absorption spectra as well as the spectra for p-polarized light are displayed within the Supplement (Figs. SI. 8 and 9).

Additionally tailoring the coupling situation

The spacer layer’s thickness \({d}_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\) defines the starting position in the standing wave pattern at normal incidence by the corresponding phase shift \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\). At normal incidence, different air gaps are necessary for different PMMA thicknesses to reach spectral resonance. Therefore, the change in cavity length \(\Delta z\) with respect to the length (for which resonance occurs at zero angle) \({L}_{cav}({\vartheta }_{res}=0)\) is used in Fig. 5 as a common axis (\(\Delta z={L}_{cav}({\vartheta }_{res})-{L}_{cav}({\vartheta }_{res}=0)\)). For decreasing cavity length compared to \({L}_{cav}({\vartheta }_{res}=0)\), the energetic resonance condition can no longer be reached, because a further increase of \(\cos (\vartheta )\) in Eq. (7) in order to compensate the phase shift compared to \(\vartheta =0\) is not possible. The simulated relationship between \(\Delta z\) and \({\vartheta }_{res}\) is displayed within Fig. 5a. On the other hand, increasing the cavity length shifts the exciton’s location relative to the field pattern towards the left side of the field maximum (left direction in Fig. 4a–c) due to the decreased \({\varphi }_{{\rm{P}}{\rm{M}}{\rm{M}}{\rm{A}}}\). By the choice of \({d}_{{\rm{PMMA}}}\), it is possible to either reduce or increase the absolute field strength at the monolayer position for increasing \(\vartheta \) as well as \(\Delta z\) (see Fig. 5b). Thus, depending on the layer’s thickness, different coupling scenarios can be reached.

Figure 5
figure 5

(a) Relationship between the change in the cavity length Δz and the resonant angle ϑres for different spacer layer’s thicknesses dPMMA for the structure in Fig. 1b. (b) Corresponding influence of Δz on the relative field strength at the excitons’ position ψex for ϑres. The reflectivity of WS2 is in those two calculations (a,b) considered, while its absorption is neglected. (c) Resulting resolvable splitting ħΩRabi at ϑres obtained by a varied Δz combined with a resonance adjustment by the angle of incidence ϑ. The splitting is determined by the difference between the maxima of two Lorentzian peaks approximating the coupled modes fitted to the calculated reflectivity spectra at Ecav (ϑres) − Eex = 0.

In Fig. 5c, the Rabi splitting for different coupling situations based on various thicknesses of the spacer layer is shown. For example, if the PMMA–air interface lies near the field maximum at normal incidence, a length tuning leads to a decreasing relative field strength and thereby coupling strength (e.g. blue and green curve in Fig. 5b for a 178.2 and 190.6 nm PMMA layer). Similarly, increasing coupling under cavity length tuning can be reached, for instance, by a thickness dPMMA larger than one half of the wavelength in PMMA (e.g. 227.9 nm or 248.6 nm). A careful choice also enables the transition between the coupling regimes. Here, the exact position of the transition between the coupling regimes is spectrally not directly detectable. No splitting in this context means, that it is either not resolvable or the system is weakly coupled and thus indeed does not feature any resonance splitting. However, a systematic analysis of the lifetime in terms of Purcell effect and Hopfield coefficients should allow locating the exceptional point. Nevertheless, the presented spacer-layer method allows a continuous tuning of the coupling strength (see supplementary video). Between no coupling at the field minimum and the maximal strong coupling at the field maximum should be a coupling strength referring to weak coupling prior to reaching the EP. Thus, weak coupling could be realized by placing the monolayer near the field minimum combined with the given tuning mechanism.


In summary, we have presented two designs for an open planar FP microcavity system, which both allow nearly material independent, active and continuous modification of the light–matter interaction in the structure according to calculations. Hence, such setups can be especially utilized as a testbed for real-time investigations on the transition between the strong and weak coupling regime. Beyond the employment of 2D-cavity systems for optoelectronic coupling, the alternative free-standing-sheet configuration could even enable sophisticated optomechanical experiments. The more practical variant utilizing a spacer layer on one of the DBRs of the FP cavity, as an alternative concept to a displaceable free-standing sheet medium, is characterized theoretically in detail with a transfer-matrix model for a tuneable variation of the coupling strength. The choice of the spacer layer’s thickness allows a precise control of the coupling situation. Thereby, an increase and decrease in the obtainable Rabi splitting as well as a modification of the coupling regime become possible scenarios while spectral resonance between cavity and exciton is realized. A setup of this kind is suitable for material systems with a sub-wavelength thickness with respect to their exciton emission wavelength, provided that a large enough oscillator strength for strong coupling is exhibited. As such, also an extension to the ultra-strong coupling regime46,47 should be possible; here, molecular materials could be a good choice, as they offer very large dipole moments at comparably low film thicknesses48,49,50. In the future, this method could be used to investigate light–matter coupling not only for 2D semiconductors, but also e.g. quantum dots, perovskite thin films, nanoplatelets or molecular films.