UltraStabLaserViaSHB · Ultra Frequency-Stable Laser via Spectral Hole Burning in Rare-earth Ion Doped Crystals

Ultra Frequency-Stable Laser via Spectral Hole Burning in Rare-earth Ion Doped Crystals

With the redefinition of the Système International d’unités (SI) base units in 2019, the unit of the second (s) sees itself in a position of significance, where now, with the exception of the mole, all other SI base units are linked to the second [1]. Currently, the second is defined by the microwave radiation at the frequency corresponding to the transition between hyperfine levels of the ground state of the cesium atom. With the first such atomic clocks produced in the 1950s, adoption of the cesium standard second has led to the development of atomic clocks which can measure the second with a relative uncertainty on the order of 1 part per 10 to the power 16. Meanwhile, clocks based on cavity-stabilized lasers probing atomic or ionic optical transition frequencies have come to surpass the accuracy in cesium atomic clocks by more than an order of magnitude [2]. The technical superiority of frequency measurement at optical transitions have effected a push towards a future redefinition of the SI second to be based on optical clocks [3]. While optical clocks have come to outperform cesium clocks, they still have not achieved their theoretical performance level set by the quantum projection noise limit. The current limiting technical noise source which diminishes their performance is the frequency fluctuations of the cavity-stabilized probe lasers [4], that is, a laser referenced to the resonant frequency of an optical cavity composed of two partially reflective mirrors held a precise distance apart. In such a system, the stability of the distance between the mirrors’ surfaces determines the frequency stability of the controlled laser. This brings us to the core subject of this research project, the development of a novel type of ultra-stable laser. For decades, ultra-stable lasers have been realized by stabilization to an optical cavity. This mature technology has approached its fundamental physical performance limit, set by thermal noise in the cavity mirrors, to a fractional frequency instability of about 1 part in 10 to the power 16 over a 1 second measurement time [5][6]. While large field cavities cooled to cryogenic temperatures have pushed this limit to several parts in 10 to the power 17, this falls short of the stability requirements of order 1 part in 10 to the power 18 needed to realize the quantum project noise limited performance of optical lattice clocks. This project explored the performance limits of a novel frequency stabilization technique via Spectral-Hole Burning (SHB). Briefly, the process of SHB involves the use of a relatively high-power pump laser to ‘burn’ or bleach a narrow transmission line at the particular wavelength(s) of the pump laser. The result is a spectral feature which is transparent to a select laser frequency in an otherwise opaque material. After pumping, we can frequency stabilize a probe laser to this spectral line. Early first experiments in SHB laser stabilization were carried out at sample temperatures of about 4 kelvin to preserve the longevity of the spectral holes. The system selected in these experiments, europium 3+ doped into an yttrium orthosilicate crystal matrix (Eu:YSO), is chosen for its desirable spectroscopic properties. These early results identified two key difficulties in achieving a performant SHB stabilized laser [7][8]: The degradation of spectral-holes under high power probing conditions limits probe beam power. This reduces available optical signal and thus accentuates detection noise and shot-noise. Temperature fluctuations in the sample crystal cryostat produced thermally induced spectral-hole line shifts translating to frequency fluctuations. In short, this project addressed these challenges by: implementing and characterizing a multi-spectral hole burning and probing scheme to reduce detection noise Installation of a dilution refrigerator for operation at sub-kelvin temperatures to reduce spectral-hole temperature sensitivity The project represented, in part, the first operation and characterization of spectral-holes at sub-kelvin temperatures, and the results will guide, not only future SHB laser stabilization research, but all sub-kelvin SHB experiments. [1] Bureau international des poids et mesures, Le Système international d’unités, 9e édition (Paris, 2019) [2] T. L. Nicholson, et al., Nature Communications 6, 6896 (2015). [3] F. Riehle, Comptes Rendus Physique 16, 506-515 (2015). [4] T. L. Nicholson, et al., Phys. Rev. Lett. 109, 230801 (2012). [5] K. Numata, et al., Phys. Rev. Lett. 93, 250602 (2004). [6] M. Notcutt, et al., Opt. Lett. 30, 1815–1817 (2005). [7] M. J. Thorpe, et al., Nature Photon. pp. 688–693 (2011). [8] N. Galland, et al., Opt. Lett. 45, 1930–1933 (2020)

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