We report metamaterial terahertz (THz) bandpass filters with tunable dual-band selectivity. The shift in the center frequency of the device is achieved by actively modifying the effective length of the resonators. This was realized by introducing vanadium dioxide (${\mathrm{VO}}_2$) bridges interconnecting specific regions of each resonator. Raising the temperature across the phase transition shifted the resonance frequency by $\sim 32\%$ due to changes in the electrical conductivity of the ${\mathrm{VO}}_2$. Measured THz transmission response of the proposed dual-band filter was in good correspondence with simulations.

We report studies of the terahertz (THz) transmission through vanadium dioxide (${\mathrm{VO}}_2$) thin films grown on $c$-, $m$-, and $r$-plane sapphire substrates. Our results revealed THz amplitude modulation as large as 84% for ${\mathrm{VO}}_2$ films grown on $r$-plane sapphire substrates upon crossing the metal–insulator phase transition temperature. Complex optical conductivity and refractive indices were determined for all investigated samples in the metallic state. Results are consistent with electrical resistivity measurements and described based on the Drude model. The real and imaginary parts of the optical conductivity and refractive index are obtained, and associations with variations in the grain morphology and crystal quality are described. We show that ${\mathrm{VO}}_2$ thin films can be used as tunable broadband THz frequency antireflecting coatings.

Vanadium dioxide (VO₂), with its characteristic metal-insulator phase transition, is a prospective active candidate to realize tunable optical devices operating at terahertz (THz) frequencies. However, the abrupt phase transition restricts its practical use in analog-like continuous applications. Incorporation of tungsten is a feasible approach to alter the phase transition properties of thin VO₂ films. We show that amplitude THz modulation depth of ∼65%, characteristic phase transition temperature of ∼40 °C, and tuning range larger than 35 °C can be achieved with W-doped VO₂ films grown on sapphire substrates. W-doped VO₂ films can also be used to suppress Fabry-Perot resonances at THz frequencies but at temperatures much lower than that observed for undoped VO₂ films. The gradual phase transition temperature window allows for precise control of the W-doped VO₂ optical properties for future analog based THz devices.

Bandpass filters are reported based on double-stacked metamaterial layers separated by an air gap for operation at terahertz frequencies. Several stacking configurations were investigated designed for a $\sim 0.5\mathrm{THz}$ center frequency. The filters exhibited improved spectral transmission properties when compared with conventional ones based on single metamaterial layers. 3 dB bandwidth of $\sim 78\mathrm{GHz}$ and sidelobe suppression ratio $>16\mathrm{dB}$ were determined when symmetric or asymmetric double layers were stacked. We demonstrate that superior frequency selectivity can be achieved when metamaterial layers with different unit cells are used. Good agreement was found between measured and simulated transmission response.

Optical images from nano-scale features were obtained by collection of leakage radiation coupled to surface plasmon polaritons excited by near-field fluorescence. Plasmonic crystals with spatial periods as small as 190 nm and non-periodic features separated by 80 nm, corresponding to ~λ/7, were clearly visible in the real plane images using this far-field technique. We show that the leaked light from the investigated samples carries detailed information to the far-field which is not present in the images obtained with conventional optical microscopy.

Materials that exhibit very high absorption and low reflectance at optical frequencies are important in a variety of light harvesting applications. We explore nano-scale topography of bulk metallic glasses to realize metallic surfaces with controllable reflectance. Diffuse reflectance as low as 1.6% in the visible range is achieved with nano-patterned Pt-based model bulk metallic glass without further optimization. Effect of pattern dimensions (size, spacing, and aspect-ratio) on the reflectance of bulk metallic glasses is investigated using a combination of simulation and experiments.