The design of a beam shaping lens with flat surfaces and ultra-thin thickness to convert a Gaussian beam to a top-hat beam

The beam shaping system to convert the Gaussian beam to top-hat beam is widely used in modem optics such as laser technologies. A general beam shaper is normally composed of a convex or concave lens in nonspherical or freeform, which is bulky in the optical path and turns to a barrier in size minorizing for a compact system. A novel beam shaper with a plane structure with flat surfaces on both bottom and top sides are provided in this paper. Taking advantages of the phase changes by the subwavelength structures and the general Fresnel principle for discrete structures, a metalens with beam shaping function is designed. The phase variation between a Gaussian beam and a top-hat beam is studied with Fourier optics and then is adopted to the layout of the beam shaping metalens. Afterwards, the finite domain time difference method is adopted to simulate the energy distribution of the modulated beam to study the effectiveness of the novel ultra-thin beam shaping metalens. Examples to convert the Gaussian beam to top-hat beam calculated with convex surface and nanopillar array with flat surfaces are illustrated in the paper to demonstrate and discuss the beam shaping results with the novel design in plane form and ultra-thin thickness. According to our study, a beam shaping lens with flat surfaces and thickness smaller than 1 um with the uniformity better than 98% can be achieved at wavelength of 790 nm. Variable beam shaping results could be obtained by the design method to figure out the phase distribution with ray optics and then design the metalens according to the desired phase modulation by arranging the subwavelength structures accordingly. Tue numerical results may pave the way for further design of metalens and offers a solution for compact systems with optical paths.