Abstract

Ballistic motion of wavepackets within resonators, that are larger than the wavepacked length has been debated for more than 25 years [1]. This topic has recently been revisited because of the wide use of cavity ringdown spectroscopy (CRDS) and by the advent of femtosecond laser which produce wavepackets (≈30 µm long) smaller than the characteristic dimensions of microcavities. Extensive theoretical studies about the transient behavior of Mie scattering from spherical microcavities have also been reported [2]. 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 µm radius containing 1g/l Coumarin 510 dye) Two femtosecond laser pulses with λ₁ = 1200 nm and λ₂ = 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 λ₁ pump beam and 2 photon-excited fluorescence of the weak λ₂ probe beam. When varying the time delay between the λ₁ and λ₂ pulses, temporal coincidences within the microdroplets are observed.

© 2000 IEEE