Small interstellar payloads

Since colonization in the future is likely to accomplished by means of
sending a microscopic "seed" of self-reproducing machinery (the
"astro-chicken"), I was wondering if there has been much thought on
what propulsion system would be best to send such a payload on an
interstellar journey.

For instance, while antimatter seems like a ridiculously expensive
proposition in the human case, for an extremely small payload it seems
conceivable that it could be cost effective.

On the matter of laser propulsion, how tightly could we focus a laser
beam at an interstellar distance? What would be the effect of having
essentially no payload mass on such a system?