## 4 Discussion  
Here, we explain how this system could be useful for STED. Resolution in STED microscopy is given by the quality of the beams used, their properties, and the ability to detect fewer and fewer photons. There is no fundamental limit for how small the point-spread function (PSF) could be, as in the case of a confocal microscope. Two issues are addressed here. Firstly, optical aberrations could generate pump beam defocusing or determine the two beams not to be perfectly collinear. In any of the cases mentioned above, it is possible that the pump beam partially illuminates a region that is not under the influence of the depletion beam, thus producing an unintended fluorescence light that will decrease the resolution. The spot shown in Figure 5b has a much greater spatial extent, and, in this way, it will decrease the possibility of unwanted fluorescence light. The second problem is the signal-to-noise ratio. Because the fluorescence light originates from a very small volume, very few photons are actually produced; therefore, the signal is weak. Furthermore, the presence of a longitudinal electric field along the optical axis for the depletion beam may decrease the signal [21]. Variation 1B of the system, as well as any configuration intermediate between 1A and 1C, may be useful to increase the signal-to-noise ratio for probe scanning along a line. While intermediate configurations also do not present a longitudinal electric field along the optical axis, the dark region becomes elliptical. This will decrease resolution in one direction, but, at the same time, will allow more photons to be produced.  
To place our work in an even broader context, we would like to mention that the MC can be replaced with any component that has the function of a radially symmetrical waveplate with the same retardance. For example, liquid crystal radially symmetric HWPs are commercially available, and radially symmetric QWPs should be readily available if practical applications are found. Since the mode conversion efficiency to a helical beam of a radially symmetric QWP is 50%, probably these were not considered useful. We first conceptualized the idea to generate vector beams with variable polarization state when we studied the potential of uniaxial crystals to generate broadband scalar helical beams and, therefore, we designed this system thinking from this perspective. However, there should be multiple different methods to generate beams with variable polarization using different types of radially symmetric QWP. The advantages of using an MC realized from a uniaxial crystal may be the broadband character and high damage threshold. MC can be realized from various other materials like magnesium fluoride or KDP (potassium di-hydrogen phosphate) and, therefore, uniaxial crystals  
Figure 8: Intensity profile near the focal plane for variation 2 of the system, using the aspheric lens with the lower numerical aperture (**a**); intensity profile and polarization state in the focal plane (**b**).  
can be used to generate cylindrical vector beams for a broad range of wavelengths from ultraviolet (UV) to far infrared (IR).  
An optical component MC similar to those designed herein could be useful for the generation of scalar helical beams with topological charge \(\pm 2\) for various applications as long as the parameters are set to result in 100% conversion efficiency (radially symmetric HWP function).