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

TL;DR

This paper presents a novel cryogenic coupling technique for high-Q optomechanical microresonators using a tapered fiber in a helium-4 cryostat, enabling efficient optical probing and thermalization at millikelvin temperatures.

Contribution

Introduction of a stable cryogenic coupling method with a cryotaper for optomechanical microresonators in a helium-4 cryostat, demonstrating effective thermalization and high optical coupling efficiency.

Findings

Mechanical mode thermalized to 1.65K

High optical coupling efficiency achieved

Stable mechanical and optical setup at cryogenic temperatures

Abstract

We developed an apparatus to couple a 50-micrometer diameter whispering-gallery silica microtoroidal resonator in a helium-4 cryostat using a straight optical tapered-fiber at 1550nm wavelength. On a top-loading probe specifically adapted for increased mechanical stability, we use a specifically-developed "cryotaper" to optically probe the cavity, allowing thus to record the calibrated mechanical spectrum of the optomechanical system at low temperatures. We then demonstrate excellent thermalization of a 63-MHz mechanical mode of a toroidal resonator down to the cryostat's base temperature of 1.65K, thereby proving the viability of the cryogenic refrigeration via heat conduction through static low-pressure exchange gas. In the context of optomechanics, we therefore provide a versatile and powerful tool with state-of-the-art performances in optical coupling efficiency, mechanical stability and cryogenic cooling.