Figure 1: Mid-IR crystalline microresonators and uncoated ChG tapered fibre.

(a) MgF₂ crystalline microresonator with a diameter of ∼5 mm. WGMs of the microresonator are excited via evanescent coupling using a ChG, that is, ChG (As₂S₃) tapered fibre. (b) Scanning electron microscope (SEM) image of the MgF₂ protrusion. Its radius of curvature, which confines the mode in the azimuthal direction, is ∼50 μm. (c) Finite element model simulations of the optical intensity profile of the fundamental WGM at . (d) SEM image of the waist of a ChG tapered fibre with subwavelength diameter of 1.2 μm. (e) Experimental set-up composed of a WGM microresonator pumped by a QCL evanescently coupled through a ChG tapered fibre (ChG TF), followed by an oscilloscope to record the transmission. An optical isolator (ISO) protects the pump laser from Fresnel reflection (∼14%) at the cleaved fibre ends. Mid-IR free space control optics (CO), including waveplates, neutral densities and a mid-IR electro-optic modulator. L1 and L2 are lenses for free space coupling into the ChG TF. PD, photodetector. Tapered fibre and microresonator are kept under a dry and inert atmosphere to preserve from degradation and physical ageing of ChG fibre⁴³. (f) Quality factor dependence of different fluoride crystals with respect to the wavelength. For mid-IR wavelengths, multiphonon absorption competes with Rayleigh scattering and strongly impacts the Q factor. The orange shading highlights increasing multiphonon absorption contribution. The inset depicts a N-phonon creation process within the multiphonon absorption of one photon.