Surface Protection and Activation of Mid-IR Plasmonic Waveguides for Spectroscopy of Liquids

Liquid spectroscopy in the mid-infrared spectral range is a very powerful, yet premature technique for selective and sensitive molecule detection. Due to the lack of suitable concepts and materials for versatile miniaturized sensors, it is often still limited to bulky systems and offline analytics. Mid-infrared plasmonics is a promising field of current research for such compact and surface-sensitive structures, enabling new pathways for much-needed photonic integrated sensors. In this work, we focus on extending the concept of Ge/Au-based mid-infrared plasmonic waveguides to enable broadband liquid detection. Through the implementation of high-quality dielectric passivation layers deposited by atomic layer deposition (ALD), we cover the weak and water-soluble Ge native oxide. We show that approximately 10 nm of e.g. Al<inline-formula><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula>O<inline-formula><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> or ZrO<inline-formula><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> can already protect the plasmonic waveguides for up to 90 min of direct water exposure. This unlocks integrated sensing schemes for broadband molecule detection based on mid-infrared plasmonics. In a proof-of-concept experiment, we further demonstrate that the ZrO<inline-formula><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> coated waveguides can be activated by surface functionalization, allowing the direct measurement of diethyl ether at a wavelength of 9.38 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m.