Needle-like focus generation by radially polarized halo beams emitted by photonic-crystal ring-cavity laser

Focused fields that possess a small spot size and long depth of focus (DOF) are expected to lead to the further development of optical applications. Here, we develop a photonic-crystal ring-cavity laser that emits a beam with a radially polarized halo shape (rinner/router > 0.9). This beam has a needle-like focus with a spot size of less than 0.4λ and a depth of focus longer than 10 λ for a 0.9 numerical aperture objective lens. We evaluate the focusing properties of the emitted beam and demonstrate that it has a longer depth of focus than conventional beams.