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

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 center dot cm at the wavelength of 3.78 mu m, 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.