# Holographic Silicon Metasurfaces for Total Angular Momentum Demultiplexing Applications in Telecom
## 2 Materials and Methods
### Fabrication  
For the fabrication of subwavelength gratings with high aspect ratio a three-step stamp process was considered. Electron-beam lithography (EBL) provides the ideal method to transfer the computational patterns from a digitally-stored format to a physical layer with high-resolution profiles. The original EBL pattern was transformed into an imprinting mold for subsequent imprinting replica and inductively coupled plasma--reactive ion etching (ICP-RIE) to achieve the final sample.  
Electron-beam lithography was performed with a JBX-6300 FS EBL machine (JEOL, Tokyo, Japan) 12 MHz, 5 nm resolution, working at 100 kV with a current of 100 pA. A thin layer of positive resist (AR-P 672.03, ALLRESIST GmbH, Strausberg, Germany) was spun at 4000 rpm obtaining a thickness  
Figure 4: **(a)** Numerical phase pattern for the demultiplexing of optical beams with OAM in the set {\(-9\), \(-6\), \(-3\), \(0\), \(+3\), \(+6\), \(+9\)] on a semicircular configuration. Pixel size: \(6.250\)\(\upmu\)m \(\times\)\(6.250\)\(\upmu\)m. 16 phase levels. Radius length: 180 pixels. **(b)** Far-field channel scheme for the given OAM set and circular polarization states. Right-handed (in blue) and left-handed (in red) circularly-polarized OAM beams are detected in far field on two distinct and complementary semicircles.  
around 130 nm, followed by a hot plate soft-baking process at 150 \({}^{\circ}\)C for 3 min. Afterwards, the sample was developed in an isopropyl alcohol (IPA):deionized water 7:3 solution for 60 s, in order to remove the exposed areas.  
To achieve the transfer from the EBL-patterned resist to the Silicon substrate, a 7-seconds stripping process in O\({}_{2}\) plasma was performed, followed by a 72-seconds etching in fluorine-based plasma with STS MESC MULTIPLEX ICP (SemiStar Corp, Morgan Hill, CA, USA).  
Next, a Thermal-NanoImprint Lithography (T-NIL) was performed with a Paul-Otto Weber hydraulic press with heating/cooling plates, for high-resolution replica [39; 40]. The process was conducted using the previously-etched EBL master as cast after a silanization process with Trichoro(1H,1H,2H,2H-perfluorooctyl)silane PFOTS (Thermo Fisher (Kandel) GmbH, Karlsruhe, Germany) [41; 42]. A layer of MR-I 7010E was deposited on a silicon wafer at 1750 rpm, achieving a thickness around 120 nm, followed by a 2-min soft bake at 140 \({}^{\circ}\)C. The sample was placed in contact with the master within a system of compliances in order to homogenize the temperature and pressure on the entire surface. The T-NIL process was performed at 100 \({}^{\circ}\)C for 10 min at 100-bar pressure. At the end of the imprinting step, a temperature decrease down to 35 \({}^{\circ}\)C occurred, maintaining the pressure fixed at 100 bar.  
After a 13-second O\({}_{2}\) treatment to remove the residual layer, a 10-nm Cr hard mask was deposited by \(e\)-gun evaporation and the transfer of the resist pattern was carried out by a lift-off process in a sonicated acetone bath for 180 s. Finally, an ICP-RIE etching was performed to remove the residual layer and hence obtain the required grating thickness. The etching time was finely adjusted to reach a final depth around 535 nm, as recommended by numerical simulations. In Figure 5, inspections at scanning electron microscopy (SEM) of the final sample are shown. In particular, the well-defined line profile is evidence of the suitability of the nanofabrication recipe for pattern transfer onto the silicon substrate.  
Figure 5: (**a**) SEM inspections of the fabricated PBOE on silicon substrate performing PDM and OAM-MDM according to the scheme in Figure 3 (heptagonal configuration). (**b–d**) Details at higher magnification. Grating period \(\Lambda\) = 290 nm, duty-cycle 0.5, thickness 535 nm, pixel size 6.250 \(\upmu\)m. 16 rotation angles.