Tunable VO2 cavity enables multispectral manipulation from visible to microwave frequencies

Optical materials capable of dynamically manipulating electromagnetic waves are an emerging field in memories, optical modulators, and thermal management. Recently, their multispectral design preliminarily attracts much attention, aiming to enhance their efficiency and integration of functionalities. However, the multispectral manipulation based on these materials is challenging due to their ubiquitous wavelength dependence restricting their capacity to narrow wavelengths. In this article, we cascade multiple tunable optical cavities with selective-transparent layers, enabling a universal approach to overcoming wavelength dependence and establishing a multispectral platform with highly integrated functions. Based on it, we demonstrate the multispectral (ranging from 400 nm to 3 cm), fast response speed (0.9 s), and reversible manipulation based on a typical phase change material, vanadium dioxide. Our platform involves tandem VO2-based Fabry–Pérot (F-P) cavities enabling the customization of optical responses at target bands independently. It can achieve broadband color-changing capacity in the visible region (a shift of ~60 nm in resonant wavelength) and is capable of freely switching between three typical optical models (transmittance, reflectance, and absorptance) in the infrared to microwave regions with drastic amplitude tunability exceeding 0.7. This work represents a state-of-art advance in multispectral optics and material science, providing a critical approach for expanding the multispectral manipulation ability of optical systems.


Introduction
The demand for advanced applications in fields such as memories, information communication, imaging, and medical health has spurred the development of optical systems operating across broadband wavelengths ranging from the visible (VIS), infrared (IR) to terahertz (THz) and microwave (MW) regions [1][2][3] .For example, the development of materials with high IR emittance and solar reflectance can motivate terrestrial surfaces to cool themselves passively by harvesting the coldness of the universe [4][5][6] .However, optical systems with fixed spectra may generate undesired outcomes in non-standard conditions, such as overcooling 7,8 and invalid concealment [9][10][11][12][13] , significantly limiting their practical applications.
Optical systems produced by phase change materials (PCMs) [14][15][16][17] and electrochromic materials (ECMs) [18][19][20] can overcome these challenges by dynamically changing optical responses under external stimulations.Overcooling can be prevented by automatically reducing the IR emittance when the temperature drops 7 .However, the strong and ubiquitous wavelength dependence observed in PCMs and ECMs restricts their unique optical properties to certain wavelengths 21 .Moreover, structures such as metasurfaces and optical cavities, which are deliberately designed to improve the efficiency of these materials, tend to exacerbate this limitation (Supplementary Fig. S1).As a result, optical systems that exhibit reversible tunability face severe challenges in achieving multispectral manipulation (Supplementary Table S1).
A significant breakthrough occurred when Kocabas et al. 21developed a multilayer graphene (MLG) based electrochemical optical platform capable of dynamically modulating reflectivity across a broad range of the swath from the VIS to MW regions.The MLG, which possesses high electrical conductivity (<50 Ω•sq −1 ), was laminated and vacuum sealed in a low-density polyethylene pouch.With optical transparency exceeding 90%, it can replace the top electrode and allows for broadband optical activity, opening up a new avenue for the development of ECMs-based optical systems.However, the ionic migration occurring inside ECMs inevitably leads to a relatively long response time in the range of dozens of seconds and additional energy consumption.The permanent tunable path from high absorption to high reflectance precludes possibilities of transmittance regulation.Despite the fair optical tunability of PCMs under external stimulations, their multispectral operations remain unreported due to the unbroken wavelength dependence (Supplementary Table S1).
In this article, we propose a universal method to breaking up the wavelength dependence of a typical PCMvanadium dioxide (VO 2 ), and demonstrate its multispectral manipulation with reversible tunability covering wavelengths ranging from the VIS to MW regions.The VO 2 -based system is capable of constructing a broadband color-changing space in the VIS region, and to the best of our knowledge, could switch freely between three typical optical models (transmittance, reflectance, and absorptance) in the IR to MW regions for the first time.Moreover, the ultrafast phase transition of VO 2 enables faster response time of 0.9 s compared to ECMs-based systems.The unparalleled performance is unachievable for any reported optical system and represents a significant breakthrough in multispectral optics.energy band change that leads to the transition from insulating to metallic behaviors (Fig. 1b).The phase transition leads to high contrast in its optical behaviors from VIS to MW regions, thereby endowing VO 2 with the potential for multispectral manipulation [22][23][24][25][26][27] 1c demonstrates the ideal multispectral manipulation in the proposed system based on the measured data: (i) broadband color-changing ability in VIS;

Operating principle and ideal spectrum
(ii) drastic transmittance tunability from zero to near unity in the IR, THz and MW regions; (iii) A dynamic absorptance region whose peak position is also adjustable in IR and longer wavelengths; (iv) ultrafast switching speed.The experimental results demonstrate that our system is the only one reported so far, achieving multispectral and dynamic manipulation with ultrafast response speed based on PCM (Fig. 1d) 18,21,[28][29][30][31][32][33][34][35][36] .

Limitations of existing VO 2 -based systems in multispectral operations
The ubiquitous wavelength dependence of PCMs and ECMs confines their functionality to specific wavelengths 21 , whose modulation efficiency deteriorates seriously at other bands.We explore the wavelength dependence of VO 2 -based optical systems by the simulated thickness-dependent VIS-MIR reflectance tunability spectra of representative VO 2 -based platforms (VO 2 /SiO 2 , VO 2 /Al, and VO 2 /HfO 2 /Al).As shown in Fig. 2, it is apparent that there exists a certain region possessing the best modulation efficiency for fixed thickness.These regions shift to a longer wavelength with the increasing VO 2 thickness and exhibit the typical sub-wavelength feature as a joint effect of the thickness-dependent interference effect 37 and the intrinsic absorptance of VO 2 .Besides the strong wavelength dependence, the poor color-changing ability of VO 2 in VIS and the shielding effect of common substrates (Al, Au, SiO 2 , et al.) in the IR to MW regions further limit the potential of VO 2 for multispectral manipulation.Therefore, there are several issues in achieving the multispectral manipulation based on VO 2 : (i) improve the color-changing ability; (ii) overcome the wavelength dependence and (iii) break the shielding effect of substrates.

Structural design for TFP operating in the VIS region
The slight permittivity discrepancy in the VIS region during the phase transition induces an inconspicuous color change of VO 2   38   .(Supplementary Fig. S2a, b) Herein, as shown in Fig. 3a, we construct a TFP structure consisting of VO 2 /HfO 2 /VO 2 /Si layers to amplify the permittivity change of VO 2 in the VIS region, and realize a reversible tuning of the reflective color.The top VO 2 film acts as the semi-transparent layer and the bottom VO 2 acts as a part of the dielectric layer.A 150-nm-thick Si layer is chosen as the substrate to block the propagation of VIS waves, while maintaining high transparency when facing IR to MW waves.(Fig. 3a and Supplementary Fig. S2c) The interactive disturbance in optical designs for TFP and BFP is simultaneously eliminated.Considering the coherent accumulation of the partial waves reflected from the F-P cavity 39,40 , the F-P resonant wavelength λ in TFP can be calculated as: where n D is the effective refractive index of dielectric layers consisting of HfO 2 and VO 2 , d D is the thickness of dielectric layers, θ is the incident angle, φ 21 and φ 23 represent the phase shift of the reflection coefficients at the top VO 2 /HfO 2 interface and the VO 2 /Si interface.According to Eq. 1, n D is the primary factor contributing to tuning λ and changing surface colors during the phase transition.Meanwhile, different combinations of d D , φ 21 and φ 23 enable the construction of a broadband colorchanging gamut theoretically.
We then investigate the color-changing capacity of TFP by simulating the temperature-dependent reflectance spectra.For preset structural parameters (10 nm VO 2 / 25 nm HfO 2 /40 nm VO 2 /150 nm Si), the amplified decrease in n D during the phase transition prompts the blue shift of the reflectance valley from 610 nm to 575 nm, with an encouraging color change observed from cyan to purple (Supplementary Fig. S2d and Fig. 3b).We further explore the potential of enriching the color-changing space by modifying structural parameters.Our calculations, presented in the CIE 1931 color space, demonstrate that it is achievable to gain diverse color-changing paths by varying the thickness of VO 2 layers (Fig. 3c-f).TFP with a top-layer VO 2 thinner than 20 nm exhibits superior dynamic color-changing capabilities across the phase transition.The ultra-thin thickness of each layer in TFP also ensures that it is optically accessible to BFP devices in the IR to MW regions.This section theoretically proves the capacity of TFP to generate broadband colors and determines an approximate thickness range for TFP 41 .

Structural design for BFP operating in the IR to MW regions
Manipulating IR waves usually changes objects' radiative properties, and further affects their heat exchange process.Thereby, the comprehensive capacity to switch between the three states of transmission, reflection, and absorption is crucial for a thermal regulator 42 .Constructing F-P cavities is the most common and efficient method to enhance the capacity of dynamic manipulating absorptance for VO 2 in the IR to MW regions.However, as mentioned before, the strong wavelength dependence confines the spectral width, and the dynamic manipulation of transmittance is not supported as well.
Herein, we propose a "two birds with one stone" method that replaces the metal reflector with a VO 2 layer, whose reflection to IR, THz and MW waves is activated in its metallic state, and therefore achieves the broadband manipulation of transmittance while maintaining the resonant absorption in an F-P cavity (Fig. 4a).Simulated spectra in Supplementary Fig. S3 find that the minimum thickness for a VO 2 layer to provide adequate reflectance in the IR region is around 300 nm.In this article, we choose a 500 nm thickness VO 2 layer as the reflector for simulations and experiments.
Then we investigate the of structural parameters in TFP on the performance of BFP.As shown in Supplementary Fig. S4, the small deviations in spectra between the two structures reveal that it is acceptable to regard the VO 2 /HfO 2 /VO 2 tri-layers in TFP as a monolayer VO 2 layer whose thickness is the sum of the thickness of two VO 2 layers.Supplementary Fig. S5 indicates that arbitrary thickness combinations of the two VO 2 layers in TFP have no impact on the IR performance when maintaining the sum of their thickness to be a constant, but meanwhile, the system could exhibit diverse surface color appearances.It represents the achievable independent spectral manipulation in divided wavelengths.Meanwhile, we discover the effect of HfO 2 thickness (varies from 0 to 2000 nm) on the IR performance of BFP and the comprehensive modulation paths covering the transmittance, reflectance and absorptance (Supplementary Fig. S6 and Fig. 4b, c).When VO 2 is insulating, the system exhibits a maximum transmittance exceeding 0.7 because of the negligible optical loss in each layer.The reflectance at regions shorter than 5 μm increases slightly as a result of the refractive mismatch in interfaces within TFP, which indicates the sub-wavelength, or wavelength-dependence effect of optical systems.After the phase transition, the completely forbidden transmission in the IR region leads to a drastic transmittance tunability of 0.7.The F-P resonance generates a strong absorption peak whose position is adjustable by varying the thickness of HfO 2 in BFP.In the non-resonant regions, the VO 2 (R) reflector enhances the reflectance significantly with maximum tunability exceeding 0.7.For potential applications requiring not only tunable amplitude but a moveable absorptance peak wavelength, it is also feasible by replacing the HfO 2 layer with dielectric materials with changeable refractive index, such as liquid crystals 43 , GST alloys 44 .
In the THz and MW regions, the dielectric constants of VO 2 can be described using the Drude model 45,46 : where ε ∞ = 12 is dielectric permittivity at the infinite frequency, ω p ðσÞ is the plasma frequency of VO 2 that dependent on conductivity,γ = 5.75 ×10 13 rad• s −1 is the collision frequency, σ is the conductivity of VO 2 (Fig. 4d and Supplementary Fig. S7).As shown in Fig. 4e, when VO 2 is insulating, BFP exhibits a completely transmissive state due to the lossless layers and the disappearance of interlayer interaction due to the wavelength dependence.After the phase transition, the transmittance drastically becomes 0, where the reflectance reaches 0.8 simultaneously.The performance may contribute to the combined applications of communications, computations and electromagnetic shielding 47,48 .In addition, systems deposited on substrates with different optical properties can bestow more manipulation paths in the IR to MW regions.Supplementary Fig.  demonstrates the simulated IR to MW spectra of systems deposited on totally absorptive/reflective substrates (Fig. 4e can be regarded as the spectrum of the system deposited on a transparent substrate).The radically different features in these spectra will activate and support much more insights into expansive multispectral applications.

Experimental demonstration and characterization
To further validate the promise of multispectral dynamic manipulation, we develop proof of concept (PoC) samples.(see Methods for details) A 300 μm thickness Si wafer with high transparency in the IR, THz and MW regions is chosen as the substrate to discover the consistency between experiments and simulations.The membrane thicknesses in TFP, from top to bottom, are 15, 35, 20, and 150 nm.The samples are labeled Sample-1, Sample-2, and Sample-3, based on the different thicknesses (0, 600, and 1200 nm) of the HfO 2 layer in BFP.The thickness of the bottom VO 2 in BFP maintains 500 nm.(Fig. 5a and Supplementary Fig. S9) Measured physical properties of the deposited VO 2 films are presented in Supplementary Fig. S10.
In the VIS region, an apparent reflective color change from purple to pink is observed in Fig. 5a and Supplementary Movie 1 when heating the samples to 100 °C.The color change path meets well with the simulation in Fig. 2e,   f.The blue shift of the F-P resonance in TFP during the phase transition is also detected in the measured temperature-dependent reflectance spectra, though errors of optical constants induce some mismatch in the intensity compared to the simulated spectra (Supplementary Fig. S11).Therefore, it is achievable to generate arbitrary colors by simply altering the layer thickness in TFP, which expands the potential of VO 2 -based optical systems in the fields of color display and adaptive camouflage.Meanwhile, the response time can be ascertained by measuring the time interval between the sample's contact with the heater and its full discoloration, which occurs in approximately 0.9 s (Supplementary Fig. S12).
In the IR region, a measured maximum transmittance difference of 0.6 across the phase transition of VO 2 in Supplementary Fig. S13a indicates the success in introducing the reversible tunability of transmittance into a multispectral optical system.The system exhibits an antireflection effect on the Si substrate so that the transmittance at around 7 μm is even higher than that of the substrate.The decreased transmittance in longer wavelengths due to the increased absorption of thicker HfO 2 can be addressed by replacing the HfO 2 dielectric layer with an IR lossless material.A large reflectance tunability of around 0.5 is also presented in Supplementary Fig. S13b.We further investigate the adjustable Si substrate 500 nm 4 Structural design for BFP with the simulation in the IR to MW regions.a Structural design for BFP with design principle; b Simulated temperature-dependent IR transmittance spectra for different HfO 2 thickness; c Simulated IR absorptance spectra for different HfO 2 thickness when VO 2 is metallic; d Measured temperature-dependent conductivity of 500 nm VO 2 film deposited on a Si substrate; e Simulated temperaturedependent transmittance/reflectance spectra in IR to MW regions of BFP.The thickness of HfO 2 in BFP is set to 400 nm absorption performance of the samples.Figure 5b shows near-perfect absorption peaks generated by the F-P resonance in BFP when VO 2 is metallic in all samples, which indicates a high absorptance tunability reaching 0.7 at resonant wavelengths, and the position can be easily adjusted within a wide range (3-11 μm) even extended to the THz and MW regions 49,50 (Supplementary Fig. S13c).In addition, we further verify the effect of substrates on the dynamic manipulation path in the IR region.We put three samples on substrates with high emittance (SiO 2 wafer) and high reflectance (Al wafer), then record the temperature-dependent IR images and videos (Fig. 5c and Supplementary Movies 2-3).The high transparency makes samples' IR performance depend on the substrate at low temperatures, while eventually coming to a substrate-independent high absorptance state after the VO 2 phase transition.Therefore, it is achievable to construct two distinctly opposite temperature-dependent absorptance tunability paths in the IR region, which can be adapted to requirements of different IR applications (Fig. 5d).
In the THz and MW regions, a sharp change of conductivity with more than three orders of magnitude is observed for a 500 nm VO 2 layer deposited on a Si substrate (Fig. 4d).Measured temperature-dependent transmittance spectra in the THz and MW regions exhibit drastic transmittance tunability exceeding 0.7 for all samples (Fig. 5e).The wavelength dependence keeps THz and MW spectra unchangeable with varying HfO 2 thicknesses.The absorptance of the Si substrate triggers a slight degeneracy in transmittance when VO 2 is insulating compared to simulations.
Finally, response time, as another critical performance of multispectral manipulation, is also crucial 21 .The ultrafast phase transition of VO 2 under the picosecond scale endows this system with an incomparable response time and has been widely confirmed 51 .Therefore, our work reports an unparalleled performance in multispectral manipulation (Fig. 1d and Supplementary Table S1).

Discussion
In summary, we propose a universal method for overcoming the wavelength dependence of tunable optical materials, and have demonstrated the exceptional potential of modified tandem VO 2 -based F-P cavities for multispectral and reversible manipulation spanning from the VIS to MW regions.The multispectral platform achieves unprecedented performance, including broadband color- changing capability in the VIS region and free switching ability among three optical states of transmittance, reflectance, and absorptance in the IR to MW regions.Moreover, the ultrafast phase transition of VO 2 enables a short response time of less than 0.9 s.Our work represents the state-of-the-art level in interdisciplinary research that interfaces multispectral optics and material science.We believe it will open up a wide range of possibilities for advanced applications in memories, thermal management, imaging, and communications.

Simulation method
Electromagnetic wave calculations are performed using FDTD Solutions, a commercially available FDTD simulation software from Lumerical Solutions, Canada.All of the simulations reported in this paper are performed in three-dimensional layouts.We use the periodic boundary conditions along the x and y axes, and use perfectly matched layers along the z axis.Mesh accuracy is set to 8, and the minimum mesh step is 0.00025 μm.The minimum auto shutoff is 10 −5 .A plane wave is chosen as the source which injects along the z axis.The transmittance and reflectance are monitored by power monitors.Optical constants of used materials at different regions can be found in the reference 52 .The physical optical models for VO 2 at metallic and insulating states can be described as Eq. ( 4) 53 ε ω VO 2 layer deposition VO 2 layers were deposited by a high-power pulsed magnetron sputtering system (MS650C) purchased from KeYou, China.During the deposition process, the Ar/O 2 ratio was fixed at 81:1.9 sccm, and the sputtering power was set to 180 W. The chamber pressure, substrate temperature, input pulse frequency and width were fixed at 0.9 Pa, 550 °C, 200 Hz and 50 μs, respectively.

HfO 2 layer deposition
HfO 2 layers were deposited by a direct current magnetron sputtering system (MS650C) purchased from KeYou, China.During the deposition process, the Ar/O 2 ratios were fixed at 81:6.8 sccm, and the sputtering powers were set to 220 W. The chamber pressure and the substrate temperature were fixed at 0.9 Pa, 400 °C, respectively.

Si layer deposition
The Si layer was deposited by a radio frequency magnetron sputtering system (MS650C) purchased from KeYou, China.During the deposition process, the Ar/O 2 ratios were fixed at 81:0 sccm, and the sputtering powers were set to 100 W. The chamber pressure and the substrate temperature were fixed at 0.9 Pa, 100 °C, respectively.

Measurements
The crystalline phases were characterized using X-ray diffraction (XRD, PANalytical B.V. Model Xpert Pro).The Raman spectrum was recorded by an inVia Laser Micro Raman spectrometer (Renishaw, UK) equipped with a confocal microscope and a 532 nm excitation laser source.The cross-sectional images were evaluated by a field emission transmission electron microscope (JEOL JEM-F200).TEM samples were prepared by a focused ion beam (FIB) instrument (Hitachi NX5000).Fourier transform infrared (FT-IR) spectroscopy was performed using an FT-IR system (VERTEX-70, Bruker) from 2.5 to 25 μm.X-ray photoelectron spectroscopy (XPS) was performed with a PHI 5700 ESCA System using Al Ka radiation (1486.6 eV).XPS data were calibrated to the C1s peak and analyzed using Casa XPS software.The IR images were recorded by thermal imager (TIX-660, FLUKE) from 7.5 to 14 μm.The conductivity was measured by Hall Effect Measurement System (Ecopia HMS-3000).

FigureFig. 1
Figure1ademonstrates the proposed tandem Fabry-Pérot (F-P) cavity structure and its fundamental operating principle.At the critical temperature of 340 K, VO 2 undergoes an ultrafast transformation from monoclinic VO 2 (M) to rutile VO 2 (R), accompanied by an

OFig. 3
Fig. 3 Structural design for TFP with the simulation in the VIS region.a Structural design for TFP with design principle; b Simulated VIS temperature-dependent reflectance spectra for 10 nm VO 2 /25 nm HfO 2 /40 nm VO 2 /150 nm Si. c Simulated surface colors in CIE 1931 space when VO 2 is insulating; d Simulated surface colors in CIE 1931 space when VO 2 is metallic; e Surface colors corresponding to specific thickness of VO 2 layers when VO 2 is insulating; f Surface colors corresponding to specific thickness of VO 2 layers when VO 2 is metallic

Fig. 5
Fig. 5 Experimental demonstration of proposed multispectral system.a Cross-section HRTEM image of Sample-2 with its color-changing capacity recorded by a camera; b The absorptance spectra of PoC samples when VO 2 is metallic; c IR images taken by an IR camera when put PoC samples on Al and SiO 2 wafers at 40 °C and 100 °C; d Distinct temperature-dependent absorptance paths for Sample-1 and Sample-3 put on SiO 2 and Al wafers; e Measured temperature-dependent THz to MW transmittance spectra of three samples.The bottom lines near to zero represent the spectra measured at 100 °C, and top lines reaching 0.7 represent the spectra measured at room temperature