Engineering metamaterials with tunable resonances from mid-infrared to near-infrared wavelengths could have far-reaching consequences for chip based optical devices, active filters, modulators, and sensors. Utilizing the metal-insulator phase transition in vanadium oxide (VO2), we demonstrate frequency-tunable metamaterials in the near-IR range, from 1.5 - 5 microns. Arrays of Ag split ring resonators (SRRs) are patterned with e-beam lithography onto planar VO2 and etched via reactive ion etching to yield Ag/VO2 hybrid SRRs. FTIR reflection data and FDTD simulation results show the resonant peak position red shifts upon heating above the phase transition temperature. We also show that, by including coupling elements in the design of these hybrid Ag/VO2 bi-layer structures, we can achieve resonant peak position tuning of up to 110 nm.
©2009 Optical Society of America
Electromagnetic resonances in subwavelength metallic resonators can be used to engineer optical responses not found in natural materials [1,2]. Metamaterials have reemerged following their initial introduction , and are the subject of intensive investigation for applications in frequency selective surfaces [4,5] and transformation optics, e.g. negative-index materials [6–9], super-lensing [10,11], and cloaking [12–14]. Active metamaterials have been demonstrated at terahertz frequencies using carrier depletion in GaAs [15,16], photoexcitation of free charge carries in Si , and the metal-insulator phase transition in vanadium oxide thin films . Typical metamaterials consist of arrays of metal structures embedded in a dielectric with feature sizes much smaller than the desired operating wavelength. In the simplest sense, the unit cell of a metamaterial array is designed to be an individual “LC” circuit with resonant frequency, ωo ~(LC)-1/2 . Split-ring resonators (SRRs) are the basis for many metamaterial designs due to ease of fabrication and modeling. Each SRR has a distributed inductance, L, and a distributed capacitance, C, arising from charge build-up at the notch. The choice of materials and the resonator dimensions set these two parameters and determine the resonant frequency of the metamaterial. Active metamaterial designs have previously focused on changing the capacitance in the SRR gap to modulate the amplitude of the resonance [15–17]. Integrating materials with tunable electrical or optical properties allows further control over the resonant response in metamaterials. Vanadium oxide is a promising candidate that exhibits a dramatic change in its complex refractive index arising from a structural phase transition from monoclinic to rutile. Here, we propose an alternative geometry consisting of self-aligned, hybrid Ag/VO2 SRR bi-layers as an approach to tuning the metamaterial response in the near-IR by controlling the resonator geometry with the phase transition. VO2 undergoes a structural transition from an insulating monoclinic phase to a metallic rutile phase at 68 °C [19,20]. This phase transition can occur on a sub-picosecond timescale  and can be induced thermally, optically  or electrically . As a result of the insulator-to-metal transition, the conductivity increases by as much as four orders of magnitude and the optical transmission in the near-IR decreases significantly . Drastic changes in the optical properties of VO2 with the phase transition enable control over the transmission and reflection properties of nanophotonic structures, such as nanoparticles [24,25], hole arrays , and metamaterials .
2. Ag/VO2 active metamaterials: simulations and experiments
2.1 Unit cell design
The active metamaterial device design presented here consists of self-aligned Ag/VO2 SRR arrays (Fig. 1a ). The VO2 thin films are grown epitaxially by pulsed laser deposition on c-plane Al2O3 substrates at 500 °C. A vanadium metal target is used as the source material and deposition takes place in 12 mTorr oxygen. 60 nm thick VO2 films are deposited with a 300 mJ laser pulse at a rate of 10 Hz. Utilizing the insulator-metal phase transition in VO2, we can control the effective dimensions of the unit cell as well as the optical properties of the hybrid-SRR arrays in both phases. Coupled asymmetric split-ring resonators are employed to reduce the spectral linewidth at the resonance frequency (Fig. 1b-d). A narrower resonant peak will increase the tuning figure of merit (FOM), the ratio of the tuning range to the full width at half maximum (FWHM) of the resonant peak. In this work, we investigate four self-aligned hybrid-SRR structures including two coupled SRR systems and one SRR coupled to a nanowire.
2.2 Active Ag SRR arrays on planar VO2 substrates
The metamaterial arrays are fabricated on a 60 nm thick planar VO2 film using standard electron beam lithography and evaporation of 150 nm of silver and 10 nm of chromium to act as an etch mask. SRR arrays are tested using the reflection mode of an FTIR microscope using a light source in the range of 1.5 – 8 microns. Reflection data were normalized using an optically thick gold standard. Electromagnetic radiation, normally incident to the SRR array (along the z-direction) with the incident E-field polarized perpendicular to the SRR arms, was used to efficiently couple to the resonators and induce circulating currents in the structures (Fig. 1a). Two resonant reflection peaks, 2.1 µm (electric resonance) and 3.6 µm (magnetic “LC” resonance), were observed as shown in Fig. 2a . The sample was subsequently heated to temperatures above the insulator-metal phase transition temperature and then cooled using a commercial heating stage comprised of a silver block with a tiny aperture milled in the center. The resulting spectra show a continuous change from two distinct metamaterial resonant peaks to a broad reflection indicative of metallic VO2. Although there is a slight hysteresis, the resonances are recovered upon cooling below the phase transition temperature. We also measured the reflection from a 60 nm thick VO2 film on sapphire substrate at room temperature and 85 °C. The reflection intensity at 3.6 µm increased from 0.36 to 0.75, indicating a drastic change in complex refractive index of the VO2 thin film upon phase change (Inset of Fig. 2a).
The resonant response was modeled using Lumerical, a commercially available finite-difference time-domain (FDTD) simulation software. A unit cell of the investigated structure is simulated using periodic boundary conditions along the x and y axes and perfectly matched layers along the axis of propagation of the electromagnetic waves (z axis). A broadband plane wave is incident on the unit cell along + z direction, and reflection is monitored by a power monitor that is placed behind the radiation source. Electric and magnetic fields are monitored by frequency profile monitors. Figure 2b shows the simulated response of an Ag SRR array on a planar VO2 thin film in both the insulator and metallic phases. To accurately simulate the metamaterial response, we measured the complex refractive index of the VO2 thin films in both phases (Fig. 2c) using variable angle spectroscopic ellipsometry. In the insulator phase, the VO2 optical constants are fit to a Tauc-Lorentz model. In the metal phase, VO2 acts like a Drude metal with strong absorption. These models were used to extrapolate the results to the near and mid-IR to aid in FDTD simulations. The simulated reflection spectra in both VO2 phases agree well with the measured data.
We calculated the magnetic field intensity, |Hz|2 at λ = 3.6 μm (inset of Fig. 2b) for the SRR array in the insulator (right) and metallic phases (left) of VO2. The resonant behavior in the insulator phase is evident from the magnetic field profile, in which the magnetic field is concentrated at the center of SRR due to the excitation of circular surface currents at the resonance frequency. However, in the metallic VO2 phase the LC circuit of each SRR is shorted and the array no longer resonates. Although the resonant behavior of the metamaterial can be switched with the phase transition of a VO2 thin film, this switch is essentially bimodal. In the metallic phase the sample is highly reflecting and behaves like a mirror at IR wavelengths, meaning that the spectral selectivity required for practical applications is not achieved. To overcome this problem, we propose a self-aligned SRR design composed of a stack of Ag and VO2 layers, whose resonant frequency could be modulated thermally.
2.3 Active Ag/VO2 hybrid-SRR metamaterials
The samples are etched using reactive ion etching to yield self-aligned hybrid-SRR arrays. The 100 Watt CF4/O2 plasma etches the VO2 film not masked by the Cr/Ag SRR structures to yield hybrid Ag/VO2 SRR elements. Figure 3a shows experimental reflection spectra for self-aligned Ag/VO2 SRR structures with dimensions of 450 × 400 nm and 150 nm width SRR arms (inset). The resonant reflection peak is observed at 2.52 µm, compared to 3.6 µm before etching. The blue shifting of the metamaterial resonance is a direct effect of replacing much of the underlying high dielectric constant VO2 substrate with air. Upon heating above the insulator-metal phase transition temperature to 100 °C, the resonant reflection response of the new hybrid-SRR structure red shifts by Δλ = 100 nm to 2.62 µm. A schematic of the self-aligned hybrid-SRR unit cell is shown in Fig. 1a.
Simulations of the metamaterial array show a similar red shift (≈100 nm) with an associated overall decrease in the reflection due to the increased absorption in the VO2 metallic phase (Fig. 3b, blue line). However, this initial simulation result does not agree well with the measurements, in which the reflection peak intensity is not drastically reduced. Additional simulations were performed to investigate the behavior observed in the experiment. The experimental results suggest that the nanostructured VO2 is not a homogeneous metallic rutile phase, but represents a nanocomposite phase where the semiconductor and metallic phases co-exist. Other researchers have reported that the VO2 phase transition occurs via nanoscale domain switching rather than a bulk homogeneous phase transformation , and the phase transition and optical properties of nanoscale VO2 structures have been shown to differ from those seen in continuous thin films . Given that the actual switching mechanism is likely inhomogeneous for patterned nanoscale structures, and that characterization of the actual domain structure is non-trivial, we employed the simplest model possible that provides good quantitative agreement with experimental observations: we assume that the heated 60 nm thick VO2 film is a bilayer composed of a 15 nm thick VO2 film in the metallic phase at the Ag/VO2 interface and a 45 nm thick VO2 film in the semiconductor phase at the VO2/sapphire interface (Fig. 3b, light red line). The increase in peak intensity for the bilayer model approaches to that of observed in the experimental data. Based on the experiments and simulations, we conclude that the optical thickness of the VO2 layer is different than the geometric thickness. It is also important to note that the optical properties of VO2 films in both phases were measured up to 2.2 µm, above which we extrapolated the complex refractive index data by fitting to the ellipsometric models as shown in Fig. 2c.
Simulated magnetic and electric field intensity plots (|Hz|2, and |Ex|2) of the self-aligned SRR structure (λ = 2.5 μm) for both the insulator and metallic VO2 phases are shown as insets in Fig. 3b. The resonant behavior is evident in the field profiles. At the resonant wavelengths, the electric field is strongly localized between the edges of the SRR arms due to the circular currents on the SRR. Since VO2 has higher losses in the metallic phase, the calculated intensities of the magnetic and electric fields are lower compared to those in the insulator phase. When the active material is in the insulator phase, each unit cell of the metamaterial array acts as a 150 nm thick Ag SRR on a 60 nm thick VO2 substrate. Upon heating above the phase transition temperature, the effective optical thickness of the resonator element changes due to the phase transition which causes the effective capacitance and inductance of the SRR element to increase and the resonant frequency to decrease. The proposed hybrid metal/VO2 design is applicable to any resonator element in which the resonant behavior depends on the effective optical thickness.
2.4 Active coupled asymmetric Ag/VO2 hybrid-SRR metamaterials
Frequency selective metamaterial surfaces could be used as modulators, filters and sensors at IR wavelengths. For practical applications, it is desirable to tune the resonant wavelength by a line-width of the resonance peak (FWHM). As seen in Fig. 3a, the resonance line-width of an ordinary SRR array is fairly broad (FWHM ≈1.25 µm) and Δλ ≈100 nm tuning is relatively small compared to the resonance bandwidth. In order to increase the FOM, the ratio of Δλ to the FWHM of the resonant peak, one could either increase the wavelength tuning range (Δλ) or decrease the line width of the resonance. Here, we demonstrate spectral line width narrowing of metamaterial resonances by introducing coupled asymmetric SRRs at near-IR wavelengths.
Figure 4 shows the experimental reflection spectra for coupled, self-aligned, Ag/VO2 hybrid-SRR structures in the near-IR range from 1.5 – 5.0 microns. First, a SRR structure coupled to a nearby nanowire (Fig. 1b) is investigated in Fig. 4a. We observe two resonant reflection peaks, λ1 and λ2, that can be attributed to electrical and magnetic resonances, respectively. Magnetic and electric field intensities in the insulator phase are plotted in the inset. The reflection peak at λ1 = 1.8 μm is due to the electric dipole resonance of the nanowire. The magnetic fields are localized around the nanowire due to the surface current flowing parallel the nanowire. At the electric dipole resonance wavelength, simulations predict strong localization of electric fields at the ends of the nanowire. The resonant wavelength, λ1, does not change when the VO2 switches phase. The wavelength of the nanowire dipole electric resonance is determined by the nanowire length and does not depend significantly on the thickness of the metal.
The resonant peak at the higher wavelength, λ2 = 3.0 μm, is due to the magnetic resonance, as evident by the characteristic magnetic resonance field responses, (i) large magnetic response at the center of SRR and (ii) strong electric field localization at the SRR gap. The magnetic resonance wavelength is red-shifted by Δλ2 = 100 nm as the VO2 changes phase from insulator to metal. Introducing nanowires as an additional element to the metamaterial unit cell resulted in narrower resonance peaks than those observed for ordinary SRR arrays. The FWHM of the resonance peaks at 1.8 and 3.0 μm were measured to be 0.5 and 0.7 μm, respectively. The FOM of ordinary SRR array was 0.08, whereas the FOM for the nanowire coupled SRRs is 0.14, due to the spectral line-width narrowing of the metamaterial resonance.
Breaking the structural symmetry of metamaterials has been shown to yield narrower metamaterial resonances at microwave frequencies . Coupling a nanowire to a single SRR is a way to introduce asymmetry to the resonant elements. Another approach is to couple two SRRs of different sizes (asymmetric SRRs) by properly designing the unit cell. Figure 4b shows the reflection spectra of two face-to-face SRRs of different dimensions (Fig. 1c) in the insulator and metallic phases. The resonant peaks for these SRRs are λ1 = 2.1 μm and λ2 = 3.1 μm. Simulated field plots (inset) show the signal at λ1 is due to resonances in both SRRs, whereas the signal at λ2 is due only to the SRR with longer arms. In the case of coupled SRR arrays, the shorter wavelength response is due to the SRR with shorter arms. The total shift upon phase change in the VO2 is Δλ2 = 110 nm. Figure 4c shows the reflection spectra for an array of back-to-back coupled SRRs (Fig. 1d) with the same dimensions as the face-to-face structures. Coupling the SRRs in this manner narrowed the wavelength separation of the two resonant peaks: λ1 red shifted by 200 nm to 2.3 μm and λ2 blue shifted by 300 nm to 2.8 μm. The change in the magnetic resonant reflection peaks for these coupled structures is Δλ1 = Δλ2 = 65 nm. We observed the hybridization of the resonant wavelength of a single SRR, λ = 2.52 µm, to shorter and longer wavelengths, λ1 and λ2. Different coupling schemes resulted in different peak positions in the reflection spectra. The resonant peak shift depends on the coupling strength of resonator elements, and in the case of strong coupling, resonant peaks show larger shifts . Since the electric fields are strongly localized at the gap of SRRs, the coupling of face-to-face SRRs is stronger than back-to-back coupled SRRs. We observed larger resonance wavelength separation in the case of face-to-face SRRs.
Metamaterial resonances can be engineered using different coupling and hybridization mechanisms as reported here. The three geometries investigated here show reflection resonances of different magnitudes, at wavelengths dependant on the size and degree of coupling between SRRs. In all cases, the magnetic resonance reflection peaks of self-aligned structures red shift as the overall geometry changes with the VO2 insulator-metal phase transition. Here, we proposed a sandwiched VO2 active layer between an Ag SRR and sapphire substrate, which caused wavelength tuning of ~100 nm. Incorporating VO2 nanostructures at different resonant metamaterial elements such as the SRR gap or SRR arms by two-step electron-beam lithography and processing could yield better device performances and higher tuning ratios. In this work the phase change was induced thermally simply by heating the metamaterial array, but optical and electrical phase changes are also possible in VO2.
In conclusion, we have demonstrated the first active metamaterial at near-IR wavelengths based on an Ag/VO2 design allowing us to change the optical geometry of the resonant element with the heat induced phase transition of VO2. Resonance hybridization with additional coupling elements such as nanowires or different size SRRs led to narrower metamaterial resonant peaks, thus increasing the frequency-tuning figure of merit.
We acknowledge financial support from the National Science Foundation under Grant DMR 0606472, and the Army Research Office; portions of this work were performed in facilities sponsored by the Center for Science and Engineering of Materials, and NSF MRSEC. We gratefully acknowledge critical support and infrastructure provided for this work by the Kavli Nanoscience Institute at Caltech. I. M. P. acknowledges the support of a National Science Foundation Graduate Fellowship. We also gratefully acknowledge the help of Professor George Rossman for IR facilities and measurements.
* These authors contributed equally to this work.
‡ Present address: Booz Allen Hamilton, Arlington VA, 22203.
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