Non-chevronic periodic columnar thin-film bilayer with a central defect layer: experiment and theory

We investigated the polarization-state-resolved optical response of non-chevronic periodic columnar thin-film (CTF) bilayers, composed of alternating layers of obliquely deposited ZnSe nanocolumns, with a central defect layer. With the thickness direction designated as parallel to the <i>z</i> axis, the nanocolumns in the first CTF in the unit cell (i.e., CTF bilayer) are tilted at an angle <i>χ</i>₁, while those in the second CTF are tilted at an angle <i>π</i>-<i>χ</i>₂ with <i>χ</i>₁≠<i>χ</i>₂, with respect to the +<i>x</i> axis in the <i>x</i><i>z</i> plane. The central defect layer is the same as the first CTF but with a different thickness. Several non-chevronic periodic CTF bilayers with and without the central defect layer were fabricated using thermal evaporation. Optical transmittances and reflectances were measured for free-space wavelength <i>λ</i>₀∈[550,850] nm and angle of incidence <i>θ</i>_{<i>i</i><i>n</i><i>c</i>} as high as 70° with respect to the +<i>z</i> axis in the <i>x</i><i>z</i> plane. The central defect layer causes the emergence of a spectral reflection hole within the polarization-state-sensitive Bragg regime. As the number of unit cells increases, the spectral reflection hole weakens and eventually disappears. Theoretical calculations closely reproduce all experimentally observed trends. The spectral reflection hole can be exploited for narrow-passband filters, provided the number of unit cells on either side of the defect layer is carefully chosen.