Implementing the Dual Functions of Switchable Broadband Absorption and Sensitive Sensing in a VO2-Based Metasurface

The metadevice with multiple functions in a fixed nanostructure is highly required. In this paper, we numerically achieved the dual functions of switchable broadband absorption and sensitive refractive index (RI) sensing in a fixed fishnet-shaped nanostructure by integrating with the phase change material vanadium dioxide (VO2). Exploiting the insulator-to-metal transition of VO2, the absorption strength could be dynamically switched from 0.07 to 0.97 in a broad terahertz (THz) band. Meanwhile, the same structure with metallic VO2 exhibits highly sensitive RI sensing performance. The sensitivity reaches 1.15 THz/RIU, which makes great progress. The proposed metasurface with dual functions will promote the development and applications of THz nanodevices.

The phase change material VO 2 could undergo the transition from insulator to metallic state when the temperature becomes higher than the phase-transition temperature T c ≈68 °C [13].Compared with phase change material GST, the phase-transition temperature of VO 2 is much lower, which is lower energy consumption in applications [11].Besides, the changes of VO 2 conductivity are almost four orders of magnitudes with the transition from insulator to metallic phase under thermal excitation [16].The enormous change of optical characteristics caused by phase transition enables VO 2 a candidate material to construct active metasurfaces.Recently, by introducing VO 2 , many active metasurfaces have realized dynamic manipulation for THz waves.For example, Huan Liu et al. obtained a broadband and switchable THz absorber by an L-shaped VO 2 resonator array, whose absorption could be dynamically switched from 0.05 to 1 [17].Furthermore, to meet the requirement of switchable functions in a fixed structure, some active metasurfaces with multiple functions have been realized.Song and Zhang et al. realized flexible function switching between broadband absorption (0.52-1.2 THz) and polarization conversion in a VO 2 -based multilayer metasurface [18].
In this paper, we proposed an active THz metasurface with the dual functions of switchable broadband absorption and sensitive refractive index (RI) sensing in a VO 2 -based fishnet-shaped nanostructure.The absorption of the metasurface maintains > 90% in the broadband from 0.53 to 2.0 THz.By changing the phase of VO 2 , the maximum absorption 0.97 could be dynamically switched to 0.07.In addition, the same nanostructure with metallic VO 2 exhibits excellent RI sensing performance.Compared with previously reported THz sensors, our structure possesses competitive sensitivity (1.15 THz/RIU).The mechanism of switchable broadband absorption and sensitive sensing are discussed by resonance mode analysis.The proposed metadevices with the dual functions of switchable absorption and sensitive RI sensing in THz band will promote the applications of THz metasurfaces in the areas of biochemical sensing, stealth detecting, and terahertz filtering.

Design and Simulations
The dual functions of switchable absorption and sensitive RI sensing are realized in a fishnet-shaped nanostructure integrated with VO 2 .The fishnet-shaped array consists of periodic cross-shaped VO 2 and Au structure on VO 2 -polymer-Au substrate as shown in Fig. 1a.Under insulator VO 2 state, the incident THz wave will penetrate the top VO 2 resonator array and interact with the cross-shaped Au structure.The composite structure of cross-shaped Au resonatormiddle polymer layer-Au substrate could be equivalent to a metal-insulator-metal (MIM) cavity.Once the environment temperature is increased above 68 °C, VO 2 will undergo the insulator-to-metal transition [19].And then, the same structure with metallic VO 2 will excite totally different resonance mode and lead to switchable EM functions.
To realize the dual functions of broadband absorption and sensitive sensing, the geometric parameters of the fishnet-shaped nanostructures are delicately optimized.The top view and side views of the unit structure in Fig. 1b, c illustrate the relative position of cross-shaped VO 2 and Au resonators.The arm length of the cross-shaped VO 2 resonator in the top layer is same as period (l = p = 210 μm), leading to the fishnet-shaped array.The width of the arm for the cross-shaped VO 2 is w 1 = 13.5 μm.The crossshaped Au structure and top VO 2 resonators are separated by a polymer spacer.The geometric dimensions of the middle cross-shaped Au structure are l 2 = 170 μm and w 2 = 32 μm.As shown in Fig. 1c, the thickness of the top VO 2 and middle Au resonators is h 1 = 47.5 μm and h 2 = 8.5 μm, respectively.
With the temperature increased above phase-transition temperature from room temperature, VO 2 shows totally different optical properties.The Drude model was used to characterize the permittivity of VO 2 [20] where ∞ = 12 is the permittivity at infinite frequency and = 5.75 × 10 13 rad/s is the collision frequency.The permit- tivity of VO 2 could be derived by plasma frequency p , which relies on conductivity [21] as shown in Eq. ( 2) (where p 0 = 1.4 × 10 15 rad/s , σ 0 = 3 × 10 5 S∕m ).The conductivities of VO 2 represent different optical properties of VO 2 .Specifically, the conductivities of 200 S/m and 2 × 10 5 S/m are used to describe the insulator and metallic VO 2 , respectively [22].The conductivity of Au is set as Au = 4.56 × 10 7 S∕m .The permittivity of polymer is set as 3.5.In simulation, we obtain the reflection and absorption spectra by using finite element method (FEM) in frequency domain in a commercial software, CST Microwave Studio 2019.The periodic boundaries are applied in x-and y-direction while open boundary is used in z-direction.Our VO 2 -based active metasurface is realistic in experiment.The fishnet-shaped nanostructure could be fabricated by magnetron sputtering and photolithography.The thermal control for the phase change of VO 2 could be realized by a heating plate. (1)

Broadband and Switchable Absorption
When VO 2 is in insulator state, the metasurface exhibits high-efficiency and broadband absorption.The reflection coefficient spectra of the proposed metasurface with insulator VO 2 are given out in Fig. 2a.The cross-polarized reflection coefficients (r yx ) are near zero in the whole band.In contrast, the co-polarized reflection coefficients (r xx ) fluctuate slightly below 0.3.Utilizing the relationship between reflection coefficients and reflection ( ), the reflection spectra are calculated and plotted in Fig. 2b.The reflection maintains the value below 0.1 in the whole band.The thickness of Au substrate is larger than the penetrable depth of incident THz waves, so the transmission is zero.Thus, the absorption (A) could be simplified into 2 [22].Furthermore, the absorption spectra of the metasurface with insulator VO 2 are given out in Fig. 2b.There are four absorption peaks with the values of 0.969, 0.988, 0.957, and 0.984 at 0.60, 0.95, 1.36, and 1.76 THz, respectively.These four absorption peaks result from different resonance modes dominantly and these distinct modes will impact on the absorption strength around the adjacent frequencies of each peak, thus leading to broadband absorption.The frequency of absorption above 0.9 spans from 0.53 to 2.0 THz, which means our metasurface could absorb almost all the incident THz waves in a broadband.The metasurface with insulator VO 2 simultaneously exhibits the advantages of high-efficiency and broadband absorption.
To further explore the mechanism of high-efficiency and broadband absorption, the electric field distributions are given out in Fig. 3a-d.The high absorption at 0.60 THz (I), 0.95 THz (II), 1.36 THz (III), and 1.76 THz (IV) results from different resonance modes.In detail, as the electric field shown in Fig. 3a, there are parallel electric fields in the first polymer layer, which is flowing outwards in the x-z plane at 0.60 THz (I).As we all know, the electric dipole resonance is characterized by parallel fields pointing from one pole to another pole [23].So the absorption at peak I originates from electric dipole resonance in the first polymer layer surrounding cross-shaped Au resonator.Similarly, the parallel fields flowing inwards in the x-z plane are enhanced at the layer of cross-shaped VO 2 resonator as shown in Fig. 3b, which means the high absorption at peak II results from electric dipole resonance but in opposite direction.Interestingly, the enhanced parallel electric fields flowing inwards in the x-z plane shift to the gap of VO 2 resonators at 1.36 THz in Fig. 3c.The electric dipole resonance between VO 2 resonators at peak III leads to high absorption.In summary, the high absorption at 0.60, 0.95, and 1.36 THz originates from the excitation of electric dipole resonances but at different relative positions of a structure.Differently, the absorption at 1.76 THz results from magnetic dipole resonance.In Fig. 3d, the vector electric fields marked by black arrows in the y-z plane form circles in the position between VO 2 resonators.
Different from electric dipole resonance, the characteristic of magnetic dipole resonance is closed circular electric fields [24].So, the resonance mode at 1.76 THz is magnetic dipole resonance, which results in the high absorption at IV region.Furthermore, the four absorption peaks are located at the frequencies between 0.6 and 1.76 THz.Except for the dominate effect for the absorption at four peaks, these specific composite resonances at peaks will contribute to the absorption at adjacent frequencies.The common effect of these composite resonance modes at four absorption peaks leads to the broadband absorption response between 0.53 and 2.0 THz.
Unitizing the phase change of VO 2 , the resonance modes at absorption peaks could be tuned.Therefore, the strength of the broadband absorption could be switched in a large range by thermally controlling VO 2 .Under the different phases of VO 2 , the absorption spectra of the metasurfaces in the frequency from 0.53 to 1.75 THz are calculated.As shown in Fig. 4a, the absorption strength is much weaker as the conductivity of VO 2 increases.When the VO 2 is insulator state (with the conductivity of 200 S/m), the metasurface behaves as a high-efficiency absorber.When VO 2 is fully transformed into its metallic phase (with conductivity of 2 × 10 5 S/m), the absorption in the 0.53-1.76THz band decreases to below 0.1.The absorption at the broadband from 0.53 to 1.76 THz could be effectively tuned from near unity to near zero.In particular, the absorption of the metasurface at 0.96 THz could be switched in the large range from 0.07 to 0.97 by external thermal excitation.
The insensitivity of incident angle to absorption plays an important role in the practical applications of oblique incident system.Here, the influence of incident angles to absorption is discussed.With the increasing incident angles from 0 to 50°, the absorption of the insulator VO 2 metasurface is plotted in Fig. 4b.The absorption maintains the relatively high value (> 0.8) with the increasing incident angles from 0° to 50° throughout the whole frequency band.The weak influence of incident angles originates from the high geometric symmetry of our fishnet structure [21].

Sensitive RI Sensing
Not only the high-efficiency and broadband absorption could be realized, the sensitive RI sensing could be achieved in the same nanostructure.As shown in Fig. 5a, the metallic VO 2 -based structure exhibits sensitive RI sensing capability by placing the analytes on the top of fishnet structure.Figure 5b shows the reflection spectra of the metallic VO 2 metasurface with different RI analytes.There is a sharp reflection dip for the metasurface without analytes (n = 1.00), whose reflection is 0.009 at 1.775 THz as the blue curve marked.The full width at half maximum (FWHM) of the reflection dip is about 0.0283 THz and the Q-factor ( Q = f 0 ∕FWHM , f 0 is the frequency of the resonance) is 62.54.The small FWHM and high Q-factor enable the metasurface accurate recognition capability for the shift of reflection peaks to RI variation of analytes.In fact, the sharp reflection curve comes from the electric dipole resonance.As the electric field distributions are exhibited in Fig. 5c, there are parallel electric fields flowing outwards in the x-z plane, which are much enhanced in the layer of VO 2 fishnet-shaped structure.Thus, by placing analytes on the top of the fishnet structure, the resonance frequency could be strongly affected by the RI change of analytes, namely, leading to the high sensitivity.
To evaluate the sensitivity of the RI sensing, Fig. 5b shows the reflection spectra with analytes' RI variation.With the RI increasing of analytes from n = 1.00 to n = 1.15 in the step of Δn = 0.025, the reflection dip under- goes remarkable shift from 1.775 to 1.621 THz, but the values of reflection dip are almost kept unchanged.The RI sensitivity is defined as S RI = Δf ∕Δn [7], where Δf is the shift of reflection peak frequency with the RI change of analytes ( Δn ).To quantitatively describe the sensitivity, the spectral positions of reflection dip with the RI variation from 1.0 to 1.15 are shown in Fig. 5d.The relationship of spectral positions and analytes' RI can be quadratically fitted by the function of y = 0.784x 2 − 2.724x + 3.704 with the fitting degree of R 2 = 0.9998 .The slope of the fitting function in Fig. 5d represents the RI sensitivity.The maximum sensitivity of the metasurface is 1.15 THz/ RIU, which is a major advance compared with previously reported sensors.
We compare the RI sensing performance of our metasurface with recently proposed sensors in Table 1.The first three nanostructures have realized RI sensing but limited to the single sensing function in a fixed frequency band.In contrast, by integrating the thermally tunable material InSb, the next two active metasurfaces achieve the multiple functions including sensitive RI sensing [25].Similarly, our metasurface enables the dual functions of switchable absorption and sensitive sensing.Note that the sensing performance of our proposed nanostructure is much improved.The sensitivity reaches 1.15 THz/RIU at 1.775 THz.Also, the FWHM (0.0283 THz) of the reflection dip is narrower compared with the proposed work, which represents the high resolution  for sensing.With the high sensitivity and high resolution, our proposed structure has huge superiority and potential in the application of THz sensing.

Summary
In this paper, we proposed an active metasurface and achieved the dual functions of switchable broadband absorption and sensitive RI sensing in terahertz range by integrating phase transition material VO 2 .By thermally controlling VO 2 , the absorption values of the proposed metasurface in the broadband of 0.45-1.76THz could be switched from nearly 0.9 to below 0.1, which shows an excellent performance as a switchable broadband absorber.Besides, the high-efficiency absorption is insensitive to the incident angles below 50°.Meanwhile, the same metasurface exhibits sensitive RI sensing in THz frequency band, whose sensitivity reaches 1.15 THz/RIU and the FWHM is 0.0283.The great sensing performance is much enhanced compared with the reported sensors.The VO 2 -based active metasurfaces with the dual functions of switchable broadband absorption and sensitive RI sensing have promising prospects in the fields of integrated THz system.

Fig. 2 Fig. 3
Fig. 2 Broadband absorption.a The polarization reflection coefficients and b reflection/absorption spectra of the metasurface with insulator VO 2

Fig. 4
Fig. 4 Dynamically switchable absorption.a The switchable absorption spectra under various conductivity of VO 2 .b The absorption spectra with various incident angles (insulator VO 2 )

Fig. 5
Fig. 5 RI sensing.a The schematic of RI sensing for the metasurface with metallic VO 2 .b The reflection spectra with different analytes.The inset is the amplified reflection spectra.c The electric field distributions at 1.775 THz.d The shift of resonant frequency with different analytes and the fitting

Table 1
Comparison with the previously published RI sensorsValues in bold indicate the sensing performance of this work to distinguish our paper from other published work