Controlled switching of photonic materials is required for a variety of devices such as modulators, switches, optical memories etc. Phase change materials like liquid crystals, chalcogenide glasses such as Ge-Sb-Te, correlated oxides like vanadium dioxide (VO₂), topological insulators such as bismuth selenide (Bi₂Se₃) have been used for optical switching. In particular, VO₂ shows a metal insulator phase transition at about 68 °C with an increase of conductivity of four orders of magnitude [1]. The phase transition in VO₂ can be caused by external stimuli such as heat or electric fields, and has been used for switching metamaterial properties [2].

Metamaterials are composite materials with sub-wavelength resonant structures, and give rise to various unique phenomena like negative refraction [3], super lenses [3], and perfect absorption [4]. Metamaterial perfect absorbers (MPA) have been projected for various applications such as bolometers, thermal emitters, sensors etc [5]. MPA typically consist of tri-layer structures with a top patterned layer that is separated from a continuous bottom conducting ground plane by a dielectric spacer layer [4]. The top patterned layer resonantly interacts with the electric field of the incident electromagnetic radiation. The resulting current distributions are mirrored in the ground plane, with the antiparallel current distributions giving rise to a magnetic resonance [6]. The impedance of the MPA can be optimized by tuning these electric and magnetic resonances to resonantly achieve near-unit absorption of light that can be independent of the incident angle [7].

Incorporating a thin film of a phase change material like VO₂ in the MPA can enable switching of the absorptivity. Use of VO₂ as the spacer layer enables switching from a high absorptive state at low temperature to a highly reflective state at high temperature [8], while its use as a ground plane enables switching from a high reflection state to an absorptive state with increase in temperature across the phase transition [9]. While even a simple thin film of VO₂ on an absorbing/ metallic substrate can give rise to a near-perfect absorption via Fabry-Pérot (F-P) like resonances [10,11], full control over parameters such as the peak wavelength, peak absorptivity at a given temperature, and bandwidth can be independently obtained through the structural resonances of the top layer in the MPA. A variety of resonant patterns such as arrays of disks [8,9], square patches [12], split ring resonators [13], Y shaped plasmonic antenna [14] or even line gratings [15] have been used for the top structured layer.

In this work, we report a novel switchable MPA with a VO₂ spacer layer and an ITO thin film as a plasmonic ground plane. ITO has negative permittivity at frequencies below 488.43 THz and has been projected for plasmonic applications at infrared (IR) frequencies [16]. The top structured layer consists of an array of gold micro-disks. This MPA shows a large contrast of 78% in the switching of reflection from the absorptive to the reflective state upon increasing the temperature across the metal-insulator transition temperature. The structure shows two pronounced absorption maxima: One, that arises from a F-P like resonance in the VO₂ layer peaked at 6 μm, and another narrower band, peaked at 9.2 μm that arises from the dipole resonance of the disk. The ITO ground plane allows multiplexing of this IR device with optical frequencies due to its transparency at visible frequencies [17]. We monitor the state of the VO₂ by optical reflectivity measurements through the glass substrate. IR emittance from the MPA is also shown to be switchable at LWIR and MWIR frequencies with low emissivity at high temperature. This gives a new dimension to engineer the surface emittance.

The schematic picture of a unit cell of the switchable absorber is depicted in Fig. 1(a). The circular gold disks have diameter of 1.5 μm and height of 100 nm and are placed on a square array with a period of 3.15 μm. The VO₂ thin film has a thickness, d = 320 nm, is deposited on a ITO coated glass substrate by chemical vapour deposition (CVD) [18]. The ITO thin film has thickness, t = 150 nm, with surface resistivity of 15–20 Ω/Sq. The phase transition temperature of the VO₂ film was measured to be about 68°C by IR reflectance measurements. The gold disks on the VO₂ film were fabricated by e-beam lithography using a thin layer of spin-coated positive resist (PMMA 495 A4). 100 nm of gold was sputtered on this patterned resist with circular holes and followed by a lift-off process using acetone. Figure 1(c) shows the scanning electron microscopy (SEM) image of the array of structured gold disks on the VO₂ surface. Atomic force microscope (AFM) scan shows an rms surface roughness about ±20 nm for the VO₂ films [Fig. 1(d)]. This roughness also manifests in the gold disk’s surface deposited on the VO₂ film. Uniform structured areas of 1mm×1mm were obtained in this manner.

 

Fig. 1 (a) Schematic diagram of the unit cell of the MPA (Au disk/VO₂/ITO) on glass with t= 150 nm, d=320 nm and h= 100 nm, a=1.5 μm and P= 3.15 μm. (b) Height profile of the disks obtained by AFM imaging. (c) SEM image of the gold disks on the film (top view, also see Visualization 1 for structural uniformity). (d) Image of an AFM scan of the VO₂ surface showing rms roughness ≈ ±20 nm (also see Visualization 2).

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The spectral response at IR frequencies from 2.5 μm to 14 μm, was measured by an IR microscope (Agilent, Cary 600) connected to a FTIR spectrometer (Agilent, Cary 660). The measured reflected intensity from the MPA was normalized with respect to the reflection from a thick gold mirror. A temperature controlled pad (Harrick, Model ATC-024-4) was used to measure the sample reflectivity from room temperature upto 80°C. The surface was illuminated at near normal incidence of light with 14° divergence due to the 10× objective and the reflected light was measured by a liquid nitrogen cooled Mercury Cadmium Telluride (MCT) detector.

The measured reflectivity of the MPA at room temperature (<68°C) and high temperature (>68°C) are highly different [Fig. 2(a)]. The low temperature state is characterised primarily by two absorption peaks (reflection minima) peaked at 6 μm and 9.2 μm. In comparison, the MPA at high temperature is mostly highly reflecting across the infra-red band. At 6 μm wavelength, the reflectivity changes from 13% at low temperature to 89% at high temperature, while at 9.2 μm wavelength, the reflectivity changes from 15% to 93% at high temperature. These changes were completely reversible over many heating and cooling cycles. The reflectivity obtained from four different regions of the MPA are shown in Fig. 2(b) (different colours) and confirm the uniformity of the metamaterial structure. The reflectivity from the sample where the top patterned layer was not present had only a single absorption band peaked at 6 μm [Fig. 2(b)]. Thus the 9.2 μm peak is identified to solely arise from the metamaterial structures, while the 6 μm peak arises from the multilayered structure of VO₂/ITO. The sharp dip in the reflectance at about 4.25 μm is due to the presence of atmospheric CO₂ that interferes with these measurements.

 

Fig. 2 (a) Reflection spectrum of the MPA at various temperatures during heating (for reflection during cooling cycle, see Visualization 3). (b) Measured reflection of the unstructured VO₂/ITO layers and from different positions of the MPA. (c) Optical reflectivity of the MPA measured through the glass substrate. (d) Calculated reflection spectrum of VO₂/ITO layers and the MPA with different disk period at room temperature and at high temp. (H. T.) (The insets show the angle dependent reflection at the peak wavelengths, 5.69 μm and 9.62 μm w.r.t the angle of incidence (A. I.)).

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We measured the spatial emittance of the MPA using thermal imaging with a long wave infrared (LWIR) camera (Thermal eye 4550AS) and a mid wave infrared (MWIR) camera (Thermosensorik, Model:InSb 640 SM) at different temperatures. The IR images in Figs. 3(b)–3(h) clearly demonstrate the switchability of the emittance of the MPA. At 50°C [Fig. 3(b)], the structures appear bright, showing the fact that the MPA behaves as a perfect emitter. As the VO₂ switches from a low temperature insulator phase to a high temperature conducting phase, the emittance of the MPA decreases dramatically and the MPA appears literally dark at 80°C [Fig. 3(d)]. In the MWIR camera image, this reduction of emittance with increasing temperature is apparent, but is not as drastic as in the LWIR images. To demonstrate multiplexing of the IR absorptive MPA with optical applications, we show the reflected spectrum of visible light incident on the MPA through the ITO coated glass substrate. For mounting the sample on the temperature controlled pad, a copper ring surrounding the MPA area was pasted by silver paint(also visible in the LWIR images) which allowed coupling of visible light through the substrate. The reflection measurements were carried out using an optical microscope (Olympus, BX51) that was fibre coupled to a spectrometer(Ocean optics HR 2000+) through the trinocular port. Figure 2(c) shows the reflected spectra where the changes in reflectivity of the MPA is clearly visible as the VO₂ goes through the insulator metal transitions. We find that the reflectance at low temperature is higher than the reflectance at high temperature. There is a peak at about 750 nm which disappears upon increasing the temperature.

 

Fig. 3 (a) Image of the sample surface with a visible camera. The square patch is the structured MPA seen through a copper ring used for thermal contact. The bright portion is highly reflecting silver paint. (b)–(d) LWIR images of the emittance at different sample temperatures. (e)–(h) show the MWIR images of the emittance at different temperatures. Results of computations on the MPA structures are given: (i) Magnetic field magnitude at λ= 5.69 μm (j) Magnetic field magnitude at 9.62 μm and the arrows in (i) and (j) indicate the distribution of electric field.

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To understand the origin of the resonant absorption, the finite element method based software, COMSOL Multiphysics, [19] was used to simulate the system. The unit cell of the MPA is depicted in Fig. 1(a). Periodic boundary conditions were applied in the plane across the unit cell in the X- and Y- directions. The Drude model was used for the permittivity of ITO, $∊(\omega )=3.95-\left[{\omega }_p^2/\omega (\omega +i\gamma )\right]$ with ω_p/2π = 488.43 THz and γ/2π =29.01 THz [20]. The frequency dependent permittivity of gold was taken from Ref. [21], and for both the low and high temperature phases of VO₂ from Ref. [22]. The COMSOL calculation confirms two absorption bands peaked at 9.62 μm and 5.69 μm for the MPA at low temperatures. Reflection from the unpatterned multilayer(VO₂/ITO) was also calculated by the transfer matrix method [23]. The transfer matrix calculations show a single peak at about 5.69 μm for VO₂/ITO layers on glass with no structured top layer [Fig. 2(d)], thereby confirming the experimental observations. For the VO₂ film assumed to be in the high temperature metallic phase, a reflectivity in excess of 80% is obtained for the MPA across the IR frequencies. We calculated the electromagnetic fields at the two absorption peaks, i.e., at 5.69 μm and 9.62 μm [Figs. 3(i) and 3(j)]. The resonance at 9.62 μm is due to a dipole resonance where the magnetic field is mainly confined within the gap between the gold patch and the underneath ITO [Fig. 3(j)]. Near-zero reflection at 5.69 μm and 8% reflection at 9.62 μm were obtained in the dielectric state of VO₂ at room temperature [Fig. 2(d)]. In contrast, for the high temperature metallic state of VO₂, reflection of over 80% in both cases, at 5.69 μm and 9.62 μm was obtained. According to the simulations, nearly 80% of amplitude modulation for the change in reflection across the phase change is obtained.

Now we discuss the mechanisms of the resonance absorption. The peak of the resonance band arises from the dipole resonance of the patch is at 9.62 μm and the nature is confirmed from the simulations. The absorption/emittance can be thermally switched as shown in Fig. 2(a) and Figs. 3(a)–3(d) as the magnetic resonance is not active at high temperature. The change in reflectivity from 15 % to over 93 % represents a modulation ratio (I_max − I_min)/(I_max + I_min) that exceeds 72 % and may be one of the largest changes reported for switchable MPAs [24]. We note that the minimum reflection is rather large ∼15 % for VO₂ thickness of 320 nm. This deviation probably arises from the scattering of radiation due to the surface roughness ∼ ±20 nm rms in our VO₂ films. The peak at 5.69 μm arises due to multiple reflections within the VO₂ in a manner reminiscent of F-P resonances. This is, in spite of the sub-wavelength thickness (∼λ/19) of the VO₂ film, and arises due to the large phase shifts at the VO₂/ITO interface. Such resonances have been seen earlier in VO₂-sapphire substrates [10] and VO₂ -metal [11] at IR frequencies, and the band critically depends on the nature of the substrate and the thickness of the VO₂ film. Typically, the substrate should be plasmonic or lossy, and here the ITO layer essentially behaves as a semi-infinite plasmonic medium. Both the principal absorption bands change little with periodicity of the patches [Fig. 2(d)]. However, we do see a secondary peak on the blue band edge of the F-P resonance. This peak at 4.64 μm for a pitch of 3.15 μm, disperses with the pitch of the array and is due to a propagating surface plasmon resonance (PSP) riding on the ITO/VO₂ interface. The dispersion of this PSP resonance, which is coupled to light by diffraction from the periodic patches is given by ${\lambda }_{\text{spp}}=\left(P/\sqrt{m^2+n^2}\right)\sqrt{\left({∊}_{\text{ITO}}{∊}_{{\text{VO}}_2}\right)/\left({∊}_{\text{ITO}}{∊}_{{\text{VO}}_2}\right)}$ and matches well for m=2 and n=2. The merging of the PSP resonance and F-P like resonances creates the appearance of a broader resonance [Fig. 2(b)]. The angular dispersion of the reflectivity at 9.62 μm and 5.69 μm (absorption peaks) with the angle of incidence are shown in the inset of Fig. 2(d). The TE polarization disperses more with the reflectivity rising to about 10 % at ±30° angle of incidence while the TM mode reflectivity remains relatively constant for the metamaterial resonance. In the case of the F-P resonance at 5.69 μm, the angular dispersion is extremely less signifying that the phase shifts are dominated by the reflection coefficients at the dielectric- ITO interface rather than the path length.

The changes in the emittance in the LWIR images are quite drastic due to the change in the emissivity (= to absorptivity) in the LWIR band. The changes in the emittance in the MWIR band are much more gradual. This is due to the reduced emissivity (high reflectivity at 3–4 μm) at low temperature caused by constructive interference of reflection from the VO₂ layer interfaces. There is a low reflectivity/ high absorption band arising from the second order band beyond this at smaller wavelengths. At high temperatures, even though the reflectivity increases over the MWIR band, it doesn’t exceed the reflectivity of the MPA at low temperatures over the 3–5 μm band. Hence the switching of emittance at MWIR wavelengths is not quite as dramatic as at LWIR frequencies where the metamaterial resonance dominates.

In conclusion, a thermally switchable metamaterial absorber with large switchability of almost 78% of the peak reflectivity has been demonstrated. The tri-layer MPA consists of gold disks separated from an IR-plasmonic thin film of ITO by a thin film of VO₂. This spacer layer of VO₂ undergoes an insulator-metal phase transition at 68°C and switches the MPA from a low reflectivity state at low temperature to a high reflectivity state at high temperature. The MPA is shown to have two principal absorption bands with an MWIR band that arises due to interference of reflection from the VO₂/ITO film interfaces, and another band in the LWIR that arises due to the metamaterial dipole resonance of the top patterned layer. Both absorption bands are switched off with increase of temperature beyond 68°C as the spacer layer VO₂ becomes metallic. The thermal IR images reveal the possibility of switching the IR emittance of the MPA from high to low as the temperature is increased beyond 68°C. The ITO ground plane in the MPA makes it transparent at visible frequencies and amenable to interfacing the MPA at IR frequencies to other applications using visible light. We have demonstrated the possibility of optically probing the state of VO₂ film through the transparent substrate.