Diffraction-limited dark laser spot produced by a hollow optical fiber

By using the diffracted field of the LP11 mode of a hollow-core optical fib er, we have produced a micrometer-sized, focused dark laser spot in the nea r field of the fiber. The minimum half-width of the dark spot is less than 1 mum. In particular, by masking the hollow core and metal coating the clad ding with a microsphere, we blocked the light propagating in the cladding a nd obtained a clean dark spot, which may be useful in atom-optical experime nts such as with atomic lenses, atom traps, and atom switches. (C) 2001 Opt ical Society of America.