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Optical Materials

Application of coating based on 5 nm octadecylamine-functionalized gold-nanoparticles for protecting the spin-polarization of atoms confined in a vacuum cell

Dimitar G. Slavov, I. Bliznakova, Yulia I. Andreeva, Zahary Y. Peshev

Published Jun 10, 2023• 5 citations
Publisher

Abstract

We report on the expressed atomic spin-polarization protection properties of thin layers of Octadecylamine capped gold-nanoparticles applied to the inner surface of an alkali metal vapor cell. The work aims to reveal the potential of using appropriately functionalized gold nanoparticles as a novel approach to forming efficient coatings on the inner surface of vacuum cells for atomic spectroscopy. The performed layer characterization uses parameters of electromagnetically induced transparency/absorption resonances observed in the Hanle configuration. We show at the Rb D2 absorption line that cells with such a coating provide conditions for achieving high-contrast and narrow, bright, and dark resonances, comparable to those reported by others for Octadecyltrichlorosilane coatings. Using visualization software for structural analysis, we represent the possible interaction scenarios between the alkali atoms and the surface building blocks. We demonstrate and discuss the main structural and functional differences introduced by covering the cell surface with passivated nanoparticles instead of directly using molecules to form monolayers. Having gold-nanoparticles in the coating allows preserving at the same time the earlier discovered advantage for high-speed control over the Rb vapor density by light-induced desorption of atoms. That enables significant problem-solving in atomic clocks and magnetometers and eases their considerable miniaturization.

DOI
10.1016/j.optmat.2023.113978

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We report on the expressed atomic spin-polarization protection properties of thin layers of Octadecylamine capped gold-nanoparticles applied to the inner surface of an alkali metal vapor cell. The work aims to reveal the potential of using appropriately functionalized gold nanoparticles as a novel approach to forming efficient coatings on the inner surface of vacuum cells for atomic spectroscopy. The performed layer characterization uses parameters of electromagnetically induced transparency/absorption resonances observed in the Hanle configuration. We show at the Rb D2 absorption line that cells with such a coating provide conditions for achieving high-contrast and narrow, bright, and dark resonances, comparable to those reported by others for Octadecyltrichlorosilane coatings. Using visualization software for structural analysis, we represent the possible interaction scenarios between the alkali atoms and the surface building blocks. We demonstrate and discuss the main structural and functional differences introduced by covering the cell surface with passivated nanoparticles instead of directly using molecules to form monolayers. Having gold-nanoparticles in the coating allows preserving at the same time the earlier discovered advantage for high-speed control over the Rb vapor density by light-induced desorption of atoms. That enables significant problem-solving in atomic clocks and magnetometers and eases their considerable miniaturization.
Antirelaxation coatings (ARC) are used in optical cells containing alkali metal vapor to reduce the depolarization of alkali atoms after collisions with the cell walls. The long-lived ground state polarization is a basis for development of atomic clocks, magnetometers, quantum memory, slow light experiments, precision measurements of fundamental symmetries etc. Another application of ARC is with Light-Induced Atomic Desorption (LIAD) for implementation optical dispensers for loading atomic devices as atomic magnetometers, atomic clocks, magneto-optical traps and their miniaturization. In this work, the Rb atomic density and the spin randomization (the number of collisions without spin depolarization) are investigated in PDMS coated vacuum cell when the density is controlled by LIAD, and compared with the results when the density is controlled by temperature. A simple method for measurement of the number of collisions without spin-randomization is used, which comprises recording of the time dependence of the fluorescence intensity of alkali atoms during exposure of the cell to resonant radiation pulses. The obtained results are discussed from point of view of LIAD applications for density control in spectroscopy investigations.
Many technologies based on cells containing alkali-metal atomic vapor benefit from the use of antirelaxation surface coatings in order to preserve atomic spin polarization. In particular, paraffin has been used for this purpose for several decades and has been demonstrated to allow an atom to experience up to 10 000 collisions with the walls of its container without depolarizing, but the details of its operation remain poorly understood. We apply modern surface and bulk techniques to the study of paraffin coatings in order to characterize the properties that enable the effective preservation of alkali spin polarization. These methods include Fourier transform infrared spectroscopy, differential scanning calorimetry, atomic force microscopy, near-edge x-ray absorption fine structure spectroscopy, and x-ray photoelectron spectroscopy. We also compare the light-induced atomic desorption yields of several different paraffin materials. Experimental results include the determination that crystallinity of the coating material is unnecessary, and the detection of C[Double Bond]C double bonds present within a particular class of effective paraffin coatings. Further study should lead to the development of more robust paraffin antirelaxation coatings, as well as the design and synthesis of new classes of coating materials.
Antirelaxation surface coatings allow long spin relaxation times in alkali-metal cells without buffer gas, enabling faster diffusion of the alkali atoms throughout the cell and giving larger signals due to narrower optical linewidths. Effective coatings were previously unavailable for operation at temperatures above 80 °C. We demonstrate that octadecyltrichlorosilane (OTS) can allow potassium or rubidium atoms to experience hundreds of collisions with the cell surface before depolarizing, and that an OTS coating remains effective up to about 170 °C for both potassium and rubidium. We consider the experimental concerns of operating without buffer gas and with minimal quenching gas at high vapor density, studying the stricter need for effective quenching of excited atoms and deriving the optical rotation signal shape for atoms with resolved hyperfine structure in the spin-temperature regime. As an example of a high-temperature application of antirelaxation coated alkali vapor cells, we operate a spin-exchange relaxation-free atomic magnetometer with sensitivity of 6 fT/Hz and magnetic linewidth as narrow as 2 Hz.
Surface relaxation rates differ for spin-polarized alkali atoms interacting with monolayer or bilayer octadecyltrichlorosilane (OTS) coatings. The morphology and composition of Rb vapor-exposed films of OTS have been studied with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). On OTS monolayers, numerous small (<500 nm wide) Rb containing islands nucleate at the boundaries of the ∼1–2 μm wide organic domains. On OTS bilayers, singular large (∼3 μm wide) Rb containing islands were found. Alkali island formation mediated by surface structure could affect the antirelaxation behavior of organic coatings used in atomic magnetometer cells.
We describe a method for characterizing self-assembled monolayers (SAMs) in terms of their performance as antirelaxation wall coatings for alkali atom vapor cells. A combination of initial surface analysis and subsequent laser spectroscopy is used to provide insight into the quality of the coating, as well as its performance under the exposure to alkalis as it occurs, for example, when used in applications such as atomic magnetometers or clocks. Fused silica plates coated with octadecyltrichlorosilane SAMs were used to make cubic R87b gas cells. The surface was characterized by x-ray diffraction, contact angle measurements, and atomic force microscopy. Measurements of hyperfine resonance linewidths and frequency shifts show that the rubidium vapor atoms collide up to 40 times with the walls of the cells before their coherence relaxes and their adsorption energy is around 0.065 eV. Chemical analysis of the cell indicates some weak reactions between the coating and the rubidium atoms.
We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cell's relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.