A multifunctional terahertz device based on vanadium dioxide metamaterials that switches between ultra-broadband absorption and ultra-high-Q narrowband absorption

Abstract

Terahertz (THz) absorbers with ultra-broadband and ultra-narrowband absorption capabilities are crucial for integrated and efficient terahertz modulation. This study proposes a dual-mode tunable terahertz absorber based on the phase transition characteristics of vanadium dioxide (VO₂), enabling dynamic switching between narrowband and broadband absorption through its insulating-to-metallic transition. In the insulating state, the excitation of quasi-bound states in the continuum (Q-BIC) resonance via geometric parameter modulation of silicon pillars is investigated, with its physical mechanism elucidated via impedance matching theory and multipole analysis. This mode demonstrates exceptional sensing performance at 8.017 THz: a refractive index sensitivity of 3.735 THz RIU⁻¹, a quality factor (Q) of 4800.89, and a figure of merit (FOM) of 3822.93 RIU⁻¹. When VO₂ is transformed into the metallic state, the device achieves more than 90% ultra-broadband absorption in the range of 3.93 THz to 9.25 THz, and its broadband absorption properties originate from the electric dipole resonance. In addition, the performance of the device remains stable at different structural parameters. Compared to existing technologies, this design integrates dual functionalities in a single-layer hybrid structure, significantly reducing fabrication complexity.

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New concepts

Terahertz metamaterials have played an important role in the development of terahertz technology, but most of today's metamaterials in the terahertz band exhibit poor performance and are mono-functional. This greatly limits the scalability and application potential of the devices. Therefore, we propose a terahertz-perfect absorber that utilizes vanadium dioxide (VO₂) with phase transition properties to achieve active switching between ultra-wideband and high-Q ultra-narrowband regions. Prior to VO₂ phase transition, the device achieves quasi-BIC resonance through geometric parameter optimization of silicon pillars. Through impedance matching theory and multipole analysis, we reveal its physical mechanism and demonstrate exceptional sensing performance at 8.017 THz: the refractive index sensitivity reaches 3.735 THz RIU⁻¹, with a quality factor of 4800.89 and a figure of merit up to 3822.93 RIU⁻¹. When VO₂ transitions to the metallic state, the device achieves ultra-broadband absorption around 4.87 THz (absorption > 95%) or 5.32 THz (absorption > 90%). The physical mechanism of broadband absorption is elucidated through electromagnetic field distribution and multipole analysis. Furthermore, we systematically investigate the influences of the structural parameters and environmental refractive index on broadband absorption performance. Compared with existing technologies, this design integrates dual functionalities through a single-layer hybrid structure, significantly reducing process complexity. It provides new insights for developing terahertz sensors, environment-adaptive devices and integrated photonic chips.

1. Introduction

Terahertz (THz) waves, occupying the unique electromagnetic spectrum between microwaves and infrared radiation, combine distinctive physical properties including low photon energy, a broad spectral range, and strong penetration capability.^1,2 These characteristics have demonstrated significant application potential in cutting-edge fields such as 6G communications, biomedical imaging, trace substance detection, and non-destructive security screening.^3,4 However, the inherent limitations of conventional materials in exhibiting substantial electromagnetic responses at terahertz frequencies pose major technical challenges for efficient electromagnetic manipulation.^5,6 Emerging metamaterial and metasurface technologies have overcome the electromagnetic response constraints of natural materials through subwavelength artificial microstructure design.^7,8 Their exceptional wavefront manipulation capabilities and compact device architectures provide innovative solutions for the miniaturization and integration of terahertz functional devices.^9,10

Landy et al. achieved 99% absorption efficiency at 11.65 GHz through rational structural design.¹¹ In recent years, terahertz metamaterial absorbers have shown promising applications in stealth technology, sensing, and spectroscopy.^12,13 Various perfect metamaterial absorbers have been successively reported, evolving from single-band to multi-band or broadband configurations.^14–16 Although absorption characteristics can be adjusted by modifying unit structure dimensions and resonant patterns, current fabrication processes face challenges in multi-material integration and precision manufacturing, resulting in inefficient production and limited accuracy.^17–19 Tunable metamaterial absorbers facilitate significant progress in both manufacturing and application by enabling performance modulation through external control without structural modifications.^20–22 However, most existing tunable schemes can only adjust the intensity or frequency for either narrowband or broadband absorption individually or require complex multi-layer stacked structures to achieve functional switching. This severely limits the potential of devices in multi-scenario applications. Consequently, there is a pressing need to develop structurally simple, multifunctional terahertz devices capable of on-demand function switching.

Vanadium dioxide (VO₂), a classic transition metal oxide, exhibits an insulator-to-metal phase transition near room temperature (≈340 K), with its property changes originating from crystal structural transformations.^23,24 Other phase-change materials such as Ge₂Sb₂Te₅ (GST) face limitations in terahertz applications requiring rapid switching due to their slower response speeds and higher phase transition temperatures.²⁵ Hence, VO₂ emerges as the preferred phase-change material for realizing high-performance dynamic terahertz devices.^26–29 In 2022, Zheng et al. designed a VO₂–SiO₂–gold hybrid terahertz absorber that achieved switchable broadband perfect absorption and total reflection over a wide frequency range.²⁷ In 2024, Niu's team developed a six-layer multifunctional terahertz absorber with mode-switching capability, realizing switching between quad-narrowband and 3.62 THz-range perfect absorption.²⁸ In 2025, Shakiba et al. leveraged VO₂'s IMT characteristics and graphene's electrical tunability to create a multifunctional absorber capable of broadband-narrowband switching, demonstrating over 80% absorption across the range of 4.28–7.55 THz.²⁹ Beyond utilizing VO₂'s phase transition properties, altering incident wave direction has also emerged as an effective approach for achieving absorption bandwidth switching.^30,31 Generally, these studies demonstrate that effective switching between broadband and narrowband absorption modes typically relies on complex stacked multilayered structures or combinations of multiple active materials. However, it is worth noting that while achieving the expected functions, this complex design brings about huge manufacturing challenges and high processing costs, thereby hindering the process of equipment miniaturization and integration. In addition, existing solutions still have room for improvement regarding the Q-factor in narrowband modes or the coverage range in broadband modes.

This study designs a terahertz absorber exploiting VO₂ insulator-to-metal transition (IMT) to achieve switchable narrowband and broadband absorption. Initially, before VO₂ phase transition, the device exhibits ultra-narrowband absorption. Adjusting the asymmetry of silicon pillars (via length L₂) excites a quasi-bound state in the continuum (Q-BIC) within the terahertz spectrum. The mechanism underpinning narrowband perfect absorption is comprehensively elucidated through impedance matching theory and Cartesian multipole decomposition. The Q-BIC resonance demonstrates an ultrahigh Q-factor and narrow linewidth, enabling high-sensitivity sensing. Analysis reveals that at 8.0175 THz, the narrowband mode achieves a refractive index sensitivity S of 3.735 THz RIU⁻¹, a full width at half maximum (FWHM) of 0.00167 THz, a figure of merit (FOM) of 3822.93 RIU⁻¹, and a Q-factor of 4800.89—performance metrics significantly surpassing existing studies. Following VO₂ phase transition, the device operates in an ultra-broadband absorption mode with a >90% absorption bandwidth around 4.87 THz. Impedance matching theory, multipole analysis, and electromagnetic field distributions collectively demonstrate that broadband absorption primarily originates from electric dipole resonance induced in the VO₂ layer. Furthermore, the influence of structural parameters and varying air refractive indices on broadband absorption is systematically investigated. This dual-mode absorber design exhibits promising applications in terahertz sensing and switching devices.

2. Design and method

Fig. 1 illustrates the three-dimensional and top-view structural schematics of the proposed multifunctional terahertz absorber. The device features a three-layer structure comprising a bottom metal reflector layer, a middle SiO₂ dielectric film, and a top hybrid pattern of silicon (Si) and VO₂. The substrate employs a highly stable gold (Au) film with a conductivity of σ_Au = 4.56 × 10⁷ S m⁻¹. The thickness of the metal substrate significantly exceeds the penetration depth of incident waves, ensuring near-zero transmission by fully suppressing electromagnetic wave transmittance.^32,33

Fig. 1 Schematic structure of a tunable multifunctional device: (a) 3D structural view, (b) cell structural view, and (c) top view of the device periodic row cell. (d) σ(ω) versus temperature curve of VO₂. The structural parameters are P_x = P_y = 35 μm, H = 10 μm, t = 0.05 μm, h = 7.5 μm, d = 2 μm, a = 10 μm, b = 4.5 μm, c = 7.5 μm, l₁ = 3 μm, l₂ = 1.5 μm, and L = 17 μm.

The middle layer employs silicon dioxide (SiO₂) with a refractive index of 1.46. In the terahertz band, SiO₂ behaves as a nearly lossless medium, allowing destructive interference through multiple reflections and transmissions at its top and bottom interfaces, thereby enabling perfect absorption. The top VO₂ synthetic pattern consists of four ellipses of the same size, intersecting a square in the middle, as shown in Fig. 1(c). The silicon component, with a dielectric constant of ε_Si = 11.6 + 0.00002i, adopts an asymmetric trapezoidal geometry along the x-axis. The design deliberately breaks the structural symmetry so that a BIC with a zero radiation loss and a near-zero linewidth can be transformed into a Q-BIC mode with an ultra-high Q and a sub-wavelength linewidth.^34,35

To analyze the absorber's performance, we employed the wave optics module in COMSOL Multiphysics to simulate absorption spectra and electromagnetic field distributions. Periodic boundary conditions were applied along the x- and y-axes, while a perfectly matched layer boundary condition was set along the z-axis.^36,37 Incident waves were configured to propagate perpendicular to the device's top surface along the z-direction. The device fabrication process involves low-pressure chemical vapor deposition and magnetron sputtering. First, a 2-μm-thick Au layer is deposited via magnetron sputtering onto the substrate, followed by the growth of a 7.5 μm SiO₂ layer through thermal oxidation. For the top-layer structure, arrays of patterned VO₂ can be generated by direct sputtering of the metal mask plate first, due to the good process tolerance of the multifunctional terahertz absorber that we have designed. The silicon layer is initially deposited using either atmospheric pressure chemical vapor deposition (APCVD) or plasma-enhanced chemical vapor deposition (PECVD), as both methods operate at relatively low temperatures, enabling the production of a low-stress material with enhanced fracture toughness. This is followed by spin-coating a photoresist layer and defining a periodic array pattern via electron beam lithography. Finally, the silicon layer is subjected to etching via inductively coupled plasma processing.^38,39

The behavior of IMT produced by temperature-induced VO₂ in the terahertz band range can be described by the Drude model, where eqn (1) expresses the relative permittivity of VO₂,⁴⁰

where ε_∞ = 12 is the dielectric constant at infinity, γ = 5.75 × 10¹³ rad s⁻¹ is the collision frequency, and ω_p is the plasma frequency with respect to the conductivity, and the relationship can be approximated as eqn (2):⁴¹

In the above equation, σ₀ = 3 × 10⁵ S m⁻¹ and ω_p(σ) = 1.4 × 10¹⁵ rad s⁻¹. The phase transition mechanism in VO₂ originates from its crystal structure transformation: at room temperature, it exhibits a monoclinic crystal structure, which transitions to a tetragonal rutile structure as temperature increases beyond the phase transition point.

Accompanying this structural reorganization, dramatic alterations occur in the electronic density of states, leading to abrupt several orders-of-magnitude changes in electrical conductivity within an ultrashort timeframe.⁴² Notably, this phase transition process is fully reversible. As shown in Fig. 1(d), the functional relationship between VO₂ conductivity and temperature was derived theoretically. Below 313 K, the material maintains low conductivity under 200 S m⁻¹, corresponding to the insulating state of VO₂. Near the critical phase transition temperature of 341 K, conductivity undergoes an abrupt three-order-of-magnitude surge. When temperature further rises above 345 K, the peak conductivity reaches 2 × 10⁵ S m⁻¹. Crucially, this dramatic five-order-of-magnitude transition fundamentally governs the evolution of the device's absorption characteristics.⁴³ While VO₂ can be transformed by manipulating the ambient temperature or by heating the device with laser pulses to realize the transition of the phase state, for example, Sun et al. used a circular ceramic heating element as a heater to heat the ambient environment around the device to achieve a non-contact, constant-temperature ambient heating to achieve conductivity switching of VO₂.⁴⁴ However, these conventional approaches face significant challenges in achieving precise conductivity regulation, posing substantial obstacles for practical device implementation and manufacturing. Therefore, this study therefore focuses on vanadium dioxide's two distinct states: the insulating state below 313 K, where the device operates in narrowband absorption mode, and the metallic state above 345 K, during which the device switches to broadband absorption mode. Critically, the multifunctional terahertz absorber maintains stable performance even when temperatures slightly deviate below 313 K or exceed 345 K, where minor conductivity fluctuations (either reduction or enhancement) exhibit a negligible impact on device characteristics. This temperature tolerance significantly reduces control complexity and associated costs.⁴⁵

3. Results and discussion

The absorption curve when the device is in the narrowband absorption mode before the VO₂ phase transition is shown in Fig. 2(a). It can be clearly found that in the narrowband absorption mode, the device achieves 99.2% absorption at f₁ = 8.0175 THz, when the FWHM is 0.00167 THz. The quality factor Q = f/FWHM,^46,47 where f is the resonance peak frequency. Thus the Q value of this peak is 47 800.98, and there is also some absorption close to 7 THz and near 9 THz, but it is low; so subsequently we will only discuss the phenomenon of interest at the f₁ frequency and its applications.⁴⁸ Fig. 2(b) shows the absorption curves in the range of 3–10 THz (after the VO₂ phase transition) when the multifunctional device is in broadband absorption mode. Simulation results show that the device absorbs more than 90% from 3.93 THz to 9.25 THz, realizing an absorption bandwidth of 5.32 THz and a relative bandwidth of 80.72% according to the bandwidth formulation. In particular, the absorption in the range of 4.33–9.2 THz is greater than 95% in all cases, with a relative bandwidth of 71.99%. Absorptions of 98.99% and 99.4% were achieved at f₂ = 6.02 THz and f₃ = 9.08 THz, respectively.

Fig. 2 (a) Device absorption profile and (c) effective impedance of the device near the f₁ frequency for the narrowband absorption mode. (b) Absorption profile and (d) effective impedance of the device in the range of 3–10 THz for the broadband absorption mode.

To elucidate the physical mechanisms behind these dual-mode perfect absorption characteristics, we employ impedance matching theory. The absorption rate and relative impedance of this multifunctional terahertz device can be expressed as follows:^49,50

where Z = Z₁/Z_air represents the relative impedance of the device, in which Z₁ and Z_air denote the effective impedances of the metasurface structure and free space, respectively. μ and ε are the relative permeability and relative dielectric constant of the absorber, respectively. S₁₁ and S₂₁ correspond to the reflection and transmission coefficients in the S-parameters. As shown in eqn (3), achieving perfect absorption requires impedance matching between the device and free space (Z₁ ≈ Z_air). The real and imaginary components of relative impedance in two operational modes were calculated using eqn (4), with results presented in Fig. 2(b) and (c). For the narrowband absorption mode, the relative impedance was analyzed within the range of 8.01–8.03 THz. At frequency f₁, the real and imaginary components reached 0.9854 and 0.0805, respectively, demonstrating effective impedance matching that enables an ultra-narrowband absorber with high absorption efficiency. Fig. 2(d) demonstrates the relative impedance of the device in the broadband absorption mode in the range of 3–10 THz, the relative impedances at f₂ and f₃ are 1.1071 + 0.0984i and 0.8939–0.0327i, both of them are close to 1 in the real part and close to 0 in the imaginary part in the range of 4–9 THz, and the effective impedance of the absorber matches well with the free-space impedance, realizing the ultra-wide range of perfect absorption.

To further investigate the physical mechanism of perfect absorption in narrowband mode, we calculated the electric and magnetic field distributions at frequency f₁. Fig. 3(a) shows the z-component of the electric field on the xy-plane, revealing strong field localization at the edges of the Si structures. And almost no electric field is excited at VO₂, which is due to the fact that VO₂ in the insulating state is similar to a dielectric material and does not undergo electromagnetic resonance with incident electromagnetic waves from outside.^51,52 This confirms that the narrowband absorption originates from high-refractive-index Si excitation. Notably, positive and negative charges concentrate at the upper and lower edges of each Si pillar, forming paired electric dipoles. This indicates activated electric dipole resonance in individual Si pillars. The antiparallel alignment of dipoles in adjacent pillars transforms them into an electric quadrupole resonance, enabling strong electromagnetic coupling that suppresses reflection and facilitates near-perfect absorption.⁵³ Further analysis of the normalized magnetic field distribution in the xz plane of Fig. 3(b) reveals that the magnetic field energy of the device is mainly concentrated in the two Si pillars and the air gap between them. In addition, the direction of displacement current is marked with arrows, and the ring current with opposite rotation directions is excited at each of the two Si columns, thus exciting two magnetic dipoles with opposite directions along the Y axis and finally forming a magnetic dipole resonance.⁵⁴

Fig. 3 Narrowband absorption modes for (a) z-component distribution of the electric field in the x–y plane. (b) Distribution of the magnetic field in the x–z plane and (c) scattering power of different types of multipoles in the range of 8–8.04 THz.

Multipole analysis can provide insight into the physical mechanisms of electromagnetic resonance and scattering. Based on this, we further explore the principle of narrow-band absorption in devices and calculate the specific multipole components directly from the displacement current density j on metamaterials.^55,56

Electric dipole (ED):

Magnetic dipole (MD):

Toroidal dipole (TD):

Electric quadrupole (EQ):

Magnetic quadrupole (MQ):

where α and β are the x and y directions of the plane of the metamaterial layer, c is the constant for the speed of light, w is the angular frequency, and r is the radial distance. In the narrowband absorption mode, as shown in Fig. 3(c), the blue curve EQ exhibits the highest peak value, followed by the purple curve of the MD, clearly demonstrating that EQ and MD resonances dominate the narrowband absorption. This phenomenon is primarily attributed to the fact that the electric quadrupole, as a higher-order multipole, relies on more complex charge distributions and electric field variations for its formation and interaction. Its excitation exhibits strong frequency selectivity—only when the frequency of incident electromagnetic waves approaches their resonant frequency can the electric quadrupole be significantly activated.⁵⁷ This enables effective coupling with incident electromagnetic waves within a narrow frequency range, resulting in strong absorption.⁵⁸ Furthermore, we calculated the individual multipole contributions at 8.0175 THz, revealing that EQ accounts for 58.49% of the total response, while the MD constitutes 40.66%, with all other multipoles collectively contributing merely 0.85%. Consequently, the combined analysis of electromagnetic field distributions and multipole decomposition demonstrates that the EQ and the MD jointly dominate the narrowband absorption.

For the ultra-narrowband absorption mode, the BIC must be converted into a Q-BIC with a high Q-factor to enable practical applications. By modifying the width of the Si pillar l₂, the BIC state is broken and transformed into a Q-BIC. As shown in Fig. 4(a), when the L₂ is 3 μm, the absorption curve shows a smooth curve at this point. This also proves that it is in the BIC state at this time, and since the BIC mode is not coupled to free space, no resonance phenomenon can be observed in the absorption spectrum.⁵⁹ To investigate the characteristics of Q-BIC resonances in the absorption spectrum, we convert the BIC mode into a leaky mode by varying the l₂. When the l₂ deviates from this critical value of 3 μm, the BIC is transformed into a Q-BIC with a finite yet high Q-factor. As the l₂ gradually shifts away from the critical value, the resonance frequency redshifts, and the linewidth broadens. This signifies an evolution of the device from a non-radiative BIC configuration toward a radiative Q-BIC regime with a high Q-factor. As shown in Fig. 4(b), as the l₂ approaches 3 μm, the Q-factor progressively increases, the FWHM decreases, and the BIC evolves into a Q-BIC with a finite but high Q-factor. This process leads to enhanced energy radiation outward.⁶⁰

Fig. 4 (a) Absorption spectrum of L₂ from 3 to 1.5 μm, (b) variation of Q, FWHM and resonance peak frequency with L₂, (c) effect of varying L on the absorption profile, (d) effect of varying H on the absorption profile.

In addition, we investigated the effect of Si geometrical parameters on the absorption properties of the proposed narrow band. As shown in Fig. 4(c) and (d), while maintaining the dimensions of Si pillars l₁ = 3 μm and l₂ = 1.5 μm, the resonance frequency of the absorption peak redshifts as the length L of the Si pillars increases. Similarly, increasing the thickness H of the Si pillars also induces a redshift in the resonance wavelength. This phenomenon may arise because the enhanced thickness of the Si material causes the magnetic dipole in Fig. 3(b) to extend further along the Z-axis, altering the device's response to incident electromagnetic waves and thereby triggering the redshift. Furthermore, modifying only L or H does not achieve the conversion from BIC to Q-BIC but allows tuning of the resonance wavelength position. This enables the resonance wavelength to be tailored to specific target bands by adjusting geometric parameters, offering a novel approach for the design and application of sensors.

For the ultra-narrowband absorption mode prior to phase transition, its narrow linewidth and high Q-factor make it highly promising for refractive index sensing applications. We simulated the absorption spectra of the device under ambient air with refractive indices ranging from 1 to 1.04. As shown in Fig. 5(a), the absorption peak exhibits a significant redshift as the environmental refractive index increases. Here, we introduce two critical performance metrics for evaluating refractive index: sensitivity S and FOM,⁶¹

Fig. 5 (a) Absorption spectra of the device in ultra-narrow band absorption mode in different refractive index media. (b) Relationships between resonance peak frequency, (c) FWHM, and (d) FOM, and the change in the refractive index of air.

The sensitivity of the device in the narrowband absorption mode can be derived from the slope of the fitted linear curve in Fig. 5(b), with an average sensitivity S of 3.735 THz RIU⁻¹. In addition, Fig. 5(c) and (d) also show the curves of the FWHM and FOM as a function of the ambient refractive index, with a gradual decrease in the FWHM as the ambient refractive index increases and a gradual increase in the FOM, with the maxima being 3822.93 RIU⁻¹ And 3822.93 RIU⁻¹, respectively. This indicates that the designed hypersurface sensor has high sensitivity and a very narrow linewidth, which has great potential for application in refractive index sensors.

For the broadband absorption mode, Fig. 6(a) and (b) demonstrate the normalized real components of the z-direction electric field distribution at two resonance frequencies. Distinctly opposite charges are observed accumulating predominantly on the Si columns and opposite sides of the elliptical VO₂ structure, forming multiple dipole resonances that enable exceptional absorption performance. At frequency f₃, while maintaining opposite-polarity charges on the elliptical VO₂ surfaces, the electric field primarily concentrates within the interior cavities of the dual Si columns. This phenomenon principally arises from dipole resonances induced by coupling effects between complex geometric patterns. Specifically, the combined optical response is generated by periodically arranged exterior cavities and interior cavities of the structural elements.⁶² Furthermore, the electric field intensity at f₃ significantly exceeds that at f₂, explaining why 99.41% absorption is achieved at f₃ compared to 98.97% at f₂. In addition, as shown in Fig. 6(c), we also utilize the multipole analysis theory to analyze it in detail. The data results show that the ED dominates the radiation, and the intensity of ED radiation is significantly higher at the high-frequency position than at the middle and low frequencies, which matches the results in Fig. 6(a) and (b), suggesting that the ultra-broadband absorption is a wide range of high absorption dominated by the electric dipole resonance. In addition, we have quantified the collective multipole contributions in the broadband range of 3.93–9.25 THz and revealed that the ED accounts for 67.25% of the total response, while the TD accounts for 15.58%, and all remaining multipoles only account for 17.17% of the total response. Therefore, it is proved that the ED mainly controls broadband absorption by combining electromagnetic field distribution analysis with multipole decomposition.⁶³

Fig. 6 Distribution of the z-component of the electric field in the x–y plane at (a) f₂ and (b) f₃ in the broadband mode, and (c) scattering power in the range of 3–10 THz for different types of multipoles.

The current structure, while simple, must still account for potential errors in actual manufacturing processes. We therefore conducted further analysis on how structural parameters affect absorption and discussed the impact of possible errors. Notably, the structural parameters of Si pillars primarily influence ultra-narrowband absorption while having no impact on ultra-wideband absorption. Fig. 7(a) demonstrates that as the major axis (a) of elliptical VO₂ increases, both the absorption intensity and bandwidth gradually expand, maintaining an absorption rate of over 90% across a bandwidth exceeding 5 THz. In Fig. 7(b), the absorption bandwidth gradually increases as the short axis (b) of the ellipse increases. When b = 4 μm, the absorption bandwidth and absorption rate are slightly lower, which is due to the fact that the slit between the four notched ellipses becomes larger at this point, resulting in the weakening of the dipole resonance.⁶⁴ Similarly, in Fig. 7(c), when b = 6.5 μm, the four notched ellipses will be connected together, and the slit is not present, which makes the local electric field decrease and the absorption bandwidth decrease. And when the slits are present (b ≥ 7 μm), the overall absorption bandwidth and absorption rate of the device remain essentially unchanged. Fig. 7(d) illustrates the absorption spectra under varying h values. The absorber shows a minimal variation in the 4–9 THz range with slightly reduced absorption rates, while the 3–4 THz region exhibits gradual bandwidth expansion and absorption enhancement as thickness h increases. These phenomena can be explained by interference theory: partial incident waves are directly reflected by surface Si and VO₂ layers, while others penetrate the dielectric layer. Multiple reflections within the dielectric layer, functioning as a Fabry–Pérot resonant cavity, create constructive/destructive interference that modulates the absorption spectrum.⁶⁵

Fig. 7 Absorption spectra for different structural parameters: (a) different elliptical long axes a, (b) different elliptical short axes b, (c) different square variable lengths c and (d) different SiO₂ layer thicknesses h.

In addition to the geometrical parameter modulation, the effects of the ambient refractive index and the refractive index of the SiO₂ medium on the absorption spectra are shown in Fig. 8(a) and (b). When the ambient refractive index is increased from 1 to 1.08, a good ambient generalization is demonstrated. The broadband absorption pattern maintains the perfect absorption of the device's ultra-broadband under refractive index perturbation, confirming the stable working ability of this multifunctional device in different environments.^66–68 In addition, Fig. 8(b) demonstrates the effect of different refractive indices of SiO₂ in the dielectric layer on the absorption spectra. The simulated data show that the absorption in the range of 9–10 THz decreases with the increase of the refractive index of the dielectric layer, but the overall absorption remains stable in the range of 4–9 THz. This multifunctional device can therefore be adapted to a wider range of electromagnetic environments and is also suitable for practical applications.

Fig. 8 Absorption spectra in broadband absorption mode for (a) different ambient refractive indices and (b) different refractive index dielectric layers.

Table 1 summarizes the performance of recent terahertz devices in narrowband absorption, broadband absorption, and broadband-narrowband switching. Compared with conventional single-function terahertz narrowband absorbers, our proposed devices exhibit excellent sensing performance when operating in ultra-narrowband absorption mode, exhibiting ultra-high sensitivity and a significantly enhanced Q-factor. This performance enhancement stems from the excitation of Q-BIC by EQ and MD resonances, which is achieved by intentionally destroying the silicon-based structure under the symmetry-protecting conditions. This synergistic effect leads to absorption enhancement and Q amplification, with 99.3% perfect absorption in the ultra-narrow band achieved at 8.0175 THz. In addition, our multifunctional device exhibits a superior absorption bandwidth when in broadband absorption mode compared to a single terahertz broadband absorber, and the device has excellent process tolerance. Compared with other broadband-narrowband switchable absorbers, our proposed single-layer hybrid structure achieves bifunctional integration through structural innovation and significantly reduces fabrication complexity without affecting performance. Notably, we have also made great progress in our research methodology, firstly, by introducing Q-BIC with a field enhancement effect to improve the Q value and enhance the absorption and secondly, by introducing the multipole analysis theory to analyze in detail the mechanism arising from the perfect absorption in both narrowband and broadband regions.

Table 1 Performance comparison with other THz devices

Principal material	Function type	S (THz RIU^−1)	Q-factor	Tunable bandwidth	FB (%)	Number of layers	Year	Ref.
A: Narrowband perfect absorber. B: Wideband perfect absorber. C: The perfect absorber that is actively switchable between wideband and narrowband regions.
Au and VO2	A	0.398	49.00	—	—	3	2024	69
MXene and graphene	A	0.400	102.9	—	—	4	2025	70
Bulk Dirac semimetals and VO2	A	0.555	44.49	—	—	3	2025	71
Graphene	B	—	—	3.28–5.24	46.00	3	2024	72
Potassium chloride and VO2	B	—	—	3.6–7.3	67.89	4	2024	73
Potassium chloride and graphene	B	—	—	7.676–9.172	17.76	5	2025	74
Graphene	C	0.5	13.23	2.16–1.27	51.89	3	2023	75
Copper and VO2	C	—	—	3.36–6.98	70.02	6	2024	28
Graphene and VO2	C	0.875	900	4.8–17.6	114.28	5	2025	76
Graphene and VO2	C	1.326	347.45	6.45–11.53	56.51	5	2025	77
Si and VO2	C	3.735	4800.89	3.93–9.25	80.72	3	2025	Pro.

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4. Conclusions

This study proposes a tunable terahertz wave absorber based on the VO₂ IMT characteristic. In the insulating state of VO₂, the device exhibits narrowband absorption characteristics. By adjusting the excitable Q-BIC of silicon pillar L₂ and employing impedance matching principles combined with multipole analysis, the physical mechanism is revealed. This ultra-narrowband mode achieves excellent sensing performance at 8.0175 THz with a quality factor Q of 4800.89 and a narrow linewidth of 0.00167 THz, demonstrating a refractive index sensitivity of 3.735 THz RIU⁻¹ and a maximum FOM of 3822.93 RIU⁻¹. When the VO₂ transitions to the metallic state, the device switches to the broadband absorption mode, showing superior broadband absorption characteristics in the range of 3.93 THz to 9.25 THz. The broadband absorption mechanism is elucidated through electromagnetic field distributions and multipole analysis theory. In addition, the effects of the refractive index of the SiO₂ dielectric layer and the ambient refractive index on the broadband absorption performance were systematically investigated. In conclusion, this study has promising applications in the fields of sensing, smart switches and thermal emitters and provides more specific ideas for the design of terahertz ultra-wideband and ultra-narrowband switching absorbers.

Author contributions

T. Liu, C. L. Wang and Z. Yi are mainly responsible for designing the simulation scheme, analyzing the data, manuscript writing and validation editing. G. L. Zou is responsible for research investigation and data analysis. J. Y. Ji is responsible for simulation analysis and interpreting the results. All authors participated in the review of the study.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data presented in this study are displayed in the figures of this paper. The original datasets are available from the corresponding author upon reasonable request. See DOI: https://doi.org/10.1039/d5nh00320b

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China under grant no. 11604252; in part by the Science and Technology Program of Shaanxi Province under Grant no. 2023-JC-YB-552; in part by the Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, Jianghan University under Grant no. JDGD-202310; and in part by the Program of the Key Laboratory of Artificial Microstructure, Ministry of Education under Grant no. 13022019af002.

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