Many of today’s most precise measurement methods such as atomic clocks rely on optical processes utilizing ultrastable cavity-stabilized laser sources. As one of the systems’ key components, the stabilizing reference cavity must meet extremely stringent requirements. While cavity end mirrors with thin film dielectric coatings yield exceptionally-low optical scattering and absorption losses [1], their high levels of mechanical (or elastic) losses lead to excess Brownian noise [2] that limits the ultimate laser stability. Mirrors based on semiconductor multilayers have proven to be an excellent alternative solution in the near-infrared spectral range for such ultrastable laser systems.
Compared to mirrors with dielectric coatings, they exhibit excellent optical properties [3], while simultaneously exhibiting a tenfold reduction in elastic losses [2]. These crystalline coatings are epitaxially grown and then integrated with standard substrates via a novel substrate-transfer process. Such supermirrors have been implemented in an ongoing ESA project meeting the requirements of both the NGGM and LISA project. However, due to increased absorption below 870 nm, the wavelength range of these state-of-the-art crystalline coatings is limited to the infrared spectral region and the implementation of a strontium lattice optical atomic frequency standard (SrLOAFS) on a free-flying platform requires optical reference cavities operating in the red spectral range. To extend the wavelength range of crystalline coatings into the visible region, novel combinations of semiconductor material compositions are required.
The objective of this work is to investigate and optimize such materials regarding their absorption and scattering characteristics in the red spectral range. Within the planned project, several designs are fabricated by metal-organic vapor-phase epitaxy (MOVPE) and then substrate-transferred to result in low loss supermirrors for the visible spectral range.
[1] Rempe et al., “Measurement of ultralow losses in an optical interferometer”, Optics Letters 17, 363-365 (1992). DOI: 10.1364/OL.17.000363
[2] Cole et al., “Tenfold reduction of Brownian noise in high-reflectivity optical coatings,” Nature Photonics 7, 644-650 (2013). DOI: 10.1038/nphoton.2013.174
[3] Cole et al., “High-performance near- and mid-infrared crystalline coatings,” Optica 3, 647-656 (2016). DOI: 10.1364/OPTICA.3.000647
[4] Priante et al., et al. "Demonstration of a 20‐W membrane‐external‐cavity surface‐emitting laser for sodium guide star applications", Electronics Letters (2021). DOI: 10.1049/ell2.12008
