Fig. 1. (a) Image of the MgF2 crystalline microresonator. (b) Calculated fundamental mode intensity profiles in resonators with 30° wedge angle at 4.78 μm. (c) Measured transmission spectrum (blue line) of the resonator and Lorentzian fit (red line) in the communication band. (d) Optical microscopic image of the As2S3 tapered fiber waist with a subwavelength diameter of ∼1.5  μm.

Fig. 2. (a) Integrated dispersion curve around the pump versus mode number μ. (b) Intracavity power as the pumping laser is tuned across a cavity resonance from the blue to red detuning. The inset shows the zoom-in for the dashed box. (c) Spectral envelope evolution corresponding to panel (b). (d) Temporal waveforms (orange) and optical spectra (blue) corresponding to the (i) MI, (ii) multi-soliton, and (iii) single-soliton states.

Fig. 3. (a) Temporal and (b) spectral evolution of the single soliton formation. (c) Evolution of intracavity average power plotted versus time. The inset shows the zoom-in for the dashed square.

Fig. 4. Experimental setup for the generation of MIR OFCs in the MgF2 resonator. QCL, quantum cascade laser (inset, photograph of QCL module); AL, aspheric lens; ChG TF, As2S3 tapered fiber; BS, beam splitter; PD, photodetector; OSC, oscilloscope; GM, gold mirror; FTIS, Fourier transform infrared spectrometer.

Fig. 5. MIR frequency comb spectrum with different pump-resonance detuning in the MgF2 resonators of (a) ∼2  mm diameter and (b) ∼9  mm diameter, and the pump QCL operated at 4.78 μm (black line). (c) MIR frequency comb spectrum with evenly spaced modes in MgF2 resonators with diameters of ∼2  mm (blue line) and ∼9  mm (red line).

Fig. 6. MIR frequency comb spanning from 3380 nm to 7760 nm. Spectrum generated by a ∼2  mm diameter MgF2 resonator under pump power of about (a) 200 mW, (b) 240 mW, and (c) 280 mW.