Self-standing quasi-random-dots fork gratings for single-order diffraction

Due to the orbital angular momentum it carries, the vortex beam generated by forked gratings is an important way to investigate physics and probe matter. However, unwanted high-order diffractions inherently introduced by traditional fork gratings may be overlaid on useful first-order diffraction, leading to errors in subsequent decomposition of the spectrum. Here, we present a single optical element with a sinusoidal transfer function, termed as quasi-random-dots fork gratings, which can effectively suppress high-order diffractions. The biggest advantage of quasi-random-dots fork gratings is that it can achieve single-order diffraction with a planar structure that can only be achieved by sinusoidal fork gratings with a three-dimensional structure. Therefore, it is a much simpler process to fabricate. Both simulation and experimental results confirm that quasi-random-dots fork gratings can effectively eliminate diffractions of second order and higher orders, and only 0th and +/- 1st orders are retained. In addition, the helical phase structure of vortex beams with multiple topological charges is also demonstrated. This optical element has potential applications in the field of particle manipulation and imaging.