Sapphire waveguides and fibers for terahertz applications

Sapphire shaped crystals are considered as a favorable material platform of the terahertz (THz) waveguide and fiber optics. Unique physical properties of sapphire, along with advantages of the Edge-defined Film-fed Growth (EFG) technique, yield fabrication of the THz waveguides and fibers with a complex cross-section geometry directly from the Al₂O₃-melt, where no labour-intensive mechanical processing is required. Wide variability of the as-grown sapphire shaped crystal geometries yields different physical mechanisms of electromagnetic waveguidance. In this review, recent advantages in the THz waveguides and fibers based on the EFG-grown sapphire shaped crystals are discussed. While possessing moderate THz-wave absorbtion and quite high dispersion, flexible sapphire fibers with a simple step-index cross-section geometry yield strong confinement of guided modes in a fiber core due to a high refractive index of sapphire in the THz range. This effect opens novel opportunities of sapphire fibers in high-resolution THz imaging, using the principles of either scanning-probe near-field optical microscopy or optical fiber bundles. In turn, antiresonant and photonic crystal hard hollow-core waveguides demonstrate advanced optical performance, along with wide capabilities in THz endoscopy and sensing in harsh environments. This review highlights that the EFG-grown sapphire shaped crystals hold strong potential in different branches of THz optics.