Ballistic trajectories of femtosecond wavepackets circulating within microcavities

Summary form only given. Ballistic motion of wavepackets within resonators, that are larger than the wavepacket length has been debated for the last quater of the 20 th century. We present here the first ever time-resolved measurement of the ballistic motion of optical femtosecond wavepackets in the equatorial portion of a pendant ethylene glycol microdroplet (670 /spl mu/m radius containing 1 g/l Coumarin 510 dye). Two femtosecond laser pulses with /spl lambda//sub 1/=1200 nm and /spl lambda//sub 2/=600 nm, respectively, are used in a pump-probe arrangement. Simultaneous absorption of both wavelengths excites the 400 nm absorption band of the Coumarin dye. The fluorescence is used as a correlation signal. Competitive processes are 3 photon-excited fluorescence of the strong /spl lambda//sub 1/ pump beam and 2 photon-excited fluorescence of the weak /spl lambda//sub 2/ probe beam. When varying the time delay between the /spl lambda//sub 1/, and /spl lambda//sub 2/ pulses, temporal coincidences within the microdroplets are observed.