Does this type of weapon exist yet?
Yes, there are several nonlethal crowd control weapons that were developed for the military. One uses a form of microwaves to heat peoples' skin, but causes no damage. Another puts out a beam of sound that is so overwhelmingly annoying that people simply must leave.
Don't look for either of these to ever be used under the current justice-for-trayvon environment. I can hear the cries of racism echoing in the future if they were used.