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Materials for Quantum Technology

Measuring optical loss in yttrium orthosilicate using a whispering gallery mode resonator

Daniel S. Norman, Farhan Azeem, Jevon J. Longdell, Harald G. L. Schwefel

Published Jan 17, 2022• 7 citations
Publisher

Abstract

Abstract The fabrication of a yttrium orthosilicate (YSO) whispering gallery mode (WGM) resonator is described, displaying quality (Q)-factors as high as 1.1 × 109. An overview of the experimental setup is presented including the procedure for spatial and spectral characterisation of the WGMs. We consider the optical losses present in such a resonator and what steps were taken in order to minimise these losses. The surface scattering in such a resonator is considered and we show that in the wavelength range of 1.5 μm, the Q-factor is not limited by scattering losses for the case in which surface inhomogeneities are weakly correlated. The high Q-factors we demonstrate have implications for rare earth ion dopant based quantum technologies because YSO is an important host material for these applications.

DOI
10.1088/2633-4356/ac4c39

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arXiv:2408.08257•Aug 15, 2024•0 citations
Abstract The fabrication of a yttrium orthosilicate (YSO) whispering gallery mode (WGM) resonator is described, displaying quality (Q)-factors as high as 1.1 × 109. An overview of the experimental setup is presented including the procedure for spatial and spectral characterisation of the WGMs. We consider the optical losses present in such a resonator and what steps were taken in order to minimise these losses. The surface scattering in such a resonator is considered and we show that in the wavelength range of 1.5 μm, the Q-factor is not limited by scattering losses for the case in which surface inhomogeneities are weakly correlated. The high Q-factors we demonstrate have implications for rare earth ion dopant based quantum technologies because YSO is an important host material for these applications.
Rare-earth crystals are promising platforms for quantum information, with potentially long coherence times and telecom-compatible optical transitions. In this work, we investigate the optical properties of erbium atoms doped into a whispering gallery mode resonator produced by polishing a yttrium orthosilicate crystal. The multimode resonator demonstrates a bare quality factor of 2.56×10 5 . We demonstrate mode interaction between atoms and the resonator, where an effective quality factor of 8.7×10 7 is induced by spectral hole burning. The Fano interference between the atomic (spectral holes) and optical modes, and the small free-spectral range achieved here, can enable the study of tuning-free linear and nonlinear collective interactions.
Slow light and Whispering Gallery Mode Resonators are two interesting and useful fields with applications in optical sensing and laser stabilisation among others. Slow light can lead to long interaction times which are useful in light-matter interactions such as sensitive non-linear optics. Whispering Gallery Mode Resonators have small mode volumes and strong beam confinement leading to high Quality Factors and intense trapped light. Whispering Gallery Mode Resonators exhibit very narrow resonance linewidths which are beneficial in frequency sensitive measurements. This thesis uses a Praseodymium REI doped Yttrium orthosilicate crystal, formed into a disc shaped Whispering Gallery Mode Resonator. The aim is to design a setup which can be used to combine the two fields, slow light and Whispering Gallery Mode Resonators, in an attempt to achieve ultra-high Quality Factors and ultra-narrow resonance linewidths. The thesis details the experimental mode coupling procedure learnt from a group at Max Planck Institute for the Science of Light in Erlangen, Germany. The thesis provides a foundation for Whispering Gallery Mode Resonator experimentation in the Quantum Information group in Lund, where the thesis work was undertaken. The resulting knowledge gained is applied to design a setup holder for use in a cryostat, which provides the necessary temperatures to create slow light structures. Two beam alignment setups are constructed and used to couple into the Whispering Gallery Mode Resonator at room temperature. The first of the two setups (free beam) was difficult to operate and control over the beam alignment and focal spot shape was poor. The resulting mode coupling had a low efficiency. The second setup (fibre coupled) provided a more user friendly setup with good control over the beam alignment and focal spot shape. Results using this setup were taken for a variety of wavelengths between 593.1 nm and 610.4 nm to investigate the effect of the ions wavelength dependent absorbance, on the losses. Due to the low efficiency of the coupling in both regimes it was not possible to isolate the cause. A mismatch of focal spot size and shape is suggested as a potential area that introduces loss and therefore requires more extensive investigation. With improved beam alignment, in addition to the focal spot geometry, it should be possible to perform the absorbance measurements again in order to determine the contribution. Suggestions for further measurements including the surface roughness and mode simulations are made in an attempt to optimise the mode coupling regime to be able to accurately estimate the Quality Factor of the Whispering Gallery Mode Resonator. This is critical in order to observe changes in the Quality Factor value at cryogenic temperatures, which can be attributed to slow light effects alone.
Yttrium orthosilicate ( \nY \n2 \nSi \nO \n5 \n, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces and shows promise for coherent microwave-optical conversion owing to its favorable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on \nY \n2 \nSi \nO \n5 \n and hence we investigate here the use of \nY \n2 \nSi \nO \n5 \n as an alternative to sapphire or silicon substrates for superconducting-hybrid-device fabrication. A \nNb \nN \n resonator with frequency 6.008 GHz and low-power quality factor \nQ \n≈ \n400 \n000 \n is fabricated on a \nY \n2 \nSi \nO \n5 \n substrate doped with isotopically enriched \n145 \nNd \n. Measurements of dielectric loss yield a loss tangent \ntan \n \nδ \n= \n4 \n× \n10 \n− \n6 \n, comparable to that of sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic \n145 \nNd \n spin and the coupling strength between the resonator and the electron spins is in the high-cooperativity regime ( \nC \n= \n30 \n). These results demonstrate that \nY \n2 \nSi \nO \n5 \n is an excellent substrate for low-loss, high- \nQ \n microwave resonators, especially in applications for coupling to optically accessible rare-earth spins.
Rare-earth ions (REIs) in solid hosts have important applications in the storage and conversion of quantum information at optical and microwave frequencies. Yttrium orthosilicate is a widely used host for REIs, with excellent optical properties, but its compatibility with superconducting resonators and circuits for quantum technology is not known. This study uses Y2{}_{2}2​SiO5{}_{5}5​ directly as a substrate for superconducting devices, revealing dielectric losses comparable to those of sapphire. This enables fabrication of high-quality microwave devices coupled to optically accessible rare-earth spins, pointing to applications in quantum storage and microwave-to-optical conversion.
We perform an investigation into the properties of Pr3+:Y2SiO5 whispering gallery mode resonators as a first step towards achieving the strong coupling regime of cavity QED with rare-earth-ion doped crystals. Direct measurement of cavity QED parameters are made using photon echoes, giving good agreement with theoretical predictions. By comparing the ions at the surface of the resonator to those in the center it is determined that the physical process of making the resonator does not negatively affect the properties of the ions. Coupling between the ions and resonator is analyzed through the observation of optical bistability and normal-mode splitting.
Of the many applications of whispering-gallery mode (WGM) microresonators, Single-atom cavity-QED poses the most extreme demands on mode-volume and quality factor. Here we present a model-based procedure for the fabrication of small mode-volume ultrahigh Q silica WGM microresonators of varying geometries, from toroidal to micro-spheres. We experimentally demonstrate WGM resonators with ultra-high qualities as high as 1.7×1081.7\times 10^81.7×108 at 780nm. We present a theoretical model that allows tailoring the recipe to attain the desired geometry of the fabricated WGM microresonators.