Mid-infrared (mid-IR) photonics covers the wavelength range of ~2–20 µm, encompassing the molecular fingerprint region (6–20 µm) and the atmospheric transmission windows (MWIR 3–5 µm, LWIR 8–12 µm). The fundamental importance of the mid-IR is that most small molecules (CO₂, CH₄, NH₃, NO, SO₂, benzene, acetone) have strong rotational-vibrational absorption lines here — orders of magnitude stronger than in the near-IR — enabling parts-per-billion concentration detection via laser absorption spectroscopy.
The dominant sources are quantum cascade lasers (QCLs), which exploit intersubband transitions in a periodic InGaAs/AlInAs or GaAs/AlGaAs heterostructure to emit at designer wavelengths from ~3–25 µm without being bound by the material bandgap. Interband cascade lasers (ICLs) extend coverage below 4 µm with lower threshold current. Detectors are dominated by Mercury Cadmium Telluride focal-plane arrays and thermoelectrically cooled HgCdTe single elements; uncooled microbolometers serve lower-sensitivity applications. Integration is the frontier: GaSb-on-Si heterogeneous bonding and quantum-dot-based mid-IR emitters on CMOS aim to replicate the scaling dynamic of Silicon Photonics in the mid-IR.
The active thesis Mid Ir Photonic Sensing maps the industrial gas-analysis and breath-diagnostics opportunity, tracking Quantum Cascade Lasers as the enabling source and chip-level integration as the cost lever that displaces NDIR and electrochemical incumbents.
Frontier
- Can GaSb/InAs-based photonic ICs reach the integration density of silicon photonics, or does the heterogeneous-on-Si bonding approach remain the practical route to chip-scale mid-IR systems?
- What is the achievable limit-of-detection for breath-based VOC diagnostics using mid-IR absorption spectroscopy before interferent gases and humidity dominate?
- Will interband cascade lasers (ICLs) displace QCLs below 5 µm emission on power budget, or does QCL wall-plug efficiency close the gap at room temperature?