Phonon-polariton assisted broadband resonant absorption in anisotropic α-phase MoO3 nanostructures

Abstract

Using finite-difference time-domain simulations, we theoretically show that trapezoid \ensuremath{\alpha}-phase molybdenum trioxide ($\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$) patch arrays can be used to achieve broadband perfect absorption due to its wide hyperbolic regions in infrared. In the merit of different reststrahlen (RS) bands for different crystal axes of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$, we further demonstrate anisotropic broadband absorption behavior by aligning an $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ trapezoidal patch in the in-plane $x$ and $y$ directions, respectively. Electromagnetic simulations reveal that the broadband absorption is elucidated by the combination of strong phonon resonances within the RS band, which dissipate energies at different wavelengths in different regions of the trapezoid. Using patterned $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ on top of reflector and lossless dielectric layers, we report average absorption values as high as 70% from 10.78 to 11.7 \textmu{}m and 12.92 to 17.19 \textmu{}m for the two orthogonal in-plane crystal directions of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$. More attractively, the spectral range and absorption bandwidth of the proposed structure can be easily engineered by changing the shapes and the thicknesses of the trapezoid patches.