[INVITED] Sensing properties of micro-cavity in-line Mach-Zehnder interferometer enhanced by reactive ion etching

In this work, we discuss an application of reactive ion etching (RIE) for enhancing the sensing properties of a micro-cavity in-line Mach-Zehnder interferometer (µIMZI). The µIMZI was fabricated using femtosecond laser micromachining in a standard single-mode fiber as a circular hole with a diameter of 54 µm. Next, the structures underwent two kinds of RIE using as reactive gases: sulfur hexafluoride (SF6) and oxygen (O2) mixtures (SF6/O2) or O2 itself. When RIE with SF6/O2 was applied, it allowed for an efficient and well-controlled etching of the fabricated structure at nanometers level observed as an increase in spectral depths of the minima in the µIMZI transmission spectrum. A similar RIE process with O2 alone was ineffective. The well-defined minima obtained with the SF6/O2 RIE significantly improved the resolution of measurements made with the µIMZI. The effect was demonstrated for high-resolution refractive index (RI) measurements of liquids in the cavity. The result of the RIE process was to clean the micro-cavity bottom, increase its depth, and smooth its sidewalls. As an additional effect, the wettability of the micro-cavity surface was improved, making the RI measurements faster and more repeatable. Moreover, we demonstrated that RIE with SF6/O2 results in more stable wettability improvement than when O2 is applied as a reactive gas.