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Defence Technology

Surface optimization and synergistic quantification method for OTS anti-relaxation coatings

Yian Xu, Hao Xia, Jiale Quan, Yuxin Cong, Yuhao Wang, Zhihong Wu, Haoying Pang, Zhuangsheng Zhu

Published Apr 25, 2026• 0 citations
Publisher

Abstract

To enhance the anti-relaxation performance of alkali metal vapor cells in quantum sensors, a synergistic optimization and quantitative evaluation strategy for surface parameters of octadecyl-trichlorosilane (OTS) anti-relaxation coatings is proposed. By systematically adjusting solution concentration and reaction time, both the polarization retention angle and the surface polarization retention cross-section are effectively optimized, achieving a maximum retention angle of 135°. A theoretical framework is subsequently established to quantify adsorption energy based on the inherent correlation between these two surface parameters. A correction method for the retention cross-section, developed through a non-uniform wetting model, enables in-situ mapping of the kinetic contribution to adsorption behavior. Through kinetic and thermodynamic parameter extraction, the adsorption energy and Gibbs free energy of the multilayer OTS coatings are determined to be 0.13 eV, which represents a substantial reduction compared to the adsorption energy of bare glass surfaces, previously reported as 2.01 eV. Furthermore, longitudinal relaxation time measurements are carried out inside vapor cells, yielding a maximum collision count of 593, which provides strong evidence of polarization preservation capability. This approach provides both theoretical guidance and an experimental pathway for optimizing coating performance and analyzing adsorption mechanisms, thereby facilitating the development of high-performance alkali metal vapor cell materials for quantum sensing applications. • Synergistic optimization of OTS coating parameters improved polarization retention, achieving a maximum retention angle of 135° by adjusting solution concentration (2-6 mM) and reaction time (60-120 min). • A heterogeneous wetting-based synergy quantification method is developed, enabling accurate evaluation of multilayer formation and local adsorption behavior by jointly analyzing retention angle and retention cross-section. • A combined thermodynamic-kinetic modeling framework establishes quantitative correlations between adsorption energy, Gibbs free energy, and coating surface characteristics, providing theoretical guidance for anti-relaxation material design. • Longitudinal relaxation tests in coated vapor cells demonstrate a maximum collision count of 593, confirming suppressed alkali-atom adsorption and strong polarization preservation consistent with the low adsorption energy of 0.13 eV.

DOI
10.1016/j.dt.2026.03.036

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To enhance the anti-relaxation performance of alkali metal vapor cells in quantum sensors, a synergistic optimization and quantitative evaluation strategy for surface parameters of octadecyl-trichlorosilane (OTS) anti-relaxation coatings is proposed. By systematically adjusting solution concentration and reaction time, both the polarization retention angle and the surface polarization retention cross-section are effectively optimized, achieving a maximum retention angle of 135°. A theoretical framework is subsequently established to quantify adsorption energy based on the inherent correlation between these two surface parameters. A correction method for the retention cross-section, developed through a non-uniform wetting model, enables in-situ mapping of the kinetic contribution to adsorption behavior. Through kinetic and thermodynamic parameter extraction, the adsorption energy and Gibbs free energy of the multilayer OTS coatings are determined to be 0.13 eV, which represents a substantial reduction compared to the adsorption energy of bare glass surfaces, previously reported as 2.01 eV. Furthermore, longitudinal relaxation time measurements are carried out inside vapor cells, yielding a maximum collision count of 593, which provides strong evidence of polarization preservation capability. This approach provides both theoretical guidance and an experimental pathway for optimizing coating performance and analyzing adsorption mechanisms, thereby facilitating the development of high-performance alkali metal vapor cell materials for quantum sensing applications. • Synergistic optimization of OTS coating parameters improved polarization retention, achieving a maximum retention angle of 135° by adjusting solution concentration (2-6 mM) and reaction time (60-120 min). • A heterogeneous wetting-based synergy quantification method is developed, enabling accurate evaluation of multilayer formation and local adsorption behavior by jointly analyzing retention angle and retention cross-section. • A combined thermodynamic-kinetic modeling framework establishes quantitative correlations between adsorption energy, Gibbs free energy, and coating surface characteristics, providing theoretical guidance for anti-relaxation material design. • Longitudinal relaxation tests in coated vapor cells demonstrate a maximum collision count of 593, confirming suppressed alkali-atom adsorption and strong polarization preservation consistent with the low adsorption energy of 0.13 eV.
Abstract Anti-relaxation coatings play an important role in enhancing the polarization lifetime of polarized alkali-metal atoms. In this study, octadecyltrichlorosilane (OTS) was coated in spherical rubidium vapor cells fabricated using borosilicate glass with two stems as anti-relaxation coatings. An orthogonal experiment method was employed to obtain the optimal conditions. The results showed that OTS concentration is the dominant factor in the number of bounces, followed by the soaking time and molar ratio/(H2O:OTS). The optimal parameters were 6 mM of OTS, 6:1 of molar ratio/(H2O:OTS), and 80 min of soaking time. The number of bounces can be up to 200. Atomic force microscope images showed that the number and coverage ratio of clusters on the anti-relaxation coating fabricated by the optimized parameters are significantly less than the number and coverage ratio of clusters formed by other conditions. This means that the clusters in the coatings significantly affect the anti-relaxation performance. In addition, five OTS-coated cells were fabricated by A3B3C5 combination. However, the variability from cell to cell using the exact same preparation procedure can be up to 43.6%. The variability may be caused by the different aging times of the OTS solution. Because only one beaker of OTS solution was prepared, the OTS solution was pumped into the cells in sequence, not at the same time.
Abstract Alkali‑metal vapor cells are critical for quantum precision measurement devices such as atomic gyroscopes, magnetometers and clocks. The anti‑relaxation coating on cell walls determines atomic spin polarization lifetime and measurement sensitivity. Conventional evaluations depend on short‑term relaxation tests and fail to characterize long‑term coating stability and microscopic degradation. This work proposes a novel evaluation method based on a cumulative relaxation damage factor. From long‑range kinetic perspectives, alkali‑metal surface dynamics are decomposed into three elementary processes quantified by adsorption energy, charge transfer and diffusion barrier. Drawing on Lusser’s reliability law, we construct this unified factor to interpret relaxation mechanisms and assess coating performance. A first‑principles computational framework with periodic slab models is established for alkali‑metal and organic/inorganic interfaces, validated using experimental interaction parameters of four typical coatings. The simulation results demonstrate that the proposed cumulative relaxation damage factor effectively distinguishes the anti-relaxation performance of different coatings and exhibits general applicability across various alkali-metal species. This study converts cumulative long‑range kinetic relaxation into microscopic coating design rules, offering a theoretical tool for wall‑collision relaxation analysis, coating screening and quantum sensing instruments optimization.
Abstract Anti-relaxation surface coatings can reduce collisions between alkali-metal atoms and cell walls, thereby prolonging the spin-polarization lifetime of alkali-metal atoms. This phenomenon can considerably improve the performance of quantum sensors. Researchers have focused on the surface topography and composition of anti-relaxation coatings in alkali-metal vapor cells and on methods that provide an increase in the relaxation time of alkali metals. We proposed a novel method to characterise the coating performance of alkali-metal cells by directly measuring the energy dissipation during collisions between the anti-relaxation coating surface and atoms in the cell. We found that the interactions between alkali-metal atoms and the anti-relaxation coating resemble elastic collisions, which can be indirectly characterised by the energy dissipation during interactions between an atomic force microscopy tip and atoms. In addition, we confirmed that the energy dissipation between collisions is positively correlated with the relaxation characteristics of alkali metals at various temperatures. This discovery is of great significance as it can be utilised to characterise the anti-relaxation coating performance of alkali-metal cells by directly analysing the energy dissipation of anti-relaxation coatings instead of measuring the relaxation time.
Spin-exchange relaxation-free (SERF) co-magnetometers exhibit broad application prospects in navigation-grade gyroscopes and fundamental physics experiments. However, their performance is constrained by finite spin relaxation times. To address this challenge, long-relaxation-time vapor cells have been realized through the introduction of dry-film anti-relaxation coatings. Such coatings form a stable hydrophobic layer on the inner walls of the vapor cell, which reduces the adsorption energy of alkali-metal atoms and suppresses wall-induced spin-destruction processes, thereby enhancing polarization retention and extending the coherence times of both electronic and nuclear spins. Nevertheless, a clear quantitative relationship between the coating properties and the resulting anti-relaxation performance has not yet been established. To resolve this issue, the present study introduces the background potential field in the Hamiltonian as an intermediate variable, linking the surface adsorption energy of the coating to atomic spin relaxation processes. From the perspective of microscopic energy structure, a theoretical mapping between coating characteristics and anti-relaxation performance is constructed, thereby filling a critical gap in the theoretical understanding of anti-relaxation coating mechanisms.
Abstract This research explores the desorption mechanism and thermal stability of octadecyltrichlorosilane (OTS, CH 3 (CH 2 ) 17 SiCl 3 ) coating on quartz slides and in alkali‐metal vapour cells. The morphological thermal‐changes, energy dissipation diversity, and anti‐relaxation characteristic of OTS coatings before and after exposure to Cs atoms by Fourier transform infrared spectroscopy, water contact angle measurement, atomic force microscopy imaging, collision energy dissipation analysis, and free induction decay of Cs atoms are measured systematically. The results show that the OTS coatings exhibit the best thermal stability under the specific process conditions, and the homogeneous and dense structure makes the adsorption of alkali‐metal atoms more stable, which effectively reduces surface energy dissipation and prolongs the relaxation time of Cs atoms. The study provides certain reference for efficient anti‐relaxation coating fabrication and coated cell application.
Anti-relaxation coatings are commonly employed in the alkali metal vapor cells of spin-exchange relaxation-free (SERF) atomic magnetometers, as they can prevent spin depolarization of alkali metal atoms. However, the anti-relaxation mechanism and the interaction process between atoms and coatings remain unclear. This lack of understanding hinders the further improvement of the performance of SERF atomic magnetometers. In this study, we employed first-principles density functional theory to comparatively investigate the microscopic interactions between Rb atoms and SiO2 substrates as well as three commonly used organic coatings. The adsorption energies and valence electron loss of Rb atoms during their adsorption processes on these coated surfaces were systematically calculated. It was found that the Rb atoms exhibited the lowest adsorption energy and the highest charge transfer on the SiO2 surface, with values of –3.55eV and 0.90 |e|, respectively. In contrast, on the 1-nonadecene surface, the adsorption energy reached only –0.13eV, with a charge transfer of merely 0.14 |e|. Similar results were observed for other organic coatings and these findings are highly consistent with previous anti-relaxation performance tests, indicating that alkali metal atoms undergo less energy dissipation and charge transfer on the surface of organic coatings, which can better maintain the electron spin polarization state. A close relationship can be established between adsorption energy, charge transfer and the anti-relaxation performance of coatings, which provides effective guidance for future theoretical research and material selection, thus accelerating the development of SERF magnetometers.