Optical Resonators – New Coating Tech Reduces Losses

High-performance mirrors are critical for constructing optical resonators in various applications in optics and photonics. Stable resonators narrow the linewidth of continuous-wave lasers, creating optical references for frequency comb stabilization and precision molecular spectroscopy. High-performance, low-loss mirrors at mid-infrared wavelengths can help researchers probe new phenomena with increased precision in chemical sensing, discrete imaging, ultracold chemistry, and fundamental physics.

Traditional physical vapor deposition techniques for fabricating high-reflectivity mirrors are not widely available in the mid-infrared region. Substrate-transferred monocrystalline interference coatings in the mid-IR spectral range offer a viable answer to these problems. Researchers have created high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings with record-low excess optical loss below ten parts per million at a central wavelength of 4.54 µm.

A novel microfabrication process realizes these high-performance mirrors for optical resonators. It differs significantly from amorphous multilayers’ production via physical vapor deposition processes, reducing scatter loss due to the low surface and interfacial roughness. Low background doping in epitaxial growth ensures strongly reduced absorption.

Researchers 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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