Motivated by the need for reliable optical sensing modalities, we fabricated a doubly chiral sculptured thin film (D-CSTF) of zinc selenide, comprising a structurally left-handed CSTF and a structurally right-handed CSTF deposited consecutively on a glass substrate. Both constituent STFs were sufficiently thick as to exhibit the circular Bragg phenomenon independently and distinctly in the 500–900-nm wavelength range. The fabrication process involved asymmetric serial bideposition through thermal evaporation. All eight circular remittances of the D-CSTF were measured using a variable-angle spectroscopic system. The direction of the incident light was varied so that the azimuthal angle ranged from 0◦ to 70◦ and the polar angle from 0° to 180°. The D-CSTF functions as a bandstop filter for RCP light as well as a bandstop filter for LCP light, regardless of the polar angle and the face that is illuminated, the two stopbands being distinct from each other.
Temporally periodic physical vapor deposition (TP-PVD) is commonly used to fabricate Bragg mirrors on planar surfaces. These devices are notionally of infinite transverse extent and homogeneity, but they have periodically nonhomogeneous electromagnetic constitutions in the thickness direction. Undertaking TP-PVD in two different formats, we found that the Bragg phenomenon can be avoided in certain thin-film applications by undertaking TP-PVD on surfaces roughened randomly on the scale of the wavelength.
A Bragg mirror is a dielectric slab that is periodically nonhomogeneous in the thickness direction. It displays the Bragg phenomenon as a high-reflectance spectral regime that depends on the direction of propagation and the polarization state of the incident light. Implicit in both theoretical treatments and fabrication procedures is the smoothness of the surface on which the Bragg mirror is fabricated. We have found that the Bragg phenomenon exhibited in the visible regime by periodically nonhomogeneous thin films grown on glass substrates can be significantly affected, even nullified, by pre-fabrication roughening of the surface of the substrate.
Columnar thin films are assemblies of parallel straight columns of nanoscale cross-sectional dimensions grown by physical vapor deposition on appropriate substrates. Sculptured thin films are similar, but the columnar shape is architected for specific objectives. Polarization-universal bandgaps with maximum transmittance under 20% were realized as sculptured thin films of two different types: (i) tightly interlaced matched ambidextrous bilayers (TIMABs) to modify the circular Bragg phenomenon and (ii) equichiral sculptured thin films (ECSTFs) that display the Bragg phenomenon. The serial bideposition technique was employed for architecting the columns. Samples characterized optically as well as with scanning electron microscopy.
Equichiral sculptured thin films (ECSTFs) with unit cells comprising a sequence of four identical columnar thin films were fabricated using asymmetric serial bideposition to exhibit the polarization-universal Bragg phenomenon. Oblique-angle optical transmission measurements of the ECSTFs showed bands of total transmittance values under 20%, regardless of the polarization state of the incident plane wave. These polarization-universal bandgaps can be tuned by adjusting the angle of incidence.
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