Optical properties of subwavelength hole arrays in vanadium dioxide thin films

We demonstrate that the transmission of far- and near-field incident light through a periodic array of subwavelength holes in a vanadium-dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ thin film is enhanced in the infrared range with respect to transmission through the unperforated film when $\mathrm{V}{\mathrm{O}}_2$ undergoes its semiconductor-to-metal transition. We explain this enhancement by analyzing the loss of transmitted intensity due to leaky evanescent waves inside the holes and scattering at the entrance and exit apertures. Numerical simulations based on the transfer-matrix formalism provide qualitative support for the model and reproduce the principal features of the experimental measurements.

Figure 1

(Color online) (a) FIB micrograph and (b) schematic of a section of a hole array in $\mathrm{V}{\mathrm{O}}_2$ on glass ($60\times 60$ holes, $\text{diameter}=250\mathrm{nm}$, $\text{pitch}=750\mathrm{nm}$, film $\text{thickness}=200\mathrm{nm}$). (c) Schematic of the optical setup used for spectral measurements. (d) Schematic of the SNOM setup used to obtain the images in Fig. 3 and this (inset) image of semiconducting-phase, visible-light $(\lambda =532\mathrm{nm})$ transmission.

Figure 2

(Color online) (a) and (b) Experimental and (c) and (d) simulated spectra of far-field, zero-order transmission $T_{00}$ through perforated (dashed lines) and plain (solid lines) $\mathrm{V}{\mathrm{O}}_2$ films, obtained under far-field illumination during (a) and (c) semiconducting and (b) and (d) metallic phases. The optical constants of $\mathrm{V}{\mathrm{O}}_2$ were extracted from Ref. 6.

Figure 3

(Color online) Schematics of light transmission through perforated metallic-phase $\mathrm{V}{\mathrm{O}}_2$ film: (a) visible and (b) near-IR wavelength. Simulated optical quantities for perforated (hatched bars) and plain (solid bars) metallic-phase $\mathrm{V}{\mathrm{O}}_2$ film: (c) visible and (d) near-IR wavelengths. According to our model, the lower dielectric contrast between the interior (air, $\epsilon =1$) and exterior (${\epsilon }^{\prime }\approx 1$; from Ref. 6) of each hole for metallic $\mathrm{V}{\mathrm{O}}_2$ in the near-IR reduces the losses from leaky-waves absorption (part of $I_{\mathit{\text{absorb}}}$) and diffuse scattering $(T_{\mathit{scatt}}+R_{\mathit{scatt}})$, to the extent that $T_{00}$ through the array exceeds $T_{00}$ through the film (direct transmission). Legend: $T_{00}=\text{zero}$-order transmission; $R_{00}=\text{specular}$ reflection; $T_{\mathit{scatt}}=\text{forward}$ diffuse scattering; $R_{\mathit{scatt}}=\text{backward}$ diffuse scattering; $I_{\mathit{\text{absorb}}}=\text{absorption}$ $\left(\text{right}\text{−}\text{side}\text{scale}\right)=100\%-T_{00}-R_{00}-T_{\mathit{scatt}}-R_{\mathit{scatt}}$.

Figure 4

(Color online) (Top half) SNOM images of far-field transmission through the $\mathrm{V}{\mathrm{O}}_2$ hole array, obtained with near-field illumination: (a) and (b) visible and (c) and (d) near-IR wavelengths, during (a) and (c) semiconducting and (b) and (d) metallic phases. (Bottom half) For each image above, intensity is plotted as a function of position along part of a row of holes and extending into the unperforated area outside of the array; the rightmost hole is indicated by an arrow in each plot.