## 1 Introduction  
Free-standing optical waveguides enable unique applications, such as sensing and inter-chip photonic interconnects (also known as photonic wire bonds). Optical sensors utilizing evanescent wave interactions with the surrounding environment are often based on dielectric or plasmonic optical waveguides [1, 2, 3]. Changes in the external medium's refractive index (from composition change [4], or thermodynamic property change [5]) alter the guided mode's propagation constant, with the accumulated phase delay being easily detected by interferometry. Surface functionalization can be applied on the core interface to the surrounding to adsorb targeted environmental elements for sensing within the evanescent wave region [6]. Chip-based evanescent waveguide sensors typically have their upper cladding exposed for interaction with the surrounding medium, with the chip platform allowing for the integration of the interferometric measurement arrangement [7, 8]. However, the drawback of this solution is that the optical mode overlap with the environment is typically small, as the mode is displaced downwards towards the lower refractive index contrast with the bottom cladding. The smaller mode overlap with the external medium reduces the propagation constant changes and hence device sensitivity. Optical microfiber-based sensors [9, 10] have the environment surrounding all sides for interaction with the propagating mode, benefitting from the low confinement factor within the core. But these fiber sensors are very fragile and not readily compatible with the interferometric measurement required for optical sensing. To address these shortcomings, a desired evanescent waveguide sensor would be photonic chip-based--for the ability to integrate interferometric circuitry, along with the robustness of the chip form factor--and conveniently coupled to an optical fiber, and yet have its entire waveguide cladding composed of the environment to be sensed. This can be achieved with complicated fabrication and etching processes to yield free-standing or nearly free-standing waveguides [11, 12, 13]. Microscale, free-standing polymer waveguides are realized using single-step digital light processing(DLP) technology [14]. Two-photon polymerization (TPP)-based direct laser writing (DLW) is a rapid prototyping technique for fabricating polymer devices, with an ability to fabricate arbitrary three-dimensional sub-micron structures, with 100 nm lateral and 300 nm axial resolutions [15]. DLW with TPP is a maskless, single-step fabrication technique which enables the realization of complicated structures that are difficult to fabricate with conventional fabrication techniques. Therefore, it is being adopted in diverse applied scientific fields, such as photonics [16; 17], micromechanics [18], and microfluidics [19]. Moreover, it can be used to write on any small substrates, which enables the fabrication of structures directly on a fiber tip. Such complex 3D structures on a fiber tip have already been realized using TPP, for example, micro lenses [20] and nano-pyramids [21]. The fabrication of air-cladded 3D polymer waveguides by DLW has been reported as a photonic wire bonding platform to interconnect nanophotonic circuits on two different chips [22], and mode field matching down tapers [23], but these free-standing waveguides have limited length/range and unknown stability. In this paper, we present the design, fabrication, and characterization of a single-mode air-cladded waveguide supported only by a narrow fin. Figure 1a-c shows the simulated mode field profiles of the proposed single-mode waveguide structures for core diameters of 1.6 \(\upmu\)m, 1.9 \(\upmu\)m, and 2.2 \(\upmu\)m. Mode field confinement inside the core increases as the core diameter of the waveguide is increased. Such fabricated waveguides can be used to realize sensors and inter-chip optical interconnects.