Optical microcavities, which can confine light by resonant recirculation, have offered great opportunities and challenges in both fundamental research and practical applications in the past decades ^[1–3]. Among all kinds of optical microcavities, whispering gallery mode (WGM) microcavities derived from typically dielectric spherical structures have attracted interest in the fields of lasers, filters, sensors, optical diodes, and comb generation for their high quality factor (Q factor), small volume, and easy integration ^[4–8].

In a WGM microcavity, waves propagate and accumulate along the surface of the microcavity by continuous total reflection ^[9]. Microcavities with diverse geometrical features have been proposed and demonstrated. Microspheres, which have typically attracted considerable attention, can support an ultrahigh Q factor due to their excellent surface smoothness ^[10]. Microcapillaries have the advantage in the fabrication process and sensing performance based on their hollow structures ^[11]. Similarly, microbottles are fabricated from silica microcapillaries by a pressure-compensated method for three-dimensional (3D) confinement of resonant modes ^[12]. Microring, microdisk, and microtoroid cavities provide platforms for the on-chip integration with devices, in the meanwhile, support high-Q factors ^[13,14]. While the output of WGM resonators based on microrings, microdisks, and microtoroids is in the parallel dimension with respect to the chip panel, rolled-up microtubular resonators offer an approach to achieve vertically emitting modes with easy integration and large-area fabrication ^[15–17]. Furthermore, based on the rolled-up technology, microtubes with a bottle-like geometry have been explored and attracted intense attention for their precisely controlled axial modes and highly enhanced Q factors ^[18]. Recently, a single-mode confinement-enhanced tubular microcavity was constructed by rolling up diamond nanomembranes, in which periodic hole arrays along the circumference were precisely engineered to realize discrete rotational symmetry ^[19]. So far, the geometrical innovation of WGM microcavities has led to remarkable development in both fundamental research and practical applications.

In addition, materials selected to construct a microcavity are not limited to silica or silicon, as a variety of materials have shown extensively excellent properties and corresponding application potentials, e.g., ZnO, LiNbO3, diamond, polymer, and high-refractive-index oxides (Y2O3 and ZrO2) ^[20–24]. In the meanwhile, the recent and intensive development in two-dimensional (2D) materials gives birth to a new focus on exceptional photonic performance for the strong interaction between light and many 2D materials ^[25]. Most importantly, 2D materials can be integrated into functional photonic devices and systems, enabling unique devices and applications with high performances. First, the 2D-material-enabled modification of WGMs offers a potential solution to much richer physical properties, such as tunable light sources ^[26]. Owing to the effect arising from small size, low-dimensional materials provide intriguing opportunities for nanostructure-dependent properties and related photonic performances as compared with their bulk counterparts ^[27]. In particular, 2D materials present a better-defined interface geometry for quantitative characterization than 0D and 1D materials, allowing novel devices based on the junction interface, e.g., gate-tunable properties ^[28]. Second, it is of great significance for the integration of 2D materials on microcavities, realizing enhanced light–matter interaction ^[29]. Thus, 2D-material-coupled WGM microcavities have stimulated intensive research and enormous progress in fundamental physics and future perspectives.

In this review, we will discuss the development of 2D-material-coupled WGM microcavities, with examples of scientific research and applications in different kinds of microcavities like microdisks, microspheres, and microtoroids. Starting with the graphene-integrated microcavity, we will mainly focus on the plasmonic hybrid mode and explore its superior property in sensitivity and tunability. For transition metal dichalcogenides (TMDCs), they have attracted growing interest due to their distinct features in high optical gain. We will present a discussion on the development of 2D materials among WS2, WSe2, and MoS2, where the appealing phenomena of excitonic lasing and spontaneous emission enhancement arise. Moreover, other kinds of emerging 2D materials and coupling schemes will also be discussed for their feasibility and future perspectives, such as black phosphorus and hexagonal boron nitride (hBN). Additionally, the integration of 2D materials based on the rolled-up technique is proposed, which would endow such a heterogeneous material microcavity with a novel coupling scheme and possible potentials in on-chip devices. The 2D material-coupled microcavities will offer promising scientific discoveries and nanophotonics technologies for future devices.

Graphene, as one kind of atomically thin material, has arisen intense interest due to its extraordinary properties in both electronics and photonics ^[30,31]. Compared with traditional materials in bulk form, graphene exhibits a variety of appealing phenomena, which arises from its dissimilar band structure near the Fermi level ^[32–35]. It is demonstrated that the ultrahigh mobility of graphene can support the study of ultrafast optoelectronic devices, such as modulators and photodetectors ^[36,37]. Besides, graphene interacts with light from microwave to ultraviolet wavelengths, which indicates its adaptability towards different applications owing to the broadband response ^[38]. It is worth noting that the tunability of a graphene-based device can be easily realized, where Fermi levels and corresponding properties can be tuned towards target applications ^[39–41]. Technically, the small size and easy integration with complementary metal oxide semiconductor (CMOS) techniques enable graphene the next-generation devices in both on-chip and flexible strategies. Based on the integration of graphene on a photonic crystal nanocavity, a strong enhancement in light–matter interaction has been realized, which opens the door to novel optical devices including hybrid systems for sensitivity improvement ^[42,43].

In early studies, graphene is demonstrated to support both localized and propagating plasmons, which is concluded as one of its semimetallic features ^[44,45]. Compared with plasmons in metals, graphene plasmons cover the region from terahertz to the midinfrared wavelengths. Besides, the plasmonic nanostructures based on noble metals suffer from high adsorption loss and low damage threshold, which limit conversion efficiency and hinder practical applications. Thus, the research on hybrid plasmonic modes in the WGM microcavity is restricted until the introduction of graphene as an optimal choice, enabling devices with high performances, such as ultrahigh sensitivity.

Based on a graphene-coated microcavity, surface plasmon polaritons propagate along the surface and interact with the whispering gallery waves, thus realizing the hybrid plasmonic modes. So far, calculations and simulations have been performed to reveal the mechanism and effects. In general, the periodic boundary condition and effective thickness are often applied to raise the efficiency and ensure the accuracy when dealing with atomically thin 2D materials ^[46]. Typically, the dynamical conductivity of graphene can be determined from the Kubo formula, which is further utilized for the optical constants as a function of chemical potential and wavelength ^[47]. Figure 1(a) exhibits the 5 nm radius InGaAs nanowire cavity coated with graphene ^[48]. In such a nanoscale configuration, the plasmonic waves are tightly confined on the interface between graphene and the free space, as demonstrated in the electromagnetic field distribution from the inset of Fig. 1(a). In addition, it is of great significance to note that the surface conductivity would strongly affect the resultant surface plasmon polaritons. Thus, one can manipulate the hybrid plasmonic modes by controlling the chemical potential through a myriad of methods, e.g., doping, gating, and strain engineering ^[39–41]. The researchers also theoretically conclude the Q factor and mode number as functions of the chemical potential ^[48]. As depicted in Fig. 1(b), the increase of chemical potential leads to the decrease in propagation loss of the plasmonic modes and the enhancement in the Q factor. The ultrasmall mode volume suggests the plasmonic WGM microcavity as a promising candidate for label-free sensors ^[48]. Besides, a graphene-based THz ring microcavity was proposed for sensing with label free and lab-on-a-chip features ^[49]. The multilayered microcavity consists of a dielectric layer Si3N4, a buffer layer SiO2, a graphene layer, and a porous cladding layer p−Al2O3. The performance of the hybrid microcavity is characterized through the fiber-coupling technique, where the light transmission shows a dip at the resonance wavelength. The refractive index of the cladding layer changes with adsorption of target molecules. Thus, the resonance wavelength shifts correspondingly, which consequently can be utilized for molecule detection. The authors also reveal the relationship between the resonance wavelength shift and the refractive index change of the cladding layer for different chemical potentials of graphene ^[49]. The decrease of chemical potential helps to enhance the sensitivity, which is dominated by better confinement of the surface plasmon mode. The sensitivity of the proposed configuration is calculated to be 46 μm/RIU, which indicates a 30-time enhancement compared with that of a whole dielectric structure. For the application potentials in integrated photonics, a graphene-sandwiched silica microdisk is proposed, as shown in Fig. 1(c), where the tunability of sensitivity is realized by electrically adjusting the permittivity of graphene ^[50]. Particularly, when the permittivity is around the epsilon-near-zero point, there would be a distinct change in the sensitivity, as shown in Fig. 1(d), which reveals its potential applications in devices with high modulation depth and high sensitivity. However, the Q factors of graphene-coated microcavities are still poor compared with those of traditional dielectric microcavities. In order to alleviate the loss for high Q factors, it is proposed that one can effectively utilize a vertically coupled monolayer graphene microcavity, via keeping their distance instead of straight contact ^[51].

Except for the sensing mechanism based on the refractive index change of the surrounding environment, it also attracted intense attention by monitoring the refractive index change of graphene based on its environment-sensitive conductivity ^[52]. When exposed to target gas, the C-C bonds would be modified with gas molecules; thus, the carrier concentration is tuned ^[53]. The complex refractive index, which is dependent on graphene’s conductivity, is sensitive to the change of the surrounding gas concentration ^[54]. Additionally, graphene and one of its derivate graphene oxides (GOs) are ideal materials for biomolecules and gas molecule adsorption for the large specific surface area and high adsorption ^[55]. So far, there are various methods employed in gas sensing based on graphene and GOs ^[56,57]. The sensing scheme based on optical microcavities indicates its advantages in high sensitivity and real-time detection. Figure 2(a) depicts the configuration of a GO-coated ring microcavity ^[54]. The authors monitored the shift of the resonant wavelength in transmission spectra, which is illustrated in Fig. 2(b) for the NH3 gas with concentrations of 0, 75, 150, 225, and 300 ppm (parts per million). Additionally, the sensitivities of NH3 and CO are different due to the adsorption energy and charge transfer ability, which indicates the potential applications in differentiating gas ^[58].

Furthermore, the novel sensing mechanism based on an optomechanical microcavity is demonstrated, which enables a graphene-based ultrasensitive gas detection ^[59]. Figure 2(c) depicts the schematic of the graphene-enhanced Brillouin microcavity, where a layer of GO film is deposited on the inner wall of the microbottle. Once the NH3 gas is introduced to the microbottle cavity with a concentration of 10 ppm, the beat note shifts from 262.0 to 260.4 MHz, which is monitored on the Brillouin and acoustic mechanical scattering. Such a hybrid microcavity enables extremely high sensitivity up to 200 kHz/ppm based on the novel “electron–phonon–photon” process. This phenomenon reveals the synergy effect of graphene and microcavity for enhanced sensitivity and ultrafast response, which hints that the combination of graphene can broaden the potential applications in sensing ^[60,61].

Graphene has attracted increasing attention as a promising tunable material thanks to the great potential in manipulating the Fermi level and carrier concentration ^[62]. While the existing microcavities suffer from a lack of tunability due to the fixed geometries and large bandgaps of constituent materials, it has become the first priority to be settled to realize the application in the fields of photonics engineering. Thus, the incorporation of graphene in microcavities is worth consideration and discussion.

Since the development of telecommunications and optical interconnections, it demands for optical modulators with exceptional performance, where ring modulators based on WGMs were successively developed with small dimensions, power consumption, and broadband response. Still, the narrow bandwidth that microcavities support limits their further applications. Since a graphene-based optical modulator has attracted strong attention, it was proposed that a novel graphene-integrated microring modulator was promising in featuring a large bandwidth through numerical simulations ^[63,64]. In 2015, the first graphene microring modulator was fabricated based on the leveraging critical coupling effects ^[65]. As depicted in Fig. 3(a), the integrated capacitor was used to modulate the loss of WGMs through the Fermi-level-dependent adsorption in graphene sheets ^[65]. By increasing the voltage, the absorption would be decreased, thus enabling the change from the undercoupled to critical coupled state. The modulation depth is over 15 dB with 10 V, as shown in Fig. 3(b), indicating the increased speed and efficiency using a graphene-integrated microring modulator. Since then, effective modulation has been achieved in a heterogeneous graphene microring with a high modulation depth, broadband response, ultrafast operation speed, and small footprint ^[66].

Moreover, considering the high value of graphene’s thermal conductivity, a thermo-optic microring modulator was demonstrated for an effective modulation featured with a small footprint ^[67]. As depicted in Fig. 3(c), the authors have transferred monolayer graphene on a microring resonator, where Joule heating was produced by electrodes. Once the heat is transferred onto the microring, there would be a change in the refractive index, thus enabling the modulation. As shown in Fig. 3(d), by applying an increasing voltage, there would be a redshift in the resonant wavelength and a decrease in the transmission drop, realizing “OFF” to “ON” state of the hybrid modulator. Further, thermo-optic all-optical devices based on the graphene-integrated microring modulator have been investigated, where the all-optical processing has advantages in power efficiency since the conversion between optical and electronic signals was no longer necessary ^[68–70]. Thus, a graphene-microcavity modulator is demonstrated as an efficient device based on mechanisms among electro-optic, electrorefractive, and thermo-optic, promising an ideal platform for applications ^[71–74].

As the tunable optical constants have been realized in the graphene heterogeneous microcavity, it can be applied in functional devices that depend on a specific relation between optical constants and resonances. Thus, the analog of electromagnetically induced transparency has been demonstrated with electrical control, showing great potential in light transmission for quantum information technology ^[75]. Additionally, the structure of the graphene-based microring was integrated with a source drain and top gating, as shown in Fig. 3(e), which enables a tunable-frequency Kerr comb by altering the Fermi level of graphene through gate tuning ^[76]. Figure 3(f) gives the primary comb lines under different gate voltages. For applied voltages of −1, −1.2, and −1.5V, the frequency offsets are observed at 2.36, 3.25, and 7.17 THz, respectively, while for the applied voltage up to −1.8V, there is no longer a frequency comb since it became harder for phase matching. Based on the gate-tunable primary comb, a series of phenomena is realized including coherent Kerr frequency combs, controllable Cherenkov radiation, and soliton states ^[77]. The electrically dynamic control of diverse comb outputs could enable the application of optical frequency combs in precision measurements and optical signal processing. Briefly, the remarkable tunability has made graphene a promising heterogeneous material for integration with microcavities as optical devices with high performance in fundamental researches and future applications of superior response speed and operation bandwidth.

Graphene surface plasma could be applied to transport and manipulate photons, which may confine the optical field and strengthen the light–matter interaction, leading to the spontaneous emission. ZnO, with wide direct bandgap (3.37 eV) and strong exciton binding energy (60 meV), has drawn intense attention in the research of short-wavelength lasers ^[78]. So far, different kinds of resonant cavities have been employed in ultraviolet lasing of ZnO, and WGM microcavities have excellent performance owing to their small volume, high Q factor, and low lasing threshold ^[79,80]. To further improve the spontaneous and stimulated emission, microcavities based on a monolayer graphene-coated ZnO microrod with hybrid modes are proposed. Graphene plasmon could help the near-field coupling in the ultraviolet range of ZnO emission, thus leading to optical confinement and lasing enhancement ^[81]. As shown in Fig. 4, the lasing intensity of graphene-coated ZnO microrods is significantly improved, which is dominated by the coupling between the graphene surface plasmon and the ZnO exciton emission ^[81]. It could be further validated through the optical images in Fig. 4, which demonstrated that a graphene/ZnO hybrid submicron microcavity can effectively confine the optical field and enhance the optical gain ^[82]. Afterwards, researchers have theoretically and experimentally investigated the lasing performance based on graphene/ZnO microcavities with the reduced lasing threshold, the increased Q factor, and the enhanced lasing peak intensity ^[82], which inspired research in low-threshold lasers using micro/nanostructures.

Additionally, the crystalline graphene layer could be employed as a nucleation layer to grow high-quality microrods via epitaxial growth. As for lasing applications, gallium nitride (GaN) microdisks have been prepared, realizing WGM lasing at room temperature ^[83]. Also, it is demonstrated that the GaN microstructures could serve as bendable light-emitting diode (LED) arrays, offering possibilities in the field of large-scale and flexible optoelectronic devices ^[84]. Besides, single-crystalline organic nanowires could also grow vertically on the surface of graphene, working as efficient optical waveguides and forming resonances ^[85]. Moreover, electrically driven nanolasers using graphene as electrodes were achieved first in 2012, based on a graphene-covered microdisk ^[86]. Graphene is expected to offer the potential in the combination of unique electronic properties and tunable optical performances. Thus, in the research of nanolasers based on single-crystalline microcavities, graphene not only helps with the epitaxy growth in a simple procedure, but also provides effective optical confinement and electric injection, realizing future on-chip light sources.

While graphene is gapless and semimetallic, TMDCs are semiconductors with a bandgap ranging from 1.0 to 2.0 eV that experience an indirect-to-direct bandgap transition as the number of layers is reduced to monolayer, creating new optical functionality in optoelectronics ^[87–91]. TMDCs offer strong light emission and high exciton binding energy due to the quantum confinement effect and reduced screening of the Coulomb force ^[92–95]. The outstanding optical properties make TMDCs potential materials for high-performance optoelectronics and photonics devices. However, the efficiency of a heterogeneous device is limited by the innate thickness of 2D materials, while integrating TMDCs with photonics structures, such as a microcavity and photonic crystal, offers a solution to enhance light–matter interaction. Thus, the coupling would benefit the research on excitonic properties of monolayer TMDCs and novel devices in 2D photonics and optoelectronics ^[96,97]. Particularly, the integration of TMDC material with WGM microcavities could support high-quality resonance, serving as a photonic light source ^[98,99].

TMDCs are promising candidates for novel optical and optoelectronic devices, which can be attributed to their direct bandgap when the number of layers is reduced to a monolayer ^[92]. The strong transition between valence and conduction bands is demonstrated to be excitonic, thus providing a feasible platform for the research on optical gain and stimulated emission. Further, excitonic lasing has been observed and studied through the integration of 2D TMDCs and optical microcavities, which allows for the significant enhancement on the interaction between the gain medium and photon ^[100–103]. First, the feasibility of lasing based on TMDC-integrated cavities was demonstrated by use of a photonic cavity, which could significantly reduce the lasing threshold of the emitter, as reported in 2015 ^[101]. In the same year, the electromagnetic environment of the TMDC emitter was further engineered through WGM microcavities ^[102,103]. Ye et al. explored a sandwiched configuration, where the optical gain medium was embedded between two dielectric layers [Si3N4/WS2/hydrogen silsesquioxane (HSQ)] with a diameter of 3.3 μm, as shown in Fig. 5(a) ^[102]. As the intensity of a 473 nm pump laser increases, the peak appears at 612.2 nm and increases sharply when the pump intensity is above 22.4mW·cm−2 [Fig. 5(b)]. To clearly demonstrate the excitonic lasing behavior, the pump intensity dependence of the photoluminescence (PL) intensity is plotted for both the cavity and background PL emission in Fig. 5(c). At low pump power, the cavity emission intensity increases linearly with the excitation power. There is an abrupt increase in the curve when the pump power is above the threshold of 20mW·cm−2, showing the evidence of excitonic lasing. A similar phenomenon was observed in the integration of a MoS2 flake at the interface between a free-standing microdisk and microsphere with room-temperature lasing performance, as shown in Fig. 5(d) ^[103]. At an excitation power of 100 μW (above threshold), such a unique microcavity could support high-Q WGMs, as depicted in Fig. 5(e) with the calculated mode position in the lower panel. The intensity shows a superlinear increase and a clear linewidth narrowing when the pump power exceeds the threshold, as shown in Fig. 5(f) ^[103]. Additionally, the near-field coupling scheme was efficiently applied on the WSe2-integrated microdisk, which could meet the demand in precisely measuring the excitonic emission for both in-plane and out-of-plane excitation ^[104]. The superior excitonic lasing performance in the visible range is attributed to the synergy effect of the large gain and the coupling between the gain medium and WGM microcavity, which effectively enhances the light–matter interaction.

Typically, the interaction between a few layers of MoTe2 and microring cavity has been studied, which indicates a feasible method to investigate and exploit the tunability of the coupling strength ^[105]. Meanwhile, the emission from TMDCs coupled to a microcavity will be influenced by many simple methods, and thus could be tuned towards the target properties. Photothermal effects are induced by photon absorption in TMDC materials when illuminated by an external light source ^[87,94]. The thermal effect was technically investigated by exploring a MoS2-coupled optical microdisk cavity ^[106]. Figure 6(a) depicts emission spectra at different power levels. It is observed that the PL intensity reaches its maximum under illumination power of 12.3 mW and starts to decrease as the power density extends the damage threshold since MoS2 is permanently damaged. Background PL spectra at these three power levels were normalized, as shown in Fig. 6(b). To demonstrate the thermal effect in the MoS2-coupled optical microdisk cavity, the PL peak position was studied, and redshifts were observed distinctively as the pump power increased from 0.47 to 12.3 mW. It is attributed to the local heating generated by nonradiative recombination processes in the MoS2 flake, which is coincident with the peak shift phenomena of monolayer TMDC dependence on the temperature ^[106]. However, there is no further redshift when the pump power is above the threshold, which can be ascribed to the permanent photoinduced damage caused by thermal oxidation of the MoS2 flakes. The result reminds researchers to avoid the thermal effect in experiments and future applications of the integrated microcavities by utilizing pulsed pump lasers to effectively minimize the heating and circumvent photothermal effects.

On the other hand, the character that emission properties could be influenced by an external factor can be exploited and applied. Mi et al. demonstrated the application potentials of MoS2 microcavities in a refractive index sensor, benefiting from the shift of emission peaks ^[107]. In their study, a straightforward fabrication method for the integration of MoS2 flakes and commercially available SiO2 microsphere resonators was first explored by the chemical vapor deposition (CVD) method ^[107]. Figure 6(c) shows that the triangular MoS2 flakes are uniformly distributed on the surface of SiO2 microspheres. Compared with the transfer method in other works, the direct deposition of TMDCs provides a better contact and thus a stronger coupling. Furthermore, the size, number of layers, and coverage of MoS2 are more controllable. The emission spectra clearly show whispering gallery resonances, which have been further validated through calculations using an explicit asymptotic formula and mode identification algorithm. The evanescent waves of WGMs are extremely sensitive to surrounding molecules, which could be monitored through the shift of the resonance peak as the refractive index varies where water serves as environment medium while certain ethanol is added to increase the refractive index. As refractive index Δn increases, resonant modes exhibit a linear redshift (Δλ), shown in Fig. 6(d), which demonstrates the high sensitivity of 150nm·RIU−1.

Thus, 2D-TMDC-integrated WGM microcavities have potential applications in optoelectronics devices ^{[99,102,103,107,108]}. In the future, more works should be carried out to develop a better understanding of exciton dissociation and quantum yield of the 2D materials, which will make sense to design structures and choose appropriate materials ^[104,109]. Van der Waals heterostructures consisting of different kinds of 2D materials exhibiting unique properties offer potential opportunities for novel photonics devices ^[110–113]. The integration of large-scale and high-quality bulk and monolayer samples is one of the critical tasks to improve the device performance.

Graphene and TMDCs are two kinds of 2D materials with great significances, and the study on their characteristics has created a new era. Still, there is a rising demand for research on new emerging 2D materials, e.g., black phosphorus and hBN. In 2019, the first cavity optomechanical system incorporating hBN was reported based on the WGM microdisk ^[114]. By positioning the hBN nanobeam in the optical near-field of a silicon microdisk, the optomechanical system is experimentally achieved and demonstrated with the observation of thermally driven mechanical resonances. Besides, the propagation of phonon polaritons in hBN potentially suggests its performance in light confinement to the deep subwavelength regime. Considering the excellent dielectric properties of hBN, a van-der-Waals-stacked hBN/WS2/hBN formed a microdisk cavity, giving rise to emission intensity and implying the feasibility of heterogeneous-material-involved devices ^[111]. While black phosphorus has a direct bandgap, it can be tuned through the number of layers from 0.3 to 2.0 eV, bridging the gap between graphene and TMDCs ^[115–117]. The in-plane anisotropic property of black phosphorus leads to its potential in nontrivial performances, e.g., light modulating and polarization tuning ^[118]. Thus, the photonic properties of black phosphorus may allow its integration with microcavities for application in novel optical components of on-chip devices. Most strikingly, it is intriguing to construct photonic devices based on many kinds of heterogeneous materials comprising different bandgaps and optical properties, realizing the highly-compatible microcavity-based devices.

Generally, there exists detailed investigation on 2D-material-integrated microring resonators as the coupling scheme, which possess on-chip superiority and fabrication convenience ^[65,67,76]. However, rolled-up tubular microcavities have their typical features in structure design, fabrication, and integration, thus enabling and optimizing the coupling with heterogeneous materials ^[119]. The rolled-up technique has been adopted to fabricate microtubes based on prestrained nanomembranes, realizing tubular mesostructures with nanoscale wall thicknesses ^[20]. In the past decade, optimizations on structure design have been adopted, which effectively improved the performance of tubular microcavities. Recently, the integration of heterogeneous materials has attracted intense attention ^[120]. Remarkably, an 8 nm thick gold layer was deposited as a cap on top of the dielectric tubular microcavity to achieve hybridized plasmon–photon modes ^[121]. It was found that the selective coupling of optical WGMs and localized surface plasmons depends on the location of the nanogap, showing a strong coupling-induced mode shift. Figure 7(a) depicts the hybridized modes, which are deduced from the lobe structure, indicating the tunable magnitude of the mode shift ^[121]. The efficient coupling of localized surface plasmons and confined resonances in the heterogeneous material microcavity is proved to be a promising strategy for the investigation of ultrasensitive detection, polarizers, and metamaterials, where light–matter interactions are involved ^[122,123].

On the other hand, with the rapid development in nanoscale fabrication and demonstration, it enables the sophisticated design on the geometry of the microtube cavity. The single-mode tubular microcavity [bottom panel of Fig. 7(b)] was proposed and realized, as demonstrated through PL measurements ^[19]. Such a microtube with aligned holes was fabricated based on the mechanical superiority of the diamond layer, as shown in Fig. 7(c). Through the coupling with the nitrogen-vacancy center emission in diamond ^[22], the PL mapping in Fig. 7(d) further verified the strongly enhanced light confinement and implied its possibility in single-mode lasing at the wavelength of 670 nm. The experimental realization on precisely tuning the subwavelength nanopatterns of the constituent layer confirms the feasibility of the rolled-up technology, which is compatible in the 2D-material--involved technique. The critically designed mesostructure in the rolled-up 2D material microtube may be able to bring a drastic breakthrough in critical light manipulation, indicating its high potential in quantum optical performance.

In summary, rolled-up technology is supposed to be an efficient and controllable method for WGM microcavities with the versatility in both heterogeneous material combination and accurate structural design. As sketched in Fig. 8(a), the rolling method would endow such heterogeneous 2D material microtubular cavities with on-chip, CMOS-compatible, and 3D-integratable properties. Recently, Yin et al. demonstrated the graphene-activated optoplasmonic microcavity by transferring the graphene layer onto the rolled-up microtube ^[124]. While the presence of graphene facilitates an enhanced sensitivity in surface detection, the photodegradation of dye molecules is monitored, paving the way for real-time and high-precision sensors. Meanwhile, graphene/oxide microtubes were constructed based on a CVD-grown monolayer graphene [Fig. 8(b)] with a transfer-free advantage, as shown in Fig. 8(c) ^[125]. The integration of graphene enables intriguing photonic phenomena, such as polarization-dependent enhancement ^[46,47], which is further demonstrated through simulation results, as shown in Figs. 8(d) and 8(e). Besides, in-plane strain would be introduced in the graphene layer during the rolling process, which indicates tunable properties towards target performances for the graphene-based microtube as optical devices. Most importantly, since the 3D configuration could support optical resonances, the coupling in graphene-integrated tubular microcavity presents high potential in expanding the optical community with novel applications. Additionally, with the nanoscale thickness tube wall, it would ensure an extremely efficient coupling between the 2D materials and microcavities through the evanescent field. We believe that such hybridized modes in rolled-up microcavities may offer a novel-type platform in lasing, sensing, and light modulating.

This review summarized recent research regarding the integration of 2D materials and optical WGM microcavities. The nontrivial band structure of graphene enables a series of appealing phenomena. The heterogeneous graphene microcavities can support hybrid plasmonic WGMs and exhibit ultrahigh sensitive performance. Most importantly, based on the synergy effect of graphene and the WGM microcavity, the heterogeneous structure features a tunable effect, paving the way in light modulation for realistic devices with compact, efficient, broadband, and fast properties. Considering the light confinement of graphene, the advantages of using graphene as both the electrode and the substrate for epitaxial growth further indicate its possible potentials in nanolasers. As another important type of 2D material, TMDCs could boost superior emission, enabling novel light sources in the visible range. Moreover, the emerging of black phosphorus and hBN would broaden the spectral range, realizing the total coverage from ultraviolet to the microwave, possessing possible potentials in nanophotonics. In conclusion, the integration of 2D materials with WGM microcavities could highly enhance the light–matter interaction, thus optimizing the performance of 2D materials towards targeted functional devices, also providing a novel and feasible solution in light control and manipulation.

With the development of information and communication in our daily life, it demands ultrafast and low-loss techniques, which makes the method of using electrons as information carriers no longer qualified. The proposal of “photon circuits,” where photons carry and transfer messages, is supposed to be a viable scheme. Therefore, research on integrated circuits with optical devices becomes essential. Meanwhile, the emerging field of 2D materials provides the traditional WGM microcavities with new schemes in light control and light–matter manipulation, implying potential applications in the emitter, modulator, and photodetector.