Near-infrared perfect absorption achieved with graphene-LiF multilayer metamaterial

Summary

Achieving perfect absorption of electromagnetic radiation across the near-infrared (NIR) to short-wave infrared spectrum is critical for various applications. While previous studies have demonstrated tunable absorption in graphene-based systems, few designs have achieved a polarization-insensitive and broadband response with dual electrical and structural tunability. Here, we present a graphene-lithium fluoride (LiF) multilayer heterostructure that achieves perfect absorption within the NIR wavelength range. By leveraging the gate-tunable conductivity of graphene in combination with the low-loss properties of LiF, our design enables broadband absorption through critical coupling and plasmonic modes. Our results demonstrate perfect absorption for both transverse magnetic (TM) and transverse electric (TE) polarizations across various incident angles. With an increasing number of graphene layers (GLs), the bandwidth of the absorption spectra broadens, and a blueshift occurs in the spectrum. This dual tunability positions the proposed graphene-LiF metamaterial as a versatile platform for dynamic control over optical responses in NIR applications.

Graphical abstract

Highlights

• •We present a graphene-lithium fluoride (LiF) multilayer heterostructure
• •Perfect absorption within the near-infrared (NIR) wavelength is achieved
• •Perfect absorption is broadband, polarization insensitive, and dual tunable

Physics; photonics; applied sciences; nanomaterials

Introduction

The control of light at the nanoscale has emerged as a significant research focus in modern photonics and optoelectronics, with light absorption being a fundamental phenomenon that has wide-ranging applications across various fields. Achieving perfect absorption of electromagnetic radiation in the near infrared (NIR) to short-wave infrared (IR) spectrum is crucial for applications such as thermal imaging, photodetection, and optical camouflage.^1,2,3 Traditional approaches to attaining perfect absorption in this regime face several challenges. Material losses in conventional metals, such as gold and silver, become pronounced at NIR wavelengths, limiting the achievable absorption efficiency.⁴ Precise nanofabrication of subwavelength structures, including metamaterials or grating couplers, is necessary to match the impedance of free space and effectively confine light.⁵ Additionally, many traditional designs are sensitive to polarization and incident angle, which restricts their practical use in broadband or wide-angle applications.^6,7 Furthermore, these structures often lack tunability once fabricated, and ohmic losses induced by thermal effects can degrade performance under high-intensity illumination. These limitations highlight the need for alternative strategies.

Achieving near-unity or perfect absorption necessitates precise impedance matching and effective dissipation control.^2,6,7,8,9,10 In this context, two-dimensional materials present promising alternatives due to their tunable plasmonic properties and strong light-matter interactions. Among these materials, graphene stands out for its exceptional electro-optical characteristics, which include gate-tunable surface conductivity, strong light-matter interactions through plasmonic effects, and atomic-scale thickness.^11,12 These attributes make graphene an ideal candidate for the development of dynamically reconfigurable optical devices. However, a monolayer graphene exhibits inherently weak light absorption (approximately 2.3%), indicating that achieving near-unity absorption critically depends on sophisticated design strategies aimed at enhancing light-matter interaction. Such strategies include critical coupling,¹³ guided resonance,¹⁴ and plasmonic excitations within integrated heterostructures.^{15,16,17,18,19,20} These approaches typically involve coupling graphene with dielectric cavities or photonic crystals to form hybrid systems that achieve a precise balance between dissipated energy and coupled energy.^21,22,23 Although previous studies have demonstrated tunable absorption in graphene-based systems, few designs have successfully achieved a polarization-insensitive and broadband response with dual electrical and structural tunability.

In this work, we propose and theoretically demonstrate a multilayer heterostructure composed of alternating layers of graphene and lithium fluoride (LiF), which enables perfect broadband absorption across the NIR spectrum. We show that this composite metamaterial supports perfect absorption for both transverse magnetic (TM) and transverse electric (TE) polarizations at the same wavelengths over a wide range of incident angles. The absorption mechanism is driven by a combination of plasmon modes and critical coupling, facilitated by the effective anisotropic permittivity of the multilayer stack. Importantly, the absorption spectrum exhibits dual tunability. It can be spectrally shifted by varying the chemical potential of graphene through electrostatic gating. Additionally, its bandwidth can be significantly broadened by increasing the number of graphene layers (GLs). The integration of graphene’s gate-tunable plasmonic properties with the low-loss dielectric characteristics of LiF results in a compact and voltage-controlled absorber, offering markedly improved bandwidth and absorption efficiency compared with traditional systems. Our proposed graphene-LiF metamaterial serves as a highly versatile platform for dynamic control over optical responses in NIR applications.^{24,25,26,27,28,29,30}

Results

Design of the graphene-LiF composite structure

The designed graphene-LiF composite structure is illustrated in Figure 1A. This structure can be fabricated by alternating ultrathin GLs with LiF spacers, using scalable techniques such as chemical vapor deposition (CVD) and sputtering.³¹ The composite metamaterial consists of monolayer graphene ($0.35\mathrm{n}\mathrm{m}$ thick) or multilayer graphene sheets separated by a $40\mathrm{n}\mathrm{m}$ LiF interlayer, forming a periodic heterostructure. The total thickness of the unit cell is given by $d_{\text{total}}=N{d}_{\text{gra}}+d_{\text{LiF}}$, where $N$ is the number of GLs, $d_{\text{gra}}$ is the thickness of a single GL, and $d_{\text{LiF}}$ is the thickness of the LiF layer. The surface conductivity of graphene incorporates both intraband Drude-like transitions and interband contributions (see STAR Methods for the calculation formulas). By adjusting the chemical potential of graphene through the application of an external gate voltage, dynamic control over its optical response can be achieved.^32,33,34 Additionally, the relative permittivity of graphene can be tuned by varying the number of GLs (see STAR Methods for details). The LiF layers, which have a relative permittivity of approximately 2.2, facilitate impedance matching with free space, minimizing reflection losses (see STAR Methods for the calculation of permittivity).⁹ Since the unit cell thickness of the composite structure is significantly smaller than the wavelength range considered in our analysis, we employ effective medium theory (EMT) to homogenize the graphene-LiF composite structure (see STAR Methods for calculation details). This approach enables us to investigate how the interplay between layer thickness and chemical potential results in tunable, perfect absorption. Furthermore, interference effects within the graphene-LiF stack enhance light-matter interactions, while the layered architecture ensures compatibility with scalable fabrication methods.

Figure 1.

Schematic of the graphene-LiF composite structure and the effective permittivity at various chemical potentials of graphene

(A) Schematic of the composite structure comprising graphene and LiF.

(B) Real part of the effective permittivity of the heterostructure with $\mathrm{G}\mathrm{L}=5$, illustrated for both parallel and perpendicular components at different chemical potentials of graphene.

(C) Imaginary part of the effective permittivity of the heterostructure with $\mathrm{G}\mathrm{L}=5$, shown for both parallel and perpendicular components at various chemical potentials of graphene.

Analysis of perfect absorption characteristics

Due to the subwavelength thickness of the unit cell within the graphene-LiF structure depicted in Figure 1A, the entire stack can be treated as a layered, homogeneous anisotropic medium characterized by an effective permittivity tensor. When a light ray enters the composite structure at an incident angle ${\theta }_i$, this effective permittivity proves particularly advantageous for achieving perfect absorption in both TM and TE polarization modes.^2,10 Figure S1 shows a schematic representation of the reflectance and transmittance of TM and TE waves at the interface between air and the metamaterial. For the TM wave, the electric field is perpendicular to the incident plane, while the magnetic field is parallel to it. Conversely, for the TE wave, the electric field is parallel to the incident plane, and the magnetic field is perpendicular to it. In both TM and TE waves, the directions of the Poynting vector align with the direction of light rays.

The anisotropic nature of the composite metamaterial facilitates impedance matching with free space, thereby minimizing reflection losses. As shown in Figure 1B, the real part of the effective permittivity governs how the composite structure interacts with the TM and TE components of electromagnetic waves, while the imaginary part, shown in Figure 1C, indicates the loss or absorption characteristics. These spectral features are significantly influenced by the chemical potential of graphene ${\mu }_c$ and exhibit resonant behavior at specific wavelengths due to plasmonic excitations.

An increase in the chemical potential ${\mu }_c$ leads to a reconfiguration of the electromagnetic response, allowing for a precise control over the absorption spectrum. As ${\mu }_c$ increases from $0.1\mathrm{e}\mathrm{V}$ to $0.9\mathrm{e}\mathrm{V}$, the effective permittivity of the composite heterostructure undergoes significant changes, as illustrated in Figures 1B and 1C. Generally, the real part of the effective permittivity decreases with increasing ${\mu }_c$, indicating a reduced capability of the material to support electric field components parallel and perpendicular to the GLs. Conversely, the imaginary part of both ${\epsilon }_{\Vert }$ and ${\epsilon }_{\perp }$ exhibits pronounced peaks at specific wavelengths, which shift as ${\mu }_c$ increases. These peaks correspond to resonant absorption phenomena. The anisotropic effective permittivity of the structure facilitates impedance matching for both TM and TE polarizations, and the changes in effective permittivity directly influence the absorption behavior of these polarizations. At wavelengths ranging from 0.75 to $2.25\mu \mathrm{m}$, doped or gated graphene effectively supports IR surface plasmon polaritons (SPPs). The LiF layer plays a dual role: it provides a high refractive index contrast with air, enhancing field confinement, and it enables interlayer coupling, leading to collective plasmonic modes across multiple periods of the graphene-LiF stacks. Such SPPs result in strong light-matter interactions.

In the graphene-LiF heterostructure with $\mathrm{G}\mathrm{L}=5$, perfect absorption is achieved through the interplay of plasmon modes,¹³ critical coupling, and impedance matching. The real and imaginary parts of the SPP wavevector, as shown in Figures 2A and 2B, respectively, illustrate how the light-matter interaction is enhanced across the NIR spectrum (see STAR Methods for details on SPP dispersion). In the proposed graphene-LiF composite system, the SPP dispersion is determined by the interaction between the permittivity of LiF and that of graphene, which is strongly dependent on the chemical potential of graphene, ${\mu }_c$. As ${\mu }_c$ increases, the real part of graphene’s conductivity rises, leading to a decrease in both $\mathrm{R}\mathrm{e}\left({\epsilon }_{\Vert }\right)$ and $\mathrm{R}\mathrm{e}\left({\epsilon }_{\perp }\right)$, as shown in Figure 1B. This tuning of the effective optical response of the entire structure is significant. As ${\mu }_c$ increases from 0.1 to $0.9\mathrm{e}\mathrm{V}$ (Figure 2A), the rising conductivity of graphene enhances the coupling between incident light and SPPs, resulting in a blue shift and deepening of absorption resonances. This behavior indicates stronger plasmonic excitation at shorter wavelengths, reflecting tighter confinement of the plasmon mode in the NIR regime. Simultaneously, the imaginary part of the SPP wavevector also increases, as shown in Figure 2B, implying higher losses due to enhanced scattering as more electrons participate in the plasmonic response. For TM polarization, absorption is directly mediated by the excitation of SPPs at the graphene-LiF interfaces, where phase-matching conditions facilitate efficient energy transfer from incoming radiation to the charge density oscillations. Conversely, TE-polarized absorption, which does not couple directly to SPPs, is governed by anisotropic guided modes and interference effects sustained within the stratified medium. These phenomena arise from the effective birefringence and periodic modulation of the heterostructure, which collectively promote broadband photon capture and dissipation. Consequently, the proposed metamaterial supports polarization-independent perfect absorption through two distinct yet complementary electromagnetic mechanisms, both intrinsic to its anisotropic design.

Figure 2.

SPP wavevector under different chemical potentials of graphene

(A) Real part of the SPP wavevector at various chemical potentials of graphene, with $\mathrm{G}\mathrm{L}=5$.

(B) Imaginary part of the SPP wavevector at different chemical potentials of graphene, with $\mathrm{G}\mathrm{L}=5$.

SPP, surface plasmon polariton.

We would like to note that, although critical coupling and SPPs typically yield narrowband resonances in single-interface systems, the multilayer architecture of the graphene-LiF stack overcomes this limitation, resulting in a broadband response. This enhancement is achieved through three interconnected mechanisms. First, the hybridization of multiple SPP modes across the stacked graphene-LiF interfaces leads to spectral broadening. Second, Fabry-Pérot-like interference within the subwavelength unit cell supports multiple overlapping resonant channels across the wavelength range in our study. Third, the increased number of GLs enhances the effective filling fraction and conductivity, which, in turn, improves interlayer plasmonic coupling, thereby widening the absorption band. Consequently, perfect absorption arises not from a single sharp mode but from the spectral overlap of multiple critically coupled resonances, making the structure functionally distinct from conventional narrowband plasmonic designs. This behavior of broadband absorption aligns with established principles in hyperbolic metamaterials,³⁵ where high k-mode density and engineered losses broaden the response beyond the limits of single resonators.

The absorption characteristics are significantly influenced by the interaction between the chemical potential ${\mu }_c$ and the effective permittivity of the composite medium, as described by the EMT outlined in Equations 6 and 7. The tunable optical response arises because ${\mu }_c$ directly modulates graphene’s conductivity ${\sigma }_{\text{gra}}$, which, in turn, alters both components of the effective permittivity tensor, ${\epsilon }_{\Vert }$ and ${\epsilon }_{\perp }$. The results shown in Figure 3 demonstrate that the heterostructure with $\mathrm{G}\mathrm{L}=5$ achieves broadband and tunable perfect absorption for both TM and TE polarizations across a wide range of incident angles. This behavior is analytically predicted by the conditions for perfect absorption: Equation 10 for TM polarization and Equation 12 for TE polarization (see STAR Methods for details). In Figures 3A and 3B, at ${\mu }_c=0.3\mathrm{e}\mathrm{V}$, perfect absorption is observed over a broad spectral range centered around $2.48\mu \mathrm{m}$, with an angular range extending up to $70°$. This phenomenon can be attributed to the effective anisotropic permittivity achieving impedance matching with free space, facilitated by plasmon coupling and Fabry-Pérot-like interference within the multilayer stack.

Figure 3.

Perfect absorption in the graphene-LiF metamaterial as a function of wavelength and incident angle for TM and TE polarizations

(A and B) Perfect absorption as a function of wavelength and incident angle for TM and TE polarizations, respectively, with the chemical potential ${\mu }_c$ fixed at $0.3\mathrm{e}\mathrm{V}$.

(C and D) Perfect absorption as a function of wavelength and incident angle for TM and TE polarizations, respectively, with the chemical potential ${\mu }_c$ fixed at $0.6\mathrm{e}\mathrm{V}$.

(E and F) Perfect absorption as a function of wavelength and incident angle for TM and TE polarizations, respectively, with the chemical potential ${\mu }_c$ fixed at $0.9\mathrm{e}\mathrm{V}$.

The color scale indicates the magnitude of reflection, with blue regions representing low reflection (approaching perfect absorption) and yellow and orange regions denoting higher reflection. Here, $\mathrm{G}\mathrm{L}=5$.

TM, transverse magnetic; TE, transverse electric.

The dynamic tunability with ${\mu }_c$ (Figures 3C–3F) highlights the versatility of the structure. Increasing ${\mu }_c$ enhances graphene’s intraband conductivity, which modifies the components of effective permittivity. Specifically, this leads to an increase in the imaginary part (representing losses) and a shift in the real part, thereby strengthening the coupling between incident light and SPPs, as described by the dispersion relation in Equation 8. For TM polarization, this results in a blue shift: at ${\mu }_c=0.6\mathrm{e}\mathrm{V}$, the peak shifts to $1.24\mu \mathrm{m}$, and at ${\mu }_c=0.9\mathrm{e}\mathrm{V}$, it shifts further to $0.82\mu \mathrm{m}$. This shift occurs because the plasmon frequency scales with ${\mu }_c$, necessitating shorter wavelengths to satisfy the SPP resonance condition. Simultaneously, the increased conductivity enhances the refractive index contrast within the heterostructure, leading to alterations in the Fabry-Pérot-like cavity response. For TE polarization, similar blue shifts are observed (Figures 3D and 3F), although the underlying mechanism differs. TE waves cannot directly excite SPPs but rely on guided modes and constructive interference within the anisotropic layered medium, which are modulated by changes in ${\epsilon }_{\Vert }$.

In Figure 4, we illustrate how absorption evolves with an increasing number of graphene layers. In Figures 4A and 4B, when $\mathrm{G}\mathrm{L}=1$ (i.e., using monolayer graphene), the system exhibits narrowband perfect absorption $(0.03\mu \mathrm{m})$ due to limited interference effects. Monolayer graphene has relatively low conductivity than multilayer graphene, which restricts the strength of the SPP modes and leads to a weaker coupling between the incident light and the composite structure. Furthermore, the refractive index contrast between the GL and the LiF spacer is not sufficiently high to support broadband absorption. Consequently, the Fabry-Pérot-like cavity response is weak, resulting in narrow resonances. Additionally, for monolayer graphene, the SPP dispersion curve is relatively narrow, indicating that SPPs can only be excited over a limited range of wavelengths due to the weakened light-matter coupling and low conductivity. As a result, the absorption peaks are confined to specific wavelengths, as shown in Figures 4A and 4B.

Figure 4.

Perfect absorption in the graphene-LiF metamaterial as a function of wavelength and incident angle for different numbers of GL

(A and B) Perfect absorption as a function of wavelength and incident angle for TM and TE polarizations, respectively, with $\mathrm{G}\mathrm{L}=1$ and the chemical potential ${\mu }_c$ fixed at $0.5\mathrm{e}\mathrm{V}$.

(C and D) Perfect absorption as a function of wavelength and incident angle for TM and TE polarizations, respectively, with $\mathrm{G}\mathrm{L}=2$ and the chemical potential ${\mu }_c$ fixed at $0.5\mathrm{e}\mathrm{V}$.

(E and F) Perfect absorption as a function of wavelength and incident angle for TM and TE polarizations, respectively, with $\mathrm{G}\mathrm{L}=5$ and the chemical potential ${\mu }_c$ fixed at $0.5\mathrm{e}\mathrm{V}$.

GL, graphene layer; TM, transverse magnetic; TE, transverse electric.

As the number of GL increases, the conductivity of graphene also rises, enhancing the plasmonic response. Both components of effective permittivity, ${\epsilon }_{\Vert }$ and ${\epsilon }_{\perp }$, as described in Equations 6 and 7, are modified, since the filling fraction and effective permittivity of graphene scale with the number of layers. This results in stronger excitation of SPPs at the graphene-LiF interfaces under TM polarization. Consequently, the SPP dispersion broadens, allowing for coupling over a wider wavelength range (Figure 2). The enhanced interlayer coupling and higher refractive index contrast improve field confinement and light-matter interaction, leading to deeper and broader TM absorption peaks. These observations align with the results shown in Figures 4C and 4E, which demonstrate that an increase in the number of GL results in broader and stronger TM absorption bands. For TE polarization, although there is no direct coupling to SPP modes, increasing the number of GLs still enhances absorption through cumulative interference effects within the multilayer stack. As more GLs are added to the unit cell of the composite structure depicted in Figure 1A, the Fabry-Pérot-like cavity formed between the LiF and GLs supports multiple internal reflections, resulting in increased constructive interference and enhanced absorption. The effective medium thickness of the unit cell increases, providing a longer optical path and more opportunities for photon absorption through interference-assisted mechanisms. This explains why TE absorption also broadens with increasing GL (Figures 4D and 4F), even though the underlying mechanism differs from that of TM polarization.

Figure 5 presents the reflection spectra of the graphene-LiF heterostructure for both TM and TE polarizations under varying chemical potentials. At ${\mu }_c=0.3\mathrm{e}\mathrm{V}$ (Figures 5A and 5B), perfect absorption (indicated by dark blue color) occurs at longer wavelengths near $2.48\mu \mathrm{m}$, exhibiting broad angular tolerance up to $70°$. This phenomenon arises due to the lower conductivity, which shifts the plasmon resonance to lower frequencies, and the conditions for perfect absorption are satisfied across a wide range because of the initial impedance matching between free space and the effective medium. As ${\mu }_c$ increases to $0.6\mathrm{e}\mathrm{V}$ (Figures 5C and 5D), the absorption peak blue-shifts to approximately $1.24\mu \mathrm{m}$, a direct consequence of the increased conductivity of graphene, which enhances the real part of the SPP wavevector, as shown in Equation 8. Consequently, a larger wavevector or a shorter wavelength is required to satisfy the momentum-matching condition for excitation, and the strengthened light-matter interaction modifies the effective permittivity tensor, resulting in a deeper absorption band. At ${\mu }_c=0.9\mathrm{e}\mathrm{V}$ (Figures 5E and 5F), the absorption further blue-shifts to around $0.82\mu \mathrm{m}$, with the higher charge carrier density significantly increasing the plasmon frequency, leading to tighter field confinement and larger losses, as evident from the imaginary part of $k_{\text{SPP}}$ shown in Figure 2B. Importantly, the anisotropic effective permittivity of the multilayer stack continues to ensure that the impedance matching condition, as described in Equations 10 and 12, is maintained across a wide range of incident angles and for both polarizations, demonstrating the robust design of the graphene-LiF metamaterial. This systematic spectral evolution in the graphene-LiF metamaterial, dictated by fundamental dispersion relations and conductivity models, conclusively showcases the active electrical tunability of the perfect absorption mechanism.

Figure 5.

Reflectance spectra for the graphene-LiF heterostructure as a function of wavelength and incident angle for TM and TE polarizations

(A and B) Reflectance spectra as a function of wavelength and incident angle for TM and TE polarizations, respectively, with the chemical potential ${\mu }_c$ fixed at $0.3\mathrm{e}\mathrm{V}$.

(C and D) Reflectance spectra as a function of wavelength and incident angle for TM and TE polarizations, respectively, with the chemical potential ${\mu }_c$ fixed at $0.6\mathrm{e}\mathrm{V}$.

(E and F) Reflectance spectra as a function of wavelength and incident angle for TM and TE polarizations, respectively, with the chemical potential ${\mu }_c$ fixed at $0.9\mathrm{e}\mathrm{V}$.

The color scale represents the magnitude of reflection, with dark blue regions indicating zero reflection and dark red regions denoting $100\%$ reflection.

TM, transverse magnetic; TE, transverse electric.

Finally, Figure 6 presents the TM/TE reflectance ratio as a function of wavelength and incident angle. In Figures 6A–6D, a clear evolution of the TM/TE reflectance ratio, ${\left|R_{\text{TM}}\right|}^2/{\left|R_{\text{TE}}\right|}^2$, is observed as ${\mu }_c$ increases. In Figure 6A, with a chemical potential of ${\mu }_c=0.3\mathrm{e}\mathrm{V}$, the TM and TE reflection ratio shows only moderate contrast, indicating limited polarization sensitivity due to low graphene conductivity and weak SPP excitation. At ${\mu }_c=0.5\mathrm{e}\mathrm{V}$ (Figure 6B), the TM/TE ratio becomes more distinct, with broader regions of high TM reflectance emerging, a result of improved conductivity and better alignment between the SPP wavevector and guided modes. As shown in Figure 6C, when the chemical potential reaches ${\mu }_c=0.7\mathrm{e}\mathrm{V}$, the contrast sharpens further, revealing extensive regions of strong TM preference across a wider spectral range, which reflects enhanced field confinement and resonant coupling. Finally, as shown in Figure 6D, at ${\mu }_c=0.9\mathrm{e}\mathrm{V}$, the TM/TE ratio reaches its maximum, exhibiting significant TM dominance over a broad range of wavelengths and incident angles. This indicates that a higher ${\mu }_c$ substantially strengthens the interaction between TM-polarized light and SPPs, while TE reflection remains minimal. The TM/TE contrast becomes most pronounced at angles above $40°$, where the system demonstrates significant polarization selectivity.

Figure 6.

TM/TE reflectance ratio as a function of incident angle and wavelength for varying chemical potentials of graphene

(A) TM/TE reflectance ratio as a function of incident angle and wavelength for ${\mu }_c=0.3\mathrm{e}\mathrm{V}$.

(B) TM/TE reflectance ratio as a function of incident angle and wavelength for ${\mu }_c=0.5\mathrm{e}\mathrm{V}$.

(C) TM/TE reflectance ratio as a function of incident angle and wavelength for ${\mu }_c=0.7\mathrm{e}\mathrm{V}$.

(D) TM/TE reflectance ratio as a function of incident angle and wavelength for ${\mu }_c=0.9\mathrm{e}\mathrm{V}$. The TM/TE reflectance ratio is defined as ${\left|R_{\text{TM}}\right|}^2/{\left|R_{\text{TE}}\right|}^2$.

TM, transverse magnetic; TE, transverse electric.

Discussion

The scientific contribution of our work lies in revealing a new functional paradigm for reconfigurable NIR photonics through a carefully engineered graphene-LiF multilayer heterostructure. Our study demonstrates that a simple, scalable stack of alternating graphene and LiF layers, without the need for complex patterning or resonant cavities, can simultaneously achieve three highly sought-after but rarely coexisting features in the NIR regime. The main characteristics of our proposed composite structure are as follows.

First, the composite structure exhibits polarization-insensitive perfect absorption for both TM and TE waves, even at oblique incidence (up to $70°$). For conventional metasurfaces, achieving polarization insensitivity typically relies on designing structures with in-plane rotational symmetry, such as circular or square unit cells. However, this type of polarization insensitivity often fails at oblique incidence and is usually effective only for normal incidence. In contrast, the polarization-insensitive perfect absorption in our design is not achieved through geometric symmetry but arises from anisotropic effective permittivity of the homogenized medium, which independently satisfies impedance-matching conditions for both TM and TE waves. This dual pathway mechanism, which is driven by SPP for TM waves and guided by anisotropic interference for TE waves, provides novel insights into how layered, 2D materials can overcome polarization limitations.

Second, the composite structure exhibits broadband absorption. In our work, broadband absorption is achieved not through a single resonance but through mode hybridization across multiple graphene-LiF interfaces and Fabry-Pérot-like interference within a deeply subwavelength unit cell. Furthermore, increasing the number of GLs transforms the response from narrowband to broadband. This dramatic enhancement is rarely reported in tunable NIR absorbers. For instance, Wu et al. demonstrated perfect absorption using graphene-hexagonal boron nitride (hBN), but this was limited to the mid-IR range and did not extend into the NIR.³⁶ Similarly, Hajati et al. achieved tunable absorption in the far-IR using graphene-LiF, but this was characterized by a narrow bandwidth and no TE absorption.⁹

Third, the composite structure demonstrates dual tunability. The center wavelength of the absorption spectrum can be electrically adjusted by modifying the chemical potential of graphene, while the bandwidth can be independently controlled by the number of GLs. This dual degree of freedom is rare in existing designs, as most studies, e.g., the work of Nazari et al.,³⁷ could achieve only electrical tuning. Our approach to decoupling the spectral position and bandwidth provides unprecedented flexibility for adaptive optical systems.

Moreover, our choice of LiF as the dielectric spacer represents a strategic innovation. Unlike the commonly used ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ or ${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$, LiF is a low-loss polar dielectric with a high permittivity $(\epsilon \approx 2.2)$, which enables strong field confinement in the NIR while avoiding absorption bands, as its Reststrahlen band lies beyond $10\mu \mathrm{m}$. This characteristic makes LiF particularly well suited for NIR plasmonics.

In contrast to existing graphene-based absorbers, which typically operate in the mid- or far-IR, are polarization sensitive, or exhibit fixed bandwidths,^9,36,37 our design addresses a critical gap by providing high-performance, dynamically reconfigurable absorption across the technologically vital NIR window. Please refer to Table 1 for a comparison between this work and other relevant publications. Therefore, while the individual components (graphene, effective medium theory, and critical coupling) are well known, their integration into a functional platform that achieves broadband, polarization-insensitive, and dual-tunable perfect absorption represents an advancement in nanophotonic design.

Table 1.

Comparison between this work and other relevant publications

Feature	This work	Wu et al. (2016)36	Hajati et al. (2021)9	Nazari et al. (2025)37
Spectral range	0.5–3.5 μm (NIR)	6.5–8.5 μm (mid-IR)	10–30 μm (far-IR)	3–5 μm
Polarization insensitivity	yes (TE and TM)	no (TM only)	no	partial (TE degrades >30°)
Dual tunability	yes (μc + GL)	μc only	μc only	switchable (binary, e.g., VO2/GST)
Broadband (NIR)	yes	no	no	yes (mid-IR)

In summary, we have demonstrated a graphene-LiF multilayer metamaterial that supports perfect absorption in the NIR range for both TM and TE polarizations. This design leverages the complementary properties of graphene, including its gate-tunable plasmonic response and strong light-matter interaction, along with the low-loss phononic characteristics and favorable permittivity of LiF. The heterostructure facilitates polarization-insensitive, wide-angle, and broadband perfect absorption through plasmon modes and critical coupling mechanisms. The absorption spectrum exhibits dual tunability: spectral shifts can be achieved by electrostatically gating the graphene chemical potential, while bandwidth broadening can be controlled by varying the number of GLs. This capability enables dynamic, post-fabrication reconfiguration of the optical response without any structural alterations. Furthermore, the effective anisotropic permittivity of the multilayer stack aids in impedance matching with free space, minimizing reflection for both TM and TE polarizations. Such performance, combined with compatibility with scalable fabrication techniques, positions the graphene-LiF metamaterial as a promising platform for advanced, reconfigurable optoelectronic devices. These devices, including IR sensors, modulators, and thermal emitters, bridge a crucial gap between tunability, efficiency, and practical applicability in next-generation photonic systems.

Limitations of the study

The design relies on effective medium theory, which assumes deeply subwavelength layer thicknesses and neglects nonlocal and quantum effects that may become relevant at atomic-scale gaps or high doping levels. Thermal stability under high-intensity illumination is not addressed, and ohmic losses in graphene could lead to performance degradation. Fabrication of uniform, large-area multilayer stacks with precise layer control remains challenging. Additionally, the tuning speed is limited by the resistance-capacitance (RC) time constant of the gating scheme, which may restrict high-frequency applications.

Resource availability

Lead contact

Requests for further information and resources should be directed to and will be fulfilled by the lead contact, Rujiang Li (rujiangli@xidian.edu.cn).

Materials availability

This study did not generate new materials.

Data and code availability

• •All data reported in this paper will be shared by the lead contact upon request.
• •All original code is available from the lead contact upon reasonable request.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Acknowledgments

We thank Dr. Shen Lian from College of Information Science and Electronic Engineering, Zhejiang University for his valuable comments and suggestions. R.L. and Y.M. were sponsored by the National Key Research and Development Program of China (grant no. 2022YFA1404902), National Natural Science Foundation of China (grant no. 12104353), and Fundamental Research Funds for the Central Universities (grant no. QTZX25086). X.H. was sponsored by the Shanxi Province Basic Research Program (202203021222250).

Author contributions

Conceptualization, M.I.; data curation, M.I., Y.M., and R.L.; formal analysis, M.I., Y.Z., and R.L.; investigation, M.I. and R.L.; methodology, M.I., X.H., Y.Z., and R.L.; validation, M.I. and Y.M.; visualization, M.I. and Y.M.; writing – original draft, M.I.; writing – review & editing, Y.M. and R.L.; project administration, Y.Z. and R.L.; resources, Y.Z. and R.L.; software, Y.Z.; supervision, Y.Z. and R.L.; funding acquisition, Y.Z. and R.L.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Software and algorithms
MATLAB R2024b	MathWorks	https://ww2.mathworks.cn/en/products/matlab.html

Method details

Conductivity of graphene

The conductivity of graphene is modeled using the random phase approximation (RPA), which accounts for contributions from both intraband and interband electronic transitions.^38,39 The intraband term predominates at low photon energies and arises from transitions within the same band (Dirac cone), primarily due to the free-carrier response. This intraband contribution can be expressed as

$${\sigma }_{\text{intra}}=\frac{\mathrm{i}{e}^2{k}_{\text{B}}T}{\pi {\hslash }^2(\omega +\mathrm{i}/\tau )}\left[\frac{{\mu }_c}{k_{\text{B}}T}+2\ln \left(e^{-{\mu }_c/k_{\text{B}}T}+1\right)\right],$$ (Equation 1)

where ${\mu }_c$ is the chemical potential (Fermi energy), $T$ is the temperature, $\tau$ is the relaxation time, $e$ is the elementary charge, $k_{\text{B}}$ is the Boltzmann constant, and $\hslash$ is the reduced Planck’s constant. The interband term describes the transitions between the valence and conduction bands and can be written as

$${\sigma }_{\text{inter}}=\frac{\mathrm{i}{e}^2(\omega +\mathrm{i}/\tau )}{\pi {\hslash }^2}{\int }_0^{\infty }\frac{f(-E)-f(E)}{{(\omega +\mathrm{i}/\tau )}^2-{(2E/\hslash )}^2}dE,$$ (Equation 2)

where $f(E)=1/\left[e^{(E-{\mu }_c)/(k_{\text{B}}T)}+1\right]$ represents the Fermi-Dirac distribution. The total conductivity of graphene is obtained by summing these two contributions:

$${\sigma }_{\text{gra}}={\sigma }_{\text{intra}}+{\sigma }_{\text{inter}}.$$ (Equation 3)

Permittivity of graphene

Graphene with a finite thickness of $d_{\text{gra}}=0.35\mathrm{n}\mathrm{m}$ allows its surface conductivity to be converted into an effective dielectric function.³⁸ The permittivity of graphene is defined as

$${\epsilon }_{\text{gra}}=1+\mathrm{i}\frac{{\sigma }_{\text{gra}}}{\omega {\epsilon }_0{d}_{\text{gra}}},$$ (Equation 4)

where ${\epsilon }_0$ is the permittivity of vacuum, $\omega$ is the angular frequency, and $d_{\text{gra}}$ is the effective thickness of graphene. In the case of multilayer graphene, $d_{\text{gra}}=N\times 0.35\mathrm{n}\mathrm{m}$, where $N$ represents the number of GL. Figure S2 illustrates the real and imaginary parts of the permittivity of graphene for different values of the chemical potential.

Permittivity of LiF

The dielectric response of LiF is modeled using a single Lorentz oscillator,⁹ which is suitable for describing phonon resonances in polar crystals. This can be expressed as

$${\epsilon }_{\text{LiF}}(\omega )={\epsilon }_{\infty ,\mathrm{L}\mathrm{i}\mathrm{F}}\left(1+\frac{{\omega }_{\text{TO}}^2-{\omega }_{\text{LO}}^2}{{\omega }_{\text{TO}}^2-{\omega }^2-\mathrm{i}\gamma \omega }\right),$$ (Equation 5)

where ${\epsilon }_{\infty ,\mathrm{L}\mathrm{i}\mathrm{F}}=2.027$ is the high-frequency dielectric constant, ${\omega }_{\text{TO}}=2\pi \cdot 9.22\times 10^{12}\mathrm{H}\mathrm{z}$ is the transverse optical phonon frequency, ${\omega }_{\text{LO}}=2\pi \cdot 19.2\times 10^{12}\mathrm{H}\mathrm{z}$ is the longitudinal optical phonon frequency, and $\gamma =2\pi \cdot 0.527\times 10^{12}\mathrm{H}\mathrm{z}$ is the damping rate. This model effectively captures the dispersion and absorption characteristics near the Reststrahlen band of LiF.

Effective permittivity of graphene-LiF composite

The electromagnetic response of the hybrid graphene-LiF structure is analyzed using effective medium theory (EMT).^2,10 This approach considers a layered composite in which graphene and LiF layers are stacked vertically. The effective permittivity components can be expressed as follows:

$$\begin{array}{ccc}\hfill {\epsilon }_{\Vert }& =\hfill & f_{\text{g}}{\epsilon }_{\text{LiF}}+(1-f_{\text{g}}){\epsilon }_{\text{gra}},\hfill \end{array}$$ (Equation 6)

$$\begin{array}{ccc}\hfill {\epsilon }_{\perp }& =\hfill & \frac{1}{\frac{f_{\text{g}}}{{\epsilon }_{\text{LiF}}}+\frac{1-f_{\text{g}}}{{\epsilon }_{\text{gra}}}},\hfill \end{array}$$ (Equation 7)

where $f_{\text{g}}=d_{\text{gra}}/\left(d_{\text{gra}}+d_{\text{LiF}}\right)$ is the filling fraction of graphene, $d_{\text{gra}}$ is the thickness of the graphene layer(s), $d_{\text{LiF}}$ is the thickness of the LiF layer, and ${\epsilon }_{\text{gra}}$ and ${\epsilon }_{\text{LiF}}$ are the permittivities of graphene and LiF, respectively. The filling fraction $f_{\text{g}}$ can be adjusted by varying the number of GL in each graphene sheet. Figures S1A and S1B illustrate the real and imaginary parts of the effective permittivity components, ${\epsilon }_{\Vert }$ and ${\epsilon }_{\perp }$, for the composite structure, specifically with five graphene layers in each sheet, i.e., $\mathrm{G}\mathrm{L}=5$.

Dispersion relation for SPPs

The dispersion relation for SPPs at the interface between two media, characterized by the permittivities of graphene and LiF, is given by

$$k_{\text{SPP}}(\omega )=k_0\sqrt{\frac{{\epsilon }_{\text{LiF}}{\epsilon }_{\text{gra}}}{{\epsilon }_{\text{LiF}}+{\epsilon }_{\text{gra}}}},$$ (Equation 8)

where $k_0=\omega /c$ is the wave number in free space. It is important to note that the tunability in our heterostructure can dynamically shift the perfect absorption condition, extending beyond the limitations of fixed-frequency resonators.

Reflection and transmission coefficients for TM and TE polarizations

Let ${\theta }_i$ represent the incident angle for a wave incident from free space to the metamaterial. The reflection and transmission coefficients for TM and TE polarizations of the proposed anisotropic graphene-LiF metamaterial, with its optical axis oriented normal to the $xy$-plane, can then be calculated.²

The reflection coefficient for TM polarization is given by

$$r_{\text{TM}}=\frac{\cos {\theta }_i-\sqrt{\left({\epsilon }_{\perp }-{\sin }^2{\theta }_i\right)/{\epsilon }_{\Vert }{\epsilon }_{\perp }}}{\cos {\theta }_i+\sqrt{\left({\epsilon }_{\perp }-{\sin }^2{\theta }_i\right)/{\epsilon }_{\Vert }{\epsilon }_{\perp }}}.$$ (Equation 9)

In this case, the wave interacts with both the in-plane $({\epsilon }_{\Vert })$ and out-of-plane $({\epsilon }_{\perp })$ components of the permittivity. The condition for perfect absorption, or zero reflection, can be derived as

$$({\epsilon }_{\perp }{\epsilon }_{\Vert }-1){\cos }^2{\theta }_i={\epsilon }_{\perp }-1.$$ (Equation 10)

For TE polarization, the reflection coefficient is given by

$$r_{\text{TE}}=\frac{\cos {\theta }_i-\sqrt{{\epsilon }_{\Vert }-{\sin }^2{\theta }_i}}{\cos {\theta }_i+\sqrt{{\epsilon }_{\Vert }-{\sin }^2{\theta }_i}}.$$ (Equation 11)

Here, the wave primarily responds to the in-plane permittivity ${\epsilon }_{\Vert }$. The condition for perfect absorption, or zero reflection, can be derived as

$${\epsilon }_{\Vert }=1.$$ (Equation 12)

The reflection coefficients for TM and TE polarizations lead to different resonance conditions for TM and TE waves. The reflectance and transmittance can be expressed as

$$\begin{array}{ccc}\hfill \left|R_{\text{TM/TE}}\right|& =\hfill & {\left|r_{\text{TM/TE}}\right|}^2,\hfill \end{array}$$ (Equation 13)

$$\begin{array}{ccc}\hfill T_{\text{TM/TE}}& =\hfill & 1-R_{\text{TM/TE}}.\hfill \end{array}$$ (Equation 14)

Please note that all the results presented in this paper are computed using custom MATLAB scripts based on the analytical models outlined in the STAR Methods section.

Quantification and statistical analysis

Our study does not include any statistical analysis.

Published: February 6, 2026