Needles of light produced with a spherical mirror.

We describe how laterally confined and axially stretched needles of light can be produced by focusing a radially polarized annular optical beam with a spherical mirror. Our analysis is based on an extension of the Richards-Wolf formalism appropriate for nonaplanetic focusing systems operated under nonparaxial conditions. While maintaining their lateral confinement near the theoretical limit of 0.36λ, the needles of light that are produced can extend axially over 1000's of λ, in full compliance with geometrical and electromagnetic considerations. Relationships are established between the thickness of the incident annular beam and the length of the needle of light.