The Mid-infrared (MIR) wavelengths (2.5 – 25 µm) hold potential important molecular spectroscopic applications, including disease diagnosis, sensing and environmental monitoring. This is attributed to the fact that most molecules in the MIR spectral region contain fingerprint absorptions. Some of the main molecular fingerprint bands are hydrogen single bonds, carbon double bonds and carbon nitrogen bonds located at the 3 – 4 µm, 5 – 7.5 µm and 8 – 11 µm wavelength regimes, respectively. In addition, special molecular groups like aromatic, organometallic and halogenated bonds located within the longer wavelength regime of 13 – 20 µm have also attracted research attention.
As one of the main MIR laser sources, quantum cascade lasers have been widely used in the wavelength region between 13 – 20 µm. However, the relatively narrow emission bandwidth of quantum cascade lasers limits their application in MIR spectroscopy with several molecular fingerprints. Consequently, MIR nonlinear crystals have a short transmission range that prevents the extension of parametric emission long-wavelength MIR regime. Solving these challenges has been the focus of current researchers.
Supercontinuum generation (SCG) could provide low-cost broadband MIR radiation with extra merits of compact size and no seed requisition. Similarly, fabricating an on-chip nonlinear waveguide by integrating CMOS compatible materials could offer a range of benefits, such as improving the efficiency, effectiveness and the design freedom of MIR emission. SCG has been extensively studied in numerous integrated systems. Specifically, the advances in CMOS integrated technology and fabrication of high-quality silicon thin films have enabled the fabrication of MIR SCG.
Unfortunately, the tradeoff between the Si transmission and wavelength beyond 8 µm hinders the extension of SC in longer MIR wavelength. Besides, there is no MIR on-chip SCG reported above the wavelength of 13 µm, despite the existence of important molecular fingerprints in this spectral regime. Recently, high-quality cadmium tellurides (CdTe) have been identified as promising candidates for long-wavelength MIR on-chip SCG owing to their remarkable properties, such as ultra-broad spectral range.
On this account, Ms. Zhe Long, Mr. Hang Yang, Mr. Yang Li, Professor Han Wu and Professor Houkun Liang from Sichuan University developed a new on-chip SCG source based on CdTe/cadmium sulfide (CdS)/silicon heterostructure to extend the functionality of the on-chip SCG beyond 13 µm. The waveguide structure comprised CdS as the intermediate cladding layer to achieve high confinement mode and low waveguide loss. A generalized nonlinear Schrödinger equation (GNLSE) was used to simulate the SC from the CdTe hetero-waveguide. Their work is currently published in the journal, Optics Express.
The researchers showed that the CdTe/CdS/Si hetero-waveguide is an excellent platform for fabricating long-wavelength MIR SCG for various sensing and molecular spectroscopic applications. A large-core CdTe waveguide was designed to generate a flat and low dispersion in a longer spectral range while at the same time balancing the effective nonlinearity for efficient coupling. When pumped at 9 µm femtosecond laser, the waveguide achieved a MIR spectrum spanning over 3.5 – 20 µm at -34dB, which could cover the whole molecular fingerprint. The MIR waveguide achieved good coherence. Associated with the propagating CdTe waveguide was the reduction in the self-compression to 1.6 optical cycles. Furthermore, the CdTe exhibited both large second and third-order nonlinearity coefficients of 109 pm/V at 1.064 µm and 5 ×10-17 m2/W at 1.55 µm, respectively, suggesting its potential application in other CdTe-based integrated devices.
In summary, the authors reported the design of the first on-chip MIR SCG with an impressive working wavelength range extending beyond 13 µm. The proposed fabrication approach is robust, practical and based on the advanced semiconductor fabrication technology, leading to the fabrication of high-quality CdTe and CdS desired for various applications. The details of the waveguide design, including parameters, wavelengths and polarizations, were provided. In a statement to Advances in Engineering, Professor Houkun Liang, the corresponding author stated that the findings provide a breakthrough for broader ultra-broadband MIR microphotonics applications.
Long, Z., Yang, H., Li, Y., Wu, H., & Liang, H. (2022). Cadmium telluride waveguide for coherent MIR supercontinuum generation covering 3.5-20 µm. Optics Express, 30(2), 2265.