Mid-Infrared ultra-high-Q resonators based on fluoride crystalline materials

Abstract

Decades ago, the losses of glasses in the near infrared (near-IR) were investigated in views of developments for optical telecommunications. Today, properties in the mid-infrared (mid-IR) are of interest for molecular spectroscopy applications. In particular, high-sensitivity spectroscopic techniques based on high-finesse mid-IR cavities hold high promise for medical applications. Due to exceptional purity and low losses, whispering gallery mode microresonators based on polished alkaline earth metal fluoride crystals (i.e the $\mathrm{X}{\mathrm{F}}_2$ family, where X $=$ Ca, Mg, Ba, Sr,...) have attained ultra-high quality (Q) factor resonances (Q$>$10${}^8$) in the near-IR and visible spectral ranges. Here we report for the first time ultra-high Q factors in the mid-IR using crystalline microresonators. Using an uncoated chalcogenide (ChG) tapered fiber, light from a continuous wave quantum cascade laser (QCL) is efficiently coupled to several crystalline microresonators at 4.4 $\mu$m wavelength. We measure the optical Q factor of fluoride crystals in the mid-IR using cavity ringdown technique. We observe that $\mathrm{Mg}{\mathrm{F}}_2$ microresonators feature quality factors that are very close to the fundamental absorption limit, as caused by the crystal's multiphonon absorption (Q$\sim$10${}^7$), in contrast to near-IR measurements far away from these fundamental limits. Due to lower multiphonon absorption in $\mathrm{Ba}{\mathrm{F}}_2$ and $\mathrm{Sr}{\mathrm{F}}_2$, we show that ultra-high quality factors of Q $⩾$ 1.4 $\times 10^8$ can be reached at 4.4 $\mu$m. This corresponds to an optical finesse of $\mathcal{F}>$4$\cdot$ 10${}^4$, the highest value achieved for any type of mid-IR resonator to date, and a more than 10-fold improvement over the state-of-the-art. Such compact ultra-high Q crystalline microresonators provide a route for narrow linewidth frequency-stabilized QCL or mid-IR Kerr comb generation.

C. Lecaplain and C. Javerzac-Galy contributed equally to this work

References in corpus (5)

• Coherent, multi-heterodyne spectroscopy using stabilized optical frequency combs
• Quantum cascade laser frequency stabilisation at the sub-Hz level
• Intrinsic fluctuations in random lasers
• Barium fluoride whispering-gallery-mode disk-resonator with one billion quality-factor
• Fluoride microresonators for mid-IR applications

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