Actively Tunable Metasurfaces via Plasmonic Nanogap Cavities with Sub-10-nm VO₂ Films

Abstract

Actively tunable optical materials integrated with engineered subwavelength structures could enable novel optoelectronic devices, including reconfigurable light sources and tunable on-chip spectral filters. The phase-change material vanadium dioxide (VO₂) provides a promising solid-state solution for dynamic tuning; however, previous demonstrations have been limited to thicker and often rough VO₂ films or require a lattice-matched substrate for growth. Here, sub-10-nm-thick VO₂ films are realized by atomic layer deposition (ALD) and integrated with plasmonic nanogap cavities to demonstrate tunable, spectrally selective absorption across 1200 nm in the near-infrared (NIR). Upon inducing the phase transition via heating, the absorption resonance is blue-shifted by as much as 60 nm. This process is reversible upon cooling and repeatable over more than ten temperature cycles. Dynamic, ultrathin VO₂ films deposited by ALD, as demonstrated here, open up new potential architectures and applications where VO₂ can be utilized to provide reconfigurability including three-dimensional, flexible and large-area structures.

Nanostructured optical materials have enabled unparalleled control over light–matter interactions for creating ultrafast single photon emitters,¹⁻³ efficient nonlinear surfaces,⁴⁻⁷ and on-chip spectral filters.⁸⁻¹⁰ Integration of active materials provides a means of tuning the optical properties in real-time promising for a new generation of multifunctional, nanoscale optoelectronic devices. Active metasurfaces have been demonstrated using a wide array of tuning mechanisms, including electrical modulation,¹¹⁻¹⁶ polarization of incident light,¹⁷ mechanical strain,^18,19 hydrogen gas flow,^20,21 UV exposure,^22,23 femtosecond laser pulses,^24,25 and heating.²⁶⁻³¹ While there have been several demonstrations of robust tuning at mid-IR and longer wavelengths,^11,16,31 active metasurfaces in the visible and near-IR have generally been limited by tuning mechanisms that are ill-suited for practical devices, for reasons, such as lack of reversibility or degradation of switching over multiple cycles.^12,18,22,32 One option for overcoming these limitations is utilizing the phase-change material vanadium dioxide (VO₂), which offers robust switching via heating above 68 °C. At this temperature, VO₂ undergoes a first-order, crystalline phase transition with an accompanying change in its refractive index, which is fully reversible by cooling below the transition temperature.³³ It has also been reported that this phase change can be induced electrically with a less than 2 ns response time,³⁴ increasing the promise of VO₂ for integration into practical, active metasurface devices.

Previously, VO₂ has been used to create active metasurfaces over a broad wavelength range,^{26,30,31,35−37} typically with bulk materials or sputtered films with thicknesses of 50–150 nm³⁸⁻⁴³ or more rarely by utilizing a lattice-matched substrate for growth of thinner films.⁴⁴ However, integrating nanometer-scale VO₂ films with nanocavity structures on non-lattice-matched substrates has remained an outstanding challenge, yet offers the promise for robust, deeply subwavelength tunable optoelectronic devices to enable, for example, nanoscale optical memories and dynamic control of quantum emitters. Furthermore, conventional techniques to deposit VO₂ thin films, such as pulsed laser deposition (PLD), are only uniform over small areas, do not deposit conformally and are incompatible with many materials. ALD provides a promising, alternative deposition technique that enables highly uniform films over wafer-scale areas, conformal coating of three-dimensional or flexible structures as well as integration of dissimilar materials that can be deposited with abrupt interfaces. Lastly, the resultant ultrathin layers may allow for heat to be removed faster which could enable devices with a faster thermal response.

Here, we demonstrate reversible tuning of near-IR absorption resonances from active plasmonic metasurfaces via the insulator-to-metal phase transition of a 9 nm VO₂ film grown by ALD. The tunable metasurfaces consist of arrays of plasmonic nanodisks on top of the VO₂ and Au films, where a single nanocavity is shown in Figure 1a. The parallel metallic interfaces in Figure 1a create a planar cavity between the two Au surfaces separated by the VO₂ film, which supports a gap-mode surface plasmon.⁴⁵ The resonance wavelength is determined by the geometry, material, and refractive index of the constituent nanocavity elements.⁴⁶⁻⁴⁹

Figure 1.

(a) Schematic of sample structure consisting of Au nanodisks (height = 30 nm) separated from an Au ground plane by 9 nm of VO₂. (b) Illustration of the phase transition of VO₂. Upon heating to ∼68 °C, the crystal structure shifts from monoclinic to rutile leading to a corresponding change in its refractive index. (c) Ellipsometry-measured real and imaginary components of the refractive index of a 9 nm VO₂ film on Au at room temperature and after heating to 80 °C. This data is plotted in terms of the real and imaginary components of ε in Figure S1.

The resonances are statically controlled by varying the nanoparticle diameter and dynamically controlled through the refractive index tuning of the VO₂ layer. Upon heating to ∼68 °C, VO₂ undergoes a phase transition which alters the corresponding crystal structure from monoclinic to rutile as depicted in Figure 1b. In the monoclinic phase, VO₂ is transparent and insulating in the IR and possesses a bandgap of 0.7 eV.⁵⁰ When transitioned to the tetragonal phase, the bandgap closes resulting in lossy metallic behavior or a metallic phase. Beyond a purely electronic transition, this insulator-to-metal transition possesses a concomitant change in the real and imaginary components of the refractive index. Figure 1c depicts the refractive index change for a 9 nm film deposited on Au and measured by ellipsometry at 24 and 80 °C for wavelengths spanning from 400 to 2400 nm. Spectrally, the largest refractive index change of −0.17 occurs at 640 nm for the real component and 0.12 at 1660 nm for the imaginary component. In the monoclinic phase, the VO₂ film exhibits normal dispersion for wavelengths longer than 500 nm indicative of the insulating or dielectric state. When in the tetragonal phase at 80 °C, the refractive index shows anomalous dispersion at wavelengths longer than 1000 nm indicative of metallic behavior. As such, the refractive index change in Figure 1c distinctly shows the insulator-to-metal transition induced upon heating. Although the real component of the permittivity of the VO₂ film is expected to go below zero around the plasma frequency, and this has been demonstrated previously for VO₂ films fabricated in a similar manner,⁵¹ this behavior is not observed in the present samples and can likely be explained by the ultrathin nature of the VO₂ film and the underlying nonlattice-matched gold substrate.

Fabrication of the structure began with growing the VO₂ on a 75 nm-thick Au film via ALD according to a process that has been extensively characterized elsewhere and been shown to produce thin films capable of undergoing a phase change⁵²⁻⁵⁴ Subsequently, the sample was annealed to form a poly crystalline film from the initially amorphous material and had the effect of ensuring that the metasurface switching was fully reversible (details in methods section). X-ray photoelectron spectroscopy (XPS) characterization of the annealed VO₂ films was performed (shown in Figure S2) and revealed V⁺⁴ and V⁺⁵ peaks with full-widths at half-maximum (fwhm) of around 1.6–1.7 eV, indicating that the film has a fairly high degree of crystallinity as amorphous films typically have widths around 2–3 eV due to slight variations in bonding strengths. While the V⁺⁴ peak is indicative of a VO₂ film, the V⁺⁵ peak is the result of some undesired contamination, likely in the form of a hydroxyl group or V–O–C bonding in addition to the V₂O₅ stoichiometry. However, previous studies have determined that the thickness of this contaminated layer is no more than 1 nm and results from the subsequent fabrication steps being performed in atmospheric conditions.⁵³ SEM images of the VO₂ film (shown in Figure S3) reveal the presence of pinholes that likely have the effect of reducing the amount of switching that can be achieved. However, such defects are common for depositions on a non-lattice-matched and polycrystalline substrate.

Next, the VO₂ films were embedded into plasmonic nanocavities by transferring Au nanodisks on top of them. The Au nanodisks were fabricated using electron-beam lithography (EBL) on a Si substrate. Because of the weak adhesion of Au to the Si substrate, the Au nanodisks could be exfoliated from the Si wafer after removing the native SiO₂ and deposited onto the VO₂ film using a PDMS stamp as shown in Figure 2a. This technique was utilized because of the sensitivity of VO₂ to electron-beam exposures and subsequent development processes (additional fabrication details in SI). In all, 13 metasurfaces were fabricated by varying the diameter of the nanodisks (from 88 to 329 nm), where SEM images of representative metasurfaces are shown in Figure 2b. The plasmonic nanocavities possess a large effective electric and magnetic response on resonance as shown in Figure 2c, which is strongly localized in the VO₂ film below the disks. As such, a sufficient fill fraction of nanocavities on the surface can result in near-perfect absorption at the plasmon resonance because of the large absorption cross section of each individual antenna element resulting in near-total effective surface coverage and absorption of incident light. Alternatively, this can be thought of as the destructive interference of the reflected wave with the multitude of plasmonic modes.^46,49

Figure 2.

(a) Fabrication process for transferring EBL-fabricated gold nanodisks onto the VO₂ film. (b) SEM images of gold nanodisks for a few of the fabricated sizes. (c) Simulated electric (left) and magnetic (right) field profiles at 24 and 80 °C for the nanogap cavity structure with 192 nm gold nanodisks. An incident plane wave with wavelength matching the room-temperature resonance of the nanocavity at 1495 nm is utilized. (d) Room-temperature reflectance spectra for metasurfaces consisting of eight different sizes of nanodisks where additional spectra are shown in Figure S4.

Because of the spectrally dependent variations in the insulator-to-metal transition, 13 spectrally distinct metasurfaces were used to assess the dispersion of the active tuning. The resonant wavelengths for these absorbing metasurfaces range from ∼900 to 2100 nm and span above and below the anomalous dispersion of the tetragonal VO₂ phase. The room-temperature reflection spectra of the metasurfaces were measured with a white light source focused through a 0.4 NA objective as shown in Figure 2d (additional spectra in Figure S4). All the metasurfaces demonstrate near-perfect absorption of incident light on resonance, with typical peak absorption values of ∼95%. The switching spectra for the 88, 150, 221, 256, and 329 nm nanodisk metasurfaces are shown in Figure 3a, while switching spectra for the remaining sizes are shown in Figure S5. In addition to producing a spectral shift in the resonance, heating the metasurfaces was also observed to modulate the peak absorbance by as much as 16%, contributing further to the dynamic nature of this structure.

Figure 3.

(a) Thermal switching spectra for metasurfaces consisting of 88, 150, 221, 256, and 329 nm diameter Au nanodisks. Spectra for the remaining sizes are shown in Figure S5. (b) Switching spectra from a control sample with a gap consisting of 9 nm of Al₂O₃ for the same size nanodisks as in panel a. (c) Comparison of the amount of switching observed experimentally for both the control sample (9 nm Al₂O₃ gap) and the VO₂ thermal switching sample (9 nm VO₂ gap). The gray dashed line at 0 nm of switching is intended to guide the reader’s eye.

A control sample was also fabricated with a gap comprised of a non-phase-change material (9 nm of Al₂O₃). The thermal switching spectra for select sizes of nanodisks on this sample (same sizes as shown in Figure 3a) are presented in Figure 3b. An average of only 3 nm of wavelength switching was observed for the control sample. A comparison of the amount of switching for each nanodisk size for both the sample with the VO₂ gap and the sample with the Al₂O₃ gap is shown in Figure 3c. We note that the full-width at half-maximum (fwhm) of the resonance on the control sample is less for the control sample. This is likely due to increased surface roughness for the VO₂ film arising from the annealing process which resulted in larger inhomogeneous broadening of the plasmon resonance.

To provide additional insight into the spectral dependence of the switching behavior, the active metasurfaces were simulated with a finite-difference solver to model the temperature-dependent reflection spectra for the various metasurfaces. The simulated spectra for the same sizes of nanodisks as in Figure 3a are shown in Supporting Figure 6. Both the simulations and experimental results show that spectral switching can be maximized for a given geometry and resonant wavelength. Furthermore, simulations show good agreement with experiment in terms of the magnitude of thermal switching that is predicted, although, for some disk sizes, simulations predict a different direction for the switching than what is observed experimentally.

For further characterization of the VO₂ switching, the amount of switching for the 192 nm nanodisks metasurface is analyzed over multiple cycles and smaller temperature steps. The temperature-dependent reflectance spectra of the 192 nm nanodisk metasurface over five cycles is shown in Figure 4a. In all, reflection spectra for this metasurface were collected over 12 temperature cycles; the shift in peak wavelength measured during each cycle is plotted in Figure 4b instead of the full spectra for clarity. For each cycle, the magnitude of shift was observed to be 36 ± 2 nm, demonstrating the repeatable nature of the thermal switching. The reflection spectra of the 192 nm nanodisk metasurface was also measured at 5 °C increments to characterize the continuity of the switching in more detail, and the results are shown in Figure 4c. The extracted resonance wavelength is shown as a function of temperature in Figure 4d. The resonance wavelength of the 192 nm metasurface displays little change until the temperature approaches 45 °C. The resonance then begins to blue-shift with increasing temperature, reaching a maximum rate of change at 60 °C. This implies a transition temperature of ∼60 °C for the fabricated VO₂ gap layer, which is slightly less than the reported value in literature for bulk VO₂.³³ Previous investigations of ALD-grown VO₂ have similarly shown a lowered transition temperature of 61 °C.⁵³ Above 80 °C in our experiments, the resonance wavelength stabilizes at a final value of 1300 nm, which is blue-shifted by 40 nm compared to the resonance wavelength at room temperature. The lowered transition temperature and gradual phase shift occurring over a 40 °C range when compared to bulk VO₂ is characteristic of the effect of boundary conditions on the VO₂ thin films. The amount of switching observed here is less than what has been measured previously with sputtered/PLD VO₂ films or bulk material and likely arises from a combination of the effects of interface strain on the ultrathin film and the presence of only a small amount of the phase-change material. Further optimization of the VO₂ film deposition process on an Au substrate, such as reducing the roughness and Au dewetting that occur during annealing as well as lessening contamination in the form of V⁺⁵, is expected to lead to a larger tuning range.

Figure 4.

Characteristics of thermal switching for a metasurface consisting of 192 nm gold nanodisks. (a) Thermal switching spectra over five cycles, which shows fully reversible switching. (b) Magnitude of switching over 12 cycles of the temperature. (c) Switching spectra over one heating cycle taken in 5 °C increments from 20 to 85 °C. (d) Extracted resonance wavelength as a function of temperature for the spectra displayed in panel c.

In summary, we have demonstrated thermally tunable nanogap absorbers, which rely upon a robust, solid-state switching mechanism and possess a range of resonant wavelengths spanning 1200 nm in the near-IR. These metasurfaces display up to 60 nm of switching that is fully reversible over more than 10 temperature cycles. With the current amount of switching, we anticipate that this structure could be utilized for controllable enhancement of processes such as fluorescence, single photon emission, and nonlinear generation, as these effects depend strongly on the overlap of the resonance with the wavelength of the process of interest.^6,55 An additional application that should be realizable with the current amount of switching is holography since the phase of the reflection coefficient varies rapidly around the resonance wavelength. Thus, a modest shift in the resonance wavelength should lead to a large shift in phase. With improvements to the VO₂ thin film deposition, such that the magnitude of switching is increased, a variety of additional deeply subwavelength, all-solid-state, optoelectronic devices could be created using this platform including optical switches and memories, tunable photodetectors, and reconfigurable displays.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c04175.

• Temperature-dependent ellipsometry data plotted in terms of the real and imaginary components of epsilon, details on the fabrication method, X-ray photoelectron spectroscopy (XPS) spectra of the VO₂ films, additional SEM images of the metasurface, room-temperature and thermal-switching spectra for the nanodisk sizes not shown in the main text, and simulated thermal switching spectra (PDF)

Author Contributions

^† A.M.B. and J.W.S. contributed equally to this work.

The authors declare no competing financial interest.