Mid‐Infrared Silicon‐on‐Lithium‐Niobate Electro‐Optic Modulators Toward Integrated Spectroscopic Sensing Systems

Abstract Mid‐infrared spectroscopy is an emerging technique in various applications such as molecule identification and label‐free chemical sensing. Integrated photonic platforms have a promising potential to perform miniaturized spectroscopic sensing with the advantage of compact footprint, low cost, and low power consumption. As an essential building block for integrated photonics, on‐chip phase shifter plays an important role in mid‐infrared spectroscopic systems, enabling signal processing and spectrum analysis. However, the implementation of effective pure phase modulation in mid‐infrared photonics remains challenging due to the unavoidable electroabsorption in the plasma dispersion effect at a long wavelength. Here, silicon photonic devices are built on lithium niobate substrates to simultaneously leverage the prominent electro‐optic effect and circumvent the absorption originating from the oxide layer. In particular, a reliable transfer printing method is presented for flexibly integrating the monocrystalline silicon waveguides with foreign substrates. In the fabricated silicon‐on‐lithium‐niobate platform, the electro‐optic performance is exploited via Mach–Zehnder interferometer for operation in the mid‐infrared regime. The modulator achieves half‐wave voltage length product of 12.3 V∙cm at the wavelength of 3.78 µm, with a maximum extinction ratio of 25.2 dB. These results indicate the potential of the proposed technology for the implementation of integrated mid‐infrared spectroscopic sensing systems.