Reflectance Coatings For Ultralow-Loss Mirrors

High-performance mirrors are employed to construct optical resonators in various applications in optics and photonics. Developing low-loss mirrors like those readily available throughout the near-infrared spectral region is a long-standing goal for optics and photonics researchers. High reflectance coatings play a significant part in the fabrication of low-loss mirrors.

The key performance metrics for any high reflectance coating technology are: low excess optical loss, combined absorption, and scatter loss. These are limiting factors for applying optical resonators in the mid-infrared spectral region.

Researchers present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of 4.54 µm with record-low excess optical loss below ten parts per million. They realized these high-performance mirrors via a novel microfabrication process. This new process enables reduced scatter loss due to the low surface and interfacial roughness. Low background doping in epitaxial growth ensures strongly reduced absorption.

They present the results of a series of optical measurements, including cavity ring-down, transmittance spectroscopy, and direct absorption tests, used to determine the optical losses of a set of prototype mirrors. They observe a unique polarization-orientation-dependent loss mechanism during these measurements, which they attribute to the elastic anisotropy of these strained epitaxial multilayers.

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