## 2 Materials and Methods  
We used a custom 2 PP setup for polymer film nanostructuring (Figure 1). Radiation from the source (Ti:Sa femtosecond laser, 800 nm, 80 MHz repetition rate, 50 fs pulse duration) passed through a pulse precompressor (PPC) to compensate for positive dispersion introduced by optical elements. A system of a halfwave plate \(\lambda/2\) in a rotational motorized stage, a Glan-Taylor prism (GTP) and a photodiode (PD) controlled the average power of incident radiation with the accuracy of 0.1 mW. Acoustooptical modulator (AOM) served as a fast shutter. The fast steering mirror (FSM) provided the beam waist movement within the sample plane (\(XY\)) with the accuracy of 1 nm in the field of 60 \(\upmu\)m \(\times\) 60 \(\upmu\)m and a piezoelectric stage moved the laser beam waist in a perpendicular direction (\(Z\)) with the accuracy of 5 nm in the travel range of 200 \(\upmu\)m. The laser beam was tightly focused by an air objective (100\(\times\), numerical aperture (NA) = 0.95). The sample was mounted in an upside-down position on a motorized microscopic table because the photonic crystal is not transparent for incident radiation. The experimental setup was completely automatic. The polymerisation process was visualized and controlled by a CMOS camera DCC1545M (Thorlabs Inc, Newton, New Jersey, USA). The photonic device height was controlled by the thickness of a polymer film. Polymer photoresist SU-8 2015 had been preliminarily diluted with a corresponding developer mr-Dev 600 to achieve necessary thickness by a standard spin-coating procedure (10 s 500 rpm, 30 s 2000 rpm). The polymer film was nanostructured in an overexposure regime, providing that the size of polymerisable volume (voxel) was much larger than polymer film thickness. This guaranteed the perfect rectangular cross-section of the BSW waveguides. The optimal exposition parameters were determined to be 11 mW of incident power, and 5 \(\upmu\)m/s of beam waist speed. The waveguide width was controlled by the number of scans with 100 nm accuracy. As a platform for BSW-supporting photonic devices, we have chosen a 1D photonic crystal consisting of 10 pairs of alternating layers of SiO\({}_{2}\) (thickness = 140 nm, \(n_{SiO_{2}}=1.53\) at \(\lambda\) = 670 nm) and Ta\({}_{2}\)O\({}_{5}\) (thickness = 98 nm, \(n_{Ta_{2}O_{5}}=2.086\) at \(\lambda\) = 670 nm) sputtered on a cover glass with the thickness of 170 \(\upmu\)m. To eliminate the excitation of BSW on the surface of bare photonic crystal, the topmost layer was \(Ta_{2}O_{5}\). According to numerical calculations, the height of BSW waveguides and thickness of the polymer film should be in the range of 140-280 nm to efficiently excite the BSW. Propagation of radiation in manufactured photonic devices was studied by leakage radiation microscopy (LRM). This simple technique allows for simultaneously visualizing the direct and Fourier plane, and analyse the efficiency of coupling as well as mode structure (Figure 2) [36]. A collimated beam from a diode laser source with 670 nm wavelength was focused by an air objective (NA = 0.5). The polarisation of incident and transmitted radiation were controlled by two Glan-Taylor prisms. The leakage radiation was collected by an oil immersion objective (NA = 1.3). The back focal plane (BFP) and sample plane were simultaneously projected on two CMOS cameras. The intermediate back focal plane image and direct plane image were additionally constructed to perform accurate elimination of intense transmitted radiation and parasitic reflections. The typical recorded intensity map of a BFP image is presented in Figure 2. It is expected to be a set of three concentric circles and a number of vertical stripes. The smallest circle corresponds to the transmitted radiation and its maximal radius is defined by the numerical aperture of the focusing objective (NA = 0.5). The largest  
Figure 1: Femtosecond laser polymerisation lithography experimental scheme.