Mid-infrared ultra-broadband optical Kerr frequency comb based on a CdTe ring microresonator: a theoretical investigation.

Microresonator Kerr frequency combs are coherent light sources that emit broadband spectrum of evenly spaced narrow lines in an optical microresonator, which provide breakthroughs in many technological areas, such as spectroscopy, metrology, optical telecommunications, and molecular sensing. The development of mid-infrared (MIR) optical frequency comb (OFC) based on microresonators could pave the way for high performance spectroscopy in the MIR "molecular fingerprint" region. However, the generation of microresonator MIR OFC, especially towards the long-wavelength MIR (>10 µm) region, is prohibited by the transmission window of the commonly used Kerr optical media such as Si and SiN, and low nonlinearity at long wavelengths. Here, we seek the possibility to realize an ultra-broadband frequency comb operating in the long-wavelength MIR region based on a cadmium telluride (CdTe) ring microresonator. CdTe features a broad transmission range covering the wavelengths of 1∼25 µm, a flat dispersion profile, and an extraordinary third-order nonlinear refractive index (∼1.4 × 10 mW at 7 µm) which is 2-order greater than that of SiN, making it a promising platform to realize MIR Kerr frequency comb. Based on the above excellent optical properties, we design a CdTe/cadmium sulfide (CdS)/Si heterojunction microring resonator to generate an ultra-broadband MIR OFC. Through the numerical simulation, the geometric parameters (width, height, and radius) of the microresonator, polarization, wavelength of the pump, and quality factor are investigated and optimized. As a result, a MIR OFC covering 3.5∼18 µm is numerically demonstrated by using the pump wavelength of 7 µm and a pump power of 500 mW. This is the first simulation demonstration of Kerr OFC with the spectral range extending beyond 10 µm, to the best of our knowledge. This work provides new opportunities for the realization of ultrabroad microresonator frequency combs based on novel Kerr optical medium, which can find important applications ranging from calibration of astronomical spectrographs to high-fidelity molecular spectroscopy.