• Thin-film research promises birefringent and chiral filters

    Based on their previous work in birefringent thin-film technology, researchers at the University of Otago in New Zealand have demonstrated circularly polarized sources, filters, and detectors using right-handed and left-handed (chiral) thin films. The results were presented by Prof. Ian Hodgkinson in July at the Optical Interference Coatings meeting of the Optical Society of America in Banff, Alberta, Canada.
    Oct. 1, 2001
    2 min read
    FIGURE 1. Ability to control substrate rotation on two axes while evaporating biaxial materials has enabled production of proof-of-concept thin films exhibiting birefringence as well as sensitivity to angles of circular polarization.
    FIGURE 1. Ability to control substrate rotation on two axes while evaporating biaxial materials has enabled production of proof-of-concept thin films exhibiting birefringence as well as sensitivity to angles of circular polarization.

    Based on their previous work in birefringent thin-film technology, researchers at the University of Otago in New Zealand have demonstrated circularly polarized sources, filters, and detectors using right-handed and left-handed (chiral) thin films. The results were presented by Prof. Ian Hodgkinson in July at the Optical Interference Coatings meeting of the Optical Society of America in Banff, Alberta, Canada.

    Prior to actually producing the chiral thin films, the researchers had to develop suitable deposition methods and biaxial materials for fabricating anisotropic films. The deposition method consisted of rotating the substrate around two axes to vary the deposition and azimuthal angles as necessary during the evaporation process (see Fig. 1). Maintaining the substrate in a fixed position during deposition resulted in a tilted deposition pattern and linear birefringence values. Rotating the substrate 180° around its azimuth after reaching specified heights of material deposition yielded symmetric, opposing deposition tilts with doubled birefringence. Slower and smaller steps of substrate rotation around the azimuth produced chiral thin films, which could exhibit either symmetry or tilt depending on the rotation schedule.

    Devices fabricated using this method included a titanium-oxide chiral reflector (see Fig. 2). The nanostructure of the coating appears similar to a double-start, left-handed screw. In agreement with predictions based on a simulation, the experimental structure actually reflected left-handed circularly polarized light within a circular Bragg resonance wavelength band centered at 633 nm. To further demonstrate the flexibility of the structure as a polarization discriminator, the researchers added a titanium oxide thin-film half-wave plate to the structure, which effectively shifted it from transmitting only TRR right circular polarization to transmitting only TRL cross polarization.
    FIGURE 2. Chiral reflector transmits left-circular polarized light around circular Bragg resonance at 633 nm. Sensitivity to cross-polarized TRL component may be remedied by refractive index matching.
    FIGURE 2. Chiral reflector transmits left-circular polarized light around circular Bragg resonance at 633 nm. Sensitivity to cross-polarized TRL component may be remedied by refractive index matching.

    The researchers also fabricated anisotropic antireflection coatings from layers of tantalum oxide and titanium oxide and used them as a linear polarizer to tune the output polarization of an open cavity HeNe laser. Other proof-of-concept devices included circularly polarized sources formed by overcoating unpolarized light-emitting diodes and laser diodes with matched chiral filters and quarter-wave plates, respectively.

    "Future developments are expected to include efficient circularly polarized light sources, more advanced filters, and solid-state detectors for circularly polarized light," the researchers wrote. "But some of these will need to await improvements in the quality of the vacuum-deposited inorganic chiral media."1

    REFERENCE

    1. I Hodgkinson and Qh Wu, Advanced Materials 13(12-13), 889 (July 4, 2001).

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    Hassaun A. Jones-Bey

    Senior Editor and Freelance Writer

    Hassaun A. Jones-Bey was a senior editor and then freelance writer for Laser Focus World.

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