Tail to tail connected double-funnel-shaped beam

The ability to fabricate inner structures in materials without surface damage is crucial for micro-nano processing. In this paper, we propose a class of double-funnel-shaped beam (DFSB). The narrow part of a funnel is referred to as the tail. The propagation trajectory of the DFSB resembles two funnels connected at their tails. The DFSB contains three distinct propagation stages: compression, stabilization, and expansion. During the compression stage, the size of the beam decreases with increasing propagation distance while maintaining a relatively low peak intensity. Subsequently, in the stabilization stage, the beam size remains constant over a long propagation distance and the peak intensity remains at a high level. Finally, in the expansion stage, the beam size gradually expands with increasing propagation distance and the peak intensity decreases to a lower level. We designed and experimentally generated the DFSB, verifying that its propagation characteristics are consistent with the design. Unlike the existing beams, which typically exhibit only the expansion stage or a combination of two stages, the DFSB encompasses all three stages. The controlled modulation of beam size and intensity during the stabilization stage makes the DFSB a promising tool for creating intricate structures, such as inner microchannels, photonic crystals, and microlens arrays, inside the material without compromising the surface.