Mechanical loss and stability analysis of NEXCERA in ultra-stable optical cavities

NEXCERA has emerged as a ceramic-based material for spacers in ultra-stable optical cavities, with a coefficient of thermal expansion that crosses zero near room temperature. In such cavities, frequency stability is ultimately limited by Brownian thermal noise in the cavity components. A key parameter in this context is the mechanical loss, which has remained unknown for NEXCERA. In this work, we investigate the mechanical loss of NEXCERA N117B at room temperature for various resonances using the gentle nodal suspension technique. We measure a promising minimum mechanical loss of $ϕ= 1.89\times 10^{-5}$, indicating the suitability of NEXCERA for low-noise optical cavities. Using this value, we calculate the thermal noise of a cavity with a NEXCERA spacer and compare its performance to established materials such as ULE and Zerodur, taking into account different mirror substrate options. Our analysis shows that NEXCERA is a strong candidate for ultra-stable cavities due to its low thermal noise. Combined with its previously reported low linear drift, it offers a highly attractive option for long-term stable optical frequency references.

💡 Research Summary

The paper presents the first systematic measurement of the mechanical loss of the ceramic material NEXCERA N117B, a promising candidate for spacers in ultra‑stable optical cavities. Using the gentle nodal suspension technique, three identical NEXCERA disks (≈50 mm diameter, 0.54 mm thickness) were suspended from a single 4.5 mm steel sphere in a high‑vacuum environment (≤10⁻⁷ mbar). The disks were electro‑statically driven (200–800 V) and their vibrations were detected optically with a 635 nm laser and a quadrant photodiode. Frequency sweeps from 400 Hz to 50 kHz revealed four resonant modes at 3.179 kHz, 7.397 kHz, 12.99 kHz and 21.16 kHz; finite‑element simulations identified the corresponding mode shapes.

Mechanical loss angles were extracted from the Lorentzian linewidths (ϕ = Δf/f₀). The lowest measured loss was ϕ = 1.894 × 10⁻⁵ (±0.041 × 10⁻⁵) at 3.2 kHz. Across different modes and samples the loss varied by up to 36 %, a scatter typical for ultra‑low‑loss measurements and attributed mainly to subtle variations in suspension geometry and internal material inhomogeneities rather than surface roughness. Thermo‑elastic loss, calculated from material parameters, was found to be ≤10⁻¹⁰ and therefore negligible.

Using the measured loss and the fluctuation‑dissipation theorem, the authors computed the Brownian thermal noise contribution of a cavity employing a NEXCERA spacer (10 cm length, 0.5 mm beam radius). Compared with conventional ULE and Zerodur spacers, the NEXCERA‑based cavity exhibits roughly 30 % lower spacer thermal noise. When fused‑silica mirror substrates are used instead of ULE substrates, the total cavity thermal noise can fall below the coating‑limited floor, reaching fractional frequency instability on the order of 10⁻¹⁶.

The paper also discusses drift rates. NEXCERA N117B shows a linear drift of 1.74 × 10⁻¹⁷ s⁻¹, about half the best reported ULE value, indicating superior long‑term dimensional stability. Combined with its high Young’s modulus (≈140 GPa) and near‑zero coefficient of thermal expansion at room temperature, NEXCERA offers a compelling combination of low thermal noise, low drift, and mechanical robustness.

In conclusion, the authors demonstrate that NEXCERA’s intrinsic mechanical loss is sufficiently low to make it a strong contender for next‑generation ultra‑stable optical cavities operating at room temperature. They propose a cavity design that leverages NEXCERA spacers and fused‑silica mirrors, predicting performance that surpasses current state‑of‑the‑art ULE‑based systems. This work paves the way for more stable optical frequency references, benefiting applications such as optical lattice clocks, gravitational‑wave detectors, and fundamental‑physics experiments requiring long integration times.