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Evanescent straight tapered-fiber coupling of ultra-high Q optomechanical micro-resonators in a low-vibration helium-4 exchange-gas cryostat
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Image of FIG. 1.
FIG. 1.

Technical layout of the helium-4 cryostat describing the commercial Oxford Instrument OptistatSXM static exchange gas cryostat (to scale), the modified top-loading cryoprobe supporting the cryogenic head, the optical fiber and electrical cable layout, the optical access and the schematic vacuum circuits.

Image of FIG. 2.
FIG. 2.

Vibration spectra of the cryohead for various regimes of the cryostat. The measurements are performed with an optical interferometer fixed to the table, probing through the front access a mirror attached to the cryohead.

Image of FIG. 3.
FIG. 3.

Photograph and technical layout of the cryohead. (a) Photograph of the cryohead without the protecting plate. The thermometer is a Lakeshore DT-670B-CO Si diode. (b) 3D rendering of the cryohead showing the piezoelectric positioners (Attocube Systems ANPx101/LT/HV, “low temperature high vacuum” version) supporting the 3-part sample holder and the cryotaper placed on the mechanical slide, held by the rigid frame. The protecting plate avoids damaging shocks during the cryoprobe insertion. (c) Off-scale symbolic drawing of the coupling mechanism. The two piezoelectric positioners displace the clamped chip in the x-y plane so as to approach the desired toroid in the near-field of the mechanical-slide supported cryotaper when the cryoprobe is inserted and cooled down in the cryostat. The z-position is manually adjusted by sliding the holder prior to cooling. (d) Scanning-electron micrograph of a typical silica microtoroidal resonator.

Image of FIG. 4.
FIG. 4.

Photograph and technical layout of the cryotaper. (a) Photograph of a finalized cryotaper. (b) On-scale 3D rendering of the cryotaper showing the groove for hosting the buffer of the optical fiber (245 μm diameter) and the sanded surface for better glue adhesion. (c) Technical layout describing the usual configuration of the cryotaper. To avoid shearing of the glass fiber during manipulation, the taper is crafted to dimensions such that the acrylate buffer is glued to the glass slide with simultaneously the uncovered central part of the glass fiber being still in contact with the support to avoid having a long suspended length subjected to large amplitude vibrations.

Image of FIG. 5.
FIG. 5.

Fabrication and installation steps of the cryotaper. The inset shows a micrograph of a single-mode tapered-fiber fabricated here to transmit only a fundamental mode at 780 nm wavelength.

Image of FIG. 6.
FIG. 6.

Thermalization in the 4He cryostat. (a) Effective temperature of the mechanical mode versus the cryostat's temperature measured with the commercial Si diode. The correspondence with the guide to the eye proves the thermalization of the mode. Inset: Mechanical displacement noise (DN) spectrum taken at 1.65 K. The red line is a fit of the mechanical spectrum with the background, both separately represented by the dashed lines. The effective temperature of the mechanical mode is extracted from the mechanical trace. (b) Split optical resonance with a loaded optical Q of ∼0.8 × 108 taken at ∼2 K with calibration sidebands. (c) Amplitude spectrum of the thermal response of the toroid covered with a layer of 4He-I and of 4He-II, across the phase transition around 2.2 K and ∼50 mbar of surrounding gaseous He.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Evanescent straight tapered-fiber coupling of ultra-high Q optomechanical micro-resonators in a low-vibration helium-4 exchange-gas cryostat