*2.1. Antiresonant Hollow-Core Fiber*

The ARHCF used for the experiments was fabricated from high purity fused silica glass (Suprasil F300) using the stack and draw technique [24]. The outer diameter of the ARHCF was Da ~310 μm, and the thickness of the solid outer cladding was ts ~54 μm. The hollow core of the ARHCF with a diameter of D ~87.6 μm was defined by the fiber cladding consisting of seven non-touching, circular capillaries with a diameter of d ~54 μm. The distance between the capillaries was between 10 and 12 μm, due to irregularities during the drawing process. The ARHCF was covered with an acrylate polymer layer with a thickness of ~120 μm to minimize mechanical damage factors. Figure 1 shows a cross-section of the fiber used in the experiments (the polymer coating was removed for the fiber cutting process).

**Figure 1.** Scanning Electron Micrograph (SEM) cross-sectional image of the ARHCF: (**a**) view of the ARHCF with an air core of 87 μm and (**b**) view of the capillaries forming the fiber cladding.

According to the ARROW (Antiresonant Reflecting Optical Waveguiding) model [25], the guided wavelength range in an ARHCF is mainly defined by the thickness of the core-wall (capillary). For the fabricated fiber, the thickness of the core-wall was tc ~1 μm, which enabled low-loss transmission of light in both near- (NIR) and mid-infrared (MIR) spectral bands, in the vicinity of 1.5 and 3.34 μm, respectively [12].
