Optical antennas, subwavelength metallic structures resonating at visible frequencies, are a relatively new branch of antenna technology being applied in science, technology and medicine. Dynamically tuning the resonances of these antennas would increase their range of application and offer potential increases in plasmonic device efficiencies. Silver nanoantenna arrays were fabricated on a thin film of the phase change material vanadium dioxide (VO2) and the resonant wavelength of these arrays was modulated by increasing the temperature of the substrate above the critical temperature (approximately 68°C). Depending on the array, wavelength modulation of up to 110 nm was observed.
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Plasmonic structures are being applied to a wide range of situations, from cancer ablation  and microsurgery [2, 3] to narrowband sensing  and enhancement of fluorescence . Plasmonic resonances result when an incident electromagnetic wave drives oscillations of the conduction electrons in a metallic structure . Localized plasmon resonances, dependent on particle geometry, dielectric environment and the materials making up the resonant structure, are fixed once the nanostructure has been fabricated. This restricts them to passive device applications. A post-fabrication, reversible, dynamic tuning mechanism in these structures, on the other hand, would enable active plasmonic structures with wider applications potential.
Tunable optical antennas - plasmonic structures designed to be resonant at optical frequencies  - and plasmonic structures resonant from the near-infrared through the terahertz range have been created using a variety of approaches. These include, but are not limited to, fabrication on AFM tips  (enabling probing of the near field around quantum dots) and flexible substrates , incorporation of liquid crystal arrays , injection/photoexcitation of charge carriers  and micro-electrical mechanical actuating devices (MEMS) . Since optical antenna arrays must by necessity be fabricated on a substrate which, in turn, modulates their resonances, a simple approach to active plasmonic resonance devices is to use a substrate incorporating a layer of a material with dynamically tunable optical properties between the substrate and the resonant antenna structures.
One such material is vanadium dioxide (VO2), which has been under intense investigation because the critical temperature of its thermally-driven phase transition is approximately 68°C . In addition to the VO2 optical properties changing significantly with the phase change , the hysteretic nature of its transition has suggested it for a variety of memory applications [14, 15]. This hysteresis has been shown to depend on the size of crystal grain within the VO2 film [16, 17] and therefore can be optimized for the intended application.
The semiconductor-to-metal transition (SMT) of VO2 leads to a decrease in optical density and a change in electrical conductivity by three to four orders of magnitude. In a thin-film geometry, the SMT is percolative , with the SMT occurring in a statistically fluctuating fashion in individual VO2 crystal grains, leading to a range of optical properties between the two extremes of the semiconducting and metallic states. The phase transition may be triggered by application of external stress , current , voltage  or directly using light . When the phase change is effected optically, the SMT in VO2 has been observed to occur on a sub-picosecond scale , suggesting that VO2-modulated optical antennas may be appropriate for ultrafast optical switching. Transmission through sub-wavelength metal-VO2 hole arrays are affected by the VO2 phase transition , and the plasmon resonance in gold nanoparticles covered by a thin film of VO2 is also modulated by the SMT .
In this work, silver nanoantenna arrays were fabricated on a thin VO2 film supported by a Si substrate and the reflectance of the arrays was measured when the VO2 was in both semiconducting and metallic phases. Changing the phase of the VO2 layer alters the dielectric environment of the nanoantennas, in turn altering the resonant wavelength of the structures. A blue-shifting of the antenna resonances is observed when the VO2 transitions from the semiconducting (ambient temperature) phase to the metallic (high temperature) phase due to the reduced relative refractive index of the thin film. We compare experimental measurements to numerical simulations and demonstrate better agreement with theory than that shown in . Most significantly, we demonstrate this active response in an open geometry that is compatible with silicon, amenable to direct electrical control of the antenna via strip lines on the VO2, and suitable for incorporation into waveguide devices and sensors.
2. Fabrication details
Vanadium dioxide films 135nm thick were deposited on a silicon substrate [(100) n-type, P-doped, resistivity 0.1-1 Ω/cm] by electron-beam evaporation of stoichiometric VO2 powder (99.5% pure). The films were subsequently annealed for 5 minutes at 450°C in an oxygen atmosphere at 250 mTorr. Atomic force microscopy in peak-force tapping mode yielded an rms roughness of approximately ± 7 nm over a 10 µm by 10 µm scan area.
Silver nanorods with nominal dimensions of 90 nm by 50nm were subsequently fabricated on these films in square arrays with a pitch of 250 nm [see Fig. 1(a)]. A Vistec EBPG 5000 electron beam lithography (EBL) system was used to write the antenna arrays in a bi-layer resist, which was selected to enhance the lift-off process. The bi-layer resist consists of methyl methacrylate and poly (methyl methacrylate) (MMA/PMMA) layers spun to give a total thickness of 300 nm, where the PMMA layer is used for the high resolution pattern and the MMA layer to provide an undercut. Following EBL exposure, the sample was developed in a 1:3 solution of methyl isobutyl ketone: isopropyl alcohol (MIBK:IPA) for 1 min, and then rinsed thoroughly in isopropanol. Electron beam evaporation was used to deposit 40 nm of silver onto the sample to provide the metal for the antennas. Lift-off was then performed using agitation in a bath of acetone leaving the antenna arrays on the VO2 surface. Upon completion of the lift-off process the sample was imaged using a scanning electron microscope (SEM), as shown in Fig. 1(b).
Due to the opacity of the silicon substrate/vanadium dioxide in the wavelength range of interest, only reflectance measurements were possible. SEM images of the fabricated structures are shown in Fig. 1(b). In this image the surface roughness of the VO2 is apparent. Multiple measurements of the dimensions of different nanorods from the SEM images gave dimensions of 99.3 ± 5.4 nm and 59.6 ± 4.7 nm for the long and short axes respectively.
The nanorod arrays were excited using normally-incident, white light from the air side linearly polarized along the long axis of the rods. Reflection measurements were taken using a white light source (HL-2000-FHSA) that was collimated using an Olympus UPlan SApo lens (10x, 0.4 NA), which was polarized by a linear polarizer (Thorlabs LPVIS050-MP). The polarized beam was focused by a 20x Olympus Plan N lens (20x, 0.4 NA) onto the sample and the reflected signal collected with the same objective. The reflected signal was coupled out through a beam splitter and focused onto the input of a fiber-coupled spectrometer (Ocean Optics QE65000), which was connected via USB to a computer for analysis. The sample was mounted on a stage attached to a resistive heater which was used to raise its temperature above that of the VO2 phase change. The resulting spectra were collected and normalized to the reflection from the adjacent substrate. This permits the isolation of the response of the antennas, which would otherwise be obscured by the dispersion of the underlying VO2. Experimentally a resonance shift of approximately 110 nm (measured from peak to peak) was observed for the rods, and is shown in Fig. 2(a).
Spectroscopic ellipsometry was performed to characterize the optical properties of the VO2 layer using a JA Woollam M-2000 ellipsometer. The data were fitted initially using a purely numerical fit which was then used as the input to create a model formed by the superposition of three Tauc-Lorentz oscillators and judged on its merit by the mean-square error (MSE), a standard figure of merit used in ellipsometry . This process resulted in an MSE of 4.085 for the ambient temperature sample and 4.887 for the heated sample, indicating a reliable fit [26, 27]. It was found during a comparison of experimental and numerical data that the results of our ellipsometry produced better agreement than what is seen in previously published data for VO2 on a sapphire substrate . This discrepancy between previously published data and the current ellipsometry data is not surprising, as every VO2 film displays a slightly different dispersion due to its granularity, age, exposure to humidity and its stoichiometry. The deposition method, substrate material and random variations across the film also have an impact on the properties of the deposited film. The refractive index dispersion curve extracted via ellipsometry and used in our modeling is shown in Fig. 2(b).
To understand the mechanism underlying the shift in plasmon resonance, the RF module of the commercial finite-element method (FEM) package COMSOL Multiphysics 4.3a  was used. This module uses the FEM to solve Maxwell's equations and can do so for arbitrary geometries and materials. A single unit cell of the fabricated particles was modeled with the same pitch in x and y directions via periodic boundary conditions. The lower boundary (in the substrate) was defined using a “scattering” boundary, adequate for absorbing the small amount of transmitted radiation with no significant reflection, and normally-incident excitation from the upper surface (the air boundary) was implemented using a port boundary condition. Given that only the zeroth diffracted order is reflected over the wavelength range of interest this port was also used to monitor the reflected field without any interference effects due to it being perfectly impedance matched to the reflected wave. Published materials data were used for Si and Ag  and data extracted from ellipsometry used for VO2. Reflected power relative to total incident power was calculated using the port boundary and normalized to the reflectance from the VO2 film without the structure present. This was done due to the dispersion of the VO2 layer, which otherwise conceals the peaks in reflectance caused by the presence of the nanostructures. This normalization method produces a normalized reflectance greater than unity, which indicates the structures are increasing the reflection from the array relative to the film itself. As expected, modeling shows the optical antenna resonances are modulated by the insulator-to-metal transition in the underlying VO2 thin film. Modeling predicted an approximately 110nm resonance shift in good agreement with experimental measurements.
There are several additional factors that have not been considered in our analysis of these structures which may additionally account for the disparity between experimental and numerical data. The porosity of the VO2 layer is one such factor. Ellipsometry on the bare films suggests that their optical thickness is significantly different to the measured physical thickness, and SEMs of the films (not shown) show structures that appear to be pores in the film surface. If the film is porous an effective medium model may be more appropriate to represent the film in our numerical model. This will require a detailed and complicated characterization of the film porosity and therefore has been excluded from the current work.
Thermoplasmonic heating expediting the premature phase transitioning of the VO2 film in the immediate neighborhood of the resonant antennas is another potential source of disagreement. Around their resonances, plasmonic structures can dissipate energy due to the high frequency oscillations of their conduction electrons. This is caused by resistive losses within them leading to joule heating, enabling them to act as nanoscale heat sources. If this occurs and causes the VO2 to transition, the dielectric environment of the nanoantennas would be made less dense optically and hence blue-shift the peak resonance wavelength from where it would be expected to occur. To investigate this, our COMSOL model was extended to couple thermal effects to the optical behavior of the VO2 with the sole heat source being optical irradiation with a linear intensity profile across the unit cell. It should be noted that in all our simulations the arrays are infinite and hence edge cooling effects are excluded from this analysis. The Microwave Heating module of COMSOL was used to simulate the fabricated structures' optical and thermal behaviors. This module solves Maxwell's equations with temperature-dependent optical properties of the constituent materials coupled to the heat equation. Inspired by Groulx and Ogoh , a temperature-dependent permittivity model was created in which a transitional temperature range was defined. Below this range the film properties were those of the semiconducting phase while above it they were those of the metallic phase. Within this range the various properties of the two phases were combined in a linear fashion, most notably the specific heat capacity [31, 32] and dielectric permittivity.
A power meter was used to measure the total power incident on the sample (maximum power = 37.6 µW) to provide an order of magnitude estimate for the calculation. For simplicity, since broadband excitation was used, a diffraction-limited spot at 500 nm (diameter = 1.525 µm) was assumed in order to calculate the maximum incident irradiance on the sample in our experiment. This was then used as the input power for the optical excitation of the array to find an upper bound on the temperature increase of the sample. The modeling showed that optical energy absorbed by the array was not sufficient to increase thetemperature of the substrate by more than a few degrees. Since the spot was assumed smaller than it actually was, the irradiance input into the model was significantly over-estimated and so the actual heating in the experiment can therefore be expected to be lower than that found in the model. Due to this over-estimation, we can be confident that any heating caused by the optical excitation of the antennas does not supply sufficient heat to transition the VO2 beneath them. Further work into coupled thermal-optical modeling of an optically-excited phase change material is the subject of ongoing work.
A final factor that may contribute to the disagreement between experiment and simulation is the temperature-dependent variation of the optical properties of the silver that comprises the antennas. Recently published research has shown that as silver is heated, its dielectric permittivity and resultant plasma frequency is altered significantly . As a result, any plasmonically-resonant structure made in silver would display a shift in the spectral position of its resonances as its temperature is raised. This effect will be incorporated into the thermal model discussed above for future work.
In conclusion, we have shown that it is possible to use thin films of vanadium dioxide to reversibly modulate the resonances of optical antenna arrays by up to 110 nm. Furthermore, we demonstrate good agreement between full-field electromagnetic modeling incorporating ellipsometric data for the optical constants of the VO2 films used and experimental reflectance measurements. This post-fabrication plasmonic resonance tuning mechanism will be useful for producing tunable optical components, such as tunable plasmonic wave-plates, lenses, waveguides, and for modulating plasmonically-coupled quantum emitters, such as quantum dots. Future papers will focus on the optimization and application of our tuning process.
The authors would like to acknowledge the financial support of the Australian Research Council (project number DP110100221), the Defense Science Institute and a Vanderbilt University/University of Melbourne collaboration grant. This work was performed in part at the Melbourne Centre for Nanofabrication and at the Swinburne node of the Australian Fabrication Facility (ANFF). R. E. M. and R. F. H. acknowledge support of this work from the National Science Foundation (DMR-1207507) and from Vanderbilt University, office of the Vice Provost for Research.
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