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Three-dimensional (3D) spiral photonic crystals (PhCs) have a periodic varied refractive index (RI) with the periodicity comparable to the wavelength of incident light. They can pass circularly polarized (CP) light with handedness opposite to their own structures’ handedness while block the polarization state with the same handedness. Three-dimensional spiral PhCs for use as circular polarizers have two main advantages over conventional circular polarizers. On the one hand, it has wide operation wavelength based on the photonic band gap caused by PhCs and the interaction between CP light and each individual spiral structure. On the other hand, the height of each spiral structure can be made within the range of several incident light’s wavelengths, therefore, compact circular polarizers can be fabricated through 3D spiral PhCs.
In this work, the Finite-difference time domain (FDTD) method was used to investigate the circular polarization selection of 3D spiral PhCs. Optical transmittance spectra of 3D spiral PhCs illuminated by two orthogonal CP lights have been calculated. Transparent materials with different RIs were adapted to demonstrate that higher RI material could have broader operation wavelength. Furthermore, dispersive materials like aluminum with different structures and pitch numbers were investigated to increase the operation wavelength of 3D spiral PhCs.
Fabrication work was aimed at high-quality 3D spiral PhCs with operation in the near-infrared range. The FDTD tool was utilized to predict the transmittance of 3D spiral PhCs based on transparent and dispersive materials.
Three-dimensional spiral PhCs were fabricated from glassy arsenic trisulfide (As
2S3) with high RI (n=2.45). This material was chosen to strongly modulate the light propagation and to obtain a broader operation band. Thermal vapor deposition was used to prepare the desired thickness of As2S3 thin films as photoresists. A laser direct writing system based on two-photon absorption was used to achieve 3D micro-structure fabrication by point-by-point laser exposure. The unexposed area was removed with appropriate development solution to reveal the 3D spiral PhCs. Large size (280μm by 280μm) spiral PhCs were fabricated.
Advisor: Natale J. Ianno