Functionalizing nanophotonic structures with 2D van der Waals materials

Abstract

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

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1. Introduction

The integration of functional materials into photonic structures is essential for building high-performance integrated optoelectronic systems¹ and an ideal platform for investigating nanophotonic phenomena.² The field of photonic integrated circuits^3–5 and nanophotonics^6,7 has achieved vibrant progress for optical communications,^8–10 artificial intelligence,^11–13 quantum technology,^14,15 imaging,^16–18 sensing,^19,20 and displays,^21–23 to name a few. However, issues such as insufficient multifunctionality and reconfigurability still await further attention. To address these challenges, novel functional materials are required beyond conventional single silicon or a silicon nitride photonics platform²⁴ to generate new functionalities for or impart versatile reconfigurability to the currently established photonic nanostructures.²⁵

Two-dimensional (2D) materials are one of such candidates with promising optoelectronic attributes^25–28 and have attracted immense research interest over the past decades since the debut of graphene.²⁹ Including graphene,³⁰ transition-metal dichalcogenides (TMDs),³¹ 2D carbides and nitrides (MXenes),^32–34 hexagonal boron nitride (h-BN),³⁵ and emergent candidates like quasi-2D halide perovskites,^36–43 a big family of 2D materials are established, with a vast collection of available bandgap values.⁴⁴ As naturally layered materials with interlayer van der Waals interactions, these materials can be exfoliated into atomic monolayers to unveil extraordinary electronic and optoelectronic properties.⁴⁵

Conventional approaches to impart functional materials rely on heteroepitaxy, which encounters lattice-matching and process compatibility constraints,⁴⁶ greatly limiting the possible material combos for heterogeneous integration in photonics. When an epilayer and a substrate have different crystal structures or the same crystal structures but with a high lattice parameter distinction, a poly-crystalline phase and defects tend to be generated above certain thickness with compromised material performance.⁴⁷ In contrast, devoid of the one-to-one chemical bonding between the material layer and the substrate, the 2D materials with signature van der Waals interfaces have made them very easy to transfer, mix, and integrate with other material platforms or prefabricated photonic architectures.⁴⁸ As prime building blocks for van der Waals integration,^48–53 the ultrathin and flexible nature of 2D materials also enable the transfer onto three-dimensional (3D) non-planar optical structures and permit wearable, implantable, and bio-compatible optical applications.^54–57

In this review, we outline recent advances in van der Waals materials-enabled nanophotonic applications.^50,58,59 We highlight the intriguing outlook on combining 2D van der Waals materials and photonic structures for infusing novel device functionality and reconfigurability to conventional photonic structures and the emergent platform to investigate nanophotonic physics.^2,60,61 Fundamental attributes of representative 2D materials and their coupling to exemplary optical structures, such as dielectric waveguides, optical fibers, photonic crystals, and metasurfaces,^10,62,63 are cataloged. Furthermore, we also underline recent advents in the van der Waals integration of 3D freestanding nanomembranes. Awaiting challenges and exciting opportunities in this field are also discussed based on current perspectives.

2. Fundamentals for 2D photonics

Nanophotonic structures can be functionalized with diverse 2D materials depending on the base function of the optical architectures and the intrinsic attributes of the 2D materials (Fig. 1).^64–67 The valuable fundamental optoelectronic properties and practical considerations on engineering their optical physical coupling are discussed in the following.⁶⁸

Fig. 1 Functionalizing nanophotonic structures with 2D materials. (a) Representative bandgap values of exemplary 2D materials for applications at different wavelengths.⁴⁴ Beside 2D monolayers, black phosphorus (BP) and quasi-2D perovskites (PVSKs) showcase useful monolayer number (n) dependent bandgaps.^114,122 (b) Synergization of various 2D materials with different functional photonic structures. Exemplary physical attributes of the nanophotonic structures are listed in blue color.

2.1. 2D material fundamentals

Frequently applied 2D van der Waals materials in nanophotonics are typically monolayer to few atomic layers in order to reveal extraordinary attributes.^27,30 Their outstanding properties along with the atomically clean and electronically keen vdW interfaces to build artificial vdW heterostructures^48,69–71 and Moiré superlattices^72–75 have rapidly burgeoned as one of the prominent research directions in materials science, photonics, and nanotechnology communities. The complementary metal-oxide semiconductor (CMOS)-compatibility and handiness for transfer have made them candidates for nanophotonic integration (Fig. 1a).⁷⁶

As a monolayer sheet is composed of carbon atoms in a honeycomb-lattice, graphene is a zero-bandgap semimetal with benefits of low density of states,³⁰ high carrier mobility,⁷⁷ and tunable optical transitions.⁷⁸ Its unique band structure also enables nonlinear optics^79,80 and gate-tunable surface plasmon polaritons (SPPs),^81–83 as well as applications for transparent electrodes,^84–86 sensors,⁸⁷ and broadband optical modulation and photodetection.⁸⁸

In contrast to graphene that lacks an electronic bandgap, 2D semiconductors such as black phosphorus with puckered atomic monolayers exhibit a broadly tunable bandgap from around 0. 3 eV to 2.0 eV depending on layer numbers from the bulk to the monolayer.^89,90 For TMD with the general formula of MX₂, where “M” represents the transition-metal atom (such as Mo and W), and “X” stands for the chalcogen atom (such as S, Se, or Te),²⁷ their bandgap changes from indirect to direct when exfoliating to monolayers.⁴⁴ Therefore, these 2D semiconductors are ideal candidates to realize active optical applications, such as light sources, optical amplification, transistors, neuromorphic computing units, and photodetection.^91–98 Moreover, a majority of 2D TMDs also possess alluring exitonic and polaritonic physical attributes that are worth studying.^74,99–102

h-BN is a wide-bandgap (∼6 eV) 2D material that was originally applied as the optimal substrate for graphene and can be used as insulating or encapsulation layers.^35,103 It can provide dangling-bond-free interfaces for photonic van der Waals integration,⁵⁰ possessing also hyperbolic phonon polaritons,^104,105 single-photon emission,¹⁰⁶ and second-order nonlinearity.¹⁰⁷

Besides conventional 2D materials, 2D MXenes refer to a set of transition metal carbides, carbonitrides and nitrides with layered lattices bound by van der Waals forces as emergent 2D candidates.^32,108,109 With the general chemical formula of M_n+1X_nT_x (n = 1–3; transition metal “M” = Sc, Ti, Zr, Hf, V, Nb, Mo, and so on; “X” = carbon and/or nitrogen and “T_x” is the surface terminations like hydroxyl, oxygen, or fluorine³³), they have useful optical applications for saturable absorbers, photodetectors, and modulators.^110–112

Metal-halide perovskites are representative organic–inorganic hybrid perovskites that have attracted tremendous research interest over the past decade due to their promising optoelectronic attributes¹¹³ for applications in photovoltaics, lasers, LEDs, photodetectors, and nonlinear optics.^39,114–121 They typically have corner-sharing BX₆⁴⁻ octahedra, where “B” stands for a divalent metal cation like Pb²⁺, Sn²⁺ or Ge²⁺, and “X” denotes a monovalent halide anion.¹¹⁷ Inspired by advents in the 2D materials community, a recent study shows that these materials can also be made into molecularly thin versions like 2D materials.¹²² With varying dimensions and layer numbers n (Fig. 1a), quasi-2D metal-halide perovskites have different electronic properties, excitonic coupling strength, and varying degrees of quantum-and dielectric-confinement effects for integration with nanophotonic devices.¹²³ For instance, they are good photovoltaic candidates. The presence of organic spacer cations in the material acts as an insulating layer, limiting carrier diffusion between stacked inorganic layers and impeding carrier extraction towards charge transport layers. Consequently, the orientation of layered 2D halide perovskite crystals on a substrate plays a crucial role in determining the overall efficiency of solar cells.^117,124,125 Quasi-2D perovskites can also exhibit a notable photoluminescence quantum yield and their photoluminescence emission color can be handily adjusted by modifying the composition or layer number n.^38,123 Consequently, 2D halide perovskites possess the capability to generate a broad spectrum of light emissions.¹²⁶ Moreover, the relatively narrow full width at half maximum of the emission peak contributes to enhanced color purity as a favorable candidate for high-performance LEDs.¹¹⁴

2.2. Photonic structures for coupling with 2D materials

In nanophotonics, judiciously designed optical structures are essential for a vast number of applications.²⁵ For instance, dielectric waveguides are the crucial building blocks in photonic integrated circuits.³ As illustrated in Fig. 1b, when 2D materials are transferred to a photonic waveguide or an optical fiber, they can evanescently couple with the guided electromagnetic modes inside to alter its effective mode index, light propagation, and dispersion.⁵⁰

Optical cavities can spatially confine light into small volumes^127,128 to boost light–matter interaction strength for nonlinear,^128–131 lasing,¹³² and sensing applications^19,133 when integrated with 2D materials. The synergy of temporal accumulation and spatial confinement of photons can drastically enhance the efficiency of encapsulated 2D materials to the optical resonators.

Photonic crystals are periodic nanostructures with dimension features similar to light wavelength. Under periodic refractive index perturbations, a photonic bandgap emerges.¹³⁴ They can also be applied as optical cavities for light enhancement and light guiding structures by intentionally introducing point and line defects, respectively.¹³⁴

In contrast, metasurfaces are engineered optical scatterer arrays with a subwavelength dimension that can achieve powerful control over the fundamental attributes of light,^{22,95,135–144} such as the phase, wave vector, amplitude, and frequency,^145–148 with widespread application such as meta-lenses,^149,150 high-efficiency holograms,^151,152 color display,^21,152–154 ultrathin cloaks,^155–157 healthcare sensors,^158–162 LiDAR,^163–165 functional meta-waveguides,^10,166–180 and nonlinear applications,^181–183 to name a few.

3. Applications on 2D photonics

Armed with the diverse 2D van der Waals materials building blocks and a large library of photonic structures, massive optical and optoelectronic applications can be prototyped (Fig. 2a–n). Exemplary preparation methods for 2D materials include chemical vapor deposition (CVD), molecular beam epitaxy (MBE), pulsed laser deposition, sputtering, mechanical exfoliation, liquid-phase exfoliation, and so on (Fig. 2o). The recent advents and opportunities are outlined below based on the primal photonic structures coupled with 2D materials.

Fig. 2 Exemplary 2D-materials-based photonic and optoelectronic applications. (a) Broadband graphene–Si optical modulator.¹⁸⁹ (b) Broadband photodetector using the MoS₂-WSe₂ van der Waals heterostructure.⁴¹⁰ (c) Near-field infrared nanoscopy using hBN.⁴¹¹ (d) Nanocavity laser based on monolayer 2D WSe₂.²¹⁹ (e) Excitonic WS₂ microdisk nanolaser.²²¹ (f) Gate tunable graphene–SiN frequency comb.²³⁶ (g) Perovskite solar cells.¹⁰ (h) Broadband graphene polarizer.²⁵¹ (i) 2D tunable meta-lens.³⁰⁵ (j) OAM detection with WTe₂.⁴¹² (k) Optical biosensor based graphene–MoS₂ layer on a prism.⁴¹³ (l) Broadband graphene image sensor array.⁴¹⁴ (m) Wearable and implantable soft bioelectronics using 2D materials.³⁴⁸ (n) Ultrafast graphene plasmonic all-optical switch.¹⁹⁷ (o) Preparation methods for 2D materials. Inspired schematics from ref. 10, 189, 197, 219, 221, 236, 251, 305, 348 and 410–414.

3.1. Optical waveguides

As the essential building block of photonic integrated circuits,³ photonic waveguides are physical structures to guide the propagation of electromagnetic waves.¹⁰ By transferring 2D materials such as graphene,^26,184 the effective mode index of the underlying dielectric bus waveguide can be controlled via the structure engineering of the waveguide,¹⁸⁵ as well as via optical pumping,¹⁸⁶ electrical gating,⁷⁸ strain,¹⁸⁷ or thermal tuning¹⁸⁸ of the graphene monolayer for optical amplitude¹⁸⁹ or phase modulators^88,190 (Fig. 2a). The giant optical nonlinearity in graphene and 2D TMD can also be harvested using 2D-laminated waveguide structures for gate-controlled optical nonlinearity.^79,191,192 The graphene-integrated waveguides can be leveraged as broadband photodetectors for harvesting guided light signals as well.^{65,77,91,193,194} For other 2D materials such as TMD and black phosphorus, similar applications are also developed while with different operation wavelengths (Fig. 2b),^195,196 and they can be more promising for active optical applications due to the varying bandgap values of these 2D materials.^27,44,101

Besides, graphene is also an excellent platform to exploit surface plasmon polaritons.^82,184 Plasmonic graphene modulators may thus have very compact footprints and fast operation speed albeit higher optical loss (Fig. 2n).^197,198 Low-loss surface plasmons can be supported in graphene/h-BN heterostructures with favorable performance in terahertz regions.^81,199–202 In contrast, other 2D materials such as TMD are more favorable to exploit nanophotonic polaritons and excitons^203,204 and h-BN for phonon polaritons (Fig. 2c). Graphene and other 2D materials can also serve to add reconfigurability to control the coupling^169,205 and dispersion²⁰⁶ of the optical waveguides to permit chip-scale optical signal processing. Applications such as plasmonic sensors,^207,208 transformation optics,²⁰⁹ nano-imaging,^80,210 and gain-modulated lasing applications^211,212 can be prototyped as well.

3.2. Micro-cavities

Optical cavities with varying formats are ideal for optical applications that require optical field enhancement and/or strong light–matter interactions.²¹³ Photons can be confined to very small volumes by resonance and recirculation. For instance, Fabry–Perot microcavities fabricated by depositing Bragg reflectors, engineered photonic crystals, and whisper-gallery resonators such as micro-spheres, micro-toroids, and micro-ring resonators with varying quality (Q) factors from thousands to billions.²¹⁴ The 2D materials such as TMD and black phosphorus can be transferred to or encapsulate into prefabricated optical cavities to harvest strong light enhancement for nano-lasers^215–221 (Fig. 2d and e), optical amplification,²²² sensing,^57,223 and photodetection^224,225 to make the most of the atomically thin semiconductor materials with an optical gain.²²⁶

Another exemplary application direction of 2D materials-coupled optical cavities is nonlinear optics.²²⁷ Engineered giant light field enhancement and nonlinear field overlap, the nonlinear optical responses in graphene, TMD, quasi-2D perovskites, black phosphorus, and so on can be boosted in the optical nanocavities^228–230 for modulation,^231–233 switching,^234,235 tunable frequency combs²³⁶ (Fig. 2f), and harmonic generation,^237,238 and solar-cells (Fig. 2g) to name a few. Alternatively, 2D material nano-sheets can also be directly patterned into nano-resonators by frequency conversion.²³⁹

3.3. Optical fibers

As the cornerstone of modern communication system, optical fibers lay the foundation of information communication technology,²⁴⁰ and have been studied for the past few decades with extensions to many fields.^241–243 Under the extensive exploration of graphene²⁴⁴ and other 2D materials,^31,245 numerous fiber optic devices based on van der Waals materials have been developed throughout the years as a result of their fascinating physical features.^64,196,246 Graphene fiber is a typical instance which incorporates graphene to an optical fiber to couple with the propagating light signals inside.²⁴⁷ Compared with the case of integrated dielectric waveguides, 2D materials-coupled optical fibers can have much a longer interaction length between the guided electromagnetic field and 2D materials^64,248 for nonlinear harmonic generation,²⁴⁹ sensors,²⁵⁰ mode convertors,²⁴⁰ and imaging.^241–243 By harvesting the giant planar anisotropy of graphene, a polarization controller based on graphene has been prototyped that profits from distinct responses upon TE and TM polarized waves by integrating graphene into the side-polished fiber (Fig. 2h),^251–253 and microfiber,^254–256 as well as other 2D materials sharing similar properties such as BP^257–259 and ReS₂.²⁶⁰

The high carrier mobility^261,262 and fast response times²⁶³ inside graphene suggest it to be a promising candidate to be used in fabricating high-efficiency photoelectric conversion devices, but graphene is not ideal for an active layer due to the lack of a bandgap and relatively low optical absorption.^193,264 Although the absorption performance can be promoted through band structure engineering²⁶⁵ or utilizing the hot electrons generated by the thermo-optic effect,²⁶⁶ another alternative strategy is combining graphene with other optical components^267,268 or using TMDs. Monolayer TMD materials can be a competitive choice applied as the active layer interacts directly with outside optical sources.²⁴⁶ Even though having the merits of miniaturized footprints and high conversion efficiency, these 2D materials-based photoelectric conversion devices generally suffer from an obvious drawback of the atomic layer thickness which limits the generation of photocarriers,²⁶⁹ and various strategies have been adopted in order to enhance the light–matter interaction, such as the photoactivation method,²⁷⁰ using heterostructures,^271,272 and certain encapsulation.²⁷³ A high-performance optical fiber-based modulator is another application direction exploiting various schemes such as thermo-optic,^274,275 electro-optic,^248,276 and nonlinear effects^277,278 based on graphene, BP, TMDs, and so on.^279,280 The 2D materials have also been widely used in fiber sensing,^250,281 fiber lasers,^253,282 and nonlinear optics.^248,283

To incorporate 2D materials into optical fibers, two approaches can be applied, namely the transferring-based method and the CVD/direct synthesis-based method.²⁸⁴ Taking graphene as an example, at the early stage of graphene-integrated fibers, the transferring method has been proven a feasible way for transferring graphene layers to the end face of an optical fiber^285,286 and the exposed area near the core of tapered^277,287 or side-polished fibers.^251,288 With the development of graphene growth technology, the CVD method gradually became a promising option featuring mass-production perspectives with high throughput, providing remarkably light–matter interaction albeit require well-controlled fabrication processes.^248,249 The possibility of growing other 2D materials on fibers through the CVD method has also been realized in recent pioneering works.^289,290

3.4. Metasurfaces

Metasurfaces with artificially engineered optical nanoantennas have showcased unprecedented degrees of freedom in controlling massive fundamental light attributes.^135,146 By judiciously altering the material and structure design of the subwavelength nanostructures, the reflected or refracted electromagnetic wave can be flexibly tailored.^291–294 As a 2D counter part of metamaterials,^295,296 intriguing nanophotonic phenomena and applications can be hatched by combining the 2D nanomaterials.^297,298

The 2D materials as an ultrathin nano-sheet can be transferred on top of the metasurface to permit electrically or optically reconfigurable or programmable meta-devices (Fig. 2i) for direct polarization or orbital angular momentum detectors (Fig. 2j),²⁹⁹ right routing,⁶¹ beam steering,³⁰⁰ lasing,³⁰¹ and sensing^302,303 (Fig. 2k). The metasurface structure can also enable enhanced light-2D material applications for spontaneous control over harmonic signal generation and beam control.^137,304 At the same time, the 2D materials can be also patterned into atomically thin metasurfaces for tunable planar optics,^305,306 light sources,^307,308 beam splitters,³⁰⁹ and so on.

For instance, light possesses multiple attributes, such as intensity, wavelength, polarization, spin, and orbital angular momentum (OAM) for multiplexing and conveying abundant information. However, conventional 2D material photodetectors only detect the light intensity of certain wavelengths which results in a loss of information during the photo-charge conversion process. In contrast, metasurfaces offer an ideal solution to functionalize 2D photodetectors to achieve multi-degree-of-freedom customized direction detection⁶⁷ on different light properties.^{137,310–314} For instance, an on-chip polarimeter comprising four metasurface-integrated graphene-silicon photodetectors can detect the full-Stokes parameters, including the intensity, orientation, and ellipticity of arbitrarily polarized incident infrared light.³¹¹ Integrated direct OAM detectors can be prototyped as well using 2D TMDs (Fig. 2j).

Besides, incorporating metasurfaces with nonlinear 2D optical materials can enhance design efficiency and convenience. A hybrid structure was proposed to combine the monolayer TMD with the plasmonic metasurface, featuring a chirped metasurface with a gradient-varying groove depth,³¹³ to achieve a coherent SHG signal across the entire visible spectrum.³¹⁴ A nonlinear synthetic metasurface with the WS₂ monolayer was also reported to entangle the phase and spin of light simultaneously as well as manipulate the nonlinear valley-locked chiral emission using 2D WS₂ at room temperature.¹³⁷ Owing to the 2D materials’ diminutive thickness and consequent feeble interaction with light, by integrating the metasurface (typically metal) and graphene into a hybrid structure, the interaction between graphene and light can be greatly enhanced, thereby enabling the in situ control of the optical response from terahertz to infrared wavelengths.^314,315 In addition to 2D materials, other low-dimensional materials are also hybridized to metasurfaces for controlled light–matter interactions, lasing, sensing, photocatalysis, polariton physics, and quantum applications.^316,317

3.5. Quantum dots and nanowires

Nanostructures such as nanowires (NWs) and quantum dots (QDs) can be optically coupled with 2D material films for active device applications. This coupling is advantageous because these structures can be grown through strain relaxation for high-crystalline structures.³¹⁸ The unique quantum confinement properties of NWs and QDs allow for precise control over the movement of electrons, making them highly promising for light emitting devices such as LEDs and laser diodes.³¹⁹ The combination of as-grown NWs and quantum dots (QDs) with transferred 2D or 3D functional nanomembranes offers the possibility of creating novel quantum structures for advanced photodetection and light sources (Fig. 2l).^320,321

Besides, we can easily spin-coat or transfer NWs (1D) or QDs (0D) onto 2D van der Waals films to achieve 0D/2D or 1D/2D hybrid structures¹⁴ to enhance the interaction between light and 2D materials. For instance, metals can function as both photonic components and electrodes to improve the transmittance and external quantum efficiency of the device.^322,323 The coupling of nanowires allows for the manipulation of single photon emission and guiding of the emitted photons through the nanowires for quantum emitters.^324,325 By combining the Si waveguide and monolayer MoS₂, one realized the effective control of directionality, polarization state and spectral emission.³²⁶ When coupled with optical fiber NWs, a higher conversion efficiency was achieved on 2D material based second harmonic generation (SHG).²⁶⁹ In addition, optical anisotropy can also be achieved by adjusting the morphology of nanostructures, which allows for the control of resonance frequency and quality factors, using, for instance, 2D material–hybrid plasmonic or optical nanowires.^325,327 Moreover, photodetectors can also benefit from 2D materials-coupled QDs with higher responsivity of the 2D films and increased light absorption thanks to the trapping and enhancement of light near the QDs.³²⁸

4. Outlook

The synergy between 2D materials and various photonic structures has proven fruitful results in numerous novel optical devices and applications. Exemplary 2D materials and their vdW heterostructure-based photodetectors are compared in Fig. 3a according to their performance indicators.^329–332 Graphene, BP, and certain 2D selenides are widely applied to develop broadband devices thanks to their wide-spectrum light absorption or engineerable bandgaps, as indicated with colored horizontal lines. For those operating wavelengths beyond the range of the horizontal axis, gradients are applied, and the maximum wavelength is directly marked out. In contrast, 2D TMDs have proven fruitful results in prototyping photodetectors with high responsivity and fast response times. Representative 3D materials-based photodetectors are also marked out (as open symbols in Fig. 3) for comprehensive comparisons, such as Si, Ge, and group III–V materials,^333–336 where they typically are free of the limitations of 2D materials in terms of the limited film thickness towards sufficient light absorption.

Fig. 3 Comparison of exemplary photodetectors and optical modulators based on 2D materials. (a) Scattering plot comparison of responsibility (top) and response time (bottom) versus wavelength for representative 2D materials-based photodetectors (filled symbols). Data are from ref. 329–332. Colored horizontal lines indicate corresponding broadband devices. Exemplary 3D materials-based photodetectors (open symbols) such as Si, Ge, and III–V materials are also marked out for benchmark.^333–336 (b) and (c) Scattering plots for optical modulators. (b) Left: comparison of modulation depth and optical operation bandwidth (calculated by |c/λ₁ – c/λ₂|, c is the light speed and λ₁ are λ₂ are start and stop operation wavelengths, respectively) for representative 2D materials-based modulators.^{280,337–342} Right: typical response time of 2D optical modulators with different schemes: all-optical (A-O), electro-optical (E-O), and thermal-optical (T-O). Data from ref. 280, 343 and 344. (c) Left: comparison of silicon-based graphene EO modulators (filled symbols).^{88,279,345,346} State-of-the-art 3D materials-based optical modulators are also marked out for comparison (open symbols).^{279,415–418} Right: operation wavelength range of certain 2D optical modulators. Data collected from ref. 280 and 338–342.

Another heavily researched direction is incorporating 2D materials for optical modulation. Typical 2D materials-based optical modulators with different operation schemes are summarized in Fig. 3b, in which the horizontal axis stands for optical operation bandwidth (detailed in figure caption) and the vertical axis stands for modulation depth.^{280,337–342} To realize high modulation depth (dB μm⁻¹), for instance, the coupling of 2D-laminated optical waveguides needs to be maximized.¹⁸⁵ The optical bandwidth, on the other hand, mostly depends on the specific 2D material and device structure (for example, resonance structures like ring resonators, high-Q photonic crystals typically leading to narrowed operation wavelength range).^196,279 Different modulation mechanisms also exist. The response time^280,343,344 and operation wavelength^{280,338–342} belonging to different modulation schemes are also drawn respectively in the right panel of Fig. 3b and c. Thermal-based 2D optical modulators typically have a comparatively lower response time with certain thermal cross-talk, while all-optical-based schemes exhibit an ultrafast modulation response time to even femtosecond level.¹⁹⁷ Electro-optical (EO) modulators have a fast operation speed between thermal- and all-optical modulators with the highest integration compatibility for photonic integrated circuits,³ which has thus drawn tremendous research interest (mostly implemented as graphene-silicon-based hybrid EO waveguide structures as the left panel of Fig. 3c).^{88,279,345,346} Emergent 3D materials such as lithium niobate and pure-Si modulators (open symbols) are marked out as well for comparison.

Despite prior vibrant research progress in 2D photonics, several practical challenges still await before enabling new opportunities.

4.1. Challenges

Despite that high-quality 2D material flakes can be produced via mechanical exfoliation,²⁹ they are typically small in size and irregular in shape, and may thus hardly meet the requirements for future industrial practical applications.^88,347 In terms of the further commercialization of 2D-materials-based photonic and optoelectronic devices,²⁸ wafer-scale scalable and reproducible 2D material growth and transfer are required.^348,349 Liquid-phase exfoliation has a low mass-production cost, but the quality and size of the 2D materials are comparatively low.³⁴⁷

Considering the yield and quality, chemical vapor deposition (CVD)-based approaches may permit potentially higher throughput and lower cost compared to mechanical exfoliation or molecular beam epitaxy.^350,351 Nevertheless, besides graphene, the direct CVD synthesis of continuous and uniform monolayer 2D materials such as TMD and h-BN is still challenging.^{350,352–355} To address this trade-off in monolayer controllability and large-scale film uniformity of high-quality 2D materials, the geometrically confined 2D material growth with pre-defined growth pockets can be considered (Fig. 4a).³⁵⁶ The ultimate goal might be the direct synthesis of high-quality 2D monolayers or 2D heterostructures, as a transfer-free method, on top of prefabricated photonic structures.^357,358 For the transfer of 2D materials, roll-to-roll scalable transfer^359–361 or robot-based precise automatic 2D flake transfer^362,363 will be favorable for industrial production.²⁸ Large-scale mechanical exfoliations using nickel³⁴⁸ or atomically flat gold layers³⁶⁴ with the aid of automatic robotic tools are also promising to produce over millimeter-scale 2D films with minimal transfer residue.

Fig. 4 Scalable manufacture perspectives of van der Waals materials with exemplary applications. (a) and (b) 2D vdW films. (a) Geometrically confined grown single-crystalline wafer-scale 2D transition metal dichalcogenides (TMD) towards future industrialized 2D photonic integrated circuits.³⁵⁶ (b) Layer-resolved splitting of wafer-scale monolayer (ML) 2D materials.³⁴⁸ (c)–(f) 3D “artificial vdW films”. (c) High-throughput manufacturing of 3D freestanding epitaxial nanomembranes via 2DLT.⁴⁰⁰ (d) Single-crystalline 3D nanomembranes on graphene nanopatterns for film exfoliation and substrate recycling.³⁸⁷ (b) Reconfigurable hetero-integrated optoelectronic systems with embedded artificial intelligence using vertically stacked optical layers like Lego bricks on a chip.³⁹⁰ (c) Top: stretchable Ga–In thin-film LEDs.³⁹⁵ Bottom: GaAs LEDs made by remote hetroepitaxy on Ge substrates.³⁸⁷ Scale bars: 1 cm (top) and 10 μm (bottom). Panels are adapted with permission from Springer Nature (a,³⁵⁶ c,⁴⁰⁰ d,³⁸⁷ e³⁹⁰ and f^387,395), from AAAS (b³⁴⁸).

4.2. Perspectives

Recently, several emerging 2D materials with exotic attributes have provided further opportunities,^38,117,122 such as 2D van der Waals materials with vanished interlayer coupling for scalable nonlinear optical applications,³⁶⁵ quasi-2D perovskites with high crystallinity,^37,40,122 and ferroelectricity and ferromagnetism in atomically thin van der Waals layered materials.^366–369 Due to their quantum confinement of charge carriers and excitons and tunable bandgap, quasi-2D halide perovskites have excellent optoelectronic properties for light sources, photovoltaics, and photodetectors.^122,370 The limited stability of conventional 3D halide perovskites has posed a significant challenge to their commercialization. Employing quasi-2D perovskites can be a substitute for their 3D bulk counterparts, where the metal-halide octahedra are better shielded against moisture and oxygen due to the presence of hydrophobic organic ligands. These organic ligands can also contribute to reducing the distortion of the metal-halide octahedra and preventing phase transitions by means of steric hindrance with comparatively suppressed anionic migration as well.

Confined growth-enabled large-scale integrated photonic devices³⁵⁶ are also promising for future industrialized 2D integrated photonic circuits (Fig. 4a).^10,357 The layer resolved splitting technique that engineers the stressor level difference of the top and bottom metal layers (typically nickel) can also be applied to produce wafer-scale big 2D monolayers from bulk 2D stacks (Fig. 4b).³⁴⁸

Besides, 2D materials that have a minuscule thickness limit their interaction length with electromagnetic waves,⁵⁰ and recent advances on remote epitaxy^371–373 and van der Waals epitaxy^374–376 also permit various three-dimensional (3D) freestanding nanomembranes (Fig. 4c),^{50,375,377–380} as well as other 3D layer exfoliation techniques by epitaxial chemical lift-off, mechanical exfoliation or laser lift-off.^68,381–386 These thin films are also made ultrathin with artificially defined van der Waals interfaces for photonic van der Waals integration,^48,50,120 on graphene-covered templates for defect-reduced heteroepitaxy (Fig. 4d) with potential to re-use the costly wafers towards film manufacture cost reduction.³⁸⁷ These 3D nanomembranes can preserve their optical attributes almost independent from film thickness^68,386 for versatile light-emitting,³⁸⁸ sensing,³⁸⁹ smart embedded optoelectronic processors (Fig. 4e),³⁹⁰ light enhancement,^391,392 new integrated photonics platforms,³⁹³ solar cells,³⁹⁴ flexible LEDs (Fig. 4f),^387,395 and other novel hetero-integrated devices.^68,396,397 Nanomembrane mass production could also be enabled with a reduced cost.^{387,398–400}

By combining the 2D and 3D van der Waals material building blocks, vast playgrounds also unfold for 2D-materials-based photonic integrated circuits^{3,4,25,66,279} (Fig. 5a), exotic nanophotonic polaritons in hybrid heterostructures^2,58,59 (Fig. 5b), flexible, wearable and implantable optoelectronic biosensors^{20,54,158,207} (Fig. 5c), and vertical 3D integrated circuits and systems^{390,401–403} (Fig. 5d). Stacked 2D vdW heterostructures with a certain precise twist angle have enabled exciting electronic attributes from the interference of Moiré superlattices.^72,75,404 The deterministic stacking of hybrid 2D and 3D vdW materials⁴⁰⁵ or nanophotonic structures^406–409 may hatch intriguing twist-induced photonic and polaritonic physics as well.⁷⁴

Fig. 5 Outlook on 2D van der Waals photonics. (a) On-chip integrated photonic and plasmonic circuits based on 2D van der Waals (vdW) materials.² (b) Novel polaritonic physics and light manipulation based on 2D and hybrid vdW heterostructures.^2,50 (c) Wearable and implantable optoelectronic biosensors.^419–421 (d) Stackable optoelectronic chip circuity for scalable and multifunctional nano-systems.³⁹⁰ Panel (a) and the right panel of (b) are adapted from ref. 2 (Springer Nature) with permission. Panel (c) right is adapted with permission from ref. 419 (Wiley). The left panels of (c) are inspired from ref. 420 and 421. Panel (d) is inspired from ref. 50 (Springer Nature).

Author contributions

All authors have reviewed and approved the final version of the manuscript. Conceptualization: Y. M., S.-H. B., and Q. X.; investigation: Y. M., H. Z., Z. X., S. H., S. K., S. P., Y. S., and M. G.; visualization: H. Z. and Y. M.; writing – original draft: Y. M., H. Z., Z. X., T. H., and J. S. K; writing – review and editing: Y. M., Q. X., and S.-H. B.; supervision: S.-H. B. and Q. X.; and project administration: S.-H. B.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Q. X. thanks support of the National Natural Science Foundation of China (62075113 and 62122040).

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Footnote

† Equal contribution.

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