Nonlinear Materials for Mid-Infrared Integrated Photonics Applications

The emergence of silicon (Si) photonics over the past two decades has established silicon as a preferred substrate platform for photonic integration. While most Si-based photonic components have so far been realized in the near-infrared (near-IR) telecommunication bands, the mid-infrared (mid-IR, 2–20-μm wavelength) band presents a significant growth opportunity for integrated photonics. Mid-IR is a technologically important wave band that (a) encompasses multiple atmospheric windows (3 - 5 μm and 8 - 14 μm) essential for thermal imaging, infrared homing, and countermeasures and (b) covers the primary absorption bands of most chemical and biological molecules as well as the fingerprint region (7 - 20 μm), both of which are of prime interest to spectroscopic sensing. However, limited by narrow mid-IR transmission window, low optical nonlinear effect, and absence of electro-optical tunability, conventional Si-based platforms preclude various attempts in the longer wavelength range and active photonic functionalities. Materials of high mid-IR transparency and nonlinear optical properties, including aluminum nitride (AlN), lithium niobate (LN), and barium titanate (BTO), are proposed to extend conventional integrated photonic applications from visible-near-IR to mid-IR region. Integrated with AlN and BTO functional layers, three mid-IR waveguide sensors are designed, fabricated, and studied. Sharp fundamental modes are clearly observed within 2.5 - 3.8 μm. By scanning the spectrum within the characteristic absorption regime, the waveguide sensors are able to perform label-free monitoring of various organic solvents in real-time. In addition, three polarization modulators based on BTO and LN are introduced. Pockels effect of the integrated photonics in the mid-IR range is exploited for the first time. The measured highest effective electro-optical coefficient is as high as 278 pm/V, and a large modulation depth of 10 dB is achieved